In geology, igneous differentiation, or magmatic differentiation, is an umbrella term for the various processes by which magmas undergo bulk chemical change during the partial melting process, cooling, emplacement, or eruption. The sequence of (usually increasingly silicic) magmas produced by igneous differentiation is known as a magma series.
Definitions
Primary melts
When a rock melts to form a liquid, the liquid is known as a primary melt.
Primary melts have not undergone any differentiation and represent the
starting composition of a magma. In nature, primary melts are rarely
seen. Some leucosomes of migmatites are examples of primary melts. Primary melts derived from the mantle are especially important and are known as primitive melts or primitive magmas.
By finding the primitive magma composition of a magma series, it is
possible to model the composition of the rock from which a melt was
formed, which is important because we have little direct evidence of the
Earth's mantle.
Parental melts
Where
it is impossible to find the primitive or primary magma composition, it
is often useful to attempt to identify a parental melt. A parental melt
is a magma composition from which the observed range of magma
chemistries has been derived by the processes of igneous
differentiation. It need not be a primitive melt.
For instance, a series of basaltlava flows is assumed to be related to one another. A composition from which they could reasonably be produced by fractional crystallization is termed a parental melt.
To prove this, fractional crystallization models would be produced to
test the hypothesis that they share a common parental melt.
Fractional crystallization and accumulation of crystals formed during
the differentiation process of a magmatic event are known as cumulate rocks,
and those parts are the first which crystallize out of the magma.
Identifying whether a rock is a cumulate or not is crucial for
understanding if it can be modelled back to a primary melt or a
primitive melt, and identifying whether the magma has dropped out
cumulate minerals is equally important even for rocks which carry no phenocrysts.
Underlying causes of differentiation
The primary cause of change in the composition of a magma is cooling,
which is an inevitable consequence of the magma being formed and
migrating from the site of partial melting into an area of lower stress -
generally a cooler volume of the crust.
Cooling causes the magma to begin to crystallize minerals from
the melt or liquid portion of the magma. Most magmas are a mixture of
liquid rock (melt) and crystalline minerals (phenocrysts).
Contamination is another cause of magma differentiation. Contamination can be caused by assimilation of wall rocks, mixing of two or more magmas or even by replenishment of the magma chamber with fresh, hot magma.
The whole gamut of mechanisms for differentiation has been
referred to as the FARM process, which stands for fractional
crystallization, assimilation, replenishment and magma mixing.
Fractional crystallization of igneous rocks
Fractional crystallization
is the removal and segregation from a melt of mineral precipitates,
which changes the composition of the melt. This is one of the most
important geochemical and physical processes operating within the
Earth's crust and mantle.
Fractional crystallization in silicate melts (magmas) is a very
complex process compared to chemical systems in the laboratory because
it is affected by a wide variety of phenomena. Prime amongst these are
the composition, temperature, and pressure of a magma during its
cooling.
The composition of a magma is the primary control on which mineral is crystallized as the melt cools down past the liquidus. For instance in mafic and ultramafic melts, the MgO and SiO2 contents determine whether forsteriteolivine is precipitated or whether enstatitepyroxene is precipitated.
Two magmas of similar composition and temperature at different
pressure may crystallize different minerals. An example is high-pressure
and high-temperature fractional crystallization of granites to produce
single-feldspar granite, and low-pressure low-temperature conditions which produce two-feldspar granites.
The partial pressure of volatile phases in silicate melts is also of prime importance, especially in near-solidus crystallization of granites.
Assimilation
Assimilation
can be broadly defined as a process where a mass of magma wholly or
partially homogenizes with materials derived from the wall rock of the
magma body.[1]
Assimilation is a popular mechanism to partly explain the felsification
of ultramafic and mafic magmas as they rise through the crust: a hot
primitive melt intruding into a cooler, felsic crust will melt the crust and mix with the resulting melt.[2]
This then alters the composition of the primitive magma. Also,
pre-existing mafic host rocks can be assimilated by very hot primitive
magmas.[3][4]
Effects of assimilation on the chemistry and evolution of magma
bodies are to be expected, and have been clearly proven in many places.
In the early 20th century there was a lively discussion on the relative
importance of the process in igneous differentiation.[5][6]
More recent research has shown, however, that assimilation has a
fundamental role in altering the trace element and isotopic composition
of magmas,[7] in formation of some economically important ore deposits,[8] and in causing volcanic eruptions.[9]
Replenishment
When
a melt undergoes cooling along the liquid line of descent, the results
are limited to the production of a homogeneous solid body of intrusive
rock, with uniform mineralogy and composition, or a partially
differentiated cumulate
mass with layers, compositional zones and so on. This behaviour is
fairly predictable and easy enough to prove with geochemical
investigations. In such cases, a magma chamber will form a close
approximation of the ideal Bowen's reaction series.
However, most magmatic systems are polyphase events, with several
pulses of magmatism. In such a case, the liquid line of descent is
interrupted by the injection of a fresh batch of hot, undifferentiated
magma. This can cause extreme fractional crystallisation because of
three main effects:
Additional heat provides additional energy to allow more vigorous convection, allows resorption
of existing mineral phases back into the melt, and can cause a
higher-temperature form of a mineral or other higher-temperature
minerals to begin precipitating
Fresh magma changes the composition of the melt, changing the
chemistry of the phases which are being precipitated. For instance, plagioclase conforms to the liquid line of descent by forming initial anorthite which, if removed, changes the equilibrium mineral composition to oligoclase or albite. Replenishment of the magma can see this trend reversed, so that more anorthite is precipitated atop cumulate layers of albite.
Fresh magma destabilises minerals which are precipitating as solid solution series or on a eutectic;
a change in composition and temperature can cause extremely rapid
crystallisation of certain mineral phases which are undergoing a
eutectic crystallisation phase.
Magma mixing
Magma
mixing is the process by which two magmas meet, comingle, and form a
magma of a composition somewhere between the two end-member magmas.
Magma mixing is a common process in volcanic magma chambers, which are open-system chambers where magmas enter the chamber,[10]
undergo some form of assimilation, fractional crystallisation and
partial melt extraction (via eruption of lava), and are replenished.
Magma mixing also tends to occur at deeper levels in the crust
and is considered one of the primary mechanisms for forming intermediate
rocks such as monzonite and andesite. Here, due to heat transfer and increased volatile flux from subduction, the silicic crust melts to form a felsic magma (essentially granitic in composition). These granitic melts are known as an underplate. Basaltic
primary melts formed in the mantle beneath the crust rise and mingle
with the underplate magmas, the result being part-way between basalt and
rhyolite; literally an 'intermediate' composition.
Other mechanisms of differentiation
Interface entrapment
Convection
in a large magma chamber is subject to the interplay of forces
generated by thermal convection and the resistance offered by friction,
viscosity and drag on the magma offered by the walls of the magma
chamber. Often near the margins of a magma chamber which is convecting,
cooler and more viscous layers form concentrically from the outside in,
defined by breaks in viscosity and temperature. This forms laminar flow, which separates several domains of the magma chamber which can begin to differentiate separately.
Flow banding
is the result of a process of fractional crystallization which occurs
by convection, if the crystals which are caught in the flow-banded
margins are removed from the melt. The friction and viscosity of the magma causes phenocrysts and xenoliths
within the magma or lava to slow down near the interface and become
trapped in a viscous layer. This can change the composition of the melt
in large intrusions, leading to differentiation.
Partial melt extraction
With
reference to the definitions, above, a magma chamber will tend to cool
down and crystallize minerals according to the liquid line of descent.
When this occurs, especially in conjunction with zonation and crystal
accumulation, and the melt portion is removed, this can change the
composition of a magma chamber. In fact, this is basically fractional
crystallization, except in this case we are observing a magma chamber
which is the remnant left behind from which a daughter melt has been
extracted.
If such a magma chamber continues to cool, the minerals it forms
and its overall composition will not match a sample liquid line of
descent or a parental magma composition.
Typical behaviours of magma chambers
It
is worth reiterating that magma chambers are not usually static single
entities. The typical magma chamber is formed from a series of
injections of melt and magma, and most are also subject to some form of
partial melt extraction.
Granite
magmas are generally much more viscous than mafic magmas and are
usually more homogeneous in composition. This is generally considered to
be caused by the viscosity of the magma, which is orders of magnitude
higher than mafic magmas. The higher viscosity means that, when melted, a
granitic magma will tend to move in a larger concerted mass and be
emplaced as a larger mass because it is less fluid and able to move.
This is why granites tend to occur as large plutons, and mafic rocks as dikes and sills.
Granites are cooler and are therefore less able to melt and
assimilate country rocks. Wholesale contamination is therefore minor and
unusual, although mixing of granitic and basaltic melts is not unknown
where basalt is injected into granitic magma chambers.
