Megatsunami

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https://en.wikipedia.org/wiki/Megatsunami 

Diagram of the 1958 Lituya Bay megatsunami, which proved the existence of megatsunamis

A megatsunami is a very large wave created by a large, sudden displacement of material into a body of water.

Megatsunamis have different features from ordinary tsunamis. Ordinary tsunamis are caused by underwater tectonic activity (movement of the earth's plates) and therefore occur along plate boundaries and as a result of earthquakes and the subsequent rise or fall in the sea floor that displaces a volume of water. Ordinary tsunamis exhibit shallow waves in the deep waters of the open ocean that increase dramatically in height upon approaching land to a maximum run-up height of around 30 metres (100 ft) in the cases of the most powerful earthquakes. By contrast, megatsunamis occur when a large amount of material suddenly falls into water or anywhere near water (such as via a landslide, meteor impact, or volcanic eruption). They can have extremely large initial wave heights in the hundreds of metres, far beyond the height of any ordinary tsunami. These giant wave heights occur because the water is "splashed" upwards and outwards by the displacement.

Examples of modern megatsunamis include the one associated with the 1883 eruption of Krakatoa (volcanic eruption), the 1958 Lituya Bay megatsunami (a landslide which caused an initial wave of 524 metres (1,719 ft)), and the Vajont Dam landslide (caused by human activity destabilizing sides of valley). Prehistoric examples include the Storegga Slide (landslide), and the Chicxulub, Chesapeake Bay, and Eltanin meteor impacts.

Overview

A megatsunami is a tsunami with an initial wave amplitude (height) measured in many tens or hundreds of metres. A megatsunami is a separate class of event from an ordinary tsunami and is caused by different physical mechanisms.

Normal tsunamis result from displacement of the sea floor due to plate tectonics. Powerful earthquakes may cause the sea floor to displace vertically on the order of tens of metres, which in turn displaces the water column above and leads to the formation of a tsunami. Ordinary tsunamis have a small wave height offshore and generally pass unnoticed at sea, forming only a slight swell on the order of 30 cm (12 in) above the normal sea surface. In deep water it is possible that a tsunami could pass beneath a ship without the crew of the vessel noticing. As it approaches land, the wave height of an ordinary tsunami increases dramatically as the sea floor slopes upward and the base of the wave pushes the water column above it upwards. Ordinary tsunamis, even those associated with the most powerful strike-slip earthquakes, typically do not reach heights in excess of 30 m (100 ft).

By contrast, megatsunamis are caused by landslides and other impact events that displace large volumes of water, resulting in waves that may exceed the height of an ordinary tsunami by tens or even hundreds of metres. Underwater earthquakes or volcanic eruptions do not normally generate megatsunamis, but landslides next to bodies of water resulting from earthquakes or volcanic eruptions can, since they cause a much larger amount of water displacement. If the landslide or impact occurs in a limited body of water, as happened at the Vajont Dam (1963) and in Lituya Bay (1958) then the water may be unable to disperse and one or more exceedingly large waves may result.

Determining a height range typical of megatsunamis is a complex and scientifically debated topic. This complexity is increased due to the fact that two different heights are often reported for tsunamis – the height of the wave itself in open water, and the height to which it surges when it encounters land. Depending upon the locale, this second or so-called "run-up height" can be several times larger than the wave's height just before reaching shore. While there is currently no minimum or average height classification for megatsunamis that is broadly accepted by the scientific community, the limited number of observed megatsunami events in recent history have all had run-up heights that exceeded 100 metres (300 ft). The megatsunami in Spirit Lake, Washington, USA that was caused by the 1980 eruption of Mount St. Helens reached 260 metres (853 ft), while the tallest megatsunami ever recorded (Lituya Bay in 1958) reached a run-up height of 520 metres (1,720 ft). It is also possible that much larger megatsunamis occurred in prehistory; researchers analyzing the geological structures left behind by prehistoric asteroid impacts have suggested that these events could have resulted in megatsunamis that exceeded 1,500 metres (4,900 ft) in height.

Recognition of the concept of megatsunami

Main article: 1958 Lituya Bay earthquake and megatsunami

Before the 1950s, scientists had theorized that tsunamis orders of magnitude larger than those observed with earthquakes could have occurred as a result of ancient geological processes, but no concrete evidence of the existence of these "monster waves" had yet been gathered. Geologists searching for oil in Alaska in 1953 observed that in Lituya Bay, mature tree growth did not extend to the shoreline as it did in many other bays in the region. Rather, there was a band of younger trees closer to the shore. Forestry workers, glaciologists, and geographers call the boundary between these bands a trim line. Trees just above the trim line showed severe scarring on their seaward side, while those from below the trim line did not. This indicated that a large force had impacted all of the elder trees above the trim line, and presumably had killed off all the trees below it. Based on this evidence, the scientists hypothesized that there had been an unusually large wave or waves in the deep inlet. Because this is a recently deglaciated fjord with steep slopes and crossed by a major fault (the Fairweather Fault), one possibility was that this wave was a landslide-generated tsunami.

On July 9, 1958, a 7.8 M_w strike-slip earthquake in southeast Alaska caused 80,000,000 metric tons (90,000,000 short tons) of rock and ice to drop into the deep water at the head of Lituya Bay. The block fell almost vertically and hit the water with sufficient force to create a wave that surged up the opposite side of the head of the bay to a height of 520 metres (1,710 feet), and was still many tens of metres high further down the bay when it carried eyewitnesses Howard Ulrich and his son Howard Jr. over the trees in their fishing boat. They were washed back into the bay and both survived.

Analysis of mechanism

The mechanism giving rise to megatsunamis was analysed for the Lituya Bay event in a study presented at the Tsunami Society in 1999; this model was considerably developed and modified by a second study in 2010.

Although the earthquake which caused the megatsunami was considered very energetic, it was determined that it could not have been the sole contributor based on the measured height of the wave. Neither water drainage from a lake, nor a landslide, nor the force of the earthquake itself were sufficient to create a megatsunami of the size observed, although all of these may have been contributing factors.

Instead, the megatsunami was caused by a combination of events in quick succession. The primary event occurred in the form of a large and sudden impulsive impact when about 40 million cubic yards of rock several hundred metres above the bay was fractured by the earthquake, and fell "practically as a monolithic unit" down the almost-vertical slope and into the bay. The rockfall also caused air to be "dragged along" due to viscosity effects, which added to the volume of displacement, and further impacted the sediment on the floor of the bay, creating a large crater. The study concluded that:

The giant wave runup of 1,720 feet (524 m) at the head of the Bay and the subsequent huge wave along the main body of Lituya Bay which occurred on July 9, 1958, were caused primarily by an enormous subaerial rockfall into Gilbert Inlet at the head of Lituya Bay, triggered by dynamic earthquake ground motions along the Fairweather Fault.

The large monolithic mass of rock struck the sediments at bottom of Gilbert Inlet at the head of the bay with great force. The impact created a large crater and displaced and folded recent and Tertiary deposits and sedimentary layers to an unknown depth. The displaced water and the displacement and folding of the sediments broke and uplifted 1,300 feet of ice along the entire front face of the Lituya Glacier at the north end of Gilbert Inlet. Also, the impact and the sediment displacement by the rockfall resulted in an air bubble and in water splashing action that reached the 1,720-foot (524 m) elevation on the other side of the head of Gilbert Inlet. The same rockfall impact, in combination with the strong ground movements, the net vertical crustal uplift of about 3.5 feet, and an overall tilting seaward of the entire crustal block on which Lituya Bay was situated, generated the giant solitary gravity wave which swept the main body of the bay.

This was the most likely scenario of the event – the "PC model" that was adopted for subsequent mathematical modeling studies with source dimensions and parameters provided as input. Subsequent mathematical modeling at the Los Alamos National Laboratory (Mader, 1999, Mader & Gittings, 2002) supported the proposed mechanism and indicated that there was indeed sufficient volume of water and an adequately deep layer of sediments in the Lituya Bay inlet to account for the giant wave runup and the subsequent inundation. The modeling reproduced the documented physical observations of runup.

