Hutton's Unconformity at Jedburgh.
Above: John Clerk of Eldin's 1787 illustration.
Below: 2003 photograph.
Above: John Clerk of Eldin's 1787 illustration.
Below: 2003 photograph.
Uniformitarianism, also known as the Doctrine of Uniformity,
is the assumption that the same natural laws and processes that operate
in our present-day scientific observations have always operated in the
universe in the past and apply everywhere in the universe. It refers to invariance in the metaphysical principles underpinning science, such as the constancy of cause and effect throughout space-time, but has also been used to describe spatiotemporal invariance of physical laws. Though an unprovable postulate that cannot be verified using the scientific method, uniformitarianism has been a key first principle of virtually all fields of science.
In geology, uniformitarianism has included the gradualistic
concept that "the present is the key to the past" and that geological
events occur at the same rate now as they have always done, though many
modern geologists no longer hold to a strict gradualism. Coined by William Whewell, it was originally proposed in contrast to catastrophism by British naturalists in the late 18th century, starting with the work of the geologist James Hutton in his many books including Theory of the Earth. Hutton's work was later refined by scientist John Playfair and popularised by geologist Charles Lyell's Principles of Geology in 1830.
Today, Earth's history is considered to have been a slow, gradual
process, punctuated by occasional natural catastrophic events.
History
18th century
Cliff at the east of Siccar Point in Berwickshire, showing the near-horizontal red sandstone layers above vertically tilted greywacke rocks.
Earlier conceptions likely had little influence on 18th-century European geological explanations for the formation of Earth. Abraham Gottlob Werner (1749–1817) proposed Neptunism, where strata represented deposits from shrinking seas precipitated onto primordial rocks such as granite. In 1785 James Hutton proposed an opposing, self-maintaining infinite cycle based on natural history and not on the Biblical account.
The solid parts of the present land appear in general, to have been composed of the productions of the sea, and of other materials similar to those now found upon the shores. Hence we find reason to conclude:
Hence we are led to conclude, that the greater part of our land, if not the whole had been produced by operations natural to this globe; but that in order to make this land a permanent body, resisting the operations of the waters, two things had been required;
- 1st, That the land on which we rest is not simple and original, but that it is a composition, and had been formed by the operation of second causes.
- 2nd, That before the present land was made, there had subsisted a world composed of sea and land, in which were tides and currents, with such operations at the bottom of the sea as now take place. And,
- Lastly, That while the present land was forming at the bottom of the ocean, the former land maintained plants and animals; at least the sea was then inhabited by animals, in a similar manner as it is at present.
- 1st, The consolidation of masses formed by collections of loose or incoherent materials;
- 2nd, The elevation of those consolidated masses from the bottom of the sea, the place where they were collected, to the stations in which they now remain above the level of the ocean.
Hutton then sought evidence to support his idea that there must have been repeated cycles, each involving deposition on the seabed, uplift with tilting and erosion, and then moving undersea again for further layers to be deposited. At Glen Tilt in the Cairngorm mountains he found granite penetrating metamorphic schists, in a way which indicated to him that the presumed primordial rock had been molten after the strata had formed. He had read about angular unconformities as interpreted by Neptunists, and found an unconformity at Jedburgh where layers of greywacke
in the lower layers of the cliff face have been tilted almost
vertically before being eroded to form a level plane, under horizontal
layers of Old Red Sandstone. In the spring of 1788 he took a boat trip along the Berwickshire coast with John Playfair and the geologist Sir James Hall, and found a dramatic unconformity showing the same sequence at Siccar Point. Playfair later recalled that "the mind seemed to grow giddy by looking so far into the abyss of time", and Hutton concluded a 1788 paper he presented at the Royal Society of Edinburgh, later rewritten as a book, with the phrase "we find no vestige of a beginning, no prospect of an end".
Both Playfair and Hall wrote their own books on the theory, and
for decades robust debate continued between Hutton's supporters and the
Neptunists. Georges Cuvier's paleontological work in the 1790s, which established the reality of extinction,
explained this by local catastrophes, after which other fixed species
repopulated the affected areas. In Britain, geologists adapted this idea
into "diluvial theory"
which proposed repeated worldwide annihilation and creation of new
fixed species adapted to a changed environment, initially identifying
the most recent catastrophe as the biblical flood.
