Geology Reference
In-Depth Information
ted to a series of gas pumps, compressors, and intensifiers that could generate a staggering
twelve thousand atmospheres of gas pressure—the pressure found twenty-five miles be-
neath Earth's surface—with a pent-up energy equivalent to the explosive power of a stick
ofdynamiteiftheapparatuseverfailedcatastrophically.Thebomb'sotheropenendaccep-
tedafoot-longrocksampleassemblyandagianthexagonalnut,sixinchesindiameter.We
sealed the device by tightening the nut with a three-foot-long, twenty-pound wrench.
The beauty of Hat Yoder's apparatus was that we could load powdered rock and mineral
samples into little gold tubes, pack the tubes into a cylindrical heater, and secure the entire
assembly inside the bomb's pressure chamber. Pump up the pressure, turn on the electric
heater, and the bomb did all the work. Each experimental run held up to six small gold
tubes; each run lasted from a few minutes to a few days. Hat Yoder's remarkable invention
was ideally suited to study how rocks evolve in Earth's crust and upper mantle.
What Hat Yoder and his colleagues found was that an incandescent melt rich in the big
six elements will typically begin to solidify by forming crystals of the magnesium silicate
olivineasitcoolsbelowabout2,700degreesFahrenheit.OnbothEarthandtheMoon,dur-
ing that long-ago cooling period, beautiful tiny green crystals began to grow in the magma
as microscopic seeds, which expanded to the size of BBs, peas, grapes. But olivine is typ-
ically denser than the liquid in which it grows, so those first crystals began to sink, faster
and faster as the crystals grew larger and larger, accumulating huge deep masses of nearly
pure crystals—forming a stunning green rock called dunite. OnEarth this rock is relatively
rare today, appearing at the surface only on the special occasions when mountain-building
activitiesofupliftanderosionexposethedistinctivedense,deep-formedolivinecumulates.
ThecontinuoussinkingofolivinecrystalsgraduallyalteredcoolingmagmasinsideEarth
and the Moon. The remaining hot melts changed composition; as they became progress-
ively depleted in magnesium, they became correspondingly more concentrated in calcium
andaluminum.OntheMoon,asthemagmaoceancontinuedtocool,asecondmineralstar-
ted to form. Anorthite, a feldspar made of calcium aluminum silicate, began to crystallize
alongside olivine, forming pale blocks. Unlike olivine, anorthite is less dense than its host
liquid,soittendstofloat.OntheMoon,immense quantities ofanorthite poppedtothesur-
face of the magma ocean to form a vast crust of floating feldspar mountain ranges rising
as much as four miles above the molten surface. These whitish-gray masses, which still
dominate 65 percent of the Moon's reflective silvery face, are called the Lunar Highlands.
Risingastheydiddirectlyfromthemagmaocean,theyaretheoldestknownformationson
the Moon. Apollo samples reveal a range of ancient ages for these distinctive anorthosites
fromasyoungas3.9billionyearstoalmost4.5billionyears,shortlyaftertheBigThwack.
OnEarth,withitswettercomposition,deepermagmaoceans,andcorrespondinglygreat-
er internal temperatures and pressures, a somewhat different scenario unfolded. A small
amount of anorthite probably crystallized early in Earth history in some near-surface,
