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34. The Changing Face of the Earth
In Chapter 32 we considered interactions among
tectonic plates that result in mountain ranges like the
Appalachians on the North American continent and the
Himalayas of Asia. Plate tectonics alone can provide us
only with the generalities—the broad locations and orientations of some landforms. The specific details of
mountains, valleys, plains, and plateaus arise from the
interaction of the plate tectonic system with the hydrologic system. As a preface to an examination of that
system, we introduce the hydrologic cycle.
(the water that flows downhill to eventually join
streams) from the discharge of the Seine. After allowing for the amount of water evaporated from the surface, he showed that there was easily enough precipitation to feed both the streams and springs in the basin. If
this fact seems contradictory to your intuition, just realize that precipitation falls over a very large area, but the
actual area occupied by streams is comparatively small.
The “water-flow budget” of the earth is depicted in
Figure 34.1.
The hydrologic cycle, shown schematically in
Figure 34.2, consists largely of those phenomena mentioned above and is powered by energy from the sun.
Note, however, that the plate tectonic system, powered
by energy from radioactive decay, plays a role in the
hydrologic cycle because subducted seafloor rock and
sediment carry H2O and CO2, which are chemically
bound in minerals, deep into the earth where metamorphism and partial melting result in their return to the
atmosphere. (Quantitatively, in cubic kilometers per
year, the plate-tectonic contribution is relatively small,
but over geologic time it has likely accounted for the
presence of enough CO2 in our atmosphere to keep the
planet warm—a very important benefit, indeed.)
The Hydrologic Cycle
If you were a traveler in space and had particularly
acute eyesight, one of the most pervasive features you
would observe as you approached the earth would be
moving water. You would see vast oceans covering
three-quarters of the planet, and throughout them the
water would be moving. On the continents water would
be flowing downhill in millions of streams, varying in
size from the very largest rivers to the smallest brooks.
Water would be slowly percolating through the pores of
rocks underground. You would see roots of plants
absorbing it from thin coatings around soil grains and
transporting it into the leaves, where it would be transpired into the atmosphere by evaporation from the surfaces of the leaves. Evaporation would move great
quantities of water into the atmosphere from the oceans,
lakes, streams, and even falling raindrops, where it
would again condense and eventually fall toward the
surface of the planet as precipitation. In a general way,
the water would circulate from the surface to the atmosphere and back again.
You are aware of the existence of rivers like the
Mississippi, the Amazon, and the Nile, and you may
wonder where all of that water comes from. Is precipitation sufficient to provide the water for large rivers like
these and for the countless smaller streams that drain the
land, as well as for springs that bring water to the surface from some unseen source? That question was
answered in the mid-17th century, before geology
became a formal science, by the French naturalist Pierre
Perrault (1611-1680). For a period of years he kept
track of the amount of precipitation that fell on the
Seine River basin and calculated the mean annual runoff
The Hydrologic System
In discussing the ways in which the hydrologic system modifies the surface of the earth, we shall consider
surface drainage, glaciers, subsurface water, wind (not
only because it carries water vapor but, more importantly, because it also has the power to erode and transport sediment), and oceans. Most of the earth’s water,
over 97 percent, is in the oceans, with about 2 percent
frozen as ice, 0.6 percent underground, and only 0.01
percent in surface water on land. Unlikely as it may
sound, that 0.01 percent has had the most widespread
effect of all on the shape of our land surfaces.
Running Water
Virtually every location on the surfaces of the earth’s
landmasses have been shaped, to some extent, by running
water. Color Plate 15 is a satellite photograph of the
Grand Canyon of Arizona, one of the more arid regions
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Precipitation
Evaporation
Precipitation
9.6
6.0
28.4
Evaporation
32.0
Runo
ff
3.6
Groundw
ater
Figure 34.1. The water-flow budget of the earth. Figures are in units of 104 km3, or tens of thousands of cubic kilometers, per year. (Each cubic kilometer is 264 billion gallons!)
Evaporation from
lakes, streams, and
soil
Evaporation
H2O and CO2
Precipitation
Transpiration
ff
Runo
Groundwater
H2 O and CO
2
H O and CO*
2
2
* Mostly as CaCO3 used
by marine organisms
to build shells.
