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_
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|
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Uncommon
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.
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Fnend
Without water, this would have been a vastly different planet—and a lifeless
one. In its beginning, the earth has been described as a hot mass of molten
_
substances. As it cooled, the heaviest elements, such as iron and nickel,
formed a molten core in the earth’s interior; the lighter elements hardened
and formed the earth’s crust.
For a time the earth is thought to have been completely covered With
clouds of water vapor and then to have cooled enough to allow condensation
of the vapor into water droplets. Rain fell, resulting in the formation of large,
freshwater seas. The clouds parted, letting in vital energy
from the sun, and
ever since that time water has played a major role in shaping the face of the
earth and in the evolution and maintenance of life
Water constantly circulates from one part of the earth to another, some—
times as water vapor in the atmosphere, sometimes as rain and snow falling
on the land and the oceans, sometimes as rivers, streams, and ground
water
owing over and under the earth’s surface, sometimes held for long periods
in
lakes or for longer periods in oceans, sometimes frozen for a day, a season, or
a millennium. Water moves in what is essentially a closed cycle, the hydrological cycle (pages 82—33), driven by solar energy. The oceans and seas con—
tain 97 percent of the earth’s circulating water and most of the remainder iS
'
held in the polar icecaps. Less than one percent is distributed as fresh water
in streams and lakes, ground water, and water vapor
in the atmosphere.
Yet
this is no small amount. Solar energy evaporates 80,000 cubic miles of water
from the oceans and 15,000 cubic miles from land surfaces and lakes annuallÿ;
and annually 24,000 cubic miles of water return to the land as rain and snow
(enough water to cover Texas to a depth of 475 feet) [2]. The amount of water
30
_
?‘E
l
|
._
|
Uncommon
,
'.
.
"..
Fnend
Without water, this would have been a vastly different planet—and a lifeless
one. In its beginning, the earth has been described as a hot mass of molten
_
substances. As it cooled, the heaviest elements, such as iron and nickel,
formed a molten core in the earth’s interior; the lighter elements hardened
and formed the earth’s crust.
For a time the earth is thought to have been completely covered With
clouds of water vapor and then to have cooled enough to allow condensation
of the vapor into water droplets. Rain fell, resulting in the formation of large,
freshwater seas. The clouds parted, letting in vital energy
from the sun, and
ever since that time water has played a major role in shaping the face of the
earth and in the evolution and maintenance of life
Water constantly circulates from one part of the earth to another, some—
times as water vapor in the atmosphere, sometimes as rain and snow falling
on the land and the oceans, sometimes as rivers, streams, and ground
water
owing over and under the earth’s surface, sometimes held for long periods
in
lakes or for longer periods in oceans, sometimes frozen for a day, a season, or
a millennium. Water moves in what is essentially a closed cycle, the hydrological cycle (pages 82—33), driven by solar energy. The oceans and seas con—
tain 97 percent of the earth’s circulating water and most of the remainder iS
'
held in the polar icecaps. Less than one percent is distributed as fresh water
in streams and lakes, ground water, and water vapor
in the atmosphere.
Yet
this is no small amount. Solar energy evaporates 80,000 cubic miles of water
from the oceans and 15,000 cubic miles from land surfaces and lakes annuallÿ;
and annually 24,000 cubic miles of water return to the land as rain and snow
(enough water to cover Texas to a depth of 475 feet) [2]. The amount of water
30