News from Afar

I've written here before about the fact that the continents are in motion, something that is only not staggering because we've all known about it since ninth grade Earth Science class.  You can easily see why it took so long to accept.  First, the motion is so slow that it was, for most of human history, beyond the limitations of the technology available at the time to measure directly.  Second, it's just hard to imagine.

Continents?  Moving in solid rock?  What?

But move they do, and it's because if you go down far enough, the rock isn't solid.  Get into the upper mantle, and it's the consistency of taffy, so it flows, pushed by subterranean convection currents.  Those currents create drag forces on the undersides of the tectonic plates, shifting them around.  Although this is an oversimplification, in general, there are three ways that plates can move relative to each other:

• Convergent zones, where plates come together.  When thin, brittle oceanic plates are pushed toward each other, one usually bends and slides under the other at a thrust fault or subduction zone; the subducted plate and the sediment riding on it eventually melt, and the hot, water-rich magma rises to form chains of volcanoes parallel to the fault.  Examples are the Japan Trench and the Sumatra Trench.  When an oceanic plate collides with a thick, cold continental plate, you still get volcanoes boring their way up through the continent -- this is the origin of the Cascade Range.  If it's two continental plates colliding, the rock simply crumples up to form mountains -- such as what is happening in the Alps and Himalayas,
• Divergent zones, where plates move apart.  This is what's happening along the Mid-Atlantic Ridge, and is why the island of Iceland is volcanic -- the eastern and western halves of the island are moving apart, and new basaltic lave bubbling up to fill the gap.

A photograph I took at Meradalir Volcano in Iceland, August 2022

• Strike-slip faults, or transform faults, which occur when plates slide in opposite directions parallel to the fault.  Examples are the San Andreas, Hayward, and Elsinore Faults in California, and the Alpine Fault in New Zealand.

All of these movements can significantly transform the shapes and positions of the continents -- you probably know that 250 million years ago, most of the Earth's land masses were assembled into a giant supercontinent (Pangaea), and the seas into a massive superocean (Panthalassa), with huge consequences to the climate.  Fascinating to realize, though, that Pangaea was only the most recent of the supercontinents; geologists believe that the same lumping-it-all-together occurred at least three or four times before then.

And the reverse can happen, too, when a divergent zone forms underneath a continent, and it tears the land mass in two.  In fact, this is the reason the topic comes up today; a paper last week in Nature Geoscience about the Afar Triple Junction, the point where three faults meet at one point (the Red Sea Rift, the Aden Ridge, and the East African Rift).  Geologists have found that underneath this region, there's a mantle plume -- an upwelling of very hot magma -- that is pulsing like a giant beating heart, driving convection that will eventually tear Africa in two, shearing off a chunk from Ethiopia to Mozambique and driving it east into the Indian Ocean.

[Image licensed under the Creative Commons Val Rime, Tectonic African-Arabian Rift System, CC BY-SA 4.0]

"We have found that the evolution of deep mantle upwellings is intimately tied to the motion of the plates above," said Derek Keir, of the University of Southampton, who co-authored the study.  "This has profound implications for how we interpret surface volcanism, earthquake activity, and the process of continental breakup...  The work shows that deep mantle upwellings can flow beneath the base of tectonic plates and help to focus volcanic activity to where the tectonic plate is thinnest.  Follow on research includes understanding how and at what rate mantle flow occurs beneath plates."

The formation of a new sea -- and the consequent turning of much of east Africa into an island -- isn't exactly what I'd call "imminent;" it's predicted that the Red Sea will breach the Afar Highlands and flood the lowest points of the rift (much of which is already below sea level) in something like five million years.  The region will be highly tectonically active throughout the process, however, and there'll be enough volcanoes and earthquakes in the meantime to keep us interested.

It's a good reminder that although mountains and oceans have been a symbol of something eternal and unchanging, 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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