A 2010 model that examined the amount of infill on the floor of the bay, which was many times larger than that of the rockfall alone, and also the energy and height of the waves, and the accounts given by eyewitnesses, concluded that there had been a "dual slide" involving a rockfall, which also triggered a release of 5 to 10 times its volume of sediment trapped by the adjacent Lituya Glacier, as an almost immediate and many times larger second slide, a ratio comparable with other events where this "dual slide" effect is known to have happened.

Examples

Prehistoric

• An astronomical object between 37 and 58 kilometres (23 and 36 mi) wide traveling at 20 kilometres (12.4 mi) per second struck the Earth 3.26 billion years ago east of what is now Johannesburg, South Africa, near South Africa's border with Swaziland, in what was then an Archean ocean that covered most of the planet, creating a crater about 500 kilometres (310 mi) wide. The impact generated a megastunami that probably extended to a depth of thousands of meters beneath the surface of the ocean and rose to the height of a skyscraper when it reached shorelines. The resultant event created the Barberton Greenstone Belt. 
• The asteroid linked to the extinction of dinosaurs, which created the Chicxulub crater in the Yucatán Peninsula approximately 66 million years ago, would have caused a megatsunami over 100 metres (330 ft) tall. The height of the tsunami was limited due to relatively shallow sea in the area of the impact; had the asteroid struck in the deep sea the megatsunami would have been 4.6 kilometres (2.9 mi) tall. Among the mechanisms triggering megatsunamis, the direct impact, shockwaves, returning water in the crater with a new push outward and seismic waves with a magnitude up to ~11 A more recent simulation of the global effects of the Chicxulub megatsunami showed an initial wave height of 1.5 kilometres (0.9 mi), with later waves up to 100 metres (330 ft) in height in the Gulf of Mexico, and up to 14 metres (46 ft) in the North Atlantic and South Pacific; the discovery of mega-ripples in Louisiana via seismic imaging data, with average wavelengths of 600 metres (2,000 ft) and average wave heights of 16 metres (52 ft), looks like to confirm it. David Shonting and Cathy Ezrailson propose an "Edgerton effect" mechanism generating the megatsunami, similar to a milk drop falling on water that triggers a crown-shape water column, with a comparable height to the Chicxulub impactor's, that means over 10–12 kilometres (6–7 mi) for the initial seawater forced outward by the explosion and blast waves; then, its collapse triggers megatsunamis changing their height according to the different water depth, raising up to 500 metres (1,600 ft). Furthermore, the initial shock wave via impact triggered seismic waves producing giant landslides and slumping around the region (the largest known event deposits on Earth) with subsequent megatsunamis of various sizes, and seiches of 10 to 100 metres (30 to 300 ft) in Tanis, 3,000 kilometres (1,900 mi) away, part of a vast inland sea at the time and directly triggered via seismic shaking by the impact within a few minutes.
• During the Messinian the coasts of northern Chile were likely struck by various megatsunamis.
• A megatsunami affected the coast of south–central Chile in the Pliocene as evidenced by the sedimentary record of Ranquil Formation.
• The Eltanin impact in the southeast Pacific Ocean 2.5 million years ago caused a megatsunami that was over 200 metres (660 ft) high in southern Chile and the Antarctic Peninsula; the wave swept across much of the Pacific Ocean.
• The northern half of the East Molokai Volcano on Molokai in Hawaii suffered a catastrophic collapse about 1.5 million years ago, generating a megatsunami, and now lies as a debris field scattered northward across the ocean bottom, while what remains on the island are the highest sea cliffs in the world. The megatsunami may have reached a height of 610 metres (2,000 ft) near its origin and reached California and Mexico.
• The existence of large scattered boulders in only one of the four marine terraces of Herradura Bay south of the Chilean city of Coquimbo has been interpreted by Roland Paskoff as the result of a mega-tsunami that occurred in the Middle Pleistocene.
• In Hawaii, a megatsunami at least 400 metres (1,312 ft) in height deposited marine sediments at a modern-day elevation of 326 metres (1,070 ft) — 375 to 425 metres (1,230 to 1,394 ft) above sea level at the time the wave struck — on Lanai about 105,000 years ago. The tsunami also deposited such sediments at an elevation of 60 to 80 metres (197 to 262 ft) on Oahu, Molokai, Maui, and the island of Hawaii.
• The collapse of the ancestral Mount Amarelo on Fogo in the Cape Verde Islands about 73,000 years ago triggered a megatsunami which struck Santiago, 55 kilometres (34 mi; 30 nmi) away, with a height of at least 170 metres (558 ft) and a run-up height of over 270 metres (886 ft).
• A major collapse of the western edge of the Lake Tahoe basin, a landslide with a volume of 12.5 cubic kilometres (3.0 cu mi) which formed McKinney Bay between 21,000 and 12,000 years ago, generated megatsunamis/seiche waves with an initial height of probably about 100 m (330 ft) and caused the lake's water to slosh back and forth for days. Much of the water in the megatsunamis washed over the lake's outlet at what is now Tahoe City, California, and flooded down the Truckee River, carrying house-sized boulders as far downstream as the California-Nevada border at what is now Verdi, California.
• In the North Sea, the Storegga Slide caused a megatsunami approximately 8,200 years ago. It is estimated to have completely flooded the remainder of Doggerland.
• Around 6370 BCE, a 25-cubic-kilometre (6 cu mi) landslide on the eastern slope of Mount Etna in Sicily into the Mediterranean Sea triggered a megatsunami in the Eastern Mediterranean with an initial wave height along the eastern coast of Sicily of 40 metres (131 ft). It struck the Neolithic village of Atlit Yam off the coast of Israel with a height of 2.5 metres (8 ft 2 in), prompting the village's abandonment.
• Around 5,650 B.C., a landslide in Greenland created a megatsunami with a run-up height on Alluttoq Island of 41 to 66 metres (135 to 217 ft).
• Around 5,350 B.C., a landslide in Greenland created a megatsunami with a run-up height on Alluttoq Island of 45 to 70 metres (148 to 230 ft).

Historic

c. 2000 BC: Réunion

• A landslide on Réunion island, to the east of Madagascar, may have caused a megatsunami.

c. 1600 BC: Santorini

Main article: Minoan eruption

• The Thera volcano erupted, the force of the eruption causing megatsunamis which affected the whole Aegean Sea and the eastern Mediterranean Sea.

Modern

1674: Ambon Island, Banda Sea

Main article: 1674 Ambon earthquake and megatsunami

On February 17, 1674, between 19:30 and 20:00 local time, an earthquake struck the Maluku Islands. Ambon Island received run-up heights of 100 metres (328 ft), making the wave far too large to be caused by the quake itself. Instead, it was probably the result of an underwater landslide triggered by the earthquake. The quake and tsunami killed 2,347 people.

1731: Storfjorden, Norway

At 10:00 p.m. on January 8, 1731, a landslide with a volume of possibly 6,000,000 cubic metres (7,800,000 cu yd) fell from the mountain Skafjell from a height of 500 metres (1,640 ft) into the Storfjorden opposite Stranda, Norway. The slide generated a megatsunami 30 metres (100 ft) in height that struck Stranda, flooding the area for 100 metres (330 ft) inland and destroying the church and all but two boathouses, as well as many boats. Damaging waves struck as far as way as Ørskog. The waves killed 17 people.

1741: Oshima-Ōshima, Sea of Japan

Main article: 1741 eruption of Oshima–Ōshima and the Kampo tsunami

An eruption of Oshima-Ōshima occurred that lasted from 18 August 1741 to 1 May 1742. On 29 August 1741, a devastating tsunami occurred. It killed at least 1,467 people along a 120-kilometre (75 mi) section of the coast, excluding native residents whose deaths were not recorded. Wave heights for Gankakezawa have been estimated at 34 metres (112 ft) based on oral histories, while an estimate of 13 metres (43 ft) is derived from written records. At Sado Island, over 350 kilometres (217 mi; 189 nmi) away, a wave height of 2 to 5 metres (6 ft 7 in to 16 ft 5 in) has been estimated based on descriptions of the damage, while oral records suggest a height of 8 metres (26 ft). Wave heights have been estimated at 3 to 4 metres (9.8 to 13.1 ft) even as far away as the Korean Peninsula. There is still no consensus in the debate as to what caused it but much evidence points to a landslide and debris avalanche along the flank of the volcano. An alternative hypothesis holds that an earthquake caused the tsunami. The event reduced the elevation of the peak of Hishiyama from 850 to 722 metres (2,789 to 2,369 ft). An estimated 2.4-cubic-kilometre (0.58 cu mi) section of the volcano collapsed onto the seafloor north of the island; the collapse was similar in size to the 2.3-cubic-kilometre (0.55 cu mi) collapse which occurred during the 1980 eruption of Mount St. Helens.

