There are-many processes of materials flow operating within and on the surface of
the "rock-
sphere." The earthquakes, volcanoes, and landslides noted earlier are associated with it, as are
the
less dramatic processes of weathering and erosion.
The lithosphere consists of three basic types of rock: igneous, sedimentary,
and
metamorphic.Originally in a molten condition, the igneous rocks have since cooled to produce
rocks such as granite and lava. Most sedimentary rocks have formed from sediments consisting of
the fragments of pre-existing rocks that were deposited by water, wind, or glacial ice. However,
some sedimentary rocks have been created through- the action of plants and animals as well as
chemical reactions. Such common rocks as sandstone, clay, coal, and limestone are typical
sedimentary rocks. The metamorphic rocks are quite different. They consist of rocks that were
originally igneous or of sedimentary origin. They have been altered—as a result of great physical
and chemical change—into a totally different type of rock. Marble, originally limestone, is a
familiar
example of a metamorphic rock.
Most of what we know about the lithosphere has been learned through the study of surface
rocks.
However, much has been learned by studying once-deeply buried rocks that are now exposed on
the surface. Ancient rocks of this kind can be seen in deeply eroded areas such as the Grand
Canyon of the Colorado River. Others have been brought to the earth's surface by great uplifts such
as mountain-building or earthquake movements. The deeper part of the lithosphere is not so well
known.
Mineral composition is very important in the study of rocks, but it does not form
the fundamental
basis for rock classification. Instead, rocks are placed in one of three rock families, according to
how they were formed. Each of these rock groups- igneous, sedimentary, and
metamorphic—makes its own special contribution to the solid matter of Earth's crust.
The igneous rocks get their name from the Latin word ignis, meaning "fire."
These "fire-formed"
rocks are the ancestors of all other rocks, for they appear to have been the first rocks to form on
Earth. They have been produced by the cooling and hardening of molten rock material called
magma.A mixture of melted minerals, magma forms deep within the earth where it accumulates in
pockets or magma reservoirs.
Some of the magma rises to the surface through volcanoes to form lava.
Lava, and other igneous
rocks that cool and solidify on the earth's surface, are called extrusive or volcanic rocks.
Others igneous rocks have formed from magma injected into rocks buried deep within
the earth.
These intrusive, or plutonic, igneous rocks are typically seen in areas that have
undergone much
erosion. Here, the intrusive masses have been raised near the surface by movements within the
earth. As time passed, they were eventually exposed as the overlying rocks were gradually
weathered and worn away. This is why you can see plutonic rocks in such contrasting places as
the deep inner gorge of the Grand Canyon or the lofty summit of Pikes Peak.
Granite, the most common igneous rock, is generally considered to be plutonic in origin.
Yet some
geologists are not so sure. They argue that certain masses of granitic rock are much too large to
have been injected into the surrounding rock. Such granite bodies—the Coast Ranges of British
Columbia, for example—are so massive that they might have formed in place by the process of
granitization. Other geologists doubt that this little-understood process even exists.
Igneous rocks become increasingly abundant in the deeper parts of the earth, for about
95 per cent
of the volume of the outermost ten miles of the crust consists of rocks of igneous origin. They are
much less common, however, on the surface of the earth.
Sedimentary rocks are composed of loose rock fragments called sediments.
These sediments
consist of weathered rock and mineral grains such as mud, sand, and gravel. With the passage of
time, the sediments have hardened into layers, or strata,of sedimentary rock. Because of the
bedded, or layered, nature of these rocks, they are also known as stratified rocks.
Sediments formed from the breakdown and decay of previously existing rocks have usually
been
moved from their place of origin. Wind, running water, and glaciers transport rock particles from one
area and deposit them elsewhere. The more common rocks around us—sandstones, clay, and
some limestones—are typical sedimentary rocks. Other sedimentary rocks consist of the remains
or products of ancient plants and animals. Coal and fossil-bearing limestones have formed in this
way. Sedimentary rocks may also be created as a result of chemical reactions. Rock salt,
gypsum, and certain other chemically produced sedimentary rocks have been precipitated from
solution in sea water.
