Volcanic ash in the sea - a valuable source of information

Scientists draw important conclusions from previous eruptions to predict future eruptions and their strength.

Large amounts of ash, gas and foreign rock are frequently hurled from the Earth's crust into the atmosphere at speeds of up to 300 km/h during highly explosive volcanic eruptions. The eruption column can extend into the stratosphere to altitudes of 20 - 40 kilometres. The ash cloud spreads out in the stratosphere, where it is sometimes caused to drift by the wind. In the vicinity of the volcano, the larger particles rain back down again. Fine ash and gas, in contrast, can remain for a long time in the stratosphere. In eruptions that occur near to the equator, the particles are sometimes distributed around the entire world.

As magmas with a very high gas content tend to ascend along the edges of the tectonic plates, explosive volcanic eruptions frequently occur near to the oceans. A portion of the ash is deposited on land, another part ends up in the sea. The shelf of a continent covered by water is usually quite flat. Fewer landslides occur on such shallow slopes than on steep slopes. In the search for depositional sequences that have been disturbed as little as possi­ble, researchers usually select areas in which there are no strong currents and where as little sediment as possible has been displaced by drifting. The prospects of finding well-preserved depositional sequences of marine sediments and intermediate volcanic ash are best in such places. These series are frequently better preserved than on land, where rainfall, wind and, last but not least, humans have often changed the volcanic loose rock by rearrangement.

Many more eruptions than previously assumed in the past

Sampling sea-floor sediment in boreholes drilled by a research ship allows reliable insights into the temporal sequence of volcanic eruptions. Valuable cores were obtained, for exam­ple, from a series of boreholes in the Cocos and Caribbean Plates off the west coast of Latin America, where tephra layers of past volcanic eruptions were well preserved. The Tephra sequence thus derived, on the one hand permitted completion of the eruption's chronology, as known from work on the mainland. This brought some previously unknown eruptions to light and made it clear that in the past 500,000 years, many more large eruptions have occurred than previously assumed. This information is important for assessing the risk of future eruptions.

On the other hand, the volumes of tephra produced can be determined more accurately with the help of the new findings. It is essential to establish the dimensions of past volcanic erup­tions to be able to interpret their strength for the prediction of future eruptions. They are also an essential prerequisite for estimating the climate-relevant greenhouse gases that were released during the eruption. While outcrops on land primarily allow documentation of the short and intermediate range of the deposit fan, mapping tephra on the seabed also makes it possible to track the particles that have travelled long distances. In this way, it was discov­ered at the Central American Volcanic Arc, that the total amount of ejected material is un­derestimated by at least 60 percent if only the deposit fans on land are taken into considera­tion.

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