1756: Langfjorden, Norway

Just before 8:00 p.m. on February 22, 1756, a landslide with a volume of 12,000,000 to 15,000,000 cubic metres (16,000,000 to 20,000,000 cu yd) travelled at high speed from a height of 400 metres (1,300 ft) on the side of the mountain Tjellafjellet into the Langfjorden about 1 kilometre (0.6 mi) west of Tjelle, Norway, between Tjelle and Gramsgrø. The slide generated three megatsunamis in the Langfjorden and the Eresfjorden with heights of 40 to 50 metres (130 to 160 ft). The waves flooded the shore for 200 metres (660 ft) inland in some areas, destroying farms and other inhabited areas. Damaging waves struck as far away as Veøy, 25 kilometres (16 mi) from the landslide — where they washed inland 20 metres (66 ft) above normal flood levels — and Gjermundnes, 40 kilometres (25 mi) from the slide. The waves killed 32 people and destroyed 168 buildings, 196 boats, large amounts of forest, and roads and boat landings.

1792: Mount Unzen, Japan

Main article: 1792 Unzen landslide and tsunami

On 21 May 1792, a flank of the Mayamaya dome of Mount Unzen collapsed after two large earthquakes. This had been preceded by a series of earthquakes coming from the mountain, beginning near the end of 1791. Initial wave heights were 100 metres (330 ft), but when they hit the other side of Ariake Bay, they were only 10 to 20 metres (33 to 66 ft) in height, though one location received 57-metre (187 ft) waves due to seafloor topography. The waves bounced back to Shimabara, which, when they hit, accounted for about half of the tsunami's victims. According to estimates, 10,000 people were killed by the tsunami, and a further 5,000 were killed by the landslide. As of 2011, it was the deadliest known volcanic event in Japan.

1853–1854: Lituya Bay, Alaska

Sometime between August 1853 and May 1854, a megatsunami occurred in Lituya Bay in what was then Russian America. Studies of Lituya Bay between 1948 and 1953 first identified the event, which probably occurred because of a large landslide on the south shore of the bay near Mudslide Creek. The wave had a maximum run-up height of 120 metres (394 ft), flooding the coast of the bay up to 230 metres (750 ft) inland.

1874: Lituya Bay, Alaska

A study of Lituya Bay in 1953 concluded that sometime around 1874, perhaps in May 1874, another megatsunami occurred in Lituya Bay in Alaska. Probably occurring because of a large landslide on the south shore of the bay in the Mudslide Creek Valley, the wave had a maximum run-up height of 24 metres (80 ft), flooding the coast of the bay up to 640 metres (2,100 ft) inland.

1883: Krakatoa, Sunda Strait

Main article: 1883 eruption of Krakatoa § Tsunamis and distant effects

The eruption of Krakatoa created pyroclastic flows which generated megatsunamis when they hit the waters of the Sunda Strait on 27 August 1883. The waves reached heights of up to 24 metres (79 feet) along the south coast of Sumatra and up to 42 metres (138 feet) along the west coast of Java. The tsunamis were powerful enough to kill over 30,000 people, and their effect was such that an area of land in Banten had its human settlements wiped out, and they never repopulated. (This area rewilded and was later declared a national park.) The steamship Berouw, a colonial gunboat, was flung over a mile (1.6 km) inland on Sumatra by the wave, killing its entire crew. Pyroclastic flows scorched several thousand people to death in southern Sumatra, and two ships reported severe winds and tephra, though they were too far away to be scorched. Two thirds of the island collapsed into the sea after the event. Groups of human skeletons were found floating on pumice numerous times, up to a year after the event. The eruption also generated what is often called the loudest sound in history, which was heard 4,800 kilometres (3,000 mi; 2,600 nmi) away on Rodrigues in the Indian Ocean.

1905: Lovatnet, Norway

On January 15, 1905, a landslide on the slope of the mountain Ramnefjellet with a volume of 350,000 cubic metres (460,000 cu yd) fell from a height of 500 metres (1,600 ft) into the southern end of the lake Lovatnet in Norway, generating three megatsunamis of up to 40.5 metres (133 ft) in height. The waves destroyed the villages of Bødal and Nesdal near the southern end of the lake, killing 61 people — half their combined population — and 261 farm animals and destroying 60 houses, all the local boathouses, and 70 to 80 boats, one of which — the tourist boat Lodalen — was thrown 300 metres (1,000 ft) inland by the last wave and wrecked. At the northern end of the 11.7-kilometre (7.3 mi) long lake, a wave measured at almost 6 metres (20 ft) destroyed a bridge.

1905: Disenchantment Bay, Alaska

On July 4, 1905, an overhanging glacier — since known as the Fallen Glacier — broke loose, slid out of its valley, and fell 300 metres (1,000 ft) down a steep slope into Disenchantment Bay in Alaska, clearing vegetation along a path 0.8 kilometres (0.5 mi) wide. When it entered the water, it generated a megatsunami which broke tree branches 34 metres (110 ft) above ground level 0.8 kilometres (0.5 mi) away. The wave killed vegetation to a height of 20 metres (65 ft) at a distance of 5 kilometres (3 mi) from the landslide, and it reached heights of from 15 to 35 metres (50 to 115 ft) at different locations on the coast of Haenke Island. At a distance of 24 kilometres (15 mi) from the slide, observers at Russell Fjord reported a series of large waves that caused the water level to rise and fall 5 to 6 metres (15 to 20 ft) for a half-hour.

1934: Tafjorden, Norway

On April 7, 1934, a landslide on the slope of the mountain Langhamaren with a volume of 3,000,000 cubic metres (3,900,000 cu yd) fell from a height of about 730 metres (2,395 ft) into the Tafjorden in Norway, generating three megatsunamis, the last and largest of which reached a height of between 62 and 63.5 metres (203 and 208 ft) on the opposite shore. Large waves struck Tafjord and Fjørå. The waves killed 23 people at Tafjord, where the last and largest wave was 17 metres (56 ft) tall and struck at an estimated speed of 160 kilometres per hour (100 mph), flooding the town for 300 metres (328 yd) inland and killing 23 people. At Fjørå, waves reached 13 metres (43 ft), destroyed buildings, removed all soil, and killed 17 people. Damaging waves struck as far as 50 kilometres (31 mi) away, and waves were detected at a distance of 100 kilometres (62 mi) from the landslide. One survivor suffered serious injuries requiring hospitalization.

1936: Lovatnet, Norway

On September 13, 1936, a landslide on the slope of the mountain Ramnefjellet with a volume of 1,000,000 cubic metres (1,300,000 cu yd) fell from a height of 800 metres (3,000 ft) into the southern end of the lake Lovatnet in Norway, generating three megatsunamis, the largest of which reached a height of 74 metres (243 ft). The waves destroyed all farms at Bødal and most farms at Nesdal — completely washing away 16 farms — as well as 100 houses, bridges, a power station, a workshop, a sawmill, several grain mills, a restaurant, a schoolhouse, and all boats on the lake. A 12.6-metre (41 ft) wave struck the southern end of the 11.7-kilometre (7.3 mi) long lake and caused damaging flooding in the Loelva River, the lake's northern outlet. The waves killed 74 people and severely injured 11.

1936: Lituya Bay, Alaska

On October 27, 1936, a megatsunami occurred in Lituya Bay in Alaska with a maximum run-up height of 150 metres (490 ft) in Crillon Inlet at the head of the bay. The four eyewitnesses to the wave in Lituya Bay itself all survived and described it as between 30 and 76 metres (100 and 250 ft) high. The maximum inundation distance was 610 metres (2,000 ft) inland along the north shore of the bay. The cause of the megatsunami remains unclear, but may have been a submarine landslide.

