Encounters of the Fourth Kind
From Etna to Vesuvius, from Mauna Loa to Mount Merapi: Fire Mountains have fascinated mankind since the beginning of time. But even today, science is still uncovering surprising news about volcanoes.
Where raw forces are at work on earth, volcanism is often not far: regardless of whether tectonic plates are separating or uniting, a clearly visible sign of their activity are ember-spitting mountains around the globe. Iceland, for example, is the fiery product of rising magmas along the Mid-Atlantic Ridge that has been pushing the New and Old Worlds further apart since the Pangea and Gondwana splits. Around the Pacific, in the notorious arc of fire, hot molten rock rises, the genesis of which lies in the collision of two plates. Their volcanoes are young relics of oceanic crust that had come under pressure and heat, which had to submit to the dominant continental plates during the collision and subsided towards the interior of the earth.
Most of Earth's mountains of fire are based on these two processes. But there are also exceptions to this rule - such as the Hawaiian Mauna Loa and Mauna Kea or the Teide on Tenerife - which appear to appear out of nowhere, pile up a mountain and then quickly disappear again after a long geological time because they have lost their underground source were cut off: the so-called hotspot. These hot spots are relatively small, fixed areas of volcanic activity, or at least strong heat flow to the Earth's surface, that can occur far from plate margins in the interiors of both oceanic and continental plates. The rise of hot rock streams called plumes from deeper layers into the uppermost mantle thins the earth's crust and creates magma chambers, which then build up and feed the volcanoes.
So far the usual explanations for volcanism. However, scientists led by Naoto Hirano from the University of California at San Diego may have discovered a fourth mechanism for the formation of volcanoes that could be significantly different from all the others. A coincidental result: During its exploration of the deep sea floor in the north-west Pacific near the Kuril and Japan Trenches, the researchers' diving robot encountered bas altic rocks that were of volcanic origin and must have formed around six million years ago. However, the age of the oceanic plate crust in this area is already 120 to 150 million years. In addition, there is no known hotspot, and no recent volcanic activity has been noted.
During further investigations around 600 kilometers to the south-east, the diving robot also brought new volcanic rocks to the sea surface which, at almost a million years old, were much younger than the first samples. Sonar images of the deep-sea floor finally showed a chain of tiny volcanic cones with a diameter of less than one kilometer and a maximum volume of one cubic kilometer - dimensions that earned them the name "Petit Spots".
But how do these dwarf volcanoes form in regions far from active plate margins and without hotspots that would have led to larger elevations? The spatial sequence of the fire mounds, although small in size, and especially their rocks, suggest a single eruption that dumped all the material. The Petit Spots consist of up to 60% cushion lava that used to be very bubble-rich, water-cooled lava bombs and marine sediments that were immediately transformed and baked on contact with the gluten. Their crystalline content and foreign rock inclusions - so-called xenoliths - such as bas alt, dolerite or gabbro provide valuable information on the origin of the magma and the course of the mountain birth.
The eruption must have happened so quickly, otherwise the heat would have completely melted and transformed the xenoliths. The magma also comes from the uppermost mantle, from where it dragged material from all the layers of the earth's crust above it on its rapid way upwards and trapped it in its core. This source, tapped at a depth of about 100 kilometers, also confirms the typical isotope ratios of the noble gases such as argon or helium trapped in the rock – they correspond to those of magma from mid-ocean ridges, which are also fed from the asthenosphere. This soft zone of the upper mantle is chemically depleted in the area of the petit spots, which ultimately speaks against plumes as the cause of volcanism, because they provide fresh and therefore "rich" material.
But since plumes are not the cause, cracks in the oceanic crust most likely played the midwife: When one plate subsides under another, powerful forces are created that cause cracks to open up through bending or stretching. The magma, which is also under pressure, can rise in these cracks like in a canal or chimney until it flows out on the sea floor and finally builds up the volcano.
They remain small, however, because there are too few supplies coming from the asthenosphere. The temperatures there are only a maximum of 1400 degrees Celsius, which is more than a hundred degrees too low to liquefy rocks sufficiently. However, water and carbon dioxide can also penetrate through the cracks – they lower the melting point of the mantle material, at least in certain areas. However, their influence remains limited, which is reflected in the volume of petit spots.
Despite their small size, the finds by Hirano and his team could still shake the previously accepted theory of much larger hotspot volcanoes. In the midst of the plates they are considered a visible expression of the plumes, but perhaps they are more likely to be the result of ruptured deep-sea floors - at least characteristic lava rocks around Hawaii or in western Samoa suggest this. However, why these mountains of fire then became so large remains completely unclear: there is sufficient material for further fascinating findings.
