Minerals and Rocks
Devils Tower, in Wyoming, shown in Figure 24, rises 264 m (867 ft)
above its base. According to an American Indian legend, the
tower’s jagged columns were formed by a giant bear scraping its
claws across the rock. The tower is actually the solidified core of a
volcano. Over millions of years, the surrounding softer rock was
worn away by the Belle Fourche River finally exposing the core.
Volcanic pipes, which are similar to volcanic cores, can be a source
of diamonds. They contain solidified magma that extends from the
mantle to Earth’s surface.
Structure and Origins of Rocks
All rocks are composed of minerals. Minerals are naturally
occurring, nonliving substances that have a composition that can
be expressed by a chemical formula. Minerals also have a definite
internal structure. Quartz, for example, is a mineral made of silicon dioxide, SiO2. It is
composed of crystals, as are most minerals.
Coal, on the other hand, is not a mineral because it is formed from decomposed plant matter.
Granite is not a mineral either; it is a rock composed of different minerals. There are about
3500 known minerals in Earth’s crust. However, no more than 20 of these are commonly found
in rocks. Together, these 20 or so minerals make up more than 95% of all the rocks in Earth’s
crust. Some of the most common of these rock-forming minerals are feldspar, pyroxene, mica,
olivine, dolomite, quartz, amphibole, and calcite.
Each combination of rock-forming minerals results in a rock with a unique set of properties.
Rocks may be porous, granular, or smooth; they may be soft or hard and have different
densities or colors. The appearance and characteristics of a rock reflect its mineral composition
and the way it formed.
Molten rock cools to form igneous rock
When molten rock cools and solidifies it forms igneous rocks. Nearly all igneous rocks are made
of crystals of various minerals, such as those shown in the granite in Figure 25A. As the rock
cools, the minerals in the rock crystallize and
grow. In general, the more quickly the rock
cools, the less the crystals grow. For instance,
obsidian, a smooth stone used by early
American Indians to make tools, is similar to
granite in composition, but it cools much more
quickly. As a result, obsidian has either very
small crystals or no crystals at all and is mostly
glass. Figure 25B shows a piece of obsidian.
Obsidian is categorized as an extrusive igneous
rock because it cools on Earth’s surface. Basalt,
a fine-grained, dark-colored rock, is the most
common extrusive igneous rock. Granite, on the other hand, is called an intrusive igneous rock
because it forms from magma that cools while trapped beneath Earth’s surface. Because the
magma is insulated by the surrounding rocks, it takes a very long time to cool—sometimes
millions of years. Because of this long cooling period, the crystals in intrusive igneous rocks are
larger than those in extrusive igneous rocks. The crystals of granite, for example, are easy to see
with the naked eye. They are much lighter in color than those of basalt. Both rocks contain
feldspar, but granite also has quartz, while basalt has pyroxene.
Remains of older rocks and organisms form sedimentary rocks
Even very hard rock with large crystals will break down over thousands of
years. The process by which rocks are broken down is called weathering.
Pieces of rock fall down hillsides due to gravity or get washed down by
wind and rain. Rivers then carry the pieces down into deltas, lakes, or the
sea. Chemical processes also knock pieces of rock away. The action of
physical and chemical weathering eventually breaks the pieces into
pebbles, sand, and even smaller pieces. As pieces of rock accumulate,
they can form another type of rock— sedimentary rocks. Think of
sedimentary rocks like those shown in Figure 26 as recycled rocks. The
sediment they are made of contains fragments of older rocks and, in
some cases, fossils.
Loose sediment forms rock in two ways
There are two ways sediment can become rock; and both require precipitation. In one, layers of
sediment get compressed from weight above, forming rock. In the second way, minerals
dissolved in water seep between bits of
sediment and “glue” them together. In
Figure 27A, the bits of rock in the
conglomerate are fused together with
material containing mostly quartz.
Sedimentary rocks are named according to
the size of the fragments they contain. As
mentioned, a rock made of pebbles is
called a conglomerate. A rock made of
sand is called sandstone. A rock made of
fine mud is usually called mudstone, but if it is flaky and breaks easily into layers, it is called
shale. Limestone, another kind of sedimentary rock, is often made of the fossils of organisms
that lived in the water, as shown in Figure 27B. Sometimes the fossilized skeletons are so small
or are broken up into such small fragments that they can’t be seen with the naked eye. Places
where limestone is found were once beneath water.
