Limestone
the lowest and
oldest layer contains fossils of
tiny creatures, showing that
100 million years ago, during
the time of the dinosaurs, the
region was under the sea.our earth changes all the time. mountains rise and wear away.
continents move, causing oceans to widen and narrow. these changes
are slow. it would take a million years to notice much difference. other
changes, such as when an earthquake shakes the land or a volcano
erupts, are sudden. geology is the study of how earth changes, how it
was formed, and the rocks that it is made of.
clues to earth’s history are hidden in its rocks. geologists survey
(map out) the land and dig down to the rocks in earth’s crust. the age
and nature of the rocks and fossils (evidence of prehistoric plants and
animals) help geologists understand the workings of earth. geologists
also help discover valuable deposits of
coal, oil, and other useful minerals.
they study the land before a large
structure such as a dam is built, to make
sure that the land can support the great
weight. geologists also warn people
about possible disasters. using special
instruments, they detect the
movement of rocks and try to
predict volcanic eruptions
and earthquakes.radioactive
dating
rocks contain
substances that decay
over millions of years,
giving off tiny amounts
of nuclear radiation.
By a process called
radioactive dating,
which measures this
radioactivity, geologists
can find out how old
the rocks are.The hisTory of geology
the ancient greeks
and hindus were the
first peoples to study and
date the rocks of earth.
during the late 18th
century, the scottish
scientist James hutton
became the first
european geologist to
realize that earth is
millions of years old and
that it changes constantly.
But his ideas were not
accepted until after his
death. in 1912, alfred Wegener, a german
meteorologist, proposed that the continents
move. But it was more than 50 years before
his idea was found to be true.231
Geology
seismic tests
special trucks strike the
ground with huge hammers,
producing shock waves,
called seismic waves, which
bounce off the layers of rock
below. computers use these
waves to draw pictures of the
layers of rock within earth.examining earth
earth’s crust is made of layer upon
layer of different kinds of rock that
have been laid down over millions
of years. the topmost layers usually
formed most recently and the
lowest layers are the oldest. By
uncovering these layers of rock,
geologists can trace back the
history of earth.aeriaL surveys
airplanes carry special
cameras that produce
three-dimensional views
of the land below, and
instruments that measure
the strength of earth’s
magnetism and gravity.rock sampLe
the layers of rock in this sample
(above) come from deep
underground.driLLing
rigs bore shafts as
deep as 10,000 ft
(3,000 m) below the
ground and bring up
samples of the rock
layers beneath.BasaLt
Lava from
a volcano formed this layer
of basalt. the land rose
from the sea, and a volcano
erupted nearby to cover
the rock below with lava.shaLe
a layer of shale
rock shows that the land must
have been beneath shallow
water. mud from a nearby
river built up and compacted,
forming shale.sandstone
the top
and youngest layer of rock
is sandstone. it sometimes
forms from desert sands. the
criss-cross pattern shows how
the wind blew sand to form
the rock.geoLogists at Work
rocks at earth’s surface reveal their
past to the expert eyes of geologists.
For example, huge cracks in layers of
rock show that powerful forces once
squeezed the rocks.In 1795, James Hutton
founded the modern
science of geology
with his book The
Theory of the Earth.Find out more
coal
continents
earth
earthquakes
Fossils
gas
oil
rocks and mineralssateLLite mapping
satellites circle earth and send back
photographs of the surface from
space. the pictures show features
of the land in great detail and help
geologists identify the rocks.
satellites have also measured
the size and shape of earth.Studying the rocks in the
ocean floor can reveal
the slow movements
of Earth’s crust.US_231_Geology.indd 231 21/01/16 4:59 pm