The Rock Review
IGNEOUS ROCKS
Granite. Granite is usually gray or pink in color. The most abundant minerals in granite
are feldspar and quartz, with smaller amounts of mica and hornblende. The minerals in
granite appear as relatively large easily visible crystals or grains.
Feldspar makes up about half of the granite and is responsible for its color. You can
identify the feldspar as relatively smooth, light-colored, block-like grains in the rock. The
quartz grains may appear as cloudy glass chips. The mica or hornblende (or both) is
commonly seen as dark specks scattered throughout the granite. Granite has a coarsegrained texture because of the large size of its crystals. As you have learned, the crystals
are large because the granite cooled slowly, deep within Earth's crust.
The crust, or bedrock, that makes up the continents (large land areas) consists mostly of
granite. In addition, many mountain ranges, such as the Rocky Mountains and the White
Mountains in New Hampshire, consist largely of granite. Because of its hardness, granite
is frequently used for buildings and monuments. The Mount Rushmore National
Memorial, located in the Black Hills-of South Dakota, is carved out of granite.
Obsidian. A lump of obsidian usually is dark brown or black in color. However, a thin
slice of obsidian appears colorless or lightly tinted. For this reason (and because the
minerals in obsidian are usually the same as those in granite), obsidian is classified with
the light-colored igneous rocks.
Obsidian is formed when molten materials cool very rapidly-too rapidly for crystals to
form. Consequently, obsidian has a glassy texture. Obsidian was used by some tribes of
Native Americans to make arrowheads and knives because of its hardness and sharp,
glasslike edges.
Pumice. At times, hot gases bubbling through lava cause the lava to froth up like
whitecaps on an ocean. When such lava cools and solidifies, a lightweight, sponge like
rock forms. This rock is called pumice. Many tiny air holes within the pumice usually
make this rock light enough to float in water. Pumice is considered to be vesicular
because of the tiny air holes. In fact, pumice is often found floating on the ocean near an
erupting volcano. Powdered pumice is a fine polishing material used in dental powders
and household scouring mixtures.
Basalt. Basalt is dark in color. The color of basalt is caused by dark-colored minerals
such as augite, olivine, and magnetite. Augite and olivine usually are dark green;
magnetite is black.
The very small size of the mineral grains in basalt gives it a texture that is almost smooth.
Basalt is the most abundant of all the igneous rocks that cooled near Earth's surface. A
great lava deposit is the Columbia River Plateau, which extends through the states of
Washington, Oregon, and Idaho.
SEDIMENTARY ROCKS
Sandstone. A sample of sandstone often feels gritty because it consists largely of grains
of quartz sand. The grains of sand are held together by natural cement: quartz. These
cements consist of minerals, such as silica, calcium carbonate, or iron compounds that are
dissolved in the water. The dissolved minerals seep into the deposits of sand, fill in many
of the open spaces, and bind the grains together. The color of sandstone is determined
mainly by the color of its cement. Rocks cemented by silica or calcium carbonate are
generally light in color; rocks cemented by iron compounds are red to reddish brown in
color.
Although the grains of sand in sandstone are cemented together, relatively large spaces
remain between the grains. For this reason, water and other liquids can soak through
sandstone.
Shale. Shale consists of clay and silt particles packed so closely together that water
carrying dissolved cements cannot freely pass between the particles. Shale crumbles
rather easily because the particles of clay and silt are cemented together weakly. Shale is
fairly easy to identify because it resembles hardened mud, and it splits easily into
relatively flat pieces. Most shales are gray, although impurities in the sediments may give
the shales other colors.
Finely ground shale is used for making bricks and tiles. Some forms of shale contain oil
locked tightly between the particles of clay and silt. At present, recovering oil from this
shale is difficult and expensive. In the future, oil shale may become an important source
of petroleum.
Conglomerate. One of the easiest rocks to identify is conglomerate. Conglomerate is
made up of rounded, pebble-sized rock fragments set in a cement-like mass of sand, silt,
or clay. A common type of conglomerate consists of smooth, rounded quartz pebbles
cemented together by finer materials. Conglomerate gets its name from a Latin word that
means "'to gather or collect together." Some conglomerates may even have boulders
cemented into their mass.