Rocks that undergo pressure and heating without melting form metamorphic rock
Heat and pressure within Earth cause changes in the texture and mineral content of rocks.
These
changes produce metamorphic rocks. The word metamorphic comes from the Greek word
metamorphosis, which means “to change form.” Limestone, a sedimentary rock, will turn into
marble, a metamorphic rock, under the effects of heat and pressure. Marble is a stone used in
buildings, such as the Taj Mahal, in India. Notice the swirling, colored bands that make marble
so attractive. These bands are the result of impurities that existed in the limestone before it
was transformed into marble. Rocks may be changed, or metamorphosed, in two ways: by heat
alone or, more commonly, by a combination of heat and pressure. In both cases, the solid rock
undergoes a chemical change over millions of years, without melting. As a result, new minerals
form in the rocks. The texture of the rocks is changed too, and any fossils in sedimentary rocks
are transformed and destroyed.
The most common types of metamorphic rock are formed by heat and pressure deep in the
crust. Slate forms in this way. It metamorphoses from mudstone or shale, as shown in Figure
29. Slate is a hard rock that can be split very easily along planes in the rock, creating large, flat
surfaces.
Old rocks in the rock cycle form new rocks
So far, you have seen some examples of one type of rock becoming another. For instance,
limestone exposed to heat and pressure becomes marble. Exposed rocks are weathered,
forming sediments. These sediments may be cemented together to make sedimentary rock.
The various types of rock are all a part of one rock system. The sequence of events in which
rocks can be weathered, melted, altered, and formed is described by the rock cycle. Figure 30
illustrates the stages of the rock cycle. Regardless of which path is taken, rock formation occurs
very slowly, often over tens of thousands to millions of years.
As magma or lava (F) cools underground, it forms igneous rock (E), such as granite. If the granite
is heated and put under pressure, it may become metamorphic rock (D); if it is exposed at the
surface of Earth, it may be weathered and become sand (B, C). The sand may be transported,
deposited, and cemented to become the sedimentary rock (A) sandstone. As more time passes,
several other layers of sediment are deposited above the sandstone. With enough heat and
pressure, the sandstone becomes a metamorphic rock (D). This metamorphic rock (D) may then
be forced deep within Earth, where it melts, forming magma (F).
How Old Are Rocks?
Rocks form and change over millions of years. It is difficult to know the exact time when a rock
formed. To determine the age of rocks on a geological time scale, several techniques have been
developed.
The relative age of rocks can be determined using the principle of superposition
Think about your hamper of dirty clothes at home. If you don’t disturb the stack of clothes in
the hamper, you can tell the relative time the clothes were placed in the hamper. In other
words, you may not know how long ago you placed a particular red shirt in the hamper, but you
can tell that the shirts above the red shirt were placed there more recently. In a similar manner,
the relative age of rocks can be determined using the principle of superposition.
The principle of superposition states the following:
Assuming no disturbance in the position of the rock layers,
the oldest will be on the bottom, and the youngest will be on top.
The principle of superposition is useful in studying the sequence of life on Earth. For instance,
the cliffside shows several sedimentary layers stacked on top of one another. The layers on the
bottom are older than the layers above them. Although the various layers of sedimentary rock
are most visible in cliffsides and canyon walls, you would also find layering if you dug down
anywhere there is sedimentary rock. By applying the principle of superposition, scientists know
that fossils in the upper layers are the remains of animals that lived more recently than the
animals that were fossilized in lower layers.
Radioactive dating can determine a more exact, or absolute, age of rocks
The chapter on nuclear changes showed that the nuclei of some isotopes decay, emitting
energy at a fairly constant rate. These isotopes are said to be radioactive. The radioactive
elements thatmake up minerals in rocks decay over billions of years. Physicists have
determined the rate at which these elements decay, and geologists can use this data to
determine the age of rocks. They measure both the amount of the original radioactive material
left undecayed in the rock and the amount of the product of the radioactive material’s decay.
