CHAPTER 1 An Introduction to Geology
16
are open systems and are far more complicated than the foregoing example. In an
open system both energy and matter flow
into and out of the system. In a weather
system such as a hurricane factors such as
the quantity of water vapor available for
cloud formation, the amount of heat
released by condensing water vapor, and
the flow of air into and out of the storm can
fluctuate a great deal. At times the storm
may strengthen; at other times it may
remain stable or weaken.
FEEDBACK MECHANISMS. Most natural
systems have mechanisms that tend to
enhance change, as well as other mechanisms that tend to resist change and thus
stabilize the system. For example, when we
get too hot, we perspire to cool down. This
cooling phenomenon works to stabilize our
body temperature and is referred to as a
negative feedback mechanism. Negative
feedback mechanisms work to maintain
the system as it is or, in other words, to
maintain the status quo. By contrast,
mechanisms that enhance or drive change
are called positive feedback mechanisms.
Most of Earth’ s systems, particularly the
climate system, contain a wide variety of
negative and positive feedback mechanisms. For example, substantial scientific
evidence indicates that Earth has entered
a period of global warming. One consequence of global warming is that some of
the world’ s glaciers and ice caps have begun
to melt. Highly reflective snow- and
ice-covered surfaces are gradually being
replaced by brown soils, green trees, or
blue oceans, all of which are darker, so they
absorb more sunlight. Therefore, as Earth
warms and some snow and ice melt, our
planet absorbs more sunlight. The result is
a positive feedback that contributes to the
warming.
On the other hand, an increase in
global temperature also causes greater
evaporation of water from Earth’ s land–sea
surface. One result of having more water
vapor in the air is an increase in cloud
cover. Because cloud tops are white and
highly reflective, more sunlight is reflected
back to space, which diminishes the
amount of sunshine reaching Earth’ s surface
and thus reduces global temperatures.
Further, warmer temperatures tend to
promote the growth of vegetation. Plants in
turn remove carbon dioxide (CO 2 ) from the air.
Since carbon dioxide is one of the atmosphere’ s
greenhouse gases, its removal has a negative impact
on global warming.*
In addition to natural processes, we must
consider the human element. Extensive cutting and
clearing of the tropical rain forests and the burning
of fossil fuels (oil, natural gas, and coal) result in an
increase in atmospheric CO 2 . Such activity is contributing to the increase in global temperature that
our planet is experiencing. One of the daunting tasks for Earth system scientists is to
predict what the climate will be like in the future by taking into account many variables,
including technological changes, population trends, and the overall impact of the numerous
competing positive and negative feedback mechanisms. Chapter 20 on “Global Climate
Change” explores this topic in some detail.
The Earth System
The Earth system has a nearly endless array of subsystems in which matter is recycled over
and over again. One example that you will learn about in Chapter 6 traces the movements
of carbon among Earth’ s four spheres. It shows us, for example, that the carbon dioxide in
the air and the carbon in living things and in certain sedimentary rocks are all part of a
subsystem described by the carbon cycle.
CYCLES IN THE EARTH SYSTEM. A more familiar loop or subsystem is the hydrologic
cycle. It represents the unending circulation of Earth’ s water among the hydrosphere,
atmosphere, biosphere, and geosphere. Water enters the atmosphere by evaporation from
Earth’ s surface and by transpiration from plants. Water vapor condenses in the atmosphere
to form clouds, which in turn produce precipitation that falls back to Earth’ s surface. Some
of the rain that falls onto the land sinks in to be taken up by plants or become groundwater, and some flows across the surface toward the ocean.
Viewed over long time spans, the rocks of the geosphere are constantly forming, changing, and reforming. The loop that involves the processes by which one rock changes to
another is called the rock cycle and will be discussed at some length in the following
section. The cycles of the Earth system, such as the hydrologic and rock cycles, are not
independent of one another. To the contrary, there are many places where they have an
interface. An interface is a common boundary where different parts of a system come in
contact and interact. For example weathering at the surface gradually disintegrates and
decomposes solid rock. The work of gravity and running water may eventually move this
material to another place and deposit it. Later, groundwater percolating through the debris
may leave behind mineral matter that cements the grains together into solid rock (a rock
that is often very different from the rock we started with). This changing of one rock into
another, which is part of the rock cycle, could not have occurred without the movement of
water through the hydrologic cycle. There are many places where one cycle or loop in the
Earth system has an interface with and is a basic part of another.
ENERGY FOR THE EARTH SYSTEM. The Earth system is powered by energy from two
sources. The Sun drives external processes that occur in the atmosphere, hydrosphere, and
at Earth’ s surface. Weather and climate, ocean circulation, and erosional processes such as
rivers, glaciers, wind, and waves are driven by energy from the Sun. Earth’ s interior is the
second source of energy. Heat remaining from when our planet formed, and heat that is
continuously generated by decay of radioactive elements, power the internal processes that
produce volcanoes, earthquakes, and mountains.
*Greenhouse gases absorb heat energy emitted by Earth and thus help keep the atmosphere warm.
D I D Y O U K N O W ?