Figure 34.2. The hydrologic cycle, consisting largely of water moving in the hydrologic system (essentially a surface
or near-surface system), and also in limited parts of the plate tectonic system (essentially a subsurface system).
of the planet. Yet, observe the intricate network of stream
valleys that dissect the plateau around the canyon.
Stream valleys, in fact, are the most abundant landform
on the continents. As water flows in them, particles of
sand and other materials carried in the current act as
“sandpaper” on the stream bed, abrading it and deepen-
ing the valley. At the same time, water moving down the
valley sides in sheets and rivulets erodes the valley walls,
causing them to recede. Valleys are lengthened by a
process called headward erosion, in which erosion is
focused at the head of a valley by convergence of runoff
from several directions (see Figs. 34.3 and 34.4).
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source of the sediment while finer particles are carried
farther. When the sediment eventually becomes sedimentary rock, its grain size provides insight into the distance of transportation of the sediment.
A little thought will convince you that there must
be a limit to the depth of stream erosion. For streams
that empty into the ocean, that limit is essentially sea
level, because a stream would have to flow uphill if it
cut more deeply than that. In order to erode at all, a
stream must flow fast enough to carry abrasive sediment, so this maximum depth of erosion must decrease
away from the sea in order that there be some slope to
the stream channel. (That is, the erosional limit must be
at higher and higher elevations further and further from
the sea.) The maximum possible depth of downcutting
is called base level, and it is the level toward which all
streams strive. They rarely attain base level, however,
because tectonic movement of the land intercedes and
changes it. Thus there is a constant competition
between the hydrologic system and the tectonic system,
the former striving to lower the land and the latter generally attempting to raise it.
Figure 34.3. Headward erosion occurs when runoff
converges on the head of a valley from several directions.
Glaciers
Glaciers are masses of ice that form either at high
elevations (in mountains) or at high latitudes (far north
or far south) where temperatures are perennially low.
The process of forming a glacier requires that snow
accumulation in the winter exceed loss in the summer,
so that the deposit becomes deeper with time.
Eventually, the snow deep in the snow field is compacted into a tough, granular form, similar texturally to the
old, grainy snow that accumulates at the side of the road
during a long winter cold spell. The final stage is the
recrystallization of the granular snow into glacial ice, a
mass of intergrown ice crystals many meters below the
surface. As more and more ice forms, the mass
becomes heavy enough to move downhill as the ice at
the bottom slowly deforms under pressure.
Glaciers that form in valleys originally carved by
streams are called valley glaciers. They move slowly
downhill both by sliding (basal slip) and by deformation
of the ice near the base (plastic flow). Continental
glaciers cover very large areas and are not confined to
valleys. Color Plate 16 shows the Vatnajökull continental glacier in Iceland with outlet glaciers that are
similar to valley glaciers near the fringe. Such glaciers
spread out from a central area in all directions, much
like pancake batter from the center of a griddle. Figures
34.5 and 34.6 are photographs of a valley glacier and a
continental glacier, respectively.
Figure 34.4. A section of the Colorado Plateau, showing a through-flowing stream with several tributary
canyons occupied by streams during periods of rain.
Headward erosion slowly extends the canyons into the
plateau.
Many stream-generated landforms are depositional
rather than erosional. As streams empty into the ocean
or lakes, they deposit sediment in deltas, fan-shaped or
fingerlike deposits that record, in part, a history of the
stream. In some areas streams do not flow to the ocean
or to lakes but end in dry basins that have no outlets. In
such places, the streams that intermittently flow from
canyons deposit their sediment on the valley floors in
alluvial fans, the subaerial analogs of the subaqueous
deltas. Streams tend to be efficient at sorting sediment
by size, so that coarser particles are deposited near the
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however, even the slowest glaciers can carry huge boulders as well as smaller material. Not only does the ice
itself abrade the land but it also carries embedded rocks
of all sizes, and thus acts very much like sandpaper.