1958: Lituya Bay, Alaska, US

Main articles: 1958 Lituya Bay earthquake and megatsunami and Lituya Bay

Damage from the 1958 Lituya Bay megatsunami can be seen in this oblique aerial photograph of Lituya Bay, Alaska as the lighter areas at the shore where trees have been stripped away. The red arrow shows the location of the landslide, and the yellow arrow shows the location of the high point of the wave sweeping over the headland.

On July 9, 1958, a giant landslide at the head of Lituya Bay in Alaska, caused by an earthquake, generated a wave that washed out trees to a maximum elevation of 520 metres (1,710 ft) at the entrance of Gilbert Inlet. The wave surged over the headland, stripping trees and soil down to bedrock, and surged along the fjord which forms Lituya Bay, destroying two fishing boats anchored there and killing two people. This was the highest wave of any kind ever recorded. The subsequent study of this event led to the establishment of the term "megatsunami," to distinguish it from ordinary tsunamis.

1963: Vajont Dam, Italy

Main article: Vajont Dam

On October 9, 1963, a landslide above Vajont Dam in Italy produced a 250 m (820 ft) surge that overtopped the dam and destroyed the villages of Longarone, Pirago, Rivalta, Villanova, and Faè, killing nearly 2,000 people. This is currently the only known example of a megatsunami that was indirectly caused by human activities.

1980: Spirit Lake, Washington, US

Main articles: Spirit Lake (Washington), 1980 eruption of Mount St. Helens, and Mount St. Helens

On May 18, 1980, the upper 400 metres (1,300 ft) of Mount St. Helens collapsed, creating a landslide. This released the pressure on the magma trapped beneath the summit bulge which exploded as a lateral blast, which then released the pressure on the magma chamber and resulted in a plinian eruption.

One lobe of the avalanche surged onto Spirit Lake, causing a megatsunami which pushed the lake waters in a series of surges, which reached a maximum height of 260 metres (850 ft) above the pre-eruption water level (about 975 m (3,199 ft) ASL). Above the upper limit of the tsunami, trees lie where they were knocked down by the pyroclastic surge; below the limit, the fallen trees and the surge deposits were removed by the megatsunami and deposited in Spirit Lake.

2000: Paatuut, Greenland

On November 21, 2000, a landslide composed of 90,000,000 cubic metres (120,000,000 cu yd) of rock with a mass of 260,000,000 tons fell from an elevation of 1,000 to 1,400 metres (3,300 to 4,600 ft) at Paatuut on the Nuussuaq Peninsula on the west coast of Greenland, reaching a speed of 140 kilometres per hour (87 mph). About 30,000,000 cubic metres (39,000,000 cu yd) of material with a mass of 87,000,000 tons entered Sullorsuaq Strait (known in Danish as Vaigat Strait), generating a megatsunami. The wave had a run-up height of 50 metres (164 ft) near the landslide and 28 metres (92 ft) at Qullissat, the site of an abandoned settlement across the strait on Disko Island, 20 kilometres (11 nmi; 12 mi) away, where it inundated the coast as far as 100 metres (328 ft) inland. Refracted energy from the tsunami created a wave that destroyed boats at the closest populated village, Saqqaq, on the southwestern coast of the Nuussuaq Peninsula 40 kilometres (25 mi) from the landslide.

2015: Taan Fiord, Alaska, US

On 9 August 2016, United States Geological Survey scientists survey run-up damage from the 17 October 2015 megatsunami in Taan Fiord. Based on visible damage to trees that remained standing, they estimated run-up heights in this area of 5 metres (16.4 ft).

Main article: Icy Bay (Alaska)

At 8:19 p.m. Alaska Daylight Time on October 17, 2015, the side of a mountain collapsed, at the head of Taan Fiord, a finger of Icy Bay in Alaska. Some of the resulting landslide came to rest on the toe of Tyndall Glacier, but about 180,000,000 short tons (161,000,000 long tons; 163,000,000 metric tons) of rock with a volume of about 50,000,000 cubic metres (65,400,000 cu yd) fell into the fjord. The landslide generated a megatsunami with an initial height of about 100 metres (330 feet) that struck the opposite shore of the fjord, with a run-up height there of 193 metres (633 feet).

Over the next 12 minutes, the wave travelled down the fjord at a speed of up to 97 kilometres per hour (60 mph), with run-up heights of over 100 metres (328 feet) in the upper fjord to between 30 and 100 metres (98 and 330 feet) or more in its middle section, and 20 metres (66 feet) or more at its mouth. Still probably 12 metres (40 feet) tall when it entered Icy Bay, the tsunami inundated parts of Icy Bay's shoreline with run-ups of 4 to 5 metres (13 to 16 feet) before dissipating into insignificance at distances of 5 kilometres (3.1 mi) from the mouth of Taan Fiord, although the wave was detected 140 kilometres (87 miles) away.

Occurring in an uninhabited area, the event was unwitnessed, and several hours passed before the signature of the landslide was noticed on seismographs at Columbia University in New York City.

2017: Karrat Fjord, Greenland

On June 17, 2017, 35,000,000 to 58,000,000 cubic metres (46,000,000 to 76,000,000 cu yd) of rock on the mountain Ummiammakku fell from an elevation of roughly 1,000 metres (3,280 ft) into the waters of the Karrat Fjord. The event was thought to be caused by melting ice that destabilised the rock. It registered as a magnitude 4.1 earthquake and created a 100-metre (328 ft) wave. The settlement of Nuugaatsiaq, 32 kilometres (20 mi) away, saw run-up heights of 9 metres (30 ft). Eleven buildings were swept into the sea, four people died, and 170 residents of Nuugaatsiaq and Illorsuit were evacuated because of a danger of additional landslides and waves. The tsunami was noted at settlements as far as 100 kilometres (62 mi) away.

2020: Elliot Creek, British Columbia, Canada

On 28 November 2020, unseasonably heavy rainfall triggered a landslide of 18,000,000 m³ (24,000,000 cu yd) into a glacial lake at the head of Elliot Creek. The sudden displacement of water generated a 100 m (330 ft) high megatsunami that cascaded down Elliot Creek and the Southgate River to the head of Bute Inlet, covering a total distance of over 60 km (37 mi). The event generated a magnitude 5.0 earthquake and destroyed over 8.5 km (5.3 mi) of salmon habitat along Elliot Creek.

Potential future megatsunamis

In a BBC television documentary broadcast in 2000, experts said that they thought that a landslide on a volcanic ocean island is the most likely future cause of a megatsunami. The size and power of a wave generated by such means could produce devastating effects, travelling across oceans and inundating up to 25 kilometres (16 mi) inland from the coast. This research was later found to be flawed. The documentary was produced before the experts' scientific paper was published and before responses were given by other geologists. There have been megatsunamis in the past, and future megatsunamis are possible but current geological consensus is that these are only local. A megatsunami in the Canary Islands would diminish to a normal tsunami by the time it reached the continents. Also, the current consensus for La Palma is that the region conjectured to collapse is too small and too geologically stable to do so in the next 10,000 years, although there is evidence for past megatsunamis local to the Canary Islands thousands of years ago. Similar remarks apply to the suggestion of a megatsunami in Hawaii.

British Columbia

Some geologists consider an unstable rock face at Mount Breakenridge, above the north end of the giant fresh-water fjord of Harrison Lake in the Fraser Valley of southwestern British Columbia, Canada, to be unstable enough to collapse into the lake, generating a megatsunami that might destroy the town of Harrison Hot Springs (located at its south end).

Canary Islands

Main article: Cumbre Vieja tsunami hazard

See also: Cumbre Vieja § Potential megatsunami

Geologists Dr. Simon Day and Dr. Steven Neal Ward consider that a megatsunami could be generated during an eruption of Cumbre Vieja on the volcanic ocean island of La Palma, in the Canary Islands, Spain. Day and Ward hypothesize that if such an eruption causes the western flank to fail, a megatsunami could be generated.

