What causes the giant Hawaiian volcanoes? Scientists have devised a way of finding out.
The Hawaiian volcanoes are higher than Mount Everest when measured from the bottom of the ocean. What creates such giant mountains?
Many geochemists believe that the Hawaiian volcanoes, like many other smaller ones in mid-oceans, are caused by hot narrow plumes of material rising from nearly 3,000km below the surface of the earth.
The earth is a boiling sphere with a thin skin at the surface, which appears solid. This skin is continuously torn into pieces and welded together again. Satellite observations watch this process from space, although the motions involved are not much faster than the growth of our fingernails. The reason for this mobility is heat rising to the surface from the earth's interior in mid-oceanic volcanoes and cold surface material sinking back into the interior at deep-sea trenches. This is the simple principle of a cooking pot.
If this hypothesis is true, there should be unusually high temperatures at great depths underneath Hawaii. How can temperature be measured at these depths? A possible way is as follows: from laboratory experiments simulating conditions at such depths, we know that with increasing pressure, molecules are packed more densely in two steps, or phase changes. Each step is accompanied by a sudden increase in density and speed of the elastic waves travelling through it.
For elastic waves, two discontinuities exist in density and speed at globally averaged depths of 410km and 660km (called the "410" and "660"). These discontinuities are usually associated with the above mentioned phase changes. With laterally increasing heat, the discontinuity at 410km depth moves downwards and the one at 660km moves upwards. The amount of depth variation is proportional to the increase in temperature. Thus the problem of measuring temperature at 410km and 660km depth is reduced to the problem of mapping with high lateral resolution the exact depth of both discontinuities.
This can be achieved. If an earthquake occurs at a large distance from a seismic recording station, elastic waves travel through the 410 and the 660 from below, before they reach the station. While passing these discontinuities, weak secondary waves are generated, which can be isolated in the records. The difficulty is to identify these weak signals. From their travel times and the known velocities in the earth, we can obtain a good estimate of the locally varying depth of the 410 and the 660. Depth differences for both discontinuities can be measured even more accurately.
In a collaborative project between the GeoForschungsZentrum Potsdam and the University of Cambridge, this technique has been applied to earthquake records from Hawaii, partly made available by the American research institution. We found an updoming of the 660, indicating a temperature 300C higher than normal about 200km southwest of Hawaii. We have interpreted this as the trace of an inclined hot plume at 660km depth, which makes it likely that the origin of the Hawaiian volcanoes is much deeper in the earth, confirming our hypothesis. We are carrying out an extended experiment with stations on the Hawaiian islands in cooperation with the Dublin Institute for Advanced Studies and the University of Hawaii. The full extent of the hot plume can, however, be determined only when a larger network, including ocean-bottom seismic stations, is installed.
Rainer Kind and Stephan V. Sobolev are based at the GeoForschungsZentrum, Potsdam, Germany.