Geology Reference
In-Depth Information
the formation of stars long before that, the key to any viable scenario is knowledge of the
immutable characteristics of the elements in question—in this instance, the physical and
chemical properties of volatile nitrogen, carbon, sulfur, and water.
Of these four ingredients, nitrogen is the easiest to deal with. It's a chemically inert gas
thatformsfewminerals,playsalmostnoroleinrockformation,andtendstoconcentratein
theatmosphere.OnlysincetheriseoflifehasthenitrogencyclehadmucheffectonEarth's
outer layers. Carbon and sulfur would also come into much greater prominence one or two
billion years into Earth's existence, when life and an oxygen-rich atmosphere transformed
the surface realm. But the fourth ingredient, water, has been central to Earth's story from
the get-go.
Water: A Short CV
Water's varied geological roles follow from the unique chemical properties of hydrogen
oxide. Recall that hydrogen is element number one, while oxygen is element eight; neither
element has a magic number of two or ten electrons. Each electron-accepting oxygen atom
seeks two more electrons to reach the magic number ten, while each hydrogen atom with
one electron to share wants one more. The molecular result is a two-to-one ratio of hydro-
gentooxygen:H
2
O.TheatomsinthiscompactunitformaVshape:thecentrallargeroxy-
gen atom is flanked by two hydrogen bumps, not unlike Mickey Mouse's ears. The oxygen
atom,havingborrowedelectronsfromtwohydrogenatoms,assumesaslightnegativeelec-
trical charge, while each hydrogen atom is correspondingly slightly positive. The result is
a polar molecule, with opposed positive and negative electrically charged parts (Mickey's
ears and chin, respectively).
Such polarity in water's molecules accounts for many of its distinctive properties. Polar
water is a super solvent, because its positive and negative ends exert strong forces that can
pull apart other molecules. Consequently, table salt, sugar, and many other ingredients dis-
solve rapidly in water. Most rocks take a bit longer to dissolve, but over millions of years,
theoceanshavebecomerichinalmostallthechemicalelements.(Asaconsequence,every
cubicmileofoceanwaterholdsaboutfourhundredpoundsofgold—morethan$10million
worthatthepreciousmetal'srecenthighvalue,ifonlywehadthetechnologytoextractit.)
This unparalleled ability of water to dissolve and transport other chemicals also makes it
an ideal medium for the origins and evolution of life. All life on Earth, and perhaps all life
in the cosmos, depends on water.
Thepolarityofwatermoleculescausesthemtobondstronglytooneanother:thepositive
side of one molecule attracts the negative sides of other molecules. Consequently, ice is an
unusually strong molecular solid (a fact you'll not soon forget if you've ever fallen hard
while ice-skating). Unusually strong intermolecular bonding also results in water's unusu-
