An Olivine Tale

by William S. Statler

In those days, rocks were mortal.

Today, rocks are engaged in a successful conspiracy to achieve immortality (or near-immortality) by building continents. Granted, they will all pass away in a far distant time, when the Sun grows to a red giant and Earth is turned to vapor. But meanwhile, any rock with a nice comfy residence in a continental land mass can expect a lifetime measured in billions of years, especially if it’s deep underground and safe from erosion. Metamorphosis, yes, that can happen. But if an ephemeral caterpillar is not unhappy about metamorphosing into a butterfly, then why should a rock worry?

Actual melting, now, that would require subduction, and nowadays it’s mostly the seafloor rocks and sediments that get subducted, and they’re a lower class of rock anyway, not the sort to be found in good continental neighborhoods. Some day, though, even the seafloor rocks will be saved (well, not the current ones, they’re doomed, but some future seafloor will be saved), because the continents will continue to grow, and plate tectonics will ultimately seize up entirely, and every rock, from lowly shale to upper crust, will be safe from subduction forever.

But in those days, the crust was young, hot, and thin, and rocks were much more mortal. Their lives were short.

What is time to a rock? Today, a deep continental rock may feel only the slow heaving of its neighbors as the continents grow and erode and crack. For a rock exposed on the surface, the years pass like waves upon the shore, and the days fall like raindrops.

There were neither waves nor shore nor raindrops in those days. Future oceans were as yet an atmosphere of high-pressure steam. The crust was a place for pioneers, tough olivines capable of solidifying from the yellow-hot magma. Here, a pile of refractory massive dunite slag from last year’s eruption: it stayed behind while the orange rivers of lava fled back into the Earth. There, a rare chunk of gem-quality peridot, crystallized slowly underground, flipped up topside by a random meteorite, carved to a wild shape by the abrasive corrosive wind: a green gem still glowing red with heat. Ah, those were the days when it really meant something to be igneous. Rhyolite? Obsidian? We got a name for pansy rocks like that ’round here, we call ’em molten, heh heh.

Of course, in those days the rocks had no names for each other, nor for the proto-Earth, nearly as large as Earth today, nor for the other small planet which shared proto-Earth’s orbit around the dim young Sun. In about 4 450 000 000 years, a squishy sentient creature would think to call the small world Theia, Mother of the Moon, who was destined to die in childbirth.

There was no one to watch Theia arrive at her stately ten kilometers a second, except the wild gem peridot and the massive dunite slag and their neighbors and kin, and even had they eyes, they would have seen little through the dusty steam and acid clouds. But perhaps they felt the tides, as the magma beneath them rose and cracked the crust, the first tides in the life of these rocks.

This had happened before. Many times. This was merely the latest, and the biggest, and the last.

It was dawn for the wild gem peridot and the massive dunite slag, but Theia arrived at sunset, on the far side of the world. She came in at a forty-five-degree angle, and sheared her top clean off, and threw her core and a vast torch of both worlds’ guts far into the night sky, and left the Earth briefly and madly egg-shaped, with one end shining brighter and hotter than the Sun’s surface.

The wild peridot and the dunite slag survived a few hours longer, even as the crust was torn apart beneath them. That day did not fall like a raindrop. It was a new day, just five hours long, and by its end the blazing sky had fallen on them, and the whole world gleamed like a tiny star.

The radiant Earth would have its own planet. The Moon slowly coalesced from the splattered and vaporized debris of two worlds — terrifyingly close, white-hot, but guaranteed by physical law never to reprise its mother’s performance.

There were no rocks to witness the birth. They came later. And they built continents.

Copyright © 2005 William S. Statler. This work is licensed under a Creative Commons License, which grants limited rights of non-commercial distribution. Please read
for details. All other rights reserved.

This is a work of fiction, but I've tried to make it at least plausible in light of current theories of lunar formation. I've drawn heavily on this article:

Canup, R. M. 2004. Simulations of a Late Lunar Forming Impact. Icarus 168, 433-456.

The author has made a preprint of this article (2MB PDF file) available on her website.

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