SEDIMENTARY ROCKS FORMED BY CHEMICAL ACTION
Limestone. Limestone consists mostly of calcium carbonate, or calcite. Limestone forms
when a calcium-containing compound, dissolved in water, separates from the water and
becomes solid particles of calcite.
Limestone can be identified easily by performing the acid test. Bubbles of carbon dioxide
gas appear on the surface of limestone when drops of dilute hydrochloric acid or vinegar
are placed on it.
Large deposits of limestone are found in Kentucky, Indiana, and Florida. Limestone is
widely used for buildings. For example, the Empire State Building in New York City and
the Pentagon in Washington, D.C., were both built with limestone.
Gypsum and Rock Salt. The minerals gyp sum and halite (rock salt) dissolve slowly in
water. During the history of Earth, enormous quantities of both of these minerals were
dissolved in ancient oceans. When these ancient oceans dried up, great deposits of
gypsum and halite were left behind.
Gypsum is composed of the elements calcium, sulfur, and oxygen combined with the
compound water. When gypsum is heated in an oven it forms plaster of Paris. A pearlywhite or pink rock made of gypsum, called alabaster, is often carved into statues and
vases. The White Sands National Monument in New Mexico consists of deposits of
beautiful, white gypsum sands.
Deposits of rock salt are usually found with deposits of gypsum because rock salt also is
left behind when seawater evaporates. The Great Salt Lake in Utah is a small remnant of
Lake Bonneville that once existed there. The great Bonneville Salt Flats that exist today
were formed when Lake Bonneville lost most of its water by evaporation. The extensive
salt deposits and the Great Salt Lake are all that remain of ancient Lake Bonneville. The
flatness of this area makes it an ideal place for testing high-speed racing cars.
FOLIATED METAMORPHIC ROCKS
Slate. When shale is compressed and heated, slate is formed. Because slate is highly
foliated, it splits easily into thin, smooth slabs. These slabs are used for roofs, sidewalks,
billiard tables, and chalkboards. Slate occurs in many colors, including black, blue, green,
and red.
Schist. The continued Metamorphism of slate or the metamorphism of basalt produces
schist. Schist consists of very thin, parallel bands, or flakes, of minerals. There are
various types of schist. Each type is named for the most abundant mineral in the schist.
For example, mica schist contains many flakes of mica. Talc schist and graphite schist
contain large amounts of talc and graphite. Schist forms the bedrock in many parts of
New England, as well as making up most of the bedrock of Manhattan Island.
Gneiss. Further metamorphism of schist or the metamorphism of granite forms gneiss.
Like schist, the minerals in gneiss appear as parallel bands. However, the coarse,
alternating bands of light and dark colored minerals are much thicker in gneiss than in
schist. Gneiss often is cut into blocks and used as a building stone.
Notice that the three foliated metamorphic rocks can be produced from the same rock:
Shale can be the source rock for slate, schist, and gneiss. The type of metamorphic rock
produced from shale depends upon the amount of heat and pressure and the length of
time that the shale undergoes metamorphism.
NON-FOLIATED METAMORPHIC ROCKS
Quartzite. When quartz sandstone under- goes metamorphism, a rock called quartzite is
formed. Quartzite is very hard and durable and resembles sandstone. However, unlike
sandstone, which is porous, there are very few open spaces between the grains in
quartzite. Water cannot seep through quartzite. Although quartzite is a hard rock, it is not
used as a building stone because it is full of cracks. The cracks result from the great
pressures that changed the sandstone to quartzite. Quartzite is used for railroad beds.
Marble. Limestone that undergoes metamorphism becomes marble. Marble is harder,
more crystalline, and coarser than the limestone from which it is formed. The mineral
grains in marble often have a somewhat rounded appearance.
Marble occurs in many colors. It often has a streaked appearance due to impurities forced
into the limestone during metamorphism. Marble is widely used for sculptures and
monuments. Highly polished marble is used as an ornamental stone in buildings in
Washington, D.C.