Since 1970, Earth’s average
surface temperature increased
by about 0.6°C (1°F). By the
end of the 21
st century, the
average global temperature
may increase by an additional
2° to 4.5°C (3.5° to 8.1°F)
16
are open systems and are far more complicated than the foregoing example. In an
open system both energy and matter flow
into and out of the system. In a weather
system such as a hurricane factors such as
the quantity of water vapor available for
cloud formation, the amount of heat
released by condensing water vapor, and
the flow of air into and out of the storm can
fluctuate a great deal. At times the storm
may strengthen; at other times it may
remain stable or weaken.
FEEDBACK MECHANISMS. Most natural
systems have mechanisms that tend to
enhance change, as well as other mechanisms that tend to resist change and thus
stabilize the system. For example, when we
get too hot, we perspire to cool down. This
cooling phenomenon works to stabilize our
body temperature and is referred to as a
negative feedback mechanism. Negative
feedback mechanisms work to maintain
the system as it is or, in other words, to
maintain the status quo. By contrast,
mechanisms that enhance or drive change
are called positive feedback mechanisms.
Most of Earth’ s systems, particularly the
climate system, contain a wide variety of
negative and positive feedback mechanisms. For example, substantial scientific
evidence indicates that Earth has entered
a period of global warming. One consequence of global warming is that some of
the world’ s glaciers and ice caps have begun
to melt. Highly reflective snow- and
ice-covered surfaces are gradually being
replaced by brown soils, green trees, or
blue oceans, all of which are darker, so they
absorb more sunlight. Therefore, as Earth
warms and some snow and ice melt, our
planet absorbs more sunlight. The result is
a positive feedback that contributes to the
warming.
On the other hand, an increase in
global temperature also causes greater
evaporation of water from Earth’ s land–sea
surface. One result of having more water
vapor in the air is an increase in cloud
cover. Because cloud tops are white and
highly reflective, more sunlight is reflected
back to space, which diminishes the
amount of sunshine reaching Earth’ s surface
and thus reduces global temperatures.
Further, warmer temperatures tend to
promote the growth of vegetation. Plants in
turn remove carbon dioxide (CO 2 ) from the air.
Since carbon dioxide is one of the atmosphere’ s
greenhouse gases, its removal has a negative impact
on global warming.*
In addition to natural processes, we must
consider the human element. Extensive cutting and
clearing of the tropical rain forests and the burning
of fossil fuels (oil, natural gas, and coal) result in an
increase in atmospheric CO 2 . Such activity is contributing to the increase in global temperature that
our planet is experiencing. One of the daunting tasks for Earth system scientists is to
predict what the climate will be like in the future by taking into account many variables,
including technological changes, population trends, and the overall impact of the numerous
competing positive and negative feedback mechanisms. Chapter 20 on “Global Climate
Change” explores this topic in some detail.
The Earth System
The Earth system has a nearly endless array of subsystems in which matter is recycled over
and over again. One example that you will learn about in Chapter 6 traces the movements
of carbon among Earth’ s four spheres. It shows us, for example, that the carbon dioxide in
the air and the carbon in living things and in certain sedimentary rocks are all part of a
subsystem described by the carbon cycle.
CYCLES IN THE EARTH SYSTEM. A more familiar loop or subsystem is the hydrologic
cycle. It represents the unending circulation of Earth’ s water among the hydrosphere,
atmosphere, biosphere, and geosphere. Water enters the atmosphere by evaporation from
Earth’ s surface and by transpiration from plants. Water vapor condenses in the atmosphere
to form clouds, which in turn produce precipitation that falls back to Earth’ s surface. Some
of the rain that falls onto the land sinks in to be taken up by plants or become groundwater, and some flows across the surface toward the ocean.
Viewed over long time spans, the rocks of the geosphere are constantly forming, changing, and reforming. The loop that involves the processes by which one rock changes to
another is called the rock cycle and will be discussed at some length in the following
section. The cycles of the Earth system, such as the hydrologic and rock cycles, are not
independent of one another. To the contrary, there are many places where they have an
interface. An interface is a common boundary where different parts of a system come in
contact and interact. For example weathering at the surface gradually disintegrates and
decomposes solid rock. The work of gravity and running water may eventually move this
material to another place and deposit it. Later, groundwater percolating through the debris
may leave behind mineral matter that cements the grains together into solid rock (a rock
that is often very different from the rock we started with). This changing of one rock into
another, which is part of the rock cycle, could not have occurred without the movement of
water through the hydrologic cycle. There are many places where one cycle or loop in the
Earth system has an interface with and is a basic part of another.
ENERGY FOR THE EARTH SYSTEM. The Earth system is powered by energy from two
sources. The Sun drives external processes that occur in the atmosphere, hydrosphere, and
at Earth’ s surface. Weather and climate, ocean circulation, and erosional processes such as
rivers, glaciers, wind, and waves are driven by energy from the Sun. Earth’ s interior is the
second source of energy. Heat remaining from when our planet formed, and heat that is
continuously generated by decay of radioactive elements, power the internal processes that
produce volcanoes, earthquakes, and mountains.
*Greenhouse gases absorb heat energy emitted by Earth and thus help keep the atmosphere warm.
D I D Y O U K N O W ?
Since 1970, Earth’s average
surface temperature increased
by about 0.6°C (1°F). By the
end of the 21
st century, the
average global temperature
may increase by an additional
2° to 4.5°C (3.5° to 8.1°F)