Since 1983 Kaj Hoernle has been studying a couple of hot spots: the Canary and Madeira Islands, two volcanic chains located in the Atlantic southwest of Gibraltar. Like other hot spots--Hawaii, for instance--these islands are thought to have been formed by narrow, pipe-shaped plumes of hot rock rising from deep in Earth’s mantle. Hoernle, a geochemist at GEOMAR, a marine geology institute in Kiel, Germany, has been measuring the chemical fingerprints of rocks on the islands. Last March he reported some bizarre results: those supposedly distinctive fingerprints look much like the ones of rocks found far away--on Mount Etna in Sicily, for instance, and as far off as Germany.
If the Madeiras and Canaries were formed by isolated plumes of mantle rock, why should their ratios of lead, strontium, and neodymium isotopes resemble ratios seen thousands of miles away? Hoernle has an idea: he thinks the hot spots are really part of a vast hot sheet of rock, 1,500 miles by 2,500 miles, that rises from the mantle under the eastern Atlantic, bends to the east under Europe and northwest Africa, and reaches within 120 miles or so of the surface under central Europe. The evidence comes not only from Hoernle’s isotope ratios but also from a seismic tomography scan--a kind of planetary CT scan that uses earthquake waves instead of X-rays to peek inside Earth--prepared by Yu-Shen Zhang, a seismologist at the University of California at Santa Cruz. On that scan the sheet is detectable, albeit only in crude outline.
Hoernle believes this one sheet, rising from a depth of hundreds of miles in the mantle and perhaps from as far as Earth’s core, feeds all the volcanoes of the region; they form where the overlying tectonic plate is thin and weak and the hot rock can punch through it. And he thinks more sheets will become visible around the world as geochemists pursue the question and seismic tomograms get sharper. My prejudice is that this is a more realistic picture of the origin of volcanism in a lot of parts of the world, says Hoernle. I don’t want to say there are no structures that look like classical plumes--you’d have a range of structures. But the inside of the Earth is more complex than our simple models can explain.