Mafic magmas are more liable to flow, and are therefore more
likely to undergo periodic replenishment of a magma chamber. Because
they are more fluid, crystal precipitation occurs much more rapidly,
resulting in greater changes by fractional crystallisation. Higher
temperatures also allow mafic magmas to assimilate wall rocks more
readily and therefore contamination is more common and better developed.
Dissolved gases
All igneous magmas contain dissolved gases (water, carbonic acid, hydrogen sulfide, chlorine, fluorine, boric acid,
etc.). Of these water is the principal, and was formerly believed to
have percolated downwards from the Earth's surface to the heated rocks
below, but is now generally admitted to be an integral part of the
magma. Many peculiarities of the structure of the plutonic rocks as
contrasted with the lavas may reasonably be accounted for by the
operation of these gases, which were unable to escape as the deep-seated
masses slowly cooled, while they were promptly given up by the
superficial effusions. The acid plutonic or intrusive rocks have never
been reproduced by laboratory experiments, and the only successful
attempts to obtain their minerals artificially have been those in which
special provision was made for the retention of the "mineralizing" gases
in the crucibles or sealed tubes employed. These gases often do not
enter into the composition of the rock-forming minerals, for most of
these are free from water, carbonic acid, etc. Hence as crystallization
goes on the residual melt must contain an ever-increasing proportion of
volatile constituents. It is conceivable that in the final stages the
still uncrystallized part of the magma has more resemblance to a
solution of mineral matter in superheated steam than to a dry igneous
fusion. Quartz,
for example, is the last mineral to form in a granite. It bears much of
the stamp of the quartz which we know has been deposited from aqueous solution in veins,
etc. It is at the same time the most infusible of all the common
minerals of rocks. Its late formation shows that in this case it arose
at comparatively low temperatures and points clearly to the special
importance of the gases of the magma as determining the sequence of
crystallization.[6]
When solidification is nearly complete the gases can no longer be
retained in the rock and make their escape through fissures towards the
surface. They are powerful agents in attacking the minerals of the
rocks which they traverse, and instances of their operation are found in
the kaolinization of granites, tourmalinization and formation of greisen,
deposition of quartz veins, and the group of changes known as
propylitization. These "pneumatolytic" processes are of the first
importance in the genesis of many ore deposits. They are a real part of the history of the magma itself and constitute the terminal phases of the volcanic sequence.[6]
Quantifying igneous differentiation
There are several methods of directly measuring and quantifying igneous differentiation processes;
Whole rock geochemistry of representative samples, to track changes and evolution of the magma systems
Investigating the contamination of magma systems by wall rock assimilation using radiogenic isotopes
In all cases, the primary and most valuable method for identifying
magma differentiation processes is mapping the exposed rocks, tracking
mineralogical changes within the igneous rocks and describing field
relationships and textural evidence for magma differentiation. Clinopyroxene thermobarometry can be used to determine pressures and temperatures of magma differentiation.
Moses Coulee in the US showing multiple flood basalt flows of the Columbia River Basalt Group. The upper basalt is Roza Member, while the lower canyon exposes Frenchmen Springs Member basalt
A flood basalt (or plateau basalt) is the result of a giant volcanic eruption or series of eruptions that covers large stretches of land or the ocean floor with basaltlava. Many flood basalts have been attributed to the onset of a hotspot reaching the surface of the Earth via a mantle plume. Flood basalt provinces such as the Deccan Traps of India are often called traps, after the Swedish word trappa (meaning "staircase"), due to the characteristic stairstep geomorphology of many associated landscapes.
Flood basalts are the most voluminous of all extrusive igneous rocks, forming enormous deposits of basaltic rock found throughout the geologic record. They are a highly distinctive form of intraplate volcanism,
set apart from all other forms of volcanism by the huge volumes of lava
erupted in geologically short time intervals. A single flood basalt
province may contain hundreds of thousands of cubic kilometers of basalt
erupted over less than a million years, with individual events each
erupting hundreds of cubic kilometers of basalt. This highly fluid basalt lava can spread laterally for hundreds of kilometers from its source vents, covering areas of tens of thousands of square kilometers.
Successive eruptions form thick accumulations of nearly horizontal
flows, erupted in rapid succession over vast areas, flooding the Earth's
surface with lava on a regional scale.
These vast accumulations of flood basalt constitute large igneous provinces. These are characterized by plateau landforms, so that flood basalts are also described as plateau basalts.
Canyons cut into the flood basalts by erosion display stair-like
slopes, with the lower parts of flows forming cliffs and the upper part
of flows or interbedded layers of sediments forming slopes. These are known in Dutch as trap or in Swedish as trappa, which has come into English as trap rock, a term particularly used in the quarry industry.
The great thickness of the basalt accumulations, often in excess of 1,000 meters (3,000 ft),
usually reflects a very large number of thin flows, varying in
thickness from meters to tens of meters, or more rarely to 100 meters
(330 ft). There are occasionally very thick individual flows. The
world's thickest basalt flow may be the Greenstone flow of the Keweenaw Peninsula of Michigan, US, which is 600 meters (2,000 ft) thick. This flow may have been part of a lava lake the size of Lake Superior.
Deep erosion of flood basalts exposes vast numbers of parallel dikes that fed the eruptions. Some individual dikes in the Columbia River Plateau are over 100 kilometers (60 mi) long. In some cases, erosion exposes radial sets of dikes with diameters of several thousand kilometers. Sills may also be present beneath flood basalts, such as the Palisades Sill of New Jersey, US. The sheet intrusions (dikes and sills) beneath flood basalts are typically diabase
that closely matches the composition of the overlying flood basalts. In
some cases, the chemical signature allows individual dikes to be
connected with individual flows.
Smaller-scale features
Flood basalt commonly displays columnar jointing,
formed as the rock cooled and contracted after solidifying from the
lava. The rock fractures into columns, typically with five to six sides,
parallel to the direction of heat flow out of the rock. This is
generally perpendicular to the upper and lower surfaces, but rainwater
infiltrating the rock unevenly can produce "cold fingers" of distorted
columns. Because heat flow out of the base of the flow is slower than
from its upper surface, the columns are more regular and larger in the
bottom third of the flow. The greater hydrostatic pressure, due to the
weight of overlying rock, also contributes to making the lower columns
larger. By analogy with Greek temple architecture, the more regular
lower columns are described as the colonnade and the more irregular upper fractures as the entablature
of the individual flow. Columns tend to be larger in thicker flows,
with columns of the very thick Greenstone flow, mentioned earlier, being
around 10 meters (30 ft) thick.
Another common small-scale feature of flood basalts is pipe-stem vesicles.
Flood basalt lava cools quite slowly, so that dissolved gases in the
lava have time to come out of solution as bubbles (vesicles) that float
to the top of the flow. Most of the rest of the flow is massive and free
of vesicles. However, the more rapidly cooling lava close to the base
of the flow forms a thin chilled margin
of glassy rock, and the more rapidly crystallized rock just above the
glassy margin contains vesicles trapped as the rock was rapidly
crystallizing. These have a distinctive appearance likened to a clay tobacco pipe stem, particularly as the vesicle is usually subsequently filled with calcite or other light-colored minerals that contrast with the surrounding dark basalt.
Petrology
At still smaller scales, the texture of flood basalts is aphanitic, consisting of tiny interlocking crystals. These interlocking crystals give trap rock its tremendous toughness and durability. Crystals of plagioclase are embedded in or wrapped around crystals of pyroxene
and are randomly oriented. This indicates rapid emplacement so that the
lava is no longer flowing rapidly when it begins to crystallize. Flood basalts are almost devoid of large phenocrysts,
larger crystals present in the lava prior to its being erupted to the
surface, which are often present in other extrusive igneous rocks.
Phenocrysts are more abundant in the dikes that fed lava to the surface.
Flood basalts are most often quartztholeiites. Olivine tholeiite (the characteristic rock of mid-ocean ridges) occurs less commonly, and there are rare cases of alkali basalts. Regardless of composition, the flows are very homogeneous and rarely contain xenoliths, fragments of the surrounding rock (country rock) that have been entrained in the lava. Because the lavas are low in dissolved gases, pyroclastic rock is extremely rare. Except where the flows entered lakes and became pillow lava, the flows are massive (featureless). Occasionally, flood basalts are associated with very small volumes of dacite or rhyolite
(much more silica-rich volcanic rock), which forms late in the
development of a large igneous province and marks a shift to more
centralized volcanism.