Unlike igneous rocks, which may originate either within the crust or on Earth's surface,
sedimentary rocks form on or very near the surface of the earth. As a result, only some 5 per cent
of the outer ten miles of the crust consists of sedimentary rocks. By contrast, sedimentary rocks
make up almost 75 per cent of the earth's surface rocks. These rocks are an important source of
mineral resources such as salt, water, sulphur, and petroleum. They also break down to form some
of our more fertile soils. The historical geologist is especially interested in sedimentary rocks, for
they commonly contain fossils. These traces of prehistoric organisms and other sedimentary
features are clues that help reveal much of the history of the earth.
The third great rock family consists of the metamorphic rocks. They get their name
from two Greek
words which literally mean "change in form." This is a very good name, for they consist of
earlier-
formed igneous or sedimentary rocks that have been changed into new and quite different rocks.
The process of metamorphism may alter sedimentary limestone into the metamorphic rock called
marble, or change sandstone into much harder quartzite.
Metamorphism occurs below the surface of the earth where rocks may be affected by
intense
pressure and heat. These changes can be produced when rocks are squeezed, bent, or broken
during crustal disturbances such as mountain-building. Deeply buried rocks can also be invaded by
mineral-bearing liquids and gases from nearby concentrations of magma. As these fluids seep into
the surrounding rock, original minerals may be dissolved and new ones will form in their place.
Some of these minerals, such as gold and silver, may enrich rocks that were originally of little
economic value.
These "made-over" rocks are especially common in areas where the rocks have
been baked by
igneous intrusions or deformed by severe crustal movements. And, because of the conditions under
which they formed, the metamorphic rocks provide evidence of some of the more violent chapters in
earth history.
A rocky "skin" forms a very thin veneer on the earth's surface. In a way
it is somewhat like the rind
of an orange which represents but a small fraction of the orange's diameter. The earth's "rind"
represents, at the very most, less than forty miles of the almost 4,000 miles to the centre of the
earth. So, on a comparative basis, Earth's "skin" is much thinner than the covering of an
orange.
The rocks in the earth's crust occur in different ways. Some rock is in the form of
loose surface
material such as sand, gravel, or soil. This is- the mantle rock. The bedrock—a
continuous mass
of solid rock that has not yet been disturbed by surface agents of weathering and erosion—lies
beneath the mantle rock.
Although they appear to differ greatly, the bedrock and mantle rock are closely related.
One may
be considered the product of the other, for mantle rock consists of rock debris derived from the
weathering and erosion of the solid bedrock. On the other hand, loose fragments of the mantle rock
may eventually become packed and cemented together to become bedrock.
Water
Even a glance at a model of the globe or a world map reveals striking divisions of the land and
water. The land, in the form of continents or islands, represents that part of the lithosphere that
is
not covered by the hydrosphere. Equally striking are the relative amounts of land and water, for
70.8 per cent of the face of the earth is covered by a world- wide sea. Averaging two miles in depth,
this far-flung body of water covers more than 140 million square miles. How much water is
contained in this vast sea? The ocean basins hold about 300 thousand million cubic miles of sea
water. To appreciate better this tremendous amount of water, look at it this way. If our planet were
perfectly smooth—no islands, continents, or mountains—there would be no dry land. Instead,
the
waters of the ocean would cover the entire earth to a depth of about two miles! Interestingly
enough, most of this sea water is concentrated south of the equator, for some 80 per cent of the
Southern Hemisphere is flooded by the sea.
Like the atmosphere, the sea is ever-moving and constantly changing. Its tides, currents,
and
restless waves affect man and his environment in many ways. It is also the source of much food,
and salt and other mineral resources which have been extracted from its waters.
Oceanographers have learned much about the oceans, but many mysteries remain. Consider,
for
example, the fact that man has travelled a quarter of a million miles to sample and study the
surface of the moon. Yet the deep ocean trenches- some seven miles beneath the sea—have
not
yet been explored.
Although 95 per cent of Earth's water is in the sea, the hydrosphere also includes
the water locked
up in the ice of glaciers, and other bodies of water make up the balance of Earth's valuable supply
of fresh water.
Astronomers believe that Earth has more water than any of the other planets in our
solar system. If
there should be no life on the other eight planets, this lack of sufficient water might be partially
responsible.
The earth's waters are not only essential to life, they enter into a number of earth
processes.
Streams of running water are actively engaged in eroding, transporting, and depositing earth
materials. In addition, the sea continually gnaws at the land, wearing it away and depositing rock
fragments on the ocean floor. And beneath the earth's surface, ground water dissolves minerals
from the subsurface formations, leaving cavities that may become caves. It is generally agreed that
water—ably assisted by atmospheric weathering—has been the major force creating the
earth's
landscapes.
