The Death of Plates

What happens to plates after they are subducted is hotly debated, and the topic points to the future of plate tectonics itself. The evidence shows that cold descending lithosphere (the slab) sinks until it matches the density of the surrounding mantle. We detect the presence of the slab in two ways: the cold, rigid rock generates earthquakes as it cracks from the stresses of the deep Earth, and studies of seismic waves show that they speed up as they move across the slab. Earthquake evidence at shallow depths is clear, but eventually slabs stop having quakes and we must rely on other clues. That evidence, based on seismic tomography, is subtle and not universally accepted.

By these means we can visualize slabs descending to around 650 kilometers depth in some cases, where there is a major discontinuity separating the upper and lower mantle. Some studies picture a few slabs going through this barrier. Other slabs appear to descend only a few hundred kilometers and level off there. Now so-called stagnant slabs are known around the world. They appear to be still relatively cool, but releasing water from high-pressure changes in their minerals over a period of around 50 million years. It is postulated that eventually, some slabs "avalanche" into the viscous lower mantle.

Do plates disappear in the mantle, or do they leave permanent traces? Is the deep Earth a mixing machine or a cemetery of old plates? It appears to be something of both. Rocks from the upper mantle have a certain amount of variation, suggesting that distinct bodies of material can be maintained for hundreds of millions, even billions of years. On the other hand, plate tectonics has gone on for nearly 3 billion years, and most of the old oceanic lithosphere has clearly been recycled. (more about the mantle)

Geochemists, seismologists, mineral physicists, paleogeographers, computer modelers and other specialists all bring to the table their own insights into the deep Earth. Tying these insights into a single unified theory of the Earth is a very difficult problem. Some of their assumptions are in conflict, and their definitions of some terms disagree. Facts that one specialty considers tentative are taken as truth by other specialists. And so the great scientific conversation will continue for many years to come.

There's a strong presumption among geologists today that below the plates the whole deep Earth has its own cycle: downwelling of old lithosphere is matched by upwelling of hot plumes from the mantle's base, and everything must be in balance. But in Earth science, we have learned again and again that geologic affairs are not what we presume.

A small but growing school of geoscientists concludes that deep plumes are not supported by the evidence. Instead, they look to plate tectonics to explain even more than it does today. With its presumptions removed, says anti-plume leader Don Anderson, "plate tectonics is a much more powerful concept than generally believed." See this introduction to the nonplume hypothesis, or dip into the ferment at the professional level at mantleplumes.org.

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