The amount of time that passed since the rock formed can be calculated from this ratio. Many
different isotopes can be analyzed when rocks are dated. Some of the most reliable are
isotopes of potassium, argon, rubidium, strontium, uranium, and lead. While the principle of
superposition gives only the relative age of rocks, radioactive dating gives the absolute age of a
rock.
Weathering and Erosion
Compared to the destructive power of an earthquake or a volcano, the force exerted by a river
may seem small. But, over time, forces such as water and wind can make vast changes in the
landscape. Parunaweep Canyon, shown in Figure 32, is one of the most magnificent examples
of how water can shape Earth’s surface.
Physical Weathering
There are two types of weathering processes: physical and
chemical. Physical, or mechanical, weathering breaks rocks
into smaller pieces but does not alter their chemical
compositions. Erosion by water or wind are examples of
physical weathering. Chemical weathering breaks down rock
by changing its chemical
composition.
Ice can break rocks
Ice can play a part in the physical or mechanical weathering of
rock. A common kind of mechanical weathering is called frost
wedging. This occurs when water seeps into cracks or joints in
rock and then freezes. When the water freezes, its volume
increases by about 10%, pushing the rock apart. Every time
the ice thaws and refreezes, it wedges farther into the rock,
and the crack in the rock widens and deepens. This process
eventually breaks off pieces of the rock or splits the rock apart.
Plants can also break rocks
The roots of plants can also act as wedges as the roots grow
into cracks in the rocks. As the plant grows, the roots exert a constant pressure on the rock. The
crack continues to deepen and widen, eventually causing a piece of the rock to break off.
Chemical Weathering
Figure 33 shows the sedimentary layers in Badlands National
Park, in South Dakota. They appear red because they contain
hematite. Hematite, Fe2O3, is one of the most common minerals
and is formed as iron reacts with oxygen in an oxidation reaction.
When certain elements, especially metals, react with oxygen, they
become oxides and their properties change. When these elements
are in minerals, oxidation can cause the mineral to decompose or
form new minerals. This is an example of chemical weathering. The
results of chemical weathering are not as easy to see as those of
physical weathering, but chemical weathering can have a great
effect on the landscape over millions of years.
Carbon dioxide can cause chemical weathering
Another common type of chemical weathering occurs when carbon
dioxide from the air dissolves in rainwater. The result is water that
contains carbonic acid, H2CO3. Although carbonic acid is a weak
acid, it reacts with some minerals. As the slightly acidic water seeps
into the ground, it can weather rock underground.
For example, calcite, the major mineral in limestone, reacts with carbonic acid to form calcium
bicarbonate. Because the calcium bicarbonate is
dissolved in water, the decomposed rock is carried
away in the water, leaving underground pockets.
The cave shown in Figure 34 resulted from the
weathering action of carbonic acid on calcite in
underground layers of limestone.
Water plays a key role in chemical weathering
Minerals react chemically with water. This reaction
changes the physical properties of minerals, and
often changes entire landscapes. Other times,
minerals dissolve completely into water and\ are
carried to a new location. Often minerals are
transported to lower layers of rock. This process is called
leaching. Some mineral ore deposits, like those mined for
aluminum, are deposited by leaching.
Water can also carry dissolved oxygen that reacts with
minerals that contain metals such as iron. This type of
chemical weathering is called oxidation. When oxygen combines with the iron found in rock, it
forms iron oxide, or rust. The red color of soil in some areas of the southeastern United States is
mainly caused by the oxidation of minerals containing iron.
Acid precipitation can slowly dissolve minerals
Rain and other forms of precipitation have a slightly acidic pH, around
5.7, because they contain carbonic acid. When fossil fuels, especially
coal, are burned, sulfur dioxide and nitrogen oxides are released and
may react with water in clouds to form nitric acid, or nitrous acid, and
sulfuric acid. These clouds form precipitation that falls to Earth as acid
precipitation. The pH value of rainwater in some northeastern United
States cities between 1940 and 1990 averaged between 4 and 5. In
some individual cases, the pH dropped below 4, to levels nearly as
acidic as vinegar.
