Into the Heart of Our World by David Whitehouse

Author:David Whitehouse
Language: eng
Format: epub
Publisher: Pegasus

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Plumes

That there is some recycling of material within the Earth is undisputed, but there is much debate about its details and how long it takes. If slabs are subducted into the Earth then something must come back up again, but from where? Is it just from the surface at mid-oceanic ridges, just below the surface, as in volcanoes, or does material come from somewhere deeper? We know what goes down but is anything coming back up?

Looking at rocks that erupted twenty million years ago on an island in the South Pacific called Mangaia, the southernmost of Polynesia’s Cook Islands, shows that the molten rock just beneath the crust varies in composition from place to place and one interpretation is that crust that once resided on the surface has altered the composition of the mantle in an uneven way. Closer inspection of the mantle in this region reveals something even more fascinating. It has traces of material that was on the surface of the Earth some two and a half billion years ago, before photosynthetic organisms filled the atmosphere with oxygen. Analysis of the Hawaiian volcano Mauna Loa shows that its lava contains material that once comprised sediments dragged into the Earth on an ancient subducting slab. That is the case in many regions. Rocks are recycled, but how? Slabs may reach their graveyard at the base of the mantle but then their material is sent back up again, and some believe this is done by so-called superplumes.

Iceland is perhaps the most volcanic country on Earth as it straddles two tectonic plates separated by a spreading centre. But its intense volcanism is greater than would be expected just from being over a spreading zone, leading to the suggestion that hot rock is reaching it from greater depths. Some have suggested it has a rising superplume beneath it.

The thought is that something happens at the sharp edges of Tuzo and Wilson, something that could force rock to rise through the lower mantle and perhaps continue to the surface. One of the clues that this could be happening comes from volcanoes. As we have seen, about 95 per cent of the world’s volcanoes are located near the boundaries of tectonic plates, and they are there because the descending slab oozes water that reduces the rock’s melting point, causing it to rise to the surface and form a volcano, but there are other types of volcanoes. The other 5 per cent are thought to be associated with so-called mantle plumes and hot spots. Mantle plumes are areas where heat and/or rocks in the mantle are rising towards the surface. A hot spot is where it reaches the surface. There is strong debate in the geophysics community about the reality of mantle plumes; like so much in that fascinating science, things are not settled.

In the 1960s, Tuzo Wilson (1908–93) noticed something remarkable about ocean islands. On a map of the Pacific basin, he found three linear chains of volcanoes and submarine volcanoes (seamounts). They were

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