19th century
Charles Lyell at the British Association meeting in Glasgow 1840
From 1830 to 1833 Charles Lyell's multi-volume Principles of Geology
was published. The work's subtitle was "An attempt to explain the
former changes of the Earth's surface by reference to causes now in
operation". He drew his explanations from field studies conducted
directly before he went to work on the founding geology text,
and developed Hutton's idea that the earth was shaped entirely by
slow-moving forces still in operation today, acting over a very long
period of time. The terms uniformitarianism for this idea, and catastrophism for the opposing viewpoint, were coined by William Whewell in a review of Lyell's book. Principles of Geology was the most influential geological work in the middle of the 19th century.
Systems of inorganic earth history
Geoscientists
support diverse systems of Earth history, the nature of which rest on a
certain mixture of views about process, control, rate, and state which
are preferred. Because geologists and geomorphologists tend to adopt
opposite views over process, rate and state in the inorganic world,
there are eight different systems of beliefs in the development of the
terrestrial sphere.
All geoscientists stand by the principle of uniformity of law. Most,
but not all, are directed by the principle of simplicity. All make
definite assertions about the quality of rate and state in the inorganic
realm.
| Methodological assumption concerning kind of process |
Substantive claim concerning state |
Substantive claim Concerning rate |
System of Inorganic Earth history |
Promoters |
|---|---|---|---|---|
| Same Kind of processes that exist today Actualism |
Steady State Non-directionalism |
Constant Rate Gradualism |
Actualistic Non-directional Gradualism |
Most of Hutton, Playfair, Lyell |
|
|
|
Changing Rate Catastrophism |
Actualistic Non-directional Catastrophism |
Hall |
|
|
Changing State Directionalism |
Constant Rate Gradualism |
Actualistic Directional Gradualism |
Small part of Hutton, Cotta, Darwin |
|
|
|
Changing Rate Catastrophism |
Actualistic Directional Catastrophism |
Hooke, Steno, Lehmann, Pallas, de Saussure, Werner and geognosists, Elis de Beaumont and followers |
| Different Kind of processes than exist today Non-Actualism |
Steady State Non-directionalism |
Constant Rate Gradualism |
Non-Actualistic Non-directional Gradualism |
Carpenter |
|
|
|
Changing Rate Catastrophism |
Non-Actualistic Non-directional Catastrophism |
Bonnet, Cuvier |
|
|
Changing State Directionalism |
Constant Rate Gradualism |
Non-Actualistic directional Gradualism |
De Mallet, Buffon |
|
|
|
Changing Rate Catastrophism |
Non-Actualistic Directional Catastrophism |
Restoration cosmogonists, English diluvialists, Scriptural geologists |
Lyell's uniformitarianism
According to Reijer Hooykaas (1963), Lyell's uniformitarianism is a family of four related propositions, not a single idea:
- Uniformity of law – the laws of nature are constant across time and space.
- Uniformity of methodology – the appropriate hypotheses for explaining the geological past are those with analogy today.
- Uniformity of kind – past and present causes are all of the same kind, have the same energy, and produce the same effects.
- Uniformity of degree – geological circumstances have remained the same over time.
None of these connotations requires another, and they are not all equally inferred by uniformitarians.
Gould explained Lyell's propositions in Time's Arrow, Time's Cycle (1987), stating that Lyell conflated two different types of propositions: a pair of methodological assumptions with a pair of substantive hypotheses. The four together make up Lyell's uniformitarianism.
Methodological assumptions
The
two methodological assumptions below are accepted to be true by the
majority of scientists and geologists. Gould claims that these
philosophical propositions must be assumed before you can proceed as a
scientist doing science. "You cannot go to a rocky outcrop and observe
either the constancy of nature's laws or the working of unknown
processes. It works the other way around." You first assume these
propositions and "then you go to the outcrop."
- Uniformity of law across time and space: Natural laws are constant across space and time. The axiom of uniformity of law is necessary in order for scientists to extrapolate (by inductive inference) into the unobservable past. The constancy of natural laws must be assumed in the study of the past; else we cannot meaningfully study it.