Like many commonplace objects, water is usually taken for granted, but few things
are more
important in our daily lives. Water is certainly abundant, for it covers 70 per cent of the earth's
surface to a depth of about 2 miles. And, like air, it is truly one of life's necessities, for without
water there would be no life. Water is many things to many people. It serves as a "thermostat"
to
temper world climates and it forms a vital part of the air that we breathe. Most of the world's
commerce moves over water, and agriculture and industry must have water to survive. Because
water reacts readily with other matter, chemists call it the "universal solvent".
The earth scientist recognizes the importance of water in yet another way. He sees
water as the
great "leveller"—the major force that has shaped Earth's face throughout geologic
time. In the past,
as today, streams have widened and deepened their channels by erosion and ground water has
passed through buried rock formations, dissolving minerals along the way. And over the ages the
sea has relentlessly attacked the land, wearing away solid rock in one area only to deposit it as
loose sediment somewhere else.
Great quantities of water, constantly on the move, are required if water is to do
all the things it
must. Thanks to the hydrologic cycle, we have a very efficient mechanism that fulfills this need.
Most of Earth's water is of meteoric origin, that is, it has been derived from the atmosphere as rain
or snow. Using energy provided by the sun, water is evaporated from the sea and condenses in the
atmosphere as clouds of water vapour. Winds (powered by solar energy) transport the water-laden
clouds to the land. There the atmospheric water may be released by precipitation in the form of rain
or snow.
Although much of the water falls back into the ocean, great amounts fall on the land.
Most of this
water is returned to the-atmosphere by evaporation or transpiration, a process whereby plants
breathe water vapour back into the air. The rest is channelled into streams or underground water-
bearing strata where it may eventually return to the sea. The water cycle has neither beginning nor
end and it has probably been in constant operation for more than four thousand million years.
During this time, water has, in one way or another, served as the unifying thread that binds the
various geologic agents together.
An estimated 22 to 30 per cent of the meteoric water is returned to the sea as run-
off. Excessive
run-off causing continued erosion can be very destructive to fertile farm lands. Luckily, most of this
water is channelled into streams. This running water can be a highly effective geologic tool.
Consider, for example, the Grand Canyon of the Colorado River in Arizona. Almost a mile deep,
about nine miles wide, and 217 miles long, this magnificent gorge is largely the work of stream
erosion. Streams wear away the land, picking up sediment and carrying it along as they flow.
Eventually, however, the stream must deposit its load. This—the process of deposition—
represents the constructional phase of the work of running water. It is not surprising, then, that
running water has done more to sculpture the landscape than all other geologic agents combined.
Water that soaks into the ground can also bring about change. As it moves downwards
in response
to gravity, ground-water may dissolve the rocks through which it flows. Consequently, areas which
are underlain by soluble rocks like the Badlands of South Dakota have been developed as a result
of erosion by running water with the help of atmospheric weathering.
Although it does its work beneath the surface, ground-water is an active geologic
agent in some
areas. Inner Space Caverns in Texas have been dissolved from limestone and decorated with many
unusual cave formations.
Limestone may become honeycombed with caverns. Yet some of the dissolved rock material
is
returned to the rocks as the mineral-laden ground-water deposits its load to form stalactites,
stalagmites, and other cave formations. The work of underground water has provided us with such
interesting features as Carlsbad Caverns, Mammoth Cave, and most of the world's great caverns.
Much more important, some of the ground-water is funnelled into water-transporting rock layers
called aquifers. These porous and permeable formations serve as invaluable reservoirs for much of
our supply of fresh water.
The never-ending battle of land and sea is almost as old as Earth itself. Anyone who
has observed
the ceaseless churning of the oceans can readily understand its power as a geologic tool. Most of
the sea's work is done by means of waves and wave- produced currents which keep erosion-
producing rock fragments in continual abrasive motion. Sooner or later the sea must deposit the
load of sediments it has produced. This marine deposition is responsible for sand bars, beaches,
and many of our lands.
In short, water—working in and on the earth's crust—is the master
tool with which Nature has
sculptured the landscape.
Rock is weathered whenever and wherever atmospheric agents come in contact with solid
earth
material. Rocks are dissolved by rain water and melting snow, scratched by wind-blown sand, and
pried apart by frost and ice. The changes that occur at the interface between the atmosphere and
lithosphere normally work slowly—but surely—to alter the earth's surface.