In 1949, an eruption occurred at three of the volcano's vents—Duraznero, Hoyo Negro, and Llano del Banco. A local geologist, Juan Bonelli-Rubio, witnessed the eruption and recorded details on various phenomenon related to the eruption. Bonelli-Rubio visited the summit area of the volcano and found that a fissure about 2.5 kilometres (1.6 mi) long had opened on the east side of the summit. As a result, the western half of the volcano—which is the volcanically active arm of a triple-armed rift—had slipped approximately 2 metres (7 ft) downwards and 1 metre (3 ft) westwards towards the Atlantic Ocean.

In 1971, an eruption occurred at the Teneguía vent at the southern end of the sub-aerial section of the volcano without any movement. The section affected by the 1949 eruption is currently stationary and does not appear to have moved since the initial rupture.

Cumbre Vieja remained dormant until an eruption began on September 19, 2021.

It is likely that several eruptions would be required before failure would occur on Cumbre Vieja. The western half of the volcano has an approximate volume of 500 cubic kilometres (120 cu mi) and an estimated mass of 1.5 trillion metric tons (1.7×10¹² short tons). If it were to catastrophically slide into the ocean, it could generate a wave with an initial height of about 1,000 metres (3,300 ft) at the island, and a likely height of around 50 metres (200 ft) at the Caribbean and the Eastern North American seaboard when it runs ashore eight or more hours later. Tens of millions of lives could be lost in the cities and/or towns of St. John's, Halifax, Boston, New York, Baltimore, Washington, D.C., Miami, Havana and the rest of the eastern coasts of the United States and Canada, as well as many other cities on the Atlantic coast in Europe, South America and Africa. The likelihood of this happening is a matter of vigorous debate.

Geologists and volcanologists are in general agreement that the initial study was flawed. The current geology does not suggest that a collapse is imminent. Indeed, it seems to be geologically impossible right now—the region conjectured as prone to collapse is too small and too stable to collapse within the next 10,000 years. A closer study of deposits left in the ocean from previous landslides suggests that a landslide would likely occur as a series of smaller collapses rather than a single landslide. A megatsunami does seem possible locally in the distant future as there is geological evidence from past deposits suggesting that a megatsunami occurred with marine material deposited 41 to 188 metres (135 to 617 ft) above sea level between 32,000 and 1.75 million years ago. This seems to have been local to Gran Canaria.

Day and Ward have admitted that their original analysis of the danger was based on several worst case assumptions. A 2008 study examined this scenario and concluded that while it could cause a megatsunami, it would be local to the Canary Islands and would diminish in height, becoming a smaller tsunami by the time it reached the continents as the waves interfered and spread across the oceans.

Hawaii

Sharp cliffs and associated ocean debris at the Kohala Volcano, Lanai and Molokai indicate that landslides from the flank of the Kilauea and Mauna Loa volcanoes in Hawaii may have triggered past megatsunamis, most recently at 120,000 BP. A tsunami event is also possible, with the tsunami potentially reaching up to about 1 kilometre (3,300 ft) in height According to the documentary National Geographic's Ultimate Disaster: Tsunami, if a big landslide occurred at Mauna Loa or the Hilina Slump, a 30-metre (98 ft) tsunami would take only thirty minutes to reach Honolulu. There, hundreds of thousands of people could be killed as the tsunami could level Honolulu and travel 25 kilometres (16 mi) inland. Also, the West Coast of America and the entire Pacific Rim could potentially be affected.

Other research suggests that such a single large landslide is not likely. Instead, it would collapse as a series of smaller landslides.

In 2018, shortly after the beginning of the 2018 lower Puna eruption, a National Geographic article responded to such claims with "Will a monstrous landslide off the side of Kilauea trigger a monster tsunami bound for California? Short answer: No."

In the same article, geologist Mika McKinnon stated:

there are submarine landslides, and submarine landslides do trigger tsunamis, but these are really small, localized tsunamis. They don't produce tsunamis that move across the ocean. In all likelihood, it wouldn't even impact the other Hawaiian islands.

Another volcanologist, Janine Krippner, added:

People are worried about the catastrophic crashing of the volcano into the ocean. There's no evidence that this will happen. It is slowly—really slowly—moving toward the ocean, but it's been happening for a very long time.

Despite this, evidence suggests that catastrophic collapses do occur on Hawaiian volcanoes and generate local tsunamis.

Norway

Although known earlier to the local population, a crack 2 metres (6.6 ft) wide and 500 metres (1,640 ft) in length in the side of the mountain Åkerneset in Norway was rediscovered in 1983 and attracted scientific attention. It since has widened at a rate of 4 centimetres (1.6 in) per year. Geological analysis has revealed that a slab of rock 62 metres (203 ft) thick and at an elevation stretching from 150 to 900 metres (492 to 2,953 ft) is in motion. Geologists assess that an eventual catastrophic collapse of 18,000,000 to 54,000,000 cubic metres (24,000,000 to 71,000,000 cu yd) of rock into Sunnylvsfjorden is inevitable and could generate megatsunamis of 35 to 100 metres (115 to 328 ft) in height on the fjord′s opposite shore. The waves are expected to strike Hellesylt with a height of 35 to 85 metres (115 to 279 ft), Geiranger with a height of 30 to 70 metres (98 to 230 ft), Tafjord with a height of 14 metres (46 ft), and many other communities in Norway's Sunnmøre district with a height of several metres, and to be noticeable even at Ålesund. The predicted disaster is depicted in the 2015 Norwegian film The Wave.

Surface-area-to-volume ratio

From Wikipedia, the free encyclopedia

Graphs of surface area, A against volume, V of the Platonic solids and a sphere, showing that the surface area decreases for rounder shapes, and the surface-area-to-volume ratio decreases with increasing volume. Their intercepts with the dashed lines show that when the volume increases 8 (2³) times, the surface area increases 4 (2²) times.

The surface-area-to-volume ratio or surface-to-volume ratio (denoted as SA:V, SA/V, or sa/vol) is the ratio between surface area and volume of an object or collection of objects.

SA:V is an important concept in science and engineering. It is used to explain the relation between structure and function in processes occurring through the surface and the volume. Good examples for such processes are processes governed by the heat equation, that is, diffusion and heat transfer by thermal conduction. SA:V is used to explain the diffusion of small molecules, like oxygen and carbon dioxide between air, blood and cells, water loss by animals, bacterial morphogenesis, organism's thermoregulation, design of artificial bone tissue, artificial lungs  and many more biological and biotechnological structures. For more examples see Glazier.

The relation between SA:V and diffusion or heat conduction rate is explained from flux and surface perspective, focusing on the surface of a body as the place where diffusion, or heat conduction, takes place, i.e., the larger the SA:V there is more surface area per unit volume through which material can diffuse, therefore, the diffusion or heat conduction, will be faster. Similar explanation appears in the literature: "Small size implies a large ratio of surface area to volume, thereby helping to maximize the uptake of nutrients across the plasma membrane", and elsewhere.

For a given volume, the object with the smallest surface area (and therefore with the smallest SA:V) is a ball, a consequence of the isoperimetric inequality in 3 dimensions. By contrast, objects with acute-angled spikes will have very large surface area for a given volume.

For solid spheres

Plot of the surface-area:volume ratio (SA:V) for a 3-dimensional ball, showing the ratio decline inversely as the radius of the ball increases.

A solid sphere or ball is a three-dimensional object, being the solid figure bounded by a sphere. (In geometry, the term sphere properly refers only to the surface, so a sphere thus lacks volume in this context.)

For an ordinary three-dimensional ball, the SA:V can be calculated using the standard equations for the surface and volume, which are, respectively, ${\displaystyle SA=4\pi r^2}$ and ${\displaystyle V=(4/3)\pi r^3}$. For the unit case in which r = 1 the SA:V is thus 3. For the general case, SA:V equals 3/r, in an inverse relationship with the radius - if the radius is doubled, the SA:V halves (see figure).

