Out of sight

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."

Large grain size = a long time spent underground.  Mineralogist Laura Cobden, who specializes in mineral crystallization rates in igneous rock, estimated that based on the inferred crystal size in the two "islands," they've been down there, relatively undisturbed, for around five hundred million years.

[Image from Deuss et al.]

So that's our cool science news from the geologists for today.  Two islands in the mantle that are stubbornly resisting melting away.  Why these structures have been so persistent is beyond the scope of this study; but as with all science, finding out something's there is the first step.  After that, the theorists can figure out how to explain it all.

Even if they never have a chance to see it.

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The rocks drawn down

The map of the world's continents is so familiar that for most of us, it seems permanent.  Even when you find out that the Earth's land masses have moved dramatically, that (geologically speaking) you don't have to go back very far to get to a time when the map would be unrecognizable, it's a little hard to fathom that the map is still changing now.  The tectonic plates are sliding as you read this, and every earthquake adjusts their positions a little bit.

Practically speaking -- and to any readers who are geologists, I will apologize for what will seem to you a drastic oversimplification -- there are three ways plates can move relative to one another:

1. Divergent plate boundaries -- where the two plates are moving apart.  This is what's happening in the middle of the Atlantic Ocean, where the Mid-Atlantic Rift Zone is upwelling magma that drags the North American and South American Plates toward the west and the European and African Plates toward the east.  New divergent boundaries can tear a continent in half, which is currently happening in eastern Africa, where the East African Rift Zone is eventually going to rip the continent in two, sending most of Ethiopia, Kenya, and Tanzania, and all of Somalia, eastward, and creating a new ocean in between.
2. Convergent plate boundaries -- where two plates are moving toward each other.  If both are thick, cool continental plates, this causes a pile-up -- i.e., non-volcanic mountains, such as the Himalayas.  If one or both is a thin oceanic plate, one will dive underneath the other and melt, creating a line of volcanoes more or less parallel to the plate boundary.  Examples include Japan, Indonesia, and the Cascades.
3. Strike-slip boundaries -- where two plates are moving alongside one another in opposite directions.  An example is the famous San Andreas Fault and the other bits and pieces of the southern California fault system.

The result is that the continents are being moved around, torn apart, and slammed back together all the time, just at a pace so slow we usually aren't aware of it.  But the reality is that the western bits of California are in the process of sliding northward along the North American coast and will eventually smash into Alaska; Australia will run into New Guinea, and north Africa into southern Europe; and Antarctica will slide northward, away from the South Pole and into what is now the southern Pacific.

[Image is in the Public Domain]

But new plate boundaries can form, as the subterranean forces in the mantle create new breaks or seams.  That's actually what brings this whole topic up; a friend and loyal reader of Skeptophilia sent me a link about research that's a few years old but that I'd somehow missed, that there's an "embryonic" subduction zone -- a convergent plate boundary -- forming off the coast of Portugal.  These things don't happen overnight, and the surmise is that this developing subduction zone is responsible for the devastating earthquake that hit Lisbon on November 1, 1755, and damn near flattened every building in the city.  It killed an estimated twenty thousand people, and geologists estimate that it had a magnitude of 8.4, more powerful than the more famous San Francisco Earthquake of 1906.

This complicates the picture in the Atlantic, however. The conventional wisdom is that the divergent boundary in the middle of the Atlantic -- shown in red in the above picture -- is moving North and South America away from Europe and Africa, but if there's a convergent boundary off the coast of Portugal, that'll eat up bits of the oceanic plate off the coast and pull Europe closer to North America.  (It also creates the possibility of Andes-type volcanoes in Portugal and Spain.)  Geologists are still investigating how, and how fast, this new convergent zone is moving, and what its capacity is for generating earthquakes and tsunamis -- we really only have the one significant data point, the 1755 earthquake, to make a stab at what the potential for seismic activity is.  And how this will affect the positions of the continents in the long term is at the moment anyone's guess.

It's endlessly fascinating to me how the face of the Earth can change -- for example, there have been at least three times that more or less all the land masses were fused together into one supercontinent (and the rest of the world was covered by one superocean).  Mountains and oceans have been a symbol of something eternal, unchanging, but in reality everything is in flux.  It recalls to mind the lines from Percy Shelley's evocative poem "Mont Blanc," which seems a fitting way to end:

Yet not a city, but a flood of ruin
Is there, that from the boundaries of the sky
Rolls its perpetual stream; vast pines are strewing
Its destin’d path, or in the mangled soil
Branchless and shatter’d stand; the rocks, drawn down
From yon remotest waste, have overthrown
The limits of the dead and living world,
Never to be reclaim’d.  The dwelling-place
Of insects, beasts, and birds, becomes its spoil;
Their food and their retreat for ever gone,
So much of life and joy is lost.  The race
Of man flies far in dread; his work and dwelling
Vanish, like smoke before the tempest’s stream,
And their place is not known.  Below, vast caves
Shine in the rushing torrents’ restless gleam,
Which from those secret chasms in tumult welling
Meet in the vale, and one majestic River,
The breath and blood of distant lands, for ever
Rolls its loud waters to the ocean-waves,
Breathes its swift vapours to the circling air.

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This week's Skeptophilia book recommendation of the week is about as cutting-edge as you can get, and is as scary as it is fascinating.  A Crack in Creation: Gene Editing and the Unthinkable Power to Control Evolution, by Jennifer Doudna and Samuel Sternberg, is a crash course in the new genetic technology called CRISPR-Cas9 -- the gene-editing protocol that Doudna herself discovered.  This technique allows increasingly precise cut-and-paste of DNA, offering promise in not just treating, but curing, deadly genetic diseases like cystic fibrosis and Huntington's disease.

But as with most new discoveries, it is not without its ethical impact.  The cautious are already warning us about "playing God," manipulating our genes not to eliminate disease, but to enhance intelligence or strength, to change personal appearance -- or personality.

A Crack in Creation is an unflinching look at the new science of gene editing, and tries to tease out the how much of what we're hearing is unwarranted fear-talk, and how much represents a genuine ethical minefield.  Doudna and Sternberg give the reader a clear understanding of what CRISPR-Cas9 is likely to be able to do, and what it won't, and maps out a direction for the discussion to take based on actual science -- neither panic and alarmism, nor a Panglossian optimism that everything will sort itself out.  It's a wonderful introduction to a topic that is sure to be much in the news over the next few years.

[Note: if you purchase this book using the image/link below, part of the proceeds goes to support Skeptophilia!]