Valley glaciers gouge and scoop out the valleys they
occupy, modifying them into U-shaped troughs. The
topography between adjacent glaciated valleys is often
angular and sharp. Continental glaciers scrape and
scour the rocks over which they flow, largely obliterating previously developed stream drainage systems.
Glaciers do not flow uphill, of course, but they may
melt faster at their lower ends than they advance. When
this occurs, we say that a glacier is receding. Because
they can carry sediment of all sizes, but are incapable of
sorting it by size as is running water, glaciers leave
deposits of rough, unsorted debris called moraines
when they melt. Moraines deposited by ancient glaciers are common landforms in some areas of the northeastern and midwestern parts of the United States.
More glaciers seem to be receding than advancing
today, but there have been times in the past when glaciation was a much more active process than it is now. There
have been several “ice ages” during geologic time, as far
back as the Precambrian and as recently as the Great Ice
Age of the Pleistocene Epoch in the Cenozoic Era. The latter involved recurring glacial and interglacial stages and
was so recent that we cannot be sure that we are not simply in another interglacial stage of the same ice age now.
Moreover, there is no general agreement about the cause of
ice ages, although it may have much to do with the positioning of continent-size landmasses at high north or south
latitudes by plate motion, and their interference with ocean
currents that facilitate the worldwide transfer of heat.
Figure 34.5. A typical valley glacier.
Groundwater
Considerably less than a percent of the total water
in the hydrologic system resides underground, but it is
of critical importance—20 percent of the freshwater
requirements of the United States is met by it. Nearly
all groundwater comes from precipitation that has
seeped into the ground. Some precipitation remains in
the upper soil layer as a film around soil particles, but
most descends to a depth at which all of the pore spaces
in the rock are filled with water—the region of groundwater. The upper surface of this region is the water
table, and its shape tends to mimic the topography
above (see Fig. 34.7). The zone of pore saturation exists
under both humid and arid areas of the earth, but it is
deeper in arid areas. In humid areas, the water table is
shallow and groundwater contributes to stream flow, so
that even during dry spells there is water in the streams.
In arid areas, the water table is deep and does not contribute to stream flow; rather, the streams “leak” and
provide water to the subsurface, so that most
streambeds in such areas contain no water during dry
Figure 34.6. Part of the continental glacier that covers
Greenland.
Like running water, glaciers generate both erosional and depositional landforms. Unlike running water,
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Zone of saturation
Water table
when the water table has been lowered by deepening of
nearby stream channels, carbonate-rich waters percolating into these caverns will deposit calcium carbonate
(CaCO3) to make beautiful stalactites, stalagmites, and
other cavern formations.
Often, where limestone beds with dissolved voids
exist near the surface, the roofs of the voids are too thin
to support themselves and they collapse, forming sinkholes (Fig. 34.8). In populated areas these cause great
damage when they collapse beneath and engulf houses,
cars, and so forth.
Figure 34.7. The water table.
Wind
periods.
Like surface water and glaciers, groundwater also
flows in response to gravity. It moves generally from
areas of high elevation of the water table to areas of
lower elevation (streams, springs, or lakes), but may
move locally upward in order to reach regions of lesser
hydrostatic pressure caused by “topography” on the
water table itself. The flow of groundwater is slower
than water flow on the surface, averaging only centimeters per day—a fortunate circumstance, for were it not
so, wells drilled into groundwater would rapidly go dry
because supply by rainfall could not keep up with depletion from pumping.
The erosional and depositional work of groundwater produces some spectacular results, as anyone who
has visited an underground cavern can attest. Rainwater
absorbs small amounts of carbon dioxide from the
atmosphere as it falls, creating a weak carbonic acid
(H2CO3) that is very effective in slowly dissolving limestone. Consequently, in areas where limestone constitutes a major part of the near-surface rock, large caverns
may be dissolved near or below the water table. Later,
Wind is incapable of carrying the heavy particles
that denser agents like running water or ice can carry,
but anyone who has been caught in a dust storm or sandstorm recognizes that wind can lift and transport very
large amounts of smaller particles. Wind plays its most
important role in shaping the face of the land in deserts
and near-desert regions, and these areas constitute about
one-fifth of the land surface of the earth. Even in most
of these, water is a very important, though infrequent,
agent of erosion.