Geochemistry
Parana traps
Flood basalts show a considerable degree of chemical uniformity across geologic time,
being mostly iron-rich tholeiitic basalts. Their major element
chemistry is similar to mid-ocean ridge basalts (MORBs), while their
trace element chemistry, particularly of the rare earth elements, resembles that of ocean island basalt. They typically have a silica content of around 52%. The magnesium number (the mol% of magnesium out of the total iron and magnesium content) is around 55, versus 60 for a typical MORB. The rare earth elements show abundance patterns suggesting that the original (primitive) magma formed from rock of the Earth's mantle that was nearly undepleted; that is, it was mantle rock rich in garnet
and from which little magma had previously been extracted. The
chemistry of plagioclase and olivine in flood basalts suggests that the
magma was only slightly contaminated with melted rock of the Earth's crust, but some high-temperature minerals had already crystallized out of the rock before it reached the surface. In other words, the flood basalt is moderately evolved. However, only small amounts of plagioclase appear to have crystallized out of the melt.
Though regarded as forming a chemically homogeneous group, flood
basalts sometimes show significant chemical diversity even with in a
single province. For example, the flood basalts of the Parana Basin
can be divided into a low phosphorus and titanium group (LPT) and a
high phosphorus and titanium group (HPT). The difference has been
attributed to inhomogeneity in the upper mantle, but strontium isotope ratios suggest the difference may arise from the LPT magma being contaminated with a greater amount of melted crust.
Formation
Plume model of flood basalt eruption
Theories of the formation of flood basalts must explain how such vast
amounts of magma could be generated and erupted as lava in such short
intervals of time. They must also explain the similar compositions and
tectonic settings of flood basalts erupted across geologic time and the
ability of flood basalt lava to travel such great distances from the
eruptive fissures before solidifying.
Generation of melt
A tremendous amount of heat is required for so much magma to be generated in so short a time. This is widely believed to have been supplied by a mantle plume impinging on the base of the Earth's lithosphere, its rigid outermost shell. The plume consists of unusually hot mantle rock of the asthenosphere, the ductile layer just below the lithosphere, that creeps upwards from deeper in the Earth's interior. The hot asthenosphere rifts
the lithosphere above the plume, allowing magma produced by
decompressional melting of the plume head to find pathways to the
surface.
The swarms of parallel dikes exposed by deep erosion of flood basalts show that considerable crustal extension
has taken place. The dike swarms of west Scotland and Iceland show
extension of up to 5%. Many flood basalts are associated with rift
valleys, are located on passive continental plate margins, or extend
into aulacogens (failed arms of triple junctions where continental rifting begins.) Flood basalts on continents are often aligned with hotspot volcanism in ocean basins. The Paraná and Etendeka traps,
located in South America and Africa on opposite sides of the Atlantic
Ocean, formed around 125 million years ago as the South Atlantic opened,
while a second set of smaller flood basalts formed near the
Triassic-Jurassic boundary in eastern North America as the North
Atlantic opened.
However, the North Atlantic flood basalts are not connected with any
hot spot traces, but seem to have been evenly distributed along the
entire divergent boundary.
Flood basalts are often interbedded with sediments, typically red beds.
The deposition of sediments begins before the first flood basalt
eruptions, so that subsidence and crustal thinning are precursors to
flood basalt activity. The surface continues to subside as basalt erupt, so that the older beds are often found below sea level. Basalt strata at depth (dipping reflectors) have been found by reflection seismology along passive continental margins.
Ascent to the surface
The
composition of flood basalts may reflect the mechanisms by which the
magma reaches the surface. The original melt formed in the upper mantle
(the primitive melt) cannot have the composition of quartz
tholeiite, the most common and typically least evolved volcanic rock of
flood basalts, because quartz tholeiites are too rich in iron relative
to magnesium to have formed in equilibrium with typical mantle rock. The
primitive melt may have had the composition of picrite basalt, but picrite basalt is uncommon in flood basalt provinces. One possibility is that a primitive melt stagnates
when it reaches the mantle-crust boundary, where it is not buoyant
enough to penetrate the lower-density crust rock. As a tholeiitic magma
differentiates (changes in composition as high-temperature minerals
crystallize and settle out of the magma) its density reaches a minimum
at a magnesium number of about 60, similar to that of flood basalts.
This restores buoyancy and permits the magma to complete its journey to
the surface, and also explains why flood basalts are predominantly
quartz tholeiites. Over half the original magma remains in the lower
crust as cumulates in a system of dikes and sills.
As the magma rises, the drop in pressure also lowers the liquidus, the temperature at which the magma is fully liquid. This likely explains the lack of phenocrysts in erupted flood basalt. The resorption
(dissolution back into the melt) of a mixture of solid olivine, augite,
and plagioclase—the high-temperature minerals likely to form as
phenocrysts—may also tend to drive the composition closer to quartz
tholeiite and help maintain buoyancy.
Eruption
Once
the magma reaches the surface, it flows rapidly across the landscape,
literally flooding the local topography. This is possible in part
because of the rapid rate of extrusion (over a cubic km per day per km
of fissure length)
and the relatively low viscosity of basaltic lava. However, the lateral
extent of individual flood basalt flows is astonishing even for so
fluid a lava in such quantities. It is likely that the lava spreads by a process of inflation in which the lava moves beneath a solid insulating crust, which keeps it hot and mobile.
Studies of the Ginkgo flow of the Columbia River Plateau, which is 30
to 70 meters (98 to 230 ft) thick, show that the temperature of the lava
dropped by just 20 °C (68 °F) over a distance of 500 kilometers
(310 mi). This demonstrates that the lava must have been insulated by a
surface crust and that the flow was laminar, reducing heat exchange with the upper crust and base of the flow. It has been estimated that the Ginkgo flow advanced 500 km in six days (a rate of advance of about 3.5 km per hour).
The lateral extent of a flood basalt flow is roughly proportional
to the cube of the thickness of the flow near its source. Thus, a flow
that is double in thickness at its source can travel roughly eight times
as far.
Flood basalt flows are predominantly pāhoehoe flows, with ʻaʻā flows much less common.
Eruption in flood basalt provinces is episodic, and each episode
has its own chemical signature. There is some tendency for lava within a
single eruptive episode to become more silica-rich with time, but there
is no consistent trend across episodes.
Large Igneous Provinces (LIPs) were originally defined as voluminous
outpourings, predominantly of basalt, over geologically very short
durations. This definition did not specify minimum size, duration,
petrogenesis, or setting. A new attempt to refine classification focuses
on size and setting. LIPs characteristically cover large areas, and the
great bulk of the magmatism occurs in less than 1 Ma. Principal LIPs in
the ocean basins include Oceanic Volcanic Plateaus (OPs) and Volcanic Passive Continental Margins. Oceanic flood basalts are LIPs distinguished from oceanic plateaus
by some investigators because they do not form morphologic plateaus,
being neither flat-topped nor elevated more than 200 m above the
seafloor. Examples include the Caribbean, Nauru, East Mariana, and
Pigafetta provinces. Continental flood basalts (CFBs) or plateau basalts
are the continental expressions of large igneous provinces.
Impact
Flood
basalts contribute significantly to the growth of continental crust.
They are also catastrophic events, which likely contributed to many mass extinctions in the geologic record.
Crust formation
The
extrusion of flood basalts, averaged over time, is comparable with the
rate of extrusion of lava at mid-ocean ridges and much higher than the
rate of extrusion by hotspots.
However, extrusion at mid-ocean ridges is relatively steady, while
extrusion of flood basalts is highly episodic. Flood basalts create new
continental crust at a rate of 0.1 to 8 cubic kilometers (0.02 to
2 cu mi) per year, while the eruptions that form oceanic plateaus
produce 2 to 20 cubic kilometers (0.5 to 5 cu mi) of crust per year.
Much of the new crust formed during flood basalt episodes takes the form of underplating, with over half the original magma crystallizing out as cumulates in sills at the base of the crust.
Mass extinctions
Siberian Traps at Red Stones Lake
The eruption of flood basalts has been linked with mass extinctions. For example, the Deccan Traps, erupted at the Cretaceous-Paleogene boundary, may have contributed to the extinction of the non-avian dinosaurs. Likewise, mass extinctions at the Permian-Triassic boundary, the Triassic-Jurassic boundary, and in the ToarcianAge of the Jurassic correspond to the ages of large igneous provinces in Siberia, the Central Atlantic Magmatic Province, and the Karoo-Ferrar flood basalt.
Some idea of the impact of flood basalts can be given by comparison with historical large eruptions. The 1783 eruption of Lakagígar
was the largest in the historical record, killing 75% of the livestock
and a quarter of the population of Iceland. However, the eruption
produced just 14 cubic kilometers (3.4 cu mi) of lava, which is tiny compared with the Roza Member of the Columbia River Plateau, erupted in the mid-Miocene, which contained at least 1,500 cubic kilometers (360 cu mi) of lava.
