Geoscience Reference
In-Depth Information
Organic Sedimentary Rocks As the name implies, organic
sedimentary rocks consist of carbon-based materials that accumu-
late in thick deposits at the surface of Earth. An example of this
process was shown in Chapter 11, specifically the accumulation
of plant and organic matter as peat in cool and moist environ-
ments. Such thick accumulations of solid organic remains have
occurred on a large scale over time, only to be buried by other
sediments. When these organic deposits are buried progressively
deeper, they slowly compact under the weight of the overlying
sediments. In addition, they begin to be heated because tempera-
ture slowly rises with increased depth of burial. Over the course of
millions of years, the amount of water and oxygen in the carbon
progressively lowers and the deposit gradually solidifies, result-
ing in coal . Sometimes these deposits are cooked so much that
they liquefy to form petroleum (crude oil). Natural gas comes
from the remains of microscopic plants that live in the surface
waters of the ocean. When they die, they settle to the bottom of
the ocean where they decompose and form gas. This form of gas
is typically about 85% methane and 10% ethane, with smaller
amounts of propane and butane. In places where organic sedi-
mentary rocks occur, they are usually found in distinct beds that
are separated by layers of limestone, sandstone, or shale. These
deposits frequently accumulate in domes that form when rocks
are bent due to compression for one reason or another; this pro-
cess will be discussed more thoroughly in Chapter 13.
In addition to coal and natural
gas, petroleum is especially vital
in modern civilization. Although
humans have used petroleum in
some way since at least the 4th
century, when the upper classes in
ancient Babylon burned it to pro-
vide light, the modern history of
oil began in the mid-1800s when
the process through which it could
be distilled to kerosene was discovered in Canada. The first oil
well in the United States was drilled in western Pennsylvania in
1859 and produced about 20 barrels of crude per day, with each
barrel containing about 160 L (42 gal) of fluid. Demand for oil
grew rapidly throughout the late 1800s, primarily for kerosene
use in oil lamps. With the advent of internal combustion en-
gines in the early 20th century, demand skyrocketed. Today it is
by far the most dominant energy source for automobiles in the
form of refined gasoline. In addition, many consumer products,
such as computers, televisions, cell phones, and plastic bottles,
to name a very few, are made with it. To meet this demand, over
80 million barrels of oil are required every day worldwide. The
United States alone consumes over 18 million barrels of oil per
day. In short, the global economy in its present form is utterly
dependent on petroleum.
Metamorphic Rocks Igneous and sedimentary rocks
can be altered after they develop. This alteration results in
metamorphic rock , which occurs when a former igneous or
sedimentary rock is subjected to intense heat or pressure within
Earth over millions of years. Thus, the name of the rock comes
from a Greek word meaning “to change form.” (A good example
of metamorphosis is when a caterpillar changes into a butterfly.)
Figure 12.16 shows the various ways in which metamorphic
rocks form. One way occurs when rocks are slowly compressed
Contact
Rocks heated by
invading magma
Contact and compression
Rocks heated by invading magma
and by compression of mountain uplift
M o lt e n r o c k
Compression
Rocks squeezed
along faults when
rock layers collide
Figure 12.16 Formation of metamorphic
rocks. Metamorphic rocks evolve due to in-
tense pressure of overlying rocks or because
rocks are heated dramatically when they come
into contact with igneous bodies.
Compression
Rocks buried under thick
layers of younger rocks
Coal A solid fossil fuel that consists of carbonized plants and
animals.
Natural gas Naturally occurring mixture of hydrocarbons
that often occurs in association with petroleum and is found in
porous geologic formations.
Petroleum Naturally occurring oily liquid that consists of
ancient hydrocarbons.
Metamorphic rocks Rocks that form when igneous, sedimentary,
or other metamorphic rocks are subjected to intense heat and pressure.
 
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