Sand dunes are probably the best known of windgenerated landforms. Color Plate 17 shows large dunes
on the Arabian Peninsula; some of them are 100 meters
high (higher than a football field on end) and up to 200
kilometers long. Depending upon the abundance of
sand, the density of plant cover, and the constancy and
strength of winds, other types of dunes with different
shapes may develop in other arid areas.
Figure 34.8. Sinkholes produced by the collapse of
underground voids in limestone. The voids develop as
weak carbonic acid, formed when rain combines with
atmospheric carbon dioxide, dissolves the limestone.
Figure 34.9. A typical sea cliff, a feature created by
marine erosion.
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Motion of sand grains
W
av
ed
ire
cti
Sand migration
on
Figure 34.10. When waves impinge obliquely on the beach, sediment is carried in from an angle, but recedes directly
downslope, resulting in migration of the sediment down a coastline.
Oceans
The crashing breakers during a storm at the
seashore demonstrate the power of the ocean to modify
the land, but of course the effects are limited to that narrow band where sea and land meet—the shoreline.
Wave action can be very effective in modifying the
shoreline, as seen in Figure 34.9, which shows a sea
cliff carved by marine erosion. Many beach cottages
that have been built well back from sea cliffs have eventually become unsafe because of receding shorelines.
The sea can create depositional landforms as well
as erosional ones. As sediment is carried by waves onto
a beach at an angle oblique to the shoreline, it washes
back toward the sea directly downslope, perpendicular
to the shoreline (Fig. 34.10). Thus sediment migrates
slowly down a coast. Such sediments are often spread
into elongate landforms, as shown in Figure 34.11.
Some of these become large enough to support buildings or communities.
The various facets of the hydrologic system do not
always work independently. Where a stream empties
into the sea, it builds a delta, but the waves and currents
of the sea may modify the form of that deposit. Within
many areas in the world, one may see the combined
effects of waves, wind, and running water; glaciers and
waves; or running water and groundwater.
Figure 34.11. An elongate landform created by the
spreading of sediment transported in the way shown in
Figure 34.10.
which it has passed since its formation. Many of the
details of this long history have yet to be worked out by
geologists, but the general outline is probably correct.
Accretion Stage
The Face of the Earth Through Time
Recall from Chapter 28 that the early history of the
solar system involved the gravitational collapse of the
condensing solar nebula into countless small clumps
and that larger clumps gradually swept up smaller ones
and grew into planetesimals. The process continued
until there emerged from the mass of gas, dust, and
While the interaction of the tectonic system and the
hydrologic system explains the appearance of the earth
today, our planet has not always appeared as it does
now—even vaguely. To conclude this brief study of the
earth, we summarize the various major stages through
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chunks a star, nine planets, their assorted moons, and
various swarms of material like meteors. One of the
planets was ours, of course, but it bore little resemblance to today’s earth. There were neither continents
nor oceans, and there was no internal structure—that is,
it was homogeneous, the same all the way from surface
to center. The earth was essentially age zero, about 4.6
billion years ago, and its temperature was around 1000
°C in the interior.
an oxide-and-silicate mantle of mostly iron-magnesium-oxygen compounds, which in turn was surrounded by a basaltlike crust. The process of forming layers
based on density is called planetary differentiation.
Because the radioactive elements were chiefly involved
in compounds with oxygen, they tended to concentrate
in the outer layers of the planet where radiogenic heat
was dissipated rapidly, and the heating therefore slowed
down. By this point 400 or 500 million years had
passed since the formation of the solar system, and the
time was about 4.2 billion years ago. The face of the
earth was still not recognizable.