For n-dimensional balls

Balls exist in any dimension and are generically called n-balls or hyperballs, where n is the number of dimensions. The same reasoning can be generalized to n-balls using the general equations for volume and surface area, which are:

${\displaystyle V=\frac{r^n{\pi }^{n/2}}{\mathrm{\Gamma }(1+n/2)}}$

${\displaystyle SA=\frac{n{r}^{n-1}{\pi }^{n/2}}{\mathrm{\Gamma }(1+n/2)}}$

So the ratio equals ${\displaystyle SA/V=n{r}^{-1}}$. Thus, the same linear relationship between area and volume holds for any number of dimensions (see figure): doubling the radius always halves the ratio.

Dimension and units

The surface-area-to-volume ratio has physical dimension inverse length (L⁻¹) and is therefore expressed in units of inverse metre (m^-1) or its prefixed unit multiples and submultiples. As an example, a cube with sides of length 1 cm will have a surface area of 6 cm² and a volume of 1 cm³. The surface to volume ratio for this cube is thus

${\displaystyle \text{SA:V}=\frac{6{\text{cm}}^2}{1{\text{cm}}^3}=6{\text{cm}}^{-1}}$.

For a given shape, SA:V is inversely proportional to size. A cube 2 cm on a side has a ratio of 3 cm⁻¹, half that of a cube 1 cm on a side. Conversely, preserving SA:V as size increases requires changing to a less compact shape.

Applications

Physical chemistry

See also: Dust explosion

Materials with high surface area to volume ratio (e.g. very small diameter, very porous, or otherwise not compact) react at much faster rates than monolithic materials, because more surface is available to react. An example is grain dust: while grain is not typically flammable, grain dust is explosive. Finely ground salt dissolves much more quickly than coarse salt.

A high surface area to volume ratio provides a strong "driving force" to speed up thermodynamic processes that minimize free energy.

Biology

Cells lining the small intestine increase the surface area over which they can absorb nutrients with a carpet of tuftlike microvilli.

The ratio between the surface area and volume of cells and organisms has an enormous impact on their biology, including their physiology and behavior. For example, many aquatic microorganisms have increased surface area to increase their drag in the water. This reduces their rate of sink and allows them to remain near the surface with less energy expenditure.

An increased surface area to volume ratio also means increased exposure to the environment. The finely-branched appendages of filter feeders such as krill provide a large surface area to sift the water for food.

Individual organs like the lung have numerous internal branchings that increase the surface area; in the case of the lung, the large surface supports gas exchange, bringing oxygen into the blood and releasing carbon dioxide from the blood. Similarly, the small intestine has a finely wrinkled internal surface, allowing the body to absorb nutrients efficiently.

Cells can achieve a high surface area to volume ratio with an elaborately convoluted surface, like the microvilli lining the small intestine.

Increased surface area can also lead to biological problems. More contact with the environment through the surface of a cell or an organ (relative to its volume) increases loss of water and dissolved substances. High surface area to volume ratios also present problems of temperature control in unfavorable environments.

The surface to volume ratios of organisms of different sizes also leads to some biological rules such as Allen's rule, Bergmann's rule and gigantothermy.

Fire spread

In the context of wildfires, the ratio of the surface area of a solid fuel to its volume is an important measurement. Fire spread behavior is frequently correlated to the surface-area-to-volume ratio of the fuel (e.g. leaves and branches). The higher its value, the faster a particle responds to changes in environmental conditions, such as temperature or moisture. Higher values are also correlated to shorter fuel ignition times, and hence faster fire spread rates.

Planetary cooling

A body of icy or rocky material in outer space may, if it can build and retain sufficient heat, develop a differentiated interior and alter its surface through volcanic or tectonic activity. The length of time through which a planetary body can maintain surface-altering activity depends on how well it retains heat, and this is governed by its surface area-to-volume ratio. For Vesta (r=263 km), the ratio is so high that astronomers were surprised to find that it did differentiate and have brief volcanic activity. The moon, Mercury and Mars have radii in the low thousands of kilometers; all three retained heat well enough to be thoroughly differentiated although after a billion years or so they became too cool to show anything more than very localized and infrequent volcanic activity. As of April 2019, however, NASA has announced the detection of a "marsquake" measured on April 6, 2019, by NASA's InSight lander. Venus and Earth (r>6,000 km) have sufficiently low surface area-to-volume ratios (roughly half that of Mars and much lower than all other known rocky bodies) so that their heat loss is minimal.

Late Devonian extinction

From Wikipedia, the free encyclopedia
https://en.wikipedia.org/wiki/Late_Devonian_extinction

The Late Devonian extinction consisted of several extinction events in the Late Devonian Epoch, which collectively represent one of the five largest mass extinction events in the history of life on Earth. The term primarily refers to a major extinction, the Kellwasser event, also known as the Frasnian-Famennian extinction, which occurred around 372 million years ago, at the boundary between the Frasnian stage and the Famennian stage, the last stage in the Devonian Period. Overall, 19% of all families and 50% of all genera became extinct. A second mass extinction called the Hangenberg event, also known as the end-Devonian extinction, occurred 359 million years ago, bringing an end to the Famennian and Devonian, as the world transitioned into the Carboniferous Period.

Although it is well established that there was a massive loss of biodiversity in the Late Devonian, the timespan of this event is uncertain, with estimates ranging from 500,000 to 25 million years, extending from the mid-Givetian to the end-Famennian. Some consider the extinction to be as many as seven distinct events, spread over about 25 million years, with notable extinctions at the ends of the Givetian, Frasnian, and Famennian stages.

By the Late Devonian, the land had been colonized by plants and insects. In the oceans, massive reefs were built by corals and stromatoporoids. Euramerica and Gondwana were beginning to converge into what would become Pangaea. The extinction seems to have only affected marine life. Hard-hit groups include brachiopods, trilobites, and reef-building organisms; the latter almost completely disappeared. The causes of these extinctions are unclear. Leading hypotheses include changes in sea level and ocean anoxia, possibly triggered by global cooling or oceanic volcanism. The impact of a comet or another extraterrestrial body has also been suggested, such as the Siljan Ring event in Sweden. Some statistical analysis suggests that the decrease in diversity was caused more by a decrease in speciation than by an increase in extinctions. This might have been caused by invasions of cosmopolitan species, rather than by any single event. Placoderms were hit hard by the Kellwasser event and completely died out in the Hangenberg event, but most other jawed vertebrates were less strongly impacted. Agnathans (jawless fish) were in decline long before the end of the Frasnian and were nearly wiped out by the extinctions.

The extinction event was accompanied by widespread oceanic anoxia; that is, a lack of oxygen, prohibiting decay and allowing the preservation of organic matter. This, combined with the ability of porous reef rocks to hold oil, has led to Devonian rocks being an important source of oil, especially in Canada and the United States.

Late Devonian world

Devonian graphical timeline
−420 — – −415 — – −410 — – −405 — – −400 — – −395 — – −390 — – −385 — – −380 — – −375 — – −370 — – −365 — – −360 — –	Paleozoic Silurian Devonian Carboniferous Early Mid Late Lochkovian Pragian Emsian Eifelian Givetian Frasnian Famennian	← Rhynie chert ← Hangenberg event, Famennian glaciation ← Kellwasser event (Late Devonian extinction) ← Widespread shrubs & trees ← Hunsrück fauna
Subdivision of the Devonian according to the ICS, as of 2021. Vertical axis scale: millions of years ago.

During the Late Devonian, the continents were arranged differently from today, with a supercontinent, Gondwana, covering much of the Southern Hemisphere. The continent of Siberia occupied the Northern Hemisphere, while an equatorial continent, Laurussia (formed by the collision of Baltica and Laurentia), was drifting towards Gondwana, closing the Rheic Ocean. The Caledonian mountains were also growing across what is now the Scottish Highlands and Scandinavia, while the Appalachians rose over America.

The biota was also very different. Plants, which had been on land in forms similar to mosses and liverworts since the Ordovician, had just developed roots, seeds, and water transport systems that allowed them to survive away from places that were constantly wet—and so grew huge forests on the highlands. Several clades had developed a shrubby or tree-like habit by the Late Givetian, including the cladoxylalean ferns, lepidosigillarioid lycopsids, and aneurophyte and archaeopterid progymnosperms. Fish were also undergoing a huge radiation, and tetrapodomorphs, such as the Frasnian-age Tiktaalik, were beginning to evolve leg-like structures.

