Tsunamis are famously caused by earthquakes or landslides. Now it turns out that microscopic creatures invisible to the eye are just as guilty, by generating staggeringly large landslides.
The role of diatoms in tsunami generation was elucidated by scientists wondering exactly how extremely shallow yet disastrous submarine landslides happen.
"Mega-slides can move thousands of cubic kilometers of material, and sometimes trigger tsunamis. Yet, remarkably, they occur on nearly flat slopes of less than 3 degrees," marvels the Geological Society of America.
How could a landslide, let alone a "mega-slide," happen on nearly flat surfaces? Marine geoscientist Morelia Urlaub of the Geomar Helmholtz Center for Ocean Research in Kiel, Germany and colleagues think they found the smoking, or in this case, oozing, gun: layers of diatom microfossils and clay, they write in GeoScience World.
Diatoms are one of the great enigmas of science, being neither plant nor animal nor, heaven forfend, fungus. They are enormously diverse in shape and form. Their cell walls contain silica, and they have been around for hundreds of millions of years. At least 100,000 types of these one-celled things have been identified.
Meanwhile, studying the geology underlying catastrophic slides has been complicated because the critical weak layers – for instance of diatom fossils or clay – usually get destroyed in the slides.
The eureka moment for Urlaub was studying a seabed core from just outside the Cap Blanc slide, a 149,000 year-old mega-slide off northwest Africa. That and other data revealed diatom-rich layers, up to ten meters thick, that had underlain the mega-slide complex.
Each diatomaceous layer proved to be topped by clay-rich sediment, which is crucial to the development of a submarine slide down a shallow slope.
"Diatom layers are very compressible and water-rich," Urlaub explains. When pressure mounts, water is squeezed from the diatom layer into the clay, which doesn't like water. The whole thing ends when the clay layer, or the interface between the clay and diatoms, fails, and the sediment and rock above them slide down, no matter how shallow the slope.
Such landslides can be tremendous. "At the Cap Blanc slide, the seafloor slopes at just 2.8 degrees. Yet when it broke loose, the slide transported over 30 cubic kilometers of material, and extended at least 35 kilometers," write Urlaub and the team.
Diatoms may even be behind the largest landslides known to Man, called the "Storegga Slides," which happened off the Norwegian coast around 6225 B.C.E. to 6170 B.C.E. – over 8,000 years ago. A vast stretch of submarine coastal rock, along 290 kilometers collapsed.the volume of rock that moved is estimated at 3,500 square kilometers. That in turn led to tsunamis in the Atlantic. The dating of this catastrophe is based on carbon-dating organic material deposited by these great waves. Until now, geologists assumed the cause of the collapse had been the explosive release of methane hydrates.
Some scientists suspect the 2011 Tohoku tsunami in Japan may have been amplified by a submarine mega-slide, that could have been caused by the failure of diatomaceous and clay interfaces.
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