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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Megatsunami | 4/10 | https://en.wikipedia.org/wiki/Megatsunami | reference | science, encyclopedia | 2026-05-05T07:35:36.838495+00:00 | kb-cron |
=== 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 Eswatini, 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 megatsunami that probably extended to a depth of thousands of meters beneath the surface of the ocean and probably 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 likely been 4.6 kilometres (2.9 mi) tall. Among the mechanisms triggering megatsunamis were 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 (ca. 7.25–ca. 5.3 million years ago) various megatsunamis likely struck the coast of northern Chile. Reservoir-induced seismicity at the end of or shortly after the Zanclean Flood (ca. 5.33 million years ago), which rapidly filled the Mediterranean Basin with water from the Atlantic Ocean, created a megatsunami with a height of nearly 100 metres (330 ft) which struck the coast of Spain near what is now Algeciras. A megatsunami affected the coast of south–central Chile in the Pliocene as evidenced by the sedimentary record of the 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 170 to 240 metres (558 to 787 ft) and a run-up height of over 270 metres (886 ft). The wave swept 770-tonne (760-long-ton; 850-short-ton) boulders 600 metres (2,000 ft) inland and deposited them 200 metres (656 ft) above sea level 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 5650 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 5350 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).