Rocks and minerals
Derived from the Greek words
lithos, meaning rock, and
sphaira (a ball), the term lithosphere graphically describes
the "ball of rock" that was mentioned earlier. It is the
lithosphere that is of greatest importance to the geologist or
earth scientist, for its rocks and minerals yield vital information
about the origin and composition of our planet. These solid earth
materials also make up the land masses and ocean basins which are
the major surface features of the earth.
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.