Acid precipitation causes damage to both living organisms and
inorganic matter. Acid rain can erode metal and rock, such as the
statue in Brooklyn, New York, shown in Figure 35. Marble and
limestone dissolve relatively rapidly even in weak acid. In 1990, the
Acid Rain Control Program was added to the Clean Air Act of 1970.
According to the program, power plants and factories were given 10
years to decrease the release of sulfur dioxide to about half the amount they emitted in 1980.
The acidity of rain has been greatly reduced since power plants have installed scrubbers that
remove the sulfur oxide gases.
Erosion
Erosion is the removal and transportation of weathered and nonweathered materials by running
water, wind, waves, ice, underground water, and gravity.
Water erosion shapes Earth’s surface
Water is the most effective physical weathering agent. Have you ever seen
a murky river? Muddy rivers carry sediment in their water. As sediment
moves along with the water, it scrapes the riverbanks and the river bottom.
As the water continues to scour the surface, it carries the new sediment
away. This process of loosening and moving sediments is known as
erosion. There is a direct relationship between the velocity of the water and
the size and amount of sediment it can carry. Quickly moving rivers can
carry away a lot of sediment, and create extraordinary canyons.
As a river becomes wider or deepens, it flows more slowly and cannot carry
as much sediment. As a result, sediment is deposited on the floor of these
calmer portions of the river or stream. The process of depositing sediment
is called deposition. Rivers eventually flow into large bodies of water, such
as seas and oceans, where the sediment is deposited along the continental
shores. As rivers slow at the continental boundary, large deposits of
sediment are laid down. These areas, called deltas, often have rich, fertile
soils, making them excellent agricultural areas. Figure 36 shows the Greenstone River delta, in
New Zealand.
Oceans also shape Earth
The oceans also have a dramatic effect on Earth’s
landscape. On seashores, the waves crash onto land,
creating tall cliffs and jagged coastlines. The Cliffs of
Moher, in western Ireland, shown in Figure 37, reach
heights of 204 m (669 ft) above the water. The cliffs
were formed partially by the force of waves in the
Atlantic Ocean eroding the rocky shale and sandstone
coast.
Glaciers erode mountains
Large masses of ice, such as the glacier shown in Figure 38A,
can exert tremendous forces on rocks. The constantly moving
ice mass carves the surface it rests on, often creating U-shaped valleys, such as the one shown
in Figure 38B. The weight of the ice and the forward movement of the glacier cause the mass
to act like a huge scouring pad. Immense boulders that are carried by the ice scrape across
other rocks, grinding them to a fine powder. Glacial meltwater streams carry the fine sediment
away from the glacier and deposit it along the banks and floors of streams or at the bottom of
glacier-formed lakes.
Wind can also shape the landscape
Just as water or glaciers can carry rocks along, scraping other
rocks as they pass, wind can also weather the Earth’s
surfaces. Have you ever been in a dust storm and felt your
skin “burn” from the swirling dust? This happens because fastmoving wind can carry sediment, just as water can. Wind that
carries sediment creates a sandblaster effect, smoothing
Earth’s surface and eroding the landscape.
The sandstone arches of Arches National Park, in Utah, are
formed partly by wind erosion. Look at Figure 39. Can you
guess how these arches might have formed? Geologists have
struggled to find a good explanation for the formation of
arches. The land in and around Arches National Park is part of
the Colorado Plateau, an area that was under a saltwater sea
more than 300 million years ago. As this sea evaporated, it
deposited a thick layer of salt that has since been covered by
many layers of sedimentary rock. The salt layer deforms more
easily than rock layers. As the salt layers warped and
deformed over the years, they created surface depressions
and bulges. Arches formed where the overlying sedimentary rocks were pushed upward by the
salt.
Figure 40 shows how one theory explains the formation of arches. As land is pushed upward in
places, small surface cracks form. These cracks are eroded by water, ice, and wind until narrow
free-standing rock formations, called fins, are formed. When these fins are exposed along their
sides, the wind wears away at the cement that holds the sediment together, causing large
pieces of the rock to fall away. Some fins collapse completely; others that are more sturdy and
balanced form arches.