During the eruption of the Siberian Traps,
some 5 to 16 million cubic kilometers (1.2 to 3.8 million cubic miles)
of magma penetrated the crust, covering an area of 5 million square
kilometres (1.9 million square miles), equal to 62% of the area of the
contiguous states of the United States. The hot magma contained vast
quantities of carbon dioxide and sulfur oxides, and released additional carbon dioxide and methane from deep petroleum reservoirs and younger coal beds in the region. The released gases created over 6400 diatreme-like pipes,
each typically over 1.6 kilometres (1 mi) in diameter. The pipes
emitted up to 160 trillion tons of carbon dioxide and 46 trillion tons
of methane. Coal ash from burning coal beds spread toxic chromium, arsenic, mercury, and lead across northern Canada. Evaporite beds heated by the magma released hydrochloric acid, methyl chloride, methyl bromide, which damaged the ozone layer and reduced ultraviolet shielding by as much as 85%. Over 5 trillion tons of sulfur dioxide
was also released. The carbon dioxide produced extreme greenhouse
conditions, with global average sea water temperatures peaking at 38 °C
(100 °F), the highest ever seen in the geologic record. Temperatures did
not drop to 32 °C (90 °F) for another 5.1 million years. Temperatures
this high are lethal to most marine organisms, and land plants have
difficulty continuing to photosynthesize at temperatures above 35 °C
(95 °F). The Earth's equatorial zone became a dead zone.
However, not all large igneous provinces are connected with extinction events.
The formation and effects of a flood basalt depend on a range of
factors, such as continental configuration, latitude, volume, rate,
duration of eruption, style and setting (continental vs. oceanic), the
preexisting climate, and the biota resilience to change.
Flood basalts are the dominant form of magmatism on the other planets and moons of the Solar System.
The maria on the Moon have been described as flood basalts composed of picritic basalt.
Individual eruptive episodes were likely similar in volume to flood
basalts of Earth, but were separated by much longer quiescent intervals
and were likely produced by different mechanisms.
Extensive flood basalts may be present on Mars.
Uses
The interlocking crystals of flood basalts, which are oriented at random, make trap rock the most durable construction aggregate of all rock types.
Intraplate volcanism is volcanism that takes place away from the margins of tectonic plates.
Most volcanic activity takes place on plate margins, and there is broad
consensus among geologists that this activity is explained well by the
theory of plate tectonics. However, the origins of volcanic activity within plates remains controversial.
Mechanisms
Mechanisms
that have been proposed to explain intraplate volcanism include mantle
plumes; non-rigid motion within tectonic plates (the plate model); and impact events. It is likely that different mechanisms accounts for different cases of intraplate volcanism.
The hypothesis
of mantle plumes has required progressive hypothesis-elaboration
leading to variant propositions such as mini-plumes and pulsing plumes.
Concepts
Mantle plumes were first proposed by J. Tuzo Wilson in 1963 and further developed by W. Jason Morgan in 1971. A mantle plume is posited to exist where hot rock nucleates at the core-mantle boundary and rises through the Earth's mantle becoming a diapir in the Earth's crust. In particular, the concept that mantle plumes are fixed relative to one
another, and anchored at the core-mantle boundary, would provide a
natural explanation for the time-progressive chains of older volcanoes
seen extending out from some such hot spots, such as the Hawaiian–Emperor seamount chain. However, paleomagnetic data show that mantle plumes can be associated with Large Low Shear Velocity Provinces (LLSVPs) and do move.
Two largely independent convective processes are proposed:
the broad convective flow associated with plate tectonics, driven primarily by the sinking of cold plates of lithosphere back into the mantle asthenosphere
the mantle plume, driven by heat exchange across the core-mantle
boundary carrying heat upward in a narrow, rising column, and postulated
to be independent of plate motions.
The plume hypothesis was studied using laboratory experiments conducted in small fluid-filled tanks in the early 1970s. Thermal or compositional fluid-dynamical plumes produced in that way were presented as models
for the much larger postulated mantle plumes. Based on these
experiments, mantle plumes are now postulated to comprise two parts: a
long thin conduit connecting the top of the plume to its base, and a
bulbous head that expands in size as the plume rises. The entire
structure is considered to resemble a mushroom. The bulbous head of
thermal plumes forms because hot material moves upward through the
conduit faster than the plume itself rises through its surroundings. In
the late 1980s and early 1990s, experiments with thermal models showed
that as the bulbous head expands it may entrain some of the adjacent
mantle into the head.
The sizes and occurrence of mushroom mantle plumes can be
predicted easily by transient instability theory developed by Tan and
Thorpe. The theory predicts mushroom shaped mantle plumes with heads of about 2000 km diameter that have a critical time of about 830 Myr for a core mantle heat flux of 20 mW/m2, while the cycle time is about 2 Gyr. The number of mantle plumes is predicted to be about 17.
When a plume head encounters the base of the lithosphere, it is
expected to flatten out against this barrier and to undergo widespread
decompression melting to form large volumes of basalt magma. It may then
erupt onto the surface. Numerical modelling predicts that melting and
eruption will take place over several million years. These eruptions have been linked to flood basalts, although many of those erupt over much shorter time scales (less than 1 million years). Examples include the Deccan traps in India, the Siberian traps of Asia, the Karoo-Ferrar basalts/dolerites in South Africa and Antarctica, the Paraná and Etendeka traps in South America and Africa (formerly a single province separated by opening of the South Atlantic Ocean), and the Columbia River basalts of North America. Flood basalts in the oceans are known as oceanic plateaus, and include the Ontong Java plateau of the western Pacific Ocean and the Kerguelen Plateau of the Indian Ocean.
The narrow vertical pipe, or conduit, postulated to connect the
plume head to the core-mantle boundary, is viewed as providing a
continuous supply of magma to a fixed location, often referred to as a
"hotspot". As the overlying tectonic plate (lithosphere) moves over this
hotspot, the eruption of magma from the fixed conduit onto the surface
is expected to form a chain of volcanoes that parallels plate motion. The Hawaiian Islands
chain in the Pacific Ocean is the type example. It has recently been
discovered that the volcanic locus of this chain has not been fixed over
time, and it thus joined the club of the many type examples that do not
exhibit the key characteristic originally proposed.
The eruption of continental flood basalts is often associated with continental rifting
and breakup. This has led to the hypothesis that mantle plumes
contribute to continental rifting and the formation of ocean basins. In
the context of the alternative "Plate model", continental breakup is a
process integral to plate tectonics, and massive volcanism occurs as a
natural consequence when it starts.
The current mantle plume theory is that material and energy from
Earth's interior are exchanged with the surface crust in two distinct
modes: the predominant, steady state plate tectonic regime driven by
upper mantle convection, and a punctuated, intermittently dominant, mantle overturn regime driven by plume convection. This second regime, while often discontinuous, is periodically significant in mountain building and continental breakup.
Chemistry, heat flow and melting
Hydrodynamic simulation of a single "finger" of the Rayleigh–Taylor instability, a possible mechanism for plume formation.
In the third and fourth frame in the sequence, the plume forms a
"mushroom cap". Note that the core is at the top of the diagram and the
crust is at the bottom.
Earth cross-section showing location of upper (3) and lower (5) mantle, D″-layer (6), and outer (7) and inner (9) core
The chemical and isotopic composition of basalts found at hotspots differs subtly from mid-ocean-ridge basalts.
These basalts, also called ocean island basalts (OIBs), are analysed in
their radiogenic and stable isotope compositions. In radiogenic isotope
systems the originally subducted material creates diverging trends,
termed mantle components.
Identified mantle components are DMM (depleted mid-ocean ridge basalt
(MORB) mantle), HIMU (high U/Pb-ratio mantle), EM1 (enriched mantle 1),
EM2 (enriched mantle 2) and FOZO (focus zone). This geochemical signature arises from the mixing of near-surface materials such as subducted slabs
and continental sediments, in the mantle source. There are two
competing interpretations for this. In the context of mantle plumes, the
near-surface material is postulated to have been transported down to
the core-mantle boundary by subducting slabs, and to have been
transported back up to the surface by plumes. In the context of the
Plate hypothesis, subducted material is mostly re-circulated in the
shallow mantle and tapped from there by volcanoes.
Stable isotopes like Fe are used to track processes that the uprising material experiences during melting.
The processing of oceanic crust, lithosphere, and sediment
through a subduction zone decouples the water-soluble trace elements
(e.g., K, Rb, Th) from the immobile trace elements (e.g., Ti, Nb, Ta),
concentrating the immobile elements in the oceanic slab (the
water-soluble elements are added to the crust in island arc volcanoes). Seismic tomography shows that subducted oceanic slabs sink as far as the bottom of the mantle transition zone
at 650 km depth. Subduction to greater depths is less certain, but
there is evidence that they may sink to mid-lower-mantle depths at about
1,500 km depth.