The work of weathering can be seen all round the world. It had a hand in the shaping
of the Grand
Canyon, has helped to round off the sharp peaks of the Alps, carved the totem-pole-like landscape
at Bryce Canyon National Park, and put the "bad" in the Badlands of South Dakota.
Some weather-produced changes are largely physical. Known as disintegration, this
type of rock
weathering simply reduces the original rock to increasingly smaller fragments. Decomposition, on
the other hand, is a form of weathering whereby rock is decayed and altered by chemical change.
Not all rocks weather at the same rate. Climate, the nature of the rock, and the elevation
of the
land all have an effect on the weathering process. The rate of weathering has also varied throughout
geologic time. Factors such as differences in the amount of protection provided by vegetation and
changing climates have been especially important.
Disintegration and decomposition do wear away the land and this can cause problems.
Weathering
does, nevertheless, play a vital and beneficial role in certain biological and geological processes.
Soil—the end result of rock weathering—more than compensates for the negative side
of
weathering. Soil is something of a "bridge" that connects the biosphere with the inorganic
spheres
of air, water, and land. Indeed, some scientists think there would be no life without soil and no soil
without life—certainly not on the land.
Glaciation
Many of the world's highest mountains are draped with massive blankets of ice. Known as glaciers,
these moving ribbons of ice originate in snow fields at higher elevation. Here the annual snowfall
and refreezing of melted snow are greater than the rate of over-all melting.
As frozen snow piles up, it gradually becomes deeper and more compact. Pressure exerted
by the
overlying snow pack eventually squeezes the lower layers together to form rounded pellets of ice.
As more snow is added, the weight on the granular ice steadily increases until the lower part of the
snow-ice pack becomes pressed together to form solid glacial ice. When sufficient glacier ice has
accumulated, the ice—reacting to the effect of gravity—slowly starts to move
downhill. At this point
a glacier is born.
Most valley (or alpine) glaciers make their way downhill by following old stream-cut
valleys. But
rather than "run" like streams of water, these powerful rivers of ice slowly creep down the
mountainside. Although the movement of the ice can vary greatly, glaciers typically travel only
inches per day or week. On steeper slopes, however, a valley glacier can move ten or even twenty
feet per day. More rarely, sudden shortlived glacial surges have been known to move glaciers as
much as 100 to 370 feet per day.
Glaciers greatly alter the rocks over which they pass. Rocks may become frozen into
the ice and
torn from the valley floor or wall. Additional rock debris may also fall on top of the glacier. Each
rock picked up by the glacier may become embedded in the ice to become a sharp "tooth" that
scratches and gouges the rocks over which it passes. As glaciation continues, the originally V-
shaped stream-cut valley is scooped out to form the typical U-shaped glacial valley.
But like other geologic agents, glaciers cannot transport their load of rock fragments
indefinitely.
Most glacial deposition occurs when the ice melts, leaving piles of glacial debris.
Yentna Glacier in Alaska is a classic example of a large alpine glacier with small
tributary glaciers
entering from either side. The dark streaks on the ice consist of rock debris that will later be
deposited when the ice melts.
Some of these sediments assume distinctive shapes which dot the landscape with telltale
traces of
past glaciation. California's Yosemite Valley, the Teton Range in Wyoming, and the Alps are
classic examples of how glacial ice has left its mark on the land.
Continental glaciers, or ice sheets, are the giants of the world of ice. The largest
of these, the
Antarctic ice sheet, blankets most of the continent of Antarctica—an area almost twice the
size of
the United States. This ice mass is as much as 13,000 feet thick in some places. The ice sheet
that covers Greenland has a surface area of about 670,000 square miles and an estimated
maximum thickness of perhaps two miles. In some regions isolated mountain peaks project above
the ice sheet causing land forms known as nunataks, an Inuit word that literally means "island"
in a
"sea" of ice. Some nunataks, like Mount Erebus in the Transantarctic Mountains, consist of
active
volcanoes.
The glaciers of today are restricted to higher elevations on the continents and cover
little more than
10 per cent of the earth's surface. But during the Ice Age, the most recent chapter in earth history,
almost one-third of the present land surface of the earth was intermittently blanketed with ice.
During these frigid times, valley glaciers dotted many of the continental mountains, the Antarctic
and Greenland ice sheets were much thicker and more widespread, and thick ice sheets spread
across Eurasia and northern North America.
