Geology Reference
In-Depth Information
morethanalittlecomplicated.Untilasrecentlyasthe1990s,theoceanswerethoughttobe
by far the largest water repository, storing about 96 percent of Earth's accessible inventory.
Ice caps and glaciers, with about 3 percent today (and perhaps no more than 5 or 6 percent
even at ice age peaks ofglacial advance), come inat adistant second. Groundwater (all the
near-subsurface H
2
O, both in well-defined aquifers and as more widely dispersed stores)
accounts for1percent, while all the lakes, rivers, streams, ponds,andthe atmosphere com-
bined represent no more than about a hundredth of a percent of Earth's near-surface water
supply.
All this water is in constant motion, shifting from one repository to another on a scale
of days to millions of years. The dynamic, life-sustaining water cycle represents the most
obvious source of change on our perpetually changing planet. Imagine the possible peram-
bulations of a single water molecule—a molecule formed from an oxygen atom and two
hydrogen atoms that have existed for many billions of years. Begin with our molecule in
the mighty Pacific Ocean, where most of Earth's near-surface water molecules spend most
of their time. A great ocean river of cold water, the California Current, sweeps that molec-
ule from near Alaska southward along the California coast to Baja and the Equator. As the
surroundingwaterwarmsandrises,themoleculereachesneartheoceansurfaceandbegins
anepicclockwisejourneyaroundtheNorthPacific—firsttheNorthEquatorialCurrentthat
flowswesttocurvepastJapan,thentheNorthPacificCurrentheadingeasttoNorthAmer-
ica. As our molecule once again moves close to California, it happens to rise to the sunlit
ocean surface and evaporates into the atmosphere, where clouds are beginning to form.
Prevailing winds sweep the thickening mass of rain clouds eastward, across the desert
Southwest,intotheelevatedterrainoftheRockyMountains.Asthecloudsriseintohigher,
cooler elevations, the rain begins to fall. Our molecule eventually descends to Earth as part
of a raindrop; it follows a sinuous path from rivulet, to creek, to stream, to a swollen river
that overflows its bank. To this point the water molecule's movements have been swift—a
year or two to cycle the entire Pacific Ocean, a day or two to join the clouds and fall as
rain, a week or so to flow across the hilly terrain. But as it soaks deeper into the ground
and merges with a vast, hidden aquifer, the molecule may spend many thousands of years
creeping through the subsurface realm.
Here human actions alter nature's ancient rhythm, for water-hungry farmers pump
out immense quantities of deep water to sustain agriculture in the semiarid Southwest.
Aquifers,minedoftheirwateratunsustainablerates,arethusdryingup.Ourmoleculesuc-
cumbs to this trend and finds itself back at the surface, splashing over a Texas cornfield,
where it quickly evaporates back into the cloudless sky and continues its eastward journey.
This story cycles without end. Some molecules are temporarily broken apart into hy-
dronium and hydroxyl ions, only to recombine into new water molecules with new atomic
companions. Other molecules become frozen into thick Antarctic ice, where they will re-