The source of mantle plumes is postulated to be the core-mantle boundary at 3,000 km depth.
Because there is little material transport across the core-mantle
boundary, heat transfer must occur by conduction, with adiabatic
gradients above and below this boundary. The core-mantle boundary is a
strong thermal (temperature) discontinuity. The temperature of the core
is approximately 1,000 degrees Celsius higher than that of the overlying
mantle. Plumes are postulated to rise as the base of the mantle becomes
hotter and more buoyant.
Plumes are postulated to rise through the mantle and begin to partially melt on reaching shallow depths in the asthenosphere by decompression melting.
This would create large volumes of magma. The plume hypothesis
postulates that this melt rises to the surface and erupts to form "hot
spots".
The lower mantle and the core
Calculated Earth's temperature vs. depth. Dashed curve: Layered mantle convection; Solid curve: Whole mantle convection.
The most prominent thermal contrast known to exist in the deep
(1000 km) mantle is at the core-mantle boundary at 2900 km. Mantle
plumes were originally postulated to rise from this layer because the
"hot spots" that are assumed to be their surface expression were thought
to be fixed relative to one another. This required that plumes were
sourced from beneath the shallow asthenosphere that is thought to be
flowing rapidly in response to motion of the overlying tectonic plates.
There is no other known major thermal boundary layer in the deep Earth,
and so the core-mantle boundary was the only candidate.
The base of the mantle is known as the D″ layer,
a seismological subdivision of the Earth. It appears to be
compositionally distinct from the overlying mantle, and may contain
partial melt.
Two very broad, large low-shear-velocity provinces, exist in the lower mantle under Africa and under the central Pacific. It is postulated that plumes rise from their surface or their edges.
Their low seismic velocities were thought to suggest that they are
relatively hot, although it has recently been shown that their low wave
velocities are due to high density caused by chemical heterogeneity.
Evidence for the theory
Various
lines of evidence have been cited in support of mantle plumes. There is
some confusion regarding what constitutes support, as there has been a
tendency to re-define the postulated characteristics of mantle plumes
after observations have been made.
Some common and basic lines of evidence cited in support of the theory are linear volcanic chains, noble gases, geophysical anomalies, and geochemistry.
An intrinsic aspect of the plume hypothesis is that the "hot
spots" and their volcanic trails have been fixed relative to one another
throughout geological time. Whereas there is evidence that the chains
listed above are time-progressive, it has, however, been shown that they
are not fixed relative to one another. The most remarkable example of
this is the Emperor chain, the older part of the Hawaii system, which
was formed by migration of volcanic activity across a geo-stationary
plate.
Many postulated "hot spots" are also lacking time-progressive
volcanic trails, e.g., Iceland, the Galapagos, and the Azores.
Mismatches between the predictions of the hypothesis and observations
are commonly explained by auxiliary processes such as "mantle wind",
"ridge capture", "ridge escape" and lateral flow of plume material.
Helium-3 is a primordial isotope that formed in the Big Bang. Very little is produced, and little has been added to the Earth by other processes since then. Helium-4 includes a primordial component, but it is also produced by the natural radioactive decay of elements such as uranium and thorium.
Over time, helium in the upper atmosphere is lost into space. Thus, the
Earth has become progressively depleted in helium, and 3He is not replaced as 4He is. As a result, the ratio 3He/4He in the Earth has decreased over time.
Unusually high 3He/4He have been observed
in some, but not all, "hot spots". In mantle plume theory, this is
explained by plumes tapping a deep, primordial reservoir in the lower
mantle, where the original, high 3He/4He ratios have been preserved throughout geologic time.
In the context of the Plate hypothesis, the high ratios are explained
by preservation of old material in the shallow mantle. Ancient, high 3He/4He ratios would be particularly easily preserved in materials lacking U or Th, so 4He was not added over time. Olivine and dunite, both found in subducted crust, are materials of this sort.
Other elements, e.g. osmium,
have been suggested to be tracers of material arising from near to the
Earth's core, in basalts at oceanic islands. However, so far conclusive
proof for this is lacking.
Geophysical anomalies
Diagram showing a cross section though the Earth's lithosphere (in yellow) with magma rising from the mantle (in red). The crust may move relative to the plume, creating a track.
The plume hypothesis has been tested by looking for the geophysical
anomalies predicted to be associated with them. These include thermal,
seismic, and elevation anomalies. Thermal anomalies are inherent in the
term "hotspot". They can be measured in numerous different ways,
including surface heat flow, petrology, and seismology. Thermal
anomalies produce anomalies in the speeds of seismic waves, but
unfortunately so do composition and partial melt. As a result, wave
speeds cannot be used simply and directly to measure temperature, but
more sophisticated approaches must be taken.
Seismic anomalies are identified by mapping variations in wave
speed as seismic waves travel through Earth. A hot mantle plume is
predicted to have lower seismic wave speeds compared with similar
material at a lower temperature. Mantle material containing a trace of
partial melt (e.g., as a result of it having a lower melting point), or
being richer in Fe, also has a lower seismic wave speed and those
effects are stronger than temperature. Thus, although unusually low wave
speeds have been taken to indicate anomalously hot mantle beneath "hot
spots", this interpretation is ambiguous.
The most commonly cited seismic wave-speed images that are used to look
for variations in regions where plumes have been proposed come from
seismic tomography. This method involves using a network of seismometers
to construct three-dimensional images of the variation in seismic wave
speed throughout the mantle.
Seismic waves
generated by large earthquakes enable structure below the Earth's
surface to be determined along the ray path. Seismic waves that have
traveled a thousand or more kilometers (also called teleseismic waves)
can be used to image large regions of Earth's mantle. They also have
limited resolution, however, and only structures at least several
hundred kilometers in diameter can be detected.
Seismic tomography images have been cited as evidence for a number of mantle plumes in Earth's mantle.[33]
There is, however, vigorous on-going discussion regarding whether the
structures imaged are reliably resolved, and whether they correspond to
columns of hot, rising rock.
The mantle plume hypothesis predicts that domal topographic
uplifts will develop when plume heads impinge on the base of the
lithosphere. An uplift of this kind occurred when the north Atlantic
Ocean opened about 54 million years ago. Some scientists have linked
this to a mantle plume postulated to have caused the breakup of Eurasia
and the opening of the north Atlantic, now suggested to underlie Iceland.
Current research has shown that the time-history of the uplift is
probably much shorter than predicted, however. It is thus not clear how
strongly this observation supports the mantle plume hypothesis.
Geochemistry
Basalts found at oceanic islands are geochemically distinct from those found at mid-ocean ridges and volcanoes associated with subduction zones (island arc basalts). "Ocean island basalt"
is also similar to basalts found throughout the oceans on both small
and large seamounts (thought to be formed by eruptions on the sea floor
that did not rise above the surface of the ocean). They are also
compositionally similar to some basalts found in the interiors of the
continents (e.g., the Snake River Plain).
In major elements, ocean island basalts are typically higher in iron (Fe) and titanium (Ti) than mid-ocean ridge basalts at similar magnesium (Mg) contents. In trace elements, they are typically more enriched in the light rare-earth elements than mid-ocean ridge basalts. Compared to island arc basalts, ocean island basalts are lower in alumina (Al2O3) and higher in immobile trace elements (e.g., Ti, Nb, Ta).
These differences result from processes that occur during the subduction of oceanic crust and mantle lithosphere.
Oceanic crust (and to a lesser extent, the underlying mantle) typically
becomes hydrated to varying degrees on the seafloor, partly as the
result of seafloor weathering, and partly in response to hydrothermal
circulation near the mid-ocean-ridge crest where it was originally
formed. As oceanic crust and underlying lithosphere subduct, water is
released by dehydration reactions, along with water-soluble elements and
trace elements. This enriched fluid rises to metasomatize
the overlying mantle wedge and leads to the formation of island arc
basalts. The subducting slab is depleted in these water-mobile elements
(e.g., K, Rb, Th, Pb)
and thus relatively enriched in elements that are not water-mobile
(e.g., Ti, Nb, Ta) compared to both mid-ocean ridge and island arc
basalts.
Ocean island basalts are also relatively enriched in immobile
elements relative to the water-mobile elements. This, and other
observations, have been interpreted as indicating that the distinct
geochemical signature of ocean island basalts results from inclusion of a
component of subducted slab material. This must have been recycled in
the mantle, then re-melted and incorporated in the lavas erupted. In the
context of the plume hypothesis, subducted slabs are postulated to have
been subducted down as far as the core-mantle boundary, and transported
back up to the surface in rising plumes. In the plate hypothesis, the
slabs are postulated to have been recycled at shallower depths – in the
upper few hundred kilometers that make up the upper mantle.
