The rocks drawn down

Imagine yourself standing on the shoreline, somewhere on the Earth, three billion years ago.

If you're picturing swamps and tree ferns and dinosaurs, you're way off.  The first trees wouldn't appear for another 2.6 billion years or so; the first dinosaurs we know about were 150 million years after that.  Three billion years ago there probably weren't even any eukaryotes -- organisms with complex cells containing organelles and nuclei, such as ours, as well as those of plants, fungi, and protists -- anywhere on Earth.  It's likely there wasn't much oxygen in the atmosphere, either.  This is before the "Great Oxidation Event," when photosynthesizing cyanobacteria reached a population sufficient to dump huge quantities of oxygen into the atmosphere, dooming most of Earth's living things (but simultaneously setting the stage for the rise of aerobic organisms such as ourselves).

So an accurate picture of what you'd experience: land that is nothing but an enormous expanse of bare rock and sand, devoid of a single living thing; murky water containing a soup of organic compounds, generated by the reducing atmosphere and frequent lightning storms; and unbreathable air mostly made of nitrogen, methane, carbon dioxide, carbon monoxide, sulfur dioxide, and ammonia.

I can just hear Mr. Spock saying, "The planet appears to be entirely inhospitable to life, Captain."

It's hard to imagine that our lush, verdant, temperate world evolved from that, but it did.  Consider, too, that the continents weren't even remotely in the same positions as they are now.  Where I sit writing this, in upstate New York, I'd have been about at the same latitude as I am now -- maybe a little bit farther south, about thirty degrees north.  But that's by far the exception.  See where you'd be on this map between about 2.5 and 1.5 billion years ago, when all of Earth's land masses were fused into a supercontinent named Columbia.  [Nota bene: this is not Pangaea.  This is two supercontinents before Pangaea.]

[Image licensed under the Creative Commons Alexandre DeZotti, Paleoglobe NO 1590 mya-vector-colors, CC BY-SA 3.0]

This was, in fact, not long after the continents formed.  We have continents because there are two basic kinds of rocks in the Earth's crust: felsic rocks, which are rich in silica, low in iron, and relatively lightweight; and mafic rocks, which are the opposite.  Most of the continental land masses are made up of felsic rocks (like granite and rhyolite), so they float in the denser rock of the mostly-mafic upper mantle.  (It's hard to imagine something as gigantic and heavy as a continental rock mass floating, but that's what it does.)  About three billion years ago was when there was sufficient separation of felsic and mafic chunks of crust that we started to see continental cratons form, and these blocks have been so stable thereafter that they're basically the same land masses we have today (albeit much cut apart and rearranged).

The reason this comes up is the discovery of evidence of what might have been one of the Earth's earliest megaquakes.  It occurred in what is now South Africa, part of the Kalahari Craton (as you can see from the map above, it'd have been in the northeast corner of the Columbia Supercontinent, at about the current latitude of Oslo, Norway).

The Barberton Greenstone Belt is one of the oldest relatively undisturbed chunks of rock in the world, and the current study, which was published two weeks ago in the journal Geology, suggests that it shows evidence of an overturned layer of chert that formed from a humongous underwater landslide of the type we see with megathrust earthquakes.  This, the researchers say, is the smoking gun that plate tectonics was already up and running three billion years ago -- that the reshuffling of continental blocks still going on today started not long after the blocks themselves formed.

The Barberton Greenstone Belt, about 350 kilometers east of Pretoria, South Africa [Image credit: Simon Lamb, Victoria University]

We think of the Earth as unchanging, don't we?  "Solid as a rock" is close to a cliché.  And yet, as we've seen, everything shifts, melts, moves; life comes and goes, evolves and falls to extinction; even the continents beneath our feet break up and recombine.  It's been going on for billions of years, and will continue for billions more.  The whole thing puts me in mind of Percy Shelley's evocative poem "Mont Blanc," which seems a fitting place 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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