Geology Reference
In-Depth Information
First Rocks
Given Earth's prodigious heat loss to space, the formation of a rocky crust was inevitable.
Somewhere, probably near one of Earth's less tidally stressed poles, the molten surface
cooled just enough for the first crystals to form. But cooling and crystallizing was far from
a simple event. Many everyday substances have a well-defined temperature at which a
cooling liquid becomes solid—the familiar freezing point. Liquid water freezes at 32 de-
grees, silvery mercury metal at -38 degrees, and ethanol (the common alcohol in booze) at
-179 degrees Fahrenheit. But magma is different. It is a curiosity of magma that it doesn't
have one single freezing point (although
freezing point
in the context of magma at more
than 2,500 degrees Fahrenheit seems something of an oxymoron).
Let's begin with the immediate post-Theia inferno 4.5 billion years ago, a time when
Earth and its Moon shared a radiant silicate vapor atmosphere at temperatures of 10,000
degrees Fahrenheit. That hellish rock gas cooled rapidly and eventually condensed into
droplets and rained magma onto the new twin worlds, as it inexorably cooled to below
5,000degrees,then4,000degrees,then3,000degrees.That'swhenthefirstcrystalsstarted
to form.
Such stories of Earth's first rocks are the scientific domain of the experimental petrolo-
gists, women and men who devise novel lab techniques to bake and squeeze rocks in order
to mimic conditions of Earth's deep interior. The quest to discover rock origins faces two
technical challenges. First, you need to control incredibly high temperatures of thousands
of degrees, far hotter than any oven or furnace in your home. To do so, scientists craft plat-
inum wire into meticulously spaced coils, through which they deliver high electrical cur-
rents to achieve temperature extremes. Even more challenging, these temperatures must be
applied while samples are subjected to crushing pressures tens or hundreds of thousands of
times that of the atmosphere. For this exacting task, researchers enlist massive hydraulic
rams and giant viselike presses.
For more than a century, the Carnegie Institution's Geophysical Laboratory, my scientif-
ichome,hasbeenacenterfortheseheroicquestsforEarth'sdeeptruths.Forashortwhile,
before his untimely death by hospital, I had the chance to work side by side with Hatten S.
Yoder, Jr., one of the pioneers of experimental petrology and the world's foremost expert
on the origins of basalt. Imposing, dynamic, enthusiastic, and attentive, Yoder was literally
a towering figure in the field. As a naval officer in World War II, he was intimately famil-
iar with gigantic metal hardware. In the 1950s, he joined the Geophysical Laboratory and
used naval surplus gun barrels and armor plating, still painted battleship gray, to build the
high-pressure lab that would frame not just his half-century career but our comprehension
of the ground we stand on.
Thecenterpiece ofYoder'sdevicewasa“bomb”—amassivesteelcylinderafootindia-
meter, twenty inches long, with an inch-diameter bore. One end of the bomb was connec-
