Geology Reference
In-Depth Information
main locked for millions of years. Still other water molecules undergo chemical reactions
to become part of clay minerals in the soil.
Life has become an integral part of the water cycle as well. Plants take up water molec-
ules and carbon dioxide and combine them, in the Sun-driven process of photosynthesis,
to manufacture roots, stems, leaves, and fruit. And when those nutrient-rich plant tissues
are eaten by animals and broken down by the metabolic miracle of respiration, the waste
products, exhaled with every breath we take, are reassembled molecules of carbon dioxide
and water.
The Deep Water Cycle
Inthemid-1980sEarthscientistsbegantothinkinearnestaboutwaterataglobalscale,for
the near-surface water cycle can't be the whole story. Because we know that magmas ori-
ginating tens or hundreds of miles down hold enough water to cause explosive volcanism,
wecanassumethatsilicatemineralscrystallizeddeepinsideourplanetmustsomehowtrap
H
2
O. There must be a deep, hidden part of the water cycle that could tell us much about
how and when Earth became the ocean-bathed planet it is today.
The experimental approach to deep water has focused on the possibility that the most
common of minerals—olivine, pyroxene, garnet, and their denser deep-Earth vari-
ants—may be able to incorporate a small amount of water at mantle conditions. The study
of water in “nominally anhydrous” minerals, which became a major focus of high-pressure
mineralogy in the 1990s, yielded astonishing results. It turns out that at high pressure and
temperature, it's relatively easy for some minerals to incorporate lots of hydrogen atoms,
which are the mineralogical equivalent of water (because hydrogen atoms combine with
oxygen in these minerals). Minerals that are invariably dry in the cooler, low-pressure en-
vironments of the shallow crust—where explosive volcanism releases any water—can be-
come rather wet in the deep mantle.
Inprinciple,theexperimentalstrategyisprettystraightforward.Takeasampleofolivine
or pyroxene, add water, heat while squeezing, and see where the water goes. In practice,
it's not so easy. In order to reproduce Earth's deeper mantle conditions, the sample must
be pressurized to hundreds of thousands of times the atmospheric pressure (equivalent to
millions of pounds per square inch) and simultaneously heated to temperatures as high as
4,000 degrees Fahrenheit. To accomplish this daunting feat, scientists employ two comple-
mentary high-pressure approaches.
Some rely on massive, room-size metal presses that exert tons of pressure on a tiny
sample—elaborate variations on Hat Yoder's pressure bomb from half a century ago. One
oft-used experimental assembly involves four nested stages like Russian dolls: each stage
surrounds and hugs the next, focusing immense pressures onto a smaller and smaller
