# Volcano Watch — Proposed experiment to discover source of Hawaiian volcanoes

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When you visit the coast to watch lava pour into the sea, do you ever wonder where the lava came from and what path it took to the surface? We earth scientists do.

When you visit the coast to watch lava pour into the sea, do you ever wonder where the lava came from and what path it took to the surface? We earth scientists do. Unfortunately lava is not very talkative, so we look for clues by studying the chemical composition of the lava, how the volcano changes in shape when magma moves within it, and the distribution of earthquakes within the volcano. From these studies we have learned that in the oceans there are many volcanic island chains like Hawaii, for example the island groups of Marquesas, Tahiti, Pitcairn, Samoa, Galapagos, Easter, Iceland, and Canary. We call these "hotspot" volcanoes. They are chemically distinct from volcanoes like Mount St. Helens, Mount Pinatubo, Mount Fiji, and Krakatau, which occur where the seafloor sinks deep into the earth. Geologists have yet to discover the cause or causes of hotspot volcanism, but we have hope that we will not have to wait much longer.

With the support of the National Science Foundation and the IRIS Consortium, seismologists, who study earthquakes and the propagation of earthquake waves through the earth, have begun to study the earthquake waves that travel deeply under active hotspot volcanoes. The seismologists place seismometers, which measure minute ground shaking, in lines hundreds of kilometers (hundreds of miles) long. When waves of a distant earthquake travel beneath the hotspot, those waves moving through hotter magma-rich rocks travel more slowly than those passing though colder magma-poor rocks. By carefully studying the relative arrival times of earthquake waves along the line of seismometers, seismologists can deduce the shape of the deep source of hotspot volcanism. This deduction process is similar to that of a CT-scan you get a hospital.

Preliminary studies have already been done by the Carnegie Institution of Washington at Iceland and by the University of Oregon at an old part of Yellowstone, which is a continental hotspot. They indicate that the minimum depth to the source is at least 200 km (130 miles) and possibly 600 km (400 miles), which is one-tenth of the distance to the center of the earth. With the support of the National Science Foundation and the IRIS Consortium, these groups hope soon to study Hawaii and the active part of Yellowstone, respectively. We hope in a few years to be able to show you a picture of the geometry of the deep volcanic source beneath Hawaii.

The staff and associates of the U.S. Geological Survey's Hawaiian Volcano Observatory (HVO) extend a warm and hearty holiday greeting to all of the faithful readers of this column. Your interest in keeping abreast of volcanic and seismic events and in being informed of geologic hazards and ways to mitigate those hazards has sustained our efforts in producing this column.

### Volcano Activity Update

There is no change in the eruptive activity of Kīlauea Volcano. Lava continues to erupt from Puu Oo and flow through a network of tubes from the vent to the sea. No surface flows from breakouts of the tube system were observed on the coastal flats. Lava is entering the ocean near Kamokuna. The public is reminded that the ocean entry areas are extremely hazardous, with explosions accompanying frequent collapses of the new land. The steam clouds are highly acidic and laced with glass particles.

There were no earthquakes reported felt since December 9, 1998.