Seismologists and volcanologists are unusual amongst scientists in that for the most part, what they're studying are things that are permanently unavailable for direct observation.
Oh, sure, they can access the results on the Earth's surface; fault lines, lava flows, uplift or subsidence from magma movement, and so on. But the actual processes -- the stuff down there that is causing it all -- is inaccessible.
The deepest hole ever dug is the Kola Superdeep Borehole, on the Kola Peninsula near the Russian border with Norway, which is an impressive twelve kilometers deep; but when you realize that's only one-thousandth of the diameter of the Earth, it puts things into perspective. Even so, it was deep enough that the bottom had a measured temperature of 180 C -- hot enough to boil water, but far from hot enough to melt rock. (It bears mention that a claim circulating last year that they'd gone down fourteen kilometers, hit temperatures of 1000 C, and could hear the screams of the damned -- because, apparently, they'd punctured a hole into hell -- was unfounded.)
So the fact remains that much of geological science is based upon inference -- not only using surface processes to infer what's happening in Earth's interior, but using data such as earthquake wave traveling speed to figure out what the mantle and core are made of, whether they're liquid or solid or somewhere in between, and how all that stuff in there is moving around. And being inferential, our understanding of deep geologic processes is constantly subject to revision.
Which brings us to a study out of Utrecht University that appeared in the journal Nature last week, about a discovery showing that deep in the Earth's mantle there are two continent-sized subterranean "islands" at least a half a billion years old -- showing that the stuff down there isn't mixing around quite the way we thought it was.
The upper mantle has been thought of as basically a big recycler. As pieces of the Earth's crust get forced down into subduction zones (marked by the oceanic trenches that neighbor some of the most tectonically-active regions on Earth), it melts and gets mixed into what's already down there. Being colder than the surrounding rock, everyone thought the process was slow; other than the bits that get hot enough to melt and then rise to the surface, causing volcanoes like the ones in the North American Cascades, Andes, Caribbean, Italy, Japan, and Indonesia, the rest just has to sit down there till it blends into the material surrounding it.
Apparently some of this will need to be rethought, because these "islands" in the mantle are still holding together despite being so old that they "should have" completely melted away by now.
One of the chunks is under Africa and the other under the Pacific Ocean, and they were located by using the paths and speeds of seismic waves, giving them the moniker of LLSVPs (Large Low Seismic Velocity Provinces). "Nobody knew what they are, and whether they are only a temporary phenomenon, or if they have been sitting there for millions or perhaps even billions of years," said Arwen Deuss, who co-authored the study. "These two large islands are surrounded by a graveyard of tectonic plates which have been transported there by subduction, where one tectonic plate dives below another plate and sinks all the way from the Earth’s surface down to a depth of almost three thousand kilometers."
You might be wondering how they figured out that they are a half a billion years old, given that they're way out of reach of direct study. That, in fact, is the most fascinating part of the study, and has to do with the fact that rocks which cool quickly (such as obsidian and basalt) have much smaller crystals than ones that cool more slowly (like granite and gabbro). The molecular reassembly that results in crystal formation takes time, especially in thick, viscous liquids like magma, so if lava is rapidly cooled on the surface it doesn't have time to form crystals.
"Grain size is much more important," Deuss said. "Subducting tectonic plates that end up in the slab graveyard consist of small grains because they recrystallize on their journey deep into the Earth. A small grain size means a larger number of grains and therefore also a larger number of boundaries between the grains. Due to the large number of grain boundaries between the grains in the slab graveyard, we find more damping, because waves lose energy at each boundary they cross. The fact that the LLSVPs show very little damping, means that they must consist of much larger grains."
One of the "cool quickly" in the 9th paragraph ought to be "cool more slowly", to make more sense, but I don't know which one. Just a nitpick.
ReplyDeleteGood catch! It's fixed.
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