Bombardment and Heating
After the initial formation of the planets there were
countless small bits of matter in the nebular disk yet to
be gravitationally swept up. Bombardment of the earth
by meteors was intense and, as each meteor hit, its
kinetic energy was transformed into internal energy—
heat. (The record of the bombardment stage is largely
absent from the earth now, but it is present on other bodies such as the moon, Mercury, Mars, and some satellites of the outer planets, none of which have had vigorous tectonic or hydrologic systems capable of obliterating it.) As gravity continued to contract the new planet,
gravitational energy was likewise transformed into heat
faster than it could be dissipated. In addition, radioactive elements (mostly uranium, thorium, and potassium), all of which must have been more abundant in the
early earth than they are now, decayed—a third source
of heat. The result of all this heating was that, by
around 4.2 to 4.5 billion years ago, the temperature rose
to the melting point of iron in a shell beginning at 400
kilometers deep and extending to 800 kilometers. (The
temperature must have been hotter deeper than this, but
the melting temperature of iron also increases with
depth. The situation is analogous to the one for the origin of the asthenosphere, and the general geometry
shown in Figure 30.9 applies—with different actual
temperatures and depths, of course.)
Onset of the Tectonic System
The heat generated by the iron catastrophe must
have repeatedly melted silicate minerals surrounding
the developing core, including those constituting the
thin primitive crust that was forming. Eventually, as the
“distillation” process of planetary differentiation proceeded, the least dense materials must have accumulated on top and formed the earliest continents, perhaps
around 4.2 billion years ago. It is difficult to know just
when the tectonic system, as we now define it, had its
beginning, but by 3.9 billion years ago there were probably thin, rather fragile plates moving comparatively
rapidly over an asthenosphere, being subducted and
recycled. These would gradually thicken, support volcanic activity and igneous intrusion, weather and erode,
be metamorphosed, and become the shields of today’s
continents. Not yet wholly familiar, the face of the earth
at least had the beginnings of familiar features.
Origin of the Atmosphere and Oceans
The original atmosphere of the earth was probably
very unlike the one we know today and was swept
away by the vigorous solar wind of the young sun. The
atmosphere we breathe is probably mostly of volcanic
origin and was produced by release of gases from the
interior. If you have seen pictures of volcanic eruptions, you might have noticed that large amounts of
gases are emitted along with lava, ash, or other volcanic emanations. Most of this gas is water vapor, with
some carbon dioxide and nitrogen, and was no doubt
produced by the early volcanism of the heated earth
just as it is today. Even at the present rate of volcanism
(and the rate in the hot, differentiating earth would
have been significantly higher), enough water would
have been produced in this way over the span of geologic time to fill the oceans, along with enough nitrogen, carbon dioxide, and other gases to create nearly all
of the atmosphere. The part of the atmosphere not produced by this process—approximately 20 percent of
it—is the oxygen on which life depends, but which is
essentially absent in volcanic gases.
The Iron Catastrophe and Differentiation
As iron in the 400-800 kilometer depth range began
to melt, it formed droplets that migrated gravitationally
toward the center of the earth, displacing the less dense
material. The process was self-accelerating: As iron
sank, it raised the temperature through friction, and
more iron melted and sank. As the temperature
increased, minerals of all sorts melted as their melting
points were reached. Most silicate minerals with low
melting points also have relatively low densities, so
they floated to the surface. The process finally elevated
the earth’s temperature to around 2000 °C, and so a
large fraction of the planet melted. This event, called
the iron catastrophe, resulted in the complete reorganization of the interior. When it was over, the metallic
iron was mostly in the center of the earth, surrounded by
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Sunlight striking the upper layers of this oxygenpoor atmosphere dissociated some of the water molecules into hydrogen and oxygen. Hydrogen is so light
that it escaped the earth’s gravity, but oxygen was carried by atmospheric turbulence downward toward the
surface of the planet, to become part of the permanent
gaseous envelope of the earth. Most of the oxygen must
eventually have been produced by photosynthesis, however, and this required plant life. The first primitive
cells could have developed in only a small fraction (perhaps as little as one five-thousandth) of the present oxygen level in the atmosphere and begun the slow, continual process of combining carbon dioxide and water to
produce carbohydrates and oxygen.