- Uniformity of process across time and space: Natural processes are constant across time and space.
- Though similar to uniformity of law, this second a priori assumption, shared by the vast majority of scientists, deals with geological causes, not physico-chemical laws. The past is to be explained by processes acting currently in time and space rather than inventing extra esoteric or unknown processes without good reason, otherwise known as parsimony or Occam's razor.
Substantive hypotheses
The substantive hypotheses were controversial and, in some cases, accepted by few.
These hypotheses are judged true or false on empirical grounds through
scientific observation and repeated experimental data. This is in
contrast with the previous two philosophical assumptions that come before one can do science and so cannot be tested or falsified by science.
- Uniformity of rate across time and space: Change is typically slow, steady, and gradual.
- Uniformity of rate (or gradualism) is what most people (including geologists) think of when they hear the word "uniformitarianism," confusing this hypothesis with the entire definition. As late as 1990, Lemon, in his textbook of stratigraphy, affirmed that "The uniformitarian view of earth history held that all geologic processes proceed continuously and at a very slow pace."
- Gould explained Hutton's view of uniformity of rate; mountain ranges or grand canyons are built by accumulation of nearly insensible changes added up through vast time. Some major events such as floods, earthquakes, and eruptions, do occur. But these catastrophes are strictly local. They neither occurred in the past, nor shall happen in the future, at any greater frequency or extent than they display at present. In particular, the whole earth is never convulsed at once.
- Uniformity of state across time and space: Change is evenly distributed throughout space and time.
- The uniformity of state hypothesis implies that throughout the history of our earth there is no progress in any inexorable direction. The planet has almost always looked and behaved as it does now. Change is continuous, but leads nowhere. The earth is in balance: a dynamic steady state.
20th century
Stephen Jay Gould's first scientific paper, Is uniformitarianism necessary? (1965), reduced these four assumptions to two.
He dismissed the first principle, which asserted spatial and temporal
invariance of natural laws, as no longer an issue of debate. He rejected
the third (uniformity of rate) as an unjustified limitation on
scientific inquiry, as it constrains past geologic rates and conditions
to those of the present. So, Lyellian uniformitarianism was unnecessary.
Uniformitarianism was proposed in contrast to catastrophism,
which states that the distant past "consisted of epochs of paroxysmal
and catastrophic action interposed between periods of comparative
tranquility"
Especially in the late 19th and early 20th centuries, most geologists
took this interpretation to mean that catastrophic events are not
important in geologic time; one example of this is the debate of the
formation of the Channeled Scablands due to the catastrophic Missoula
glacial outburst floods. An important result of this debate and others
was the re-clarification that, while the same principles operate in
geologic time, catastrophic events that are infrequent on human
time-scales can have important consequences in geologic history.
Derek Ager has noted that "geologists do not deny uniformitarianism in
its true sense, that is to say, of interpreting the past by means of the
processes that are seen going on at the present day, so long as we
remember that the periodic catastrophe is one of those processes. Those
periodic catastrophes make more showing in the stratigraphical record
than we have hitherto assumed."
Even Charles Lyell thought that ordinary geological processes would cause Niagara Falls to move upstream to Lake Erie within 10,000 years, leading to catastrophic flooding of a large part of North America.
Modern geologists do not apply uniformitarianism in the same way
as Lyell. They question if rates of processes were uniform through time
and only those values measured during the history of geology are to be
accepted. The present may not be a long enough key to penetrate the deep lock of the past.
Geologic processes may have been active at different rates in the past
that humans have not observed. "By force of popularity, uniformity of
rate has persisted to our present day. For more than a century, Lyell's
rhetoric conflating axiom with hypotheses has descended in unmodified
form. Many geologists have been stifled by the belief that proper
methodology includes an a priori commitment to gradual change, and by a
preference for explaining large-scale phenomena as the concatenation of
innumerable tiny changes."
The current consensus is that Earth's history is a slow, gradual process punctuated by occasional natural catastrophic events that have affected Earth and its inhabitants.
In practice it is reduced from Lyell's conflation, or blending, to
simply the two philosophical assumptions. This is also known as the
principle of geological actualism, which states that all past geological
action was like all present geological action. The principle of actualism is the cornerstone of paleoecology.
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