Extinction patterns

The Kellwasser event and most other Later Devonian pulses primarily affected the marine community, and had a greater effect on shallow warm-water organisms than on cool-water organisms. The Kellwasser event's effects were also stronger at low latitudes than high ones. Large differences are observed between the biotas before and after the Frasnian-Famennian boundary, demonstrating the extinction event's magnitude.

Reef destruction

Side view of a stromatoporoid showing laminae and pillars; Columbus Limestone (Devonian) of Ohio

The most hard-hit biological category affected by the Kellwasser event were the calcite-based reef-builders of the great Devonian reef-systems, including the stromatoporoid sponges and the rugose and tabulate corals. It left communities of beloceratids and manticoceratids devastated. Following the Kellwasser event, reefs of the Famennian were primarily dominated by siliceous sponges and calcifying bacteria, producing structures such as oncolites and stromatolites, although there is evidence this shift in reef composition began prior to the Frasnian-Famennian boundary. The collapse of the reef system was so stark that it would take until the Mesozoic for reefs to recover their Middle Devonian extent. Mesozoic and modern reefs are based on scleractinian ("stony") corals, which would not evolve until the Triassic period. Devonian reef-builders are entirely extinct in the modern day: Stromatoporoids died out in the end-Devonian Hangenberg event, while rugose and tabulate corals went extinct at the Permian-Triassic extinction.

Marine invertebrates

Further taxa to be starkly affected include the brachiopods, trilobites, ammonites, conodonts, acritarch and graptolites. Cystoids disappeared during this event. The surviving taxa show morphological trends through the event. Atrypid and strophomenid brachiopods became rarer, replaced in many niches by productids, whose spiny shells made them more resistant to predation and environmental disturbances. Trilobites evolved smaller eyes in the run-up to the Kellwasser event, with eye size increasing again afterwards. This suggests vision was less important around the event, perhaps due to increasing water depth or turbidity. The brims of trilobites (i.e. the rims of their heads) also expanded across this period. The brims are thought to have served a respiratory purpose, and the increasing anoxia of waters led to an increase in their brim area in response. The shape of conodonts' feeding apparatus varied with the oxygen isotope ratio, and thus with the sea water temperature; this may relate to their occupying different trophic levels as nutrient input changed. As with most extinction events, specialist taxa occupying small niches were harder hit than generalists. Marine invertebrates that lived in warmer ecoregions were devastated more compared to those living in colder biomes.

Vertebrates

Tiktaalik, an early air-breathing elpistostegalian. They were among the vertebrates which died out due to the Kellwasser event

Vertebrates were not strongly affected by the Kellwasser event, but still experienced some diversity loss. Around half of placoderm families died out, primarily species-poor bottom-feeding groups. More diverse placoderm families survived the event only to succumb in the Hangenberg event at the end of the Devonian. Most lingering agnathan (jawless fish) groups, such as osteostracans, galeaspids, and heterostracans, also went extinct by the end of the Frasnian. The jawless thelodonts only barely survived, succumbing early in the Famennian. Among freshwater and shallow marine tetrapodomorph fish, the tetrapod-like elpistostegalians (such as Tiktaalik) disappeared at the Frasnian-Famennian boundary. True tetrapods (defined as four-limbed vertebrates with digits) survived and experienced an evolutionary radiation following the Kellwasser extinction, though their fossils are rare until the mid-to-late Famennian.

Magnitude of diversity loss

The late Devonian crash in biodiversity was more drastic than the familiar extinction event that closed the Cretaceous. A recent survey (McGhee 1996) estimates that 22% of all the 'families' of marine animals (largely invertebrates) were eliminated. The family is a great unit, and to lose so many signifies a deep loss of ecosystem diversity. On a smaller scale, 57% of genera and at least 75% of species did not survive into the Carboniferous. These latter estimates need to be treated with a degree of caution, as the estimates of species loss depend on surveys of Devonian marine taxa that are perhaps not well enough known to assess their true rate of losses, so it is difficult to estimate the effects of differential preservation and sampling biases during the Devonian.

Duration and timing

Extinction rates appear to have been higher than the background rate for an extended interval covering the last 20–25 million years of the Devonian. During this time, about eight to ten distinct events can be seen, of which two, the Kellwasser and the Hangenberg events, stand out as particularly severe. The Kellwasser event was preceded by a longer period of prolonged biodiversity loss.

The Kellwasser event, named for its type locality, the Kellwassertal in Lower Saxony, Germany, is the term given to the extinction pulse that occurred near the Frasnian–Famennian boundary (372.2 ± 1.6 Ma). Most references to the "Late Devonian extinction" are in fact referring to the Kellwasser, which was the first event to be detected based on marine invertebrate record and was the most severe of the extinction crises of the Late Devonian. There may in fact have been two closely spaced events here, as shown by the presence of two distinct anoxic shale layers.

There is evidence that the Kellwasser event was a two-pulsed event, with the two extinction pulses being separated by an interval of approximately 800,000 years. The second pulse was more severe than the first.

Potential causes

Since the Kellwasser-related extinctions occurred over such a long time, it is difficult to assign a single cause, and indeed to separate cause from effect. From the end of the Middle Devonian (382.7±1.6 Ma), into the Late Devonian (382.7±1.6 Ma to 358.9±0.4 Ma), several environmental changes can be detected from the sedimentary record, which directly affected organisms and caused extinction. What caused these changes is somewhat more open to debate. Possible triggers for the Kellwasser event are as follows:

Weathering and anoxia

During the Late Silurian and Devonian, land plants, assisted by fungi, underwent a hugely significant phase of evolution known as the Silurian-Devonian Terrestrial Revolution. Their maximum height went from 30 cm at the start of the Devonian, to 30 m archaeopterids, at the end of the period. This increase in height was made possible by the evolution of advanced vascular systems, which permitted the growth of complex branching and rooting systems, facilitating their ability to colonise drier areas previously off limits to them. In conjunction with this, the evolution of seeds permitted reproduction and dispersal in areas which were not waterlogged, allowing plants to colonise previously inhospitable inland and upland areas. The two factors combined to greatly magnify the role of plants on the global scale. In particular, Archaeopteris forests expanded rapidly during the closing stages of the Devonian. These tall trees required deep rooting systems to acquire water and nutrients, and provide anchorage. These systems broke up the upper layers of bedrock and stabilized a deep layer of soil, which would have been of the order of metres thick. In contrast, early Devonian plants bore only rhizoids and rhizomes that could penetrate no more than a few centimeters. The mobilization of a large portion of soil had a huge effect: soil promotes weathering, the chemical breakdown of rocks, releasing ions which are nutrients for plants and algae.

The relatively sudden input of nutrients into river water as rooted plants expanded into upland regions may have caused eutrophication and subsequent anoxia. For example, during an algal bloom, organic material formed at the surface can sink at such a rate that decomposition of dead organisms uses up all available oxygen, creating anoxic conditions and suffocating bottom-dwelling fish. The fossil reefs of the Frasnian were dominated by stromatoporoids and (to a lesser degree) corals—organisms which only thrive in low-nutrient conditions. Therefore, the postulated influx of high levels of nutrients may have caused an extinction. Anoxic conditions correlate better with biotic crises than phases of cooling, suggesting anoxia may have played the dominant role in extinction. Evidence exists of a rapid increase in the rate of organic carbon burial and for widespread anoxia in oceanic bottom waters. Signs of anoxia in shallow waters have also been described from a variety of localities. Good evidence has been found for high-frequency sea-level changes around the Frasnian–Famennian Kellwasser event, with one sea-level rise associated with the onset of anoxic deposits; marine transgressions likely helped spread deoxygenated waters. Evidence exists for the modulation of the intensity of anoxia by Milankovitch cycles as well. Negative δ²³⁸U excursions concomitant with both the Lower and Upper Kellwasser events provide direct evidence for an increase in anoxia. Photic zone euxinia, documented by concurrent negative ∆¹⁹⁹Hg and positive δ²⁰²Hg excursions, occurred in the North American Devonian Seaway. Elevated molybdenum concentrations further support widespread euxinic waters.