For a long time the atmosphere was too poor in oxygen to support complex forms of life; hence, the
Precambrian is represented by few fossils, virtually all
soft-bodied organisms. However, as simple plants
(chiefly algae) built the oxygen supply, increasingly
complex organisms developed, and the attainment of
some critical level of oxygen concentration may have
led to the almost explosive flowering of shelled forms
(and hence fossils) at the beginning of the Paleozoic Era.
That the chemistry of our atmosphere is highly
dynamic and has evolved significantly over time is a
fascinatingly troubling concept. To be sure, we are the
beneficiaries of that long course of development, but we
are also now its modifiers—and it is clear that it can be
modified. By our industrial and cultural activities, we
are adding to the levels of some noxious compounds in
the air (such as carbon monoxide and sulfur dioxide),
and we appear to be decreasing the abundance of at least
one crucially necessary substance, ozone. Such tampering is not without consequences, and hence the current concern with control of atmospheric pollution. One
may argue over the methods and priorities suggested by
those on various sides of the issue, but its importance
and urgency cannot be in doubt.
reconstruct some of the plate interactions that occurred
before Pangaea, although the details become less and less
distinct as we look further back in time. For example, the
Ural Mountains, which constitute the traditional geographic separation between Europe and Asia, testify of an
ancient collision between those two continents that welded them into a single large landmass.
It appears that there have been six periods of very
intense and widespread mountain-building activity during the earth’s history. Some of these have been longer
and more intense than others, but they may each represent a time when continents were converging to form a
supercontinent. The times are about 2600, 2100, 1700,
1100, 650, and 250 million years ago. The 400- to 600million-year periodicity in these events has led some to
suggest that there is a plate-tectonic cycle consisting of
the repeated assembly and fragmentation of supercontinents, caused largely by the way in which heat builds up
under large land masses. Regardless of whether this is
the case, it is clear that mountain ranges have been built
during several events—not only those mentioned above
but also numerous more local events at other times—
only to be leveled by the hydrologic system.
Since Pangaea
The division of the geologic account into periods
before and after the break-up of Pangaea is somewhat artificial, and post-Pangaea history was included in the section on general continental evolution in Chapter 32. Here
we mention only a major Cenozoic event that changed the
face of much of the land—the Great Ice Age of the
Pleistocene Epoch. For nearly two-million years, great
sheets of ice up to two kilometers thick episodically
advanced over northern North America and northern
Europe, and then retreated during interglacial periods,
exposing the scraped and scoured topography typical of
continental glaciation. The ice obliterated stream
drainage systems and left deposits of coarse, unsorted sediment. It created myriad lakes and clusters of low, streamlined hills (one of them Hill Cumorah near Palmyra, New
York). It gouged out stream valleys to make the magnificent fjords of Scandinavia and lowered sea level to facilitate the carving of some immense canyons on the nowsubmerged continental shelves. While intense glaciation
is by no means unique in the history of the earth, the
Pleistocene Ice Age was recent enough to have clearly left
its mark on the world we inhabit. In fact, it is quite possible that we are now living during an interglacial period,
merely awaiting the return of the ice.
Toward Pangaea
In Chapter 31 we examined some of the evidence for
the existence of the supercontinent Pangaea. We recognized that Pangaea was not the initial event in the history
of plate motion, but simply a part of the continuum of
plate tectonic history that spans nearly four billion years.
The existence of two or more continental masses makes
it virtually inevitable that there will be eventual continental collisions, and these produce large ranges of fold
mountains. Even when such mountains have been completely eroded, their “roots” are still discernible through
careful geologic mapping, and the large amounts of sediment shed from them show up as thick sequences of sedimentary rock layers that become thinner away from the
sites of the former mountains. By understanding the significance of such geologic features, it is possible to
Summary
The hydrologic system consists of all fluids
that move near the surface of the earth. Its various
agents (running water, groundwater, wind, waves, and
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ice) have eroded and deposited material to shape the
face of the earth for many millions of years, but always
in concert with the tectonic system. As plate motion has
raised mountains, the hydrologic system has worked to
wear them down. The conflict between the two systems, one driven by heat from radioactive decay within
the earth and the other by the heat of solar radiation
from outside the earth, has yielded the variety and beauty of our planet. The application of physical and chemical principles we have learned previously to the preferred model of the origin of the solar system leads us
through a series of predictable stages for a developing
earth, ending with a planet whose face is now familiar
to us but which we realize is still undergoing slow and
inexorable change.
