Understanding Earth: A Dynamic and Evolving Planet - The Changing Earth: Exploring Geology and Evolution - page 22

Mid-oceanic
ridge

Weathering
and erosion

Oceanic
crust

Trench

Deposition

Sediment

Continental
shelf

Upwelling

Metamorphism

Asthenosphere

Magma and
igneous activity

Upper
mantle

Melting

Continental
crust

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Figure 1.16 Plate Tectonics and the Rock Cycle Plate movement provides the driving mechanism
that recycles Earth materials. The cross section shows how the three major rock groups—igneous,
sedimentary, and metamorphic—are recycled through both the continental and oceanic regions.
Subducting plates are partially melted to produce magma, which rises and either crystallizes beneath
Earth's surface as intrusive igneous rock or spills out on the surface, solidifying as extrusive igneous
rock. Rocks exposed at the surface are weathered and eroded to produce sediments that are
transported and eventually lithifi ed into sedimentary rocks. Metamorphic rocks result from pressure
generated along converging plates or adjacent to rising magma.

record allows geologists to interpret physical events and con-
ditions in the geologic past, fossils , which are the remains or
traces of once-living organisms, not only provide evidence
that evolution has occurred, but also demonstrate that Earth
has a history extending beyond that recorded by humans.
The succession of fossils in the rock record provides geolo-
gists with a means for dating rocks and allowed for a relative
geologic time scale to be constructed in the 1800s.

time. Although they may have disastrous effects on the human
species, global warming and cooling are part of a larger cycle
that has resulted in numerous glacial advances and retreats
during the past 1.8 million years. Because of their geologic
perspective on time and how the various Earth subsystems
and cycles are interrelated, geologists can make valuable con-
tributions to many of the current environmental debates such
as those involving global warming and sea-level changes.
The geologic time scale subdivides geologic time into
a hierarchy of increasingly shorter time intervals; each time
subdivision has a specifi c name. The geologic time scale re-
sulted from the work of many 19th-century geologists who
pieced together information from numerous rock exposures
and constructed a chronology based on changes in Earth's
biota through time. Subsequently, with the discovery of ra-
dioactivity in 1895 and the development of various radio-
metric dating techniques, geologists have been able to assign
numerical ages (also known as absolute ages) in years to the
subdivisions of the geologic time scale (

GEOLOGIC TIME AND
UNIFORMITARIANISM
An appreciation of the immensity of geologic time is
central to understanding the evolution of Earth and its
biota. Indeed, time is one of the main aspects that sets
geology apart from the other sciences, except astronomy.
Most people have difficulty comprehending geologic
time because they tend to think in terms of the human
perspective—seconds, hours, days, and years. Ancient
history is what occurred hundreds or even thousands
of years ago. When geologists talk of ancient geologic
history, however, they are referring to events that hap-
pened hundreds of millions or even billions of years
ago. To a geologist, recent geologic events are those that
occurred within the last million years or so.
It is also important to remember that Earth goes through
cycles of much longer duration than the human perspective of

Figure 1.17).
One of the cornerstones of geology is the principle of
uniformitarianism , which is based on the premise that
present-day processes have operated throughout geologic
time. Therefore, to understand and interpret geologic events
from evidence preserved in rocks, we must fi rst understand
present-day processes and their results. In fact, uniformitari-
anism fi ts in completely with the system approach that we
are following for the study of Earth.

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