However, the plate hypothesis is inconsistent with both the
geochemistry of shallow asthenosphere melts (i.e., Mid-ocean ridge
basalts) and with the isotopic compositions of ocean island basalts.
Seismology
In 2015, based on data from 273 large earthquakes, researchers compiled a model based on full waveform tomography, requiring the equivalent of 3 million hours of supercomputer time.
Due to computational limitations, high-frequency data still could not
be used, and seismic data remained unavailable from much of the
seafloor. Nonetheless, vertical plumes, 400 C hotter than the surrounding rock, were visualized under many hotspots, including the Pitcairn, Macdonald, Samoa, Tahiti, Marquesas, Galapagos, Cape Verde, and Canary hotspots.
They extended nearly vertically from the core-mantle boundary (2900 km
depth) to a possible layer of shearing and bending at 1000 km. They were detectable because they were 600–800 km wide, more than three times the width expected from contemporary models. Many of these plumes are in the large low-shear-velocity provinces
under Africa and the Pacific, while some other hotspots such as
Yellowstone were less clearly related to mantle features in the model.
The unexpected size of the plumes leaves open the possibility
that they may conduct the bulk of the Earth's 44 terawatts of internal
heat flow from the core to the surface, and means that the lower mantle
convects less than expected, if at all. It is possible that there is a
compositional difference between plumes and the surrounding mantle that
slows them down and broadens them.
Suggested mantle plume locations
An example of plume locations suggested by one recent group. Figure from Foulger (2010).
Many different localities have been suggested to be underlain by
mantle plumes, and scientists cannot agree on a definitive list. Some
scientists suggest that several tens of plumes exist, whereas others suggest that there are none.
The theory was really inspired by the Hawaiian volcano system. Hawaii
is a large volcanic edifice in the center of the Pacific Ocean, far from
any plate boundaries. Its regular, time-progressive chain of islands
and seamounts superficially fits the plume theory well. However, it is
almost unique on Earth, as nothing as extreme exists anywhere else. The
second strongest candidate for a plume location is often quoted to be
Iceland, but according to opponents of the plume hypothesis its massive
nature can be explained by plate tectonic forces along the mid-Atlantic
spreading center.
Mantle plumes have been suggested as the source for flood basalts.
These extremely rapid, large scale eruptions of basaltic magmas have
periodically formed continental flood basalt provinces on land and
oceanic plateaus in the ocean basins, such as the Deccan Traps, the Siberian Traps, the Karoo-Ferrar flood basalts of Gondwana, and the largest known continental flood basalt, the Central Atlantic magmatic province (CAMP).
Many continental flood basalt events coincide with continental rifting.
This is consistent with a system that tends toward equilibrium: as
matter rises in a mantle plume, other material is drawn down into the
mantle, causing rifting.
Plate theory
The hypothesis
of mantle plumes from depth is not universally accepted as explaining
all such volcanism. It has required progressive hypothesis-elaboration
leading to variant propositions such as mini-plumes and pulsing plumes.
Another hypothesis for unusual volcanic regions is the plate theory. This proposes shallower, passive leakage of magma
from the mantle onto the Earth's surface where extension of the
lithosphere permits it, attributing most volcanism to plate tectonic
processes, with volcanoes far from plate boundaries resulting from
intraplate extension.
Schematic
of the plate theory. Mid-blue: lithosphere; light-blue/green:
inhomogeneous upper mantle; yellow: lower mantle; orange/red:
core-mantle boundary. Lithospheric extension enables pre-existing melt
(red) to rise.
The plate theory attributes all volcanic activity on Earth, even that which appears superficially to be anomalous, to the operation of plate tectonics. According to the plate theory, the principal cause of volcanism is extension of the lithosphere. Extension of the lithosphere is a function of the lithospheric stress field.
The global distribution of volcanic activity at a given time reflects
the contemporaneous lithospheric stress field, and changes in the
spatial and temporal distribution of volcanoes reflect changes in the
stress field. The main factors governing the evolution of the stress
field are:
An illustration of competing models of crustal recycling
and the fate of subducted slabs. The plume hypothesis invokes deep
subduction (right), while the plate hypothesis focuses on shallow
subduction (left).
Beginning in the early 2000s, dissatisfaction with the state of the evidence for mantle plumes and the proliferation of ad hoc hypotheses drove a number of geologists, led by Don L. Anderson, Gillian Foulger, and Warren B. Hamilton,
to propose a broad alternative based on shallow processes in the upper
mantle and above, with an emphasis on plate tectonics as the driving
force of magmatism.
The plate hypothesis
suggests that "anomalous" volcanism results from lithospheric extension
that permits melt to rise passively from the asthenosphere beneath. It
is thus the conceptual inverse of the plume hypothesis because the plate
hypothesis attributes volcanism to shallow, near-surface processes
associated with plate tectonics, rather than active processes arising at
the core-mantle boundary.
Lithospheric extension is attributed to processes related to
plate tectonics. These processes are well understood at mid-ocean
ridges, where most of Earth's volcanism occurs. It is less commonly
recognised that the plates themselves deform internally, and can permit
volcanism in those regions where the deformation is extensional.
Well-known examples are the Basin and Range Province in the western USA,
the East African Rift valley, and the Rhine Graben.
Under this hypothesis, variable volumes of magma are attributed to
variations in chemical composition (large volumes of volcanism
corresponding to more easily molten mantle material) rather than to
temperature differences.
While not denying the presence of deep mantle convection and
upwelling in general, the plate hypothesis holds that these processes do
not result in mantle plumes, in the sense of columnar vertical features
that span most of the Earth's mantle, transport large amounts of heat,
and contribute to surface volcanism.
Under the umbrella of the plate hypothesis, the following
sub-processes, all of which can contribute to permitting surface
volcanism, are recognised:
Continental break-up;
Fertility at mid-ocean ridges;
Enhanced volcanism at plate boundary junctions;
Small-scale sublithospheric convection;
Oceanic intraplate extension;
Slab tearing and break-off;
Shallow mantle convection;
Abrupt lateral changes in stress at structural discontinuities;
Continental intraplate extension;
Catastrophic lithospheric thinning;
Sublithospheric melt ponding and draining.
Lithospheric extension enables pre-existing melt in the crust and mantle to escape to the surface. If extension is severe and thins the lithosphere to the extent that the asthenosphere rises, then additional melt is produced by decompression upwelling.
A major virtue of the plate theory is that it extends plate
tectonics into a unifying account of the Earth's volcanism which
dispenses with the need to invoke extraneous hypotheses designed to
accommodate instances of volcanic activity which appear superficially to
be exceptional.
Origins of the plate theory
Developed
during the late 1960s and 1970s, plate tectonics provided an elegant
explanation for most of the Earth's volcanic activity. At spreading
boundaries where plates move apart, the asthenosphere decompresses and
melts to form new oceanic crust. At subduction zones, slabs of oceanic crust sink into the mantle, dehydrate, and release volatiles which lower the melting temperature and give rise to volcanic arcs and back-arc
extensions. Several volcanic provinces, however, do not fit this simple
picture and have traditionally been considered exceptional cases which
require a non-plate-tectonic explanation.
Just prior to the development of plate tectonics in the early 1960s, the Canadian GeophysicistJohn Tuzo Wilson suggested that chains of volcanic islands form from movement of the seafloor over relatively stationary hotspots in stable centres of mantle convection cells. In the early 1970s, Wilson's idea was revived by the American geophysicist W. Jason Morgan.
In order to account for the long-lived supply of magma that some
volcanic regions seemed to require, Morgan modified the hypothesis,
shifting the source to a thermal boundary layer.
Because of the perceived fixity of some volcanic sources relative to
the plates, he proposed that this thermal boundary was deeper than the
convecting upper mantle on which the plates ride and located it at the core-mantle boundary,
3,000 km beneath the surface. He suggested that narrow convection
currents rise from fixed points at this thermal boundary and form
conduits which transport abnormally hot material to the surface.
This, the mantle plume theory, became the dominant explanation for apparent volcanic anomalies for the remainder of the 20th century. Testing the hypothesis, however, is beset with difficulties. A central
tenet of the plume theory is that the source of melt is significantly
hotter than the surrounding mantle, so the most direct test is to
measure the source temperature of magmas. This is difficult as the petrogenesis of magmas is extremely complex, rendering inferences from petrology or geochemistry to source temperatures unreliable. Seismic data used to provide additional constraints on source temperatures are highly ambiguous.
In addition to this, several predictions of the plume theory have
proved unsuccessful at many locations purported to be underlain by
mantle plumes, and there are also significant theoretical reasons to doubt the hypothesis.