8.
9.
10.
11.
STUDY GUIDE
Chapter 34: The Changing Face of the Earth
12.
A. FUNDAMENTAL PRINCIPLES: No new fundamental principles.
13.
B. MODELS, IDEAS, QUESTIONS, AND APPLICATIONS
1. What is the hydrologic system? What are the main
elements of the system and what influence do they
have on the features of the surface of the earth?
2. What has happened and what is now happening to
determine the main features observed on the surface of the earth?
3. Sketch the geologic and biological history of the
earth with approximate dates as we now understand
it from interpretation of the physical data.
14.
15.
16.
17.
C. GLOSSARY
1. Alluvial Fan: A fan-shaped deposit formed when
a stream that flows intermittently from a canyon
deposits its sediments on the valley floor.
2. Base Level: The maximum possible depth to
which a stream can cut by erosion; the level to
which each stream strives.
3. Continental Glacier: Large dollar shaped glacier
covering an extended land mass (Greenland,
Antarctica). Ice accumulates near the center and
flows to the periphery.
4. Delta: A fan-shaped or fingerlike deposit formed
when a stream or river empties into an ocean or
lake.
5. Evaporation: The changing of water from its liquid state to its gaseous state.
6. Groundwater: Water which exists in the subsurface of the earth’s crust, usually in the pore spaces
of the rock.
7. Headward Erosion: A process by which valleys
are lengthened because erosion is focused at the
head of a valley by convergence of runoff from sev-
eral directions.
Hydrological Cycle: The cyclic movement of the
earth’s water supply as it moves among the oceans,
the atmosphere, and the land.
Hydrologic System: The methods by which the
water moves through the hydrological cycle, i.e.,
running water, glaciers, subsurface water, wind,
evaporation, etc.
Iron Catastrophe: An early event in the earth’s
history (about 4.2 billion years ago) during which
the interior melted rather quickly and extensively
and the denser elements (iron and nickel) moved to
the center, displacing less dense oxides and silicates to the mantle and crust.
Moraine: A deposit of rough, unsorted debris left
by a glacier when it melts.
Planetary Differentiation: The process by which
the materials of a forming planet organize themselves into layers, with the more dense materials on
the inside and the less dense materials on the outside. See Chapter 30.
Precipitation: Water which condenses into liquid
or solid form and falls toward the surface of the
earth.
Sand Dune: A wind-generated deposit of sand,
commonly found in arid areas.
Sinkhole: A hole or depression in the ground
formed by the collapse of the roof above an underground void in limestone.
Water Table: The top surface of the underground
region where all of the pore spaces in the rock are
filled with water.
Valley Glacier: A glacier (mass of moving ice and
snow) that forms in a valley originally carved by a
stream.
D. FOCUS QUESTIONS
1. Describe three main elements of the hydrologic
system. Use examples to explain how individual
elements of the cycle influence the appearance of
the earth.
2. Describe the various stages in the history of the
earth from its formation as a planet to its present
stage according to current geologic thought.
Include at least five important dates in your outline.
E. EXERCISES
34.1. Discuss the general features of the hydrologic cycle.
34.2. The hydrologic system causes the most
extensive changes in the surface of the earth through
(a) running water.
(b) glaciers.
(c) shoreline processes.
(d) wind.
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34.3. Why is the central region of a large continental mass never eroded all the way to sea level?
34.4. Ice, running water, and wind are all able to
transport sediment from a source area to an area of
deposition. List these three in order of their ability to
carry the largest particles.
(a) Wind, ice, running water
(b) Running water, wind, ice
(c) Ice, wind, running water
(d) Ice, running water, wind
34.5. It is believed that volcanic eruptions were the
major source of most of the atmosphere of the earth.
What major component would not have been contributed in this way, and where did it come from?
34.6. How do mountain ranges provide evidence
that much plate tectonic activity occurred prior to the
assembly of Pangaea?
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