The timing, magnitude, and causes of Kellwasser anoxia remain poorly understood. Anoxia was not omnipresent across the globe; in some regions, such as South China, the Frasnian-Famennian boundary instead shows evidence of increased oxygenation of the seafloor. Trace metal proxies in black shales from New York state point to anoxic conditions only occurring intermittently, being interrupted by oxic intervals, further indicating that anoxia was not globally synchronous, a finding also supported by the prevalence of cyanobacterial mats in the Holy Cross Mountains in the time period around the Kellwasser event. Evidence from various European sections reveals that Kellwasser anoxia was relegated to epicontinental seas and developed as a result of upwelling of poorly oxygenated waters within ocean basins into shallow waters rather than a global oceanic anoxic event that intruded into epicontinental seas.

Global cooling

A positive δ¹⁸O excursion is observed across the Frasnian-Famennian boundary in brachiopods from North America, Germany, Spain, Morocco, Siberia, and China; conodont apatite δ¹⁸O excursions also occurred at this time. A similar positive δ¹⁸O excursion in phosphates is known from the boundary, corresponding to a removal of atmospheric carbon dioxide and a global cooling event. This oxygen isotope excursion is known from time-equivalent strata in South China and in the western Palaeotethys, suggesting it was a globally synchronous climatic change. The concomitance of the drop in global temperatures and the swift decline of metazoan reefs indicates the blameworthiness of global cooling in precipitating the extinction event.

The "greening" of the continents during the Silurian-Devonian Terrestrial Revolution that led to them being covered with massive photosynthesizing land plants in the first forests reduced CO₂ levels in the atmosphere. Since CO₂ is a greenhouse gas, reduced levels might have helped produce a chillier climate, in contrast to the warm climate of the Middle Devonian. The biological sequestration of carbon dioxide may have ultimately led to the beginning of the Late Palaeozoic Ice Age during the Famennian, which has been suggested as a cause of the Hangenberg event.

The weathering of silicate rocks also draws down CO₂ from the atmosphere, and CO₂ sequestration by mountain building has been suggested as a cause of the decline in greenhouse gases during the Frasnian-Famennian transition. This mountain-building may have also enhanced biological sequestration through an increase in nutrient runoff. The combination of silicate weathering and the burial of organic matter to decreased atmospheric CO₂ concentrations from about 15 to three times present levels. Carbon in the form of plant matter would be produced on prodigious scales, and given the right conditions, could be stored and buried, eventually producing vast coal measures (e.g. in China) which locked the carbon out of the atmosphere and into the lithosphere. This reduction in atmospheric CO₂ would have caused global cooling and resulted in at least one period of late Devonian glaciation (and subsequent sea level fall), probably fluctuating in intensity alongside the 40ka Milankovic cycle. The continued drawdown of organic carbon eventually pulled the Earth out of its greenhouse state during the Famennian into the icehouse that continued throughout the Carboniferous and Permian.

Volcanism

Magmatism was suggested as a cause of the Late Devonian extinction in 2002. The end of the Devonian Period had extremely widespread trap magmatism and rifting in the Russian and Siberian platforms, which were situated above the hot mantle plumes and suggested as a cause of the Frasnian / Famennian and end-Devonian extinctions. The Viluy Large igneous province, located in the Vilyuysk region on the Siberian Craton, covers most of the present day north-eastern margin of the Siberian Platform. The triple-junction rift system was formed during the Devonian Period; the Viluy rift is the western remaining branch of the system and two other branches form the modern margin of the Siberian Platform. Volcanic rocks are covered with post Late Devonian–Early Carboniferous sediments. Volcanic rocks, dyke belts, and sills that cover more than 320,000 km², and a gigantic amount of magmatic material (more than 1 million km³) formed in the Viluy branch. The Viluy and Pripyat-Dnieper-Donets large igneous provinces were suggested to correlate with the Frasnian / Famennian extinction, with the Kola and Timan-Pechora magmatic provinces being suggested to be related to the Hangenberg event at the Devonian-Carboniferous boundary. Viluy magmatism may have injected enough CO₂ and SO₂ into the atmosphere to have generated a destabilised greenhouse and ecosystem, causing rapid global cooling, sea-level falls, and marine anoxia to occur during Kellwasser black shale deposition. Viluy Traps activity may have also enabled euxinia by fertilising the oceans with sulphate, increasing rates of microbial sulphate reduction.

Recent studies have confirmed a correlation between Viluy traps in the Vilyuysk region on the Siberian Craton and the Kellwasser extinction by ⁴⁰Ar/³⁹Ar dating. Ages show that the two volcanic phase hypotheses are well supported and the weighted mean ages of each volcanic phase are 376.7±3.4 and 364.4±3.4 Ma, or 373.4±2.1 and 363.2±2.0 Ma, which the first volcanic phase is in agreement with the age of 372.2±3.2 Ma proposed for the Kellwasser event. However, the second volcanic phase is slightly older than Hangenberg event, which is dated to around 358.9±1.2 Ma.

Coronene and mercury enrichment has been found in deposits dating back to the Kellwasser event, with similar enrichments found in deposits coeval with the Frasnes event at the Givetian-Frasnian boundary and in ones coeval with the Hangenberg event. Because coronene enrichment is only known in association with large igneous province emissions and extraterrestrial impacts and the fact that there is no confirmed evidence of the latter occurring in association with the Kellwasser event, this enrichment strongly suggests a causal relationship between volcanism and the Kellwasser extinction event. However, not all sites show evidence of mercury enrichment across the Frasnian-Famennian boundary, leading other studies to reject volcanism as an explanation for the crisis.

Another overlooked contributor to the Kellwasser mass extinction could be the now extinct Cerberean Caldera which was active in the Late Devonian period and thought to have undergone a supereruption approximately 374 million years ago. Remains of this caldera can be found in the modern day state of Victoria, Australia. Eovariscan volcanic activity in present-day Europe may have also played a role in conjunction with the Viluy Traps.

Impact event

Bolide impacts can be dramatic triggers of mass extinctions. An asteroid impact was proposed as the prime cause of this faunal turnover. The impact that created the Siljan Ring either was just before the Kellwasser event or coincided with it. Most impact craters, such as the Kellwasser-aged Alamo, cannot generally be dated with sufficient precision to link them to the event; others dated precisely are not contemporaneous with the extinction. Although some evidence of meteoric impact have been observed in places, including iridium anomalies and microspherules, these were probably caused by other factors. Some lines of evidence suggest that the meteorite impact and its associated geochemical signals postdate the extinction event. Modelling studies have ruled out a single impact as entirely inconsistent with available evidence, although a multiple impact scenario may still be viable.

Supernova

Near-Earth supernovae have been speculated as possible drivers of mass extinctions due to their ability to cause ozone depletion. A recent explanation suggests that a nearby supernova explosion was the cause for the specific Hangenberg event, which marks the boundary between the Devonian and Carboniferous periods. This could offer a possible explanation for the dramatic drop in atmospheric ozone during the Hangenberg event that could have permitted massive ultraviolet damage to the genetic material of lifeforms, triggering a mass extinction. Recent research offers evidence of ultraviolet damage to pollen and spores over many thousands of years during this event as observed in the fossil record and that, in turn, points to a possible long-term destruction of the ozone layer. A supernova explosion is an alternative explanation to global temperature rise, that could account for the drop in atmospheric ozone. Because very high mass stars, required to produce a supernova, tend to form in dense star-forming regions of space and have short lifespans lasting only at most tens of millions of years, it is likely that if a supernova did occur, multiple others also did within a few million years of it. Thus, supernovae have also been speculated to have been responsible for the Kellwasser event, as well as the entire sequence of environmental crises covering several millions of years towards the end of the Devonian period. Detecting either of the long-lived, extra-terrestrial radioisotopes ¹⁴⁶Sm or ²⁴⁴Pu in one or more end-Devonian extinction strata would confirm a supernova origin. However, there is currently no direct evidence for this hypothesis.

Other hypotheses

Other mechanisms put forward to explain the extinctions include tectonic-driven climate change, sea-level change, and oceanic overturning. These have all been discounted because they are unable to explain the duration, selectivity, and periodicity of the extinctions.