The foregoing issues have inspired a growing number of geoscientists, led by American geophysicist Don L. Anderson and British geophysicist Gillian R. Foulger,
to pursue other explanations for volcanic activity not easily accounted
for by plate tectonics. Rather than introducing another extraneous
theory, these explanations essentially expand the scope of plate
tectonics in ways that can accommodate volcanic activity previously
thought to be outside its remit. The key modification to the basic
plate-tectonic model here is a relaxation of the assumption that plates
are rigid. This implies that lithospheric extension occurs not only at
spreading plate boundaries but throughout plate interiors, a phenomenon
that is well supported both theoretically and empirically.
Over the last two decades, the plate theory has developed into a
cohesive research programme, attracting many adherents, and occupying
researchers in several subdisciplines of Earth science. It has also been the focus of several international conferences and many peer-reviewed papers and is the subject of two major Geological Society of America edited volumes and a textbook.
Since 2003, discussion and development of the plate theory has been fostered by the Durham University(UK)-hosted website mantleplumes.org, a major international forum with contributions from geoscientists working in a wide variety of specialties.
Lithospheric extension
Global-scale
lithospheric extension is a necessary consequence of the non-closure of
plate motion circuits and is equivalent to an additional slow-spreading
boundary. Extension results principally from the following three
processes.
Changes in the configuration of plate boundaries. These can
result from various processes including the formation or annihilation of
plates and boundaries and slab rollback (vertical sinking of subducting
slabs causing oceanward migration of trenches).
Thermal contraction, which sums to the largest amount across large plates such as the Pacific.
Extension resulting from these processes manifests in a variety of structures including continental rift zones (e.g., the East African Rift), diffuse oceanic plate boundaries (e.g., Iceland), continental back-arc extensional regions (e.g., the Basin and Range Province in the Western United States), oceanic back-arc basins (e.g., the Manus Basin in the Bismarck Sea off Papua New Guinea), fore-arc regions (e.g., the western Pacific), and continental regions undergoing lithospheric delamination (e.g., New Zealand).
Continental breakup begins with rifting. When extension is
persistent and entirely compensated by magma from asthenospheric
upwelling, oceanic crust is formed, and the rift becomes a spreading
plate boundary. If extension is isolated and ephemeral it is classified
as intraplate. Rifting can occur in both oceanic and continental crust
and ranges from minor to amounts approaching those seen at spreading
boundaries. All can give rise to magmatism.
Various extensional styles are seen in the northeast Atlantic. Continental rifting began in the late Paleozoic and was followed by catastrophic destabilisation in the late Cretaceous and early Paleocene.
The latter was possibly caused by rollback of the Alpine slab, which
generated extension throughout Europe. More severe rifting occurred
along the Caledonian Suture, a zone of pre-existing weakness where the Iapetus Ocean closed around 420 Ma. As extension became localised, oceanic crust began to form around 54 Ma, with diffuse extension persisting around Iceland.
Some intracontinental rifts are essentially failed continental
breakup axes, and some of these form triple junctions with plate
boundaries. The East African Rift, for example, forms a triple junction
with the Red Sea and the Gulf of Aden,
both of which have progressed to the seafloor spreading stage.
Likewise, the Mid-American Rift constitutes two arms of a triple
junction along with a third which separated the Amazonian Craton from Laurentia around 1.1 Ga.
Diverse volcanic activity resulting from lithospheric extension has occurred throughout the western United States. The Cascade Volcanoes are a back-arc volcanic chain extending from British Columbia to Northern California. Back-arc extension continues to the east in the Basin and Range Province, with small-scale volcanism distributed throughout the region.
The Pacific Plate is the largest tectonic plate on Earth, covering about one third of Earth's surface. It undergoes considerable extension and shear deformation due to thermal contraction of the lithosphere. Shear deformation is greatest in the area between Samoa and the Easter Microplate, an area replete with volcanic provinces such as the Cook-Austral chain, the Marquesas and Society Islands, the Tuamotu Archipelago, the Fuca and Pukapuka ridges and Pitcairn Island.
Magma source
The
volume of magma that is intruded and/or erupted in a given area of
lithospheric extension depends on two variables: (1) the availability of
pre-existing melt in the crust and mantle; and (2) the amount of
additional melt supplied by decompression upwelling. The latter depends
on three factors: (a) lithospheric thickness; (b) the amount of
extension; and (c) fusibility and temperature of the source.
There is abundant pre-existing melt throughout both the crust and
the mantle. In the crust, melt is stored under active volcanoes in
shallow reservoirs which are fed by deeper ones. In the asthenosphere, a
small amount of partial melt is thought to provide a weak layer that
acts as lubrication for the movement of tectonic plates. The presence of
pre-existing melt means that magmatism can occur even in areas where
lithospheric extension is modest such as the Cameroon and Pitcairn-Gambier volcanic lines.
The rate of magma formation from decompression of the
asthenosphere depends on how high the asthenosphere can rise, which in
turn depends on the thickness of the lithosphere. From numerical
modelling it is evident that the formation of melt in the largest flood
basalts cannot be concurrent with its emplacement. This means that melt is formed over a longer period, stored in reservoirs – most likely located at the lithosphere-asthenosphere boundary
– and released by lithospheric extension. That large volumes of magma
are stored at the base of the lithosphere is evinced in observations of
large magmatic provinces such as the Great Dyke in Zimbabwe and the Bushveld Igneous Complex in South Africa.
There, thick lithosphere remained intact during large-volume magmatism,
so decompression upwelling on the scale required can be ruled out,
implying that large volumes of magma must have pre-existed.
If extension is severe and results in significant thinning of the
lithosphere, the asthenosphere can rise to shallow depths, inducing
decompression melting and producing larger volumes of melt. At mid-ocean
ridges, where the lithosphere is thin, decompression upwelling produces
a modest rate of magmatism. The same process can also produce
small-volume magmatism on or near slowly extending continental rifts.
Beneath continents, the lithosphere is up to 200 km thick. If
lithosphere this thick undergoes severe and persistent extension, it can
rupture, and the asthenosphere can upwell to the surface, producing
tens of millions of cubic kilometres of melt along axes hundreds of
kilometres long. This occurred, for example, during the opening of the
North Atlantic Ocean when the asthenosphere rose from base of the Pangaean lithosphere to the surface.
Examples
The
vast majority of volcanic provinces which are thought to be anomalous in
the context of rigid plate tectonics have now been explained using the
plate theory. The type examples of this kind of volcanic activity are Iceland, Yellowstone, and Hawaii. Iceland is the type example of a volcanic anomaly situated on a plate boundary. Yellowstone, together with the Eastern Snake River Plain to its west, is the type example of an intra-continental volcanic anomaly. Hawaii, along with the related Hawaiian-Emperor seamount chain, is the type example of an intra-oceanic volcanic anomaly.
Iceland
Regional
map of the North East Atlantic. Bathymetry shown in colour; land
topography in grey. RR: Reykjanes Ridge; KR: Kolbeinsey Ridge; JMMC: Jan
Mayen Microcontinent; AR: Aegir Ridge; FI: Faroe Islands. Red lines:
boundaries of the Caledonian orogen and associated thrusts, dashed where
extrapolated into younger Atlantic Ocean.
Iceland is a 1 km high, 450x300 km basaltic shield on the mid-ocean
ridge in the northeast Atlantic Ocean. It comprises over 100 active or
extinct volcanoes and has been extensively studied by Earth scientists
for several decades.
Iceland must be understood in the context of the broader structure and tectonic history of the northeast Atlantic. The northeast Atlantic formed in the early Cenozoic when, after an extensive period of rifting, Greenland separated from Eurasia as Pangaea
began to break up. To the north of Iceland's present location, the
breakup axis propagated south along the Caledonian Suture. To the south,
the breakup axis propagated north. The two axes were separated by
around 100 km from east to west and 300 km from north to south. When the
two axes developed to full seafloor spreading, the 100x300 km
continental region between the two rifts formed the Iceland microcontinent which underwent diffuse extension and shear along several north-oriented rift axes, and basaltic
lavas were emplaced in and on the stretched continental crust. This
style of extension persists across parallel rift zones which frequently
become extinct and are replaced with new ones.
This model explains several distinct characteristics of the region:
Persistence of a subaerial land-bridge from Greenland to the
Faroe Islands which was broken up when the northeast Atlantic was around
1,000 km wide, older parts of which now form a shallow submarine ridge.
The instability and decoupling of spreading ridges to the north and south. To the north, the Aegir Ridge became extinct around 31-28 Ma and extension transferred to the Kolbeinsey Ridge
around 400 km to the west. In the Reykjanes Ridge to the south, after
around 16 million years of spreading perpendicular to the ridge strike,
the direction of extension changed, and the ridge became a ridge-transform system which later migrated eastward.
Properties of the crust beneath the Greenland-Iceland-Faroe Ridge.
Here the crust is mostly 30–40 km thick. Its combination of low seismic
wave speed and high density defy classification as thick oceanic crust
and indicate instead that it is magma-inflated continental crust. This
suggests that Iceland is the result of persistent extension of
continental crust which was structurally resistant to continued
propagation of the new oceanic ridges. As a result, continental
extension continued for an exceptionally long period and has not yet
given way to true ocean spreading. Melt production is similar to the
adjacent mid-ocean ridges which produces oceanic crust around 10 km
thick, though under Iceland, rather than forming oceanic crust, melt is
emplaced into and on top of stretched continental crust.
Iceland's unusual petrology and geochemistry, which is around 10%
silicic and intermediate, with geochemistry similar to such flood
basalts as Karoo and Deccan which have undergone silicic assimilation of, or contamination by, continental crust.
Yellowstone
Geological
map of northwest USA showing Basin and Range faults and basalts and
rhyolites <17 Ma. Blue lines represent approximate age contours of
silicic volcanic centres across the Eastern Snake River Plain and a
contemporaneous trend of oppositely propagating silicic volcanism across
central Oregon.
Yellowstone and the Eastern Snake River Plain to the west comprise a
belt of large, silicic caldera volcanoes that get progressively younger
to the east, culminating in the currently active Yellowstone Caldera in northwest Wyoming.
The belt, however, is covered with basaltic lavas that display no time
progression. Being located on a continental interior, it has been
studied extensively, though research has consisted largely of seismology
and geochemistry aimed at locating sources deep in the mantle. These
methods are not suitable for developing a plate theory, which holds that
volcanism is associated with processes at shallow depths.
As with Iceland, volcanism in the Yellowstone-Eastern Snake River
Plain region must be understood in its broader tectonic context. The
tectonic history of the western United States is heavily influenced by
the subduction of the East Pacific Rise under the North American Plate
beginning around 17 Ma. A change in the plate boundary from subduction
to shear induced extension across the western United States. This
brought about widespread volcanism, commencing with the Columbia River Basalt Group which erupted through a 250-km-long zone of dikes
that broadened the crust by several kilometres. The Basin and Range
province then formed via normal faulting, producing scattered volcanism
with especially abundant eruptions in three east–west zones: the
Yellowstone-Eastern Snake River Plain, Valles,
and St. George volcanic zones. Compared with the others, the
Yellowstone-Eastern Snake River Plain zone is considered unusual because
of its time-progressive silicic volcano chain and striking geothermal
features.
The volcanoes’ silicic composition indicates a lower crustal
source. If volcanism resulted from lithospheric extension, then
extension along the Yellowstone-Eastern Snake River Plain zone must have
migrated from west to east during the last 17 million years.
There is evidence that this is the case. Accelerated motion on nearby
normal faults, which indicates extension in the Basin and Range
province, migrates east coincidentally with migration of the silicic
volcanism. This is corroborated by measurements of recent deformation
from GPS surveying, which finds the most intense zones of extension in
the Basin and Range province in the far east and far west and little
extension in the central 500 km.
The Yellowstone-Eastern Snake River Plain zone, therefore, likely
reflects a locus of extension that has migrated from west to east.
This is further supported by analogous extension-driven silicic
magmatism elsewhere in the Western United States, for example in the Coso Hot Springs and Long Valley Caldera in California.
That persistent basaltic volcanism results from simultaneous
extension along the entire length of the Yellowstone-Eastern Snake River
Plain zone is evident in GPS measurements recorded between 1987 and
2003, which record extension to both the north and south of the zone. Evidence of historic extension can be found in northwest-oriented dike-fed rift zones responsible for basalt flows.
Analogy with similar volcanic activity in Iceland and on mid-ocean
ridges indicates that periods of extension are brief and thus that
basaltic volcanism along the Yellowstone-Eastern Snake River Plain zone
occurs in short bursts of activity in between long inactive periods.
Hawaii
The
Hawaii-Emperor volcanic system is notoriously difficult to study. It is
thousands of kilometres from any major continental landmass and
surrounded by deep ocean, very little of it is above sea level, and it
is covered in thick basalt which obscures its deeper structure. It is
situated within the Cretaceous Magnetic Quiet Zone, a relatively long period of normal polarity in the Earth's magnetic field,
so age variations in the lithosphere are difficult to determine with
accuracy. Reconstructing the tectonic history of the Pacific Ocean more
generally is problematic because earlier plates and plate boundaries,
including the spreading ridge where the Emperor chain began, have been
subducted. Because of these issues, geoscientists are yet to produce a
fully developed theory of the system's origins which can be positively
tested.
Observations that must be accounted for by any such theory include:
Hawaii's position in almost the exact geometric centre of the
Pacific Plate, that is, at the middle point of a line dividing the
western Pacific which is surrounded mainly by subduction zones and the
eastern Pacific which is surrounded mainly by spreading ridges.
The increasing volume of melt. Over the last 50 million years, the
rate of melt production has increased from a mere 0.001 km³ per year to
0.25 km³ per year, a factor of around 250. The current rate of magmatism
responsible for the formation of the Big Island has been in operation
for only 2 million years.
Non-movement of the volcanic centre relative to both the geomagnetic
pole and geometry of the Pacific Plate for around 50 million years.
Continuity of the Hawaiian chain with the Emperor chain via a 60°
“bend”. The latter formed over a 30-million-year period during which the
volcanic centre migrated south-southeast. Migration ceased at the
beginning of the Hawaiian chain. The 60° bend cannot be accounted for by
a change in plate direction because no such change occurred.
The lack of any regional heatflow anomaly detected around the extinct islands and seamounts indicates that the volcanoes are local thermal features.
According to the plate theory, the Hawaiian-Emperor system formed at a
region of extension in the Pacific Plate. Extension in the plate is a
consequence of deformation at plate boundaries, thermal contraction, and
isostatic adjustment. Extension originated at a spreading ridge around
80 Ma. The plate's stress field evolved over the next 30 million years,
causing the region of extension and consequent volcanism to migrate
south-southeast. Around 50 Ma, the stress field stabilised and the
region of extension became almost stationary. At the same time, the
north-westerly motion of the Pacific Plate increased, and over the next
50 million years, the Hawaiian chain formed as the plate moved across a
near-stationary region of extension.
The increasing rate of volcanic activity in the Hawaiian-Emperor
system reflects the availability of melt in the crust and mantle. The
oldest volcanoes in the Emperor chain formed on young, and therefore
thin, oceanic lithosphere. The size of the seamounts increases with the
age of the seafloor, indicating that the availability of melt increases
with the thickness of the lithosphere. This suggests that decompression
melting may contribute, as this, too, is expected to increase with
lithospheric thickness. The significant increase in magmatism during the
last 2 million years indicates a major increase in melt availability,
implying that either a larger reservoir of pre-existing melt or an
exceptionally fusible source region has become available. Petrological
and geochemical evidence suggests that this source may be old
metamorphosed oceanic crust in the asthenosphere, highly fusible
material which would produce far greater magma volumes than mantle
rocks.
The impact hypothesis
In addition to these processes, impact events such as ones that created the Addams crater on Venus and the Sudbury Igneous Complex
in Canada are known to have caused melting and volcanism. In the impact
hypothesis, it is proposed that some regions of hotspot volcanism can
be triggered by certain large-body oceanic impacts which are able to
penetrate the thinner oceanic lithosphere, and flood basalt volcanism can be triggered by converging seismic energy focused at the antipodal point opposite major impact sites.
Impact-induced volcanism has not been adequately studied and comprises a
separate causal category of terrestrial volcanism with implications for
the study of hotspots and plate tectonics.
Comparison of the hypotheses
In
1997 it became possible using seismic tomography to image submerging
tectonic slabs penetrating from the surface all the way to the
core-mantle boundary.
For the Hawaii hotspot,
long-period seismic body wave diffraction tomography provided evidence
that a mantle plume is responsible, as had been proposed as early as
1971. For the Yellowstone hotspot,
seismological evidence began to converge from 2011 in support of the
plume model, as concluded by James et al., "we favor a lower mantle
plume as the origin for the Yellowstone hotspot." Data acquired through Earthscope, a program collecting high-resolution seismic data throughout the contiguous United States has accelerated acceptance of a plume underlying Yellowstone.
Although there is strong evidence that at least two deep mantle plumes
rise to the core-mantle boundary, confirmation that other hypotheses
can be dismissed may require similar tomographic evidence for other hot
spots.
-1.svg)
