GEO 200: Physical Geography
Introduction to Landform Study
Big topic
• The Earth’s size is difficult for humans to
comprehend.
– Diameter: 12,800 km
– Circumference: 40,000 km
• Geographers can, however, largely ignore the
interior and focus on the surface.
The unknown interior, part 1
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Humans have not penetrated more than onethousandth of the Earth’s radius.
We have inferential knowledge of the interior,
through monitoring shock waves transmitted
through Earth from earthquakes or from humanmade explosions.
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Our knowledge is incomplete.
Scientist have deduced that the Earth has a heavy
inner core surrounded by three concentric layers of
various composition and density.
The unknown interior, part 2
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Inferential knowledge (continued)
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The four regions are the crust, mantle, outer core,
and inner core.
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The crust is the outermost solid layer of Earth, consisting of
a broad mixture of rock types.
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On average, the crust is three times as thick under continents
as under the ocean.
It makes up less than 1 percent of Earth’s volume.
The Mohorovičić discontinuity (Moho) is the boundary
between the crust and mantle.
It is thought to be a narrow zone with a significant change in
mineral composition.
The unknown interior, part 3
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Inferential knowledge (continued)
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Four regions (continued)
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The mantle is that portion of the Earth beneath the crust and
surrounding the outer core, to a depth of about 2,900 km.
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It has the largest volume of all four shells.
Scientists believe there are three zones within the mantle:
lithosphere, asthenosphere, mesosphere.
The lithosphere is the uppermost zone of mantle and the crust
together. (The term also refers to tectonic plates consisting of
the crust and upper rigid mantle. It is also sometimes used as a
general term for the entire solid Earth.)
The unknown interior, part 4
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Inferential knowledge (continued)
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Four regions (continued)
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The mantle (continued)
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The asthenosphere is a plastic layer of the upper mantle that
underlies the lithosphere. Its rock is very hot and therefore
weak and easily deformed.
The mesosphere is the rigid part of the deep mantle, below the
asthenosphere. (The term also refers to the atmospheric layer
above the stratopause).
The outer core is the (molten) liquid shell beneath the
mantle that encloses Earth’s inner core.
The inner core is the supposedly solid, dense, innermost
portion of Earth, believed to consist largely of iron/nickel or
iron/silicate.
The unknown interior, part 5
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Inferential knowledge (continued)
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Understanding of crust and upper mantle has
fundamentally changed in last three decades.
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Continental drift is the theory that proposes that the present
continents were originally connected as one or two large
landmasses that have broken up and literally drifted apart
over the last several million years.
Plate tectonics, a descendant of continental drift, is a
coherent theory of massive crustal rearrangement based on
the movement of continent-sized lithospheric plates.
Focus on the surface
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Geographers are interested in the Earth’s interior
only as it influences topography and other
surface features, how it affects the nature and
characteristics of surface.
Composition of the crust, part 1
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A mineral is a naturally formed inorganic solid
substance that has an unvarying chemical
composition and characteristic crystal structure.
– About 3,500 minerals are known, with about 50 new
types discovered each year.
– Fewer than 20 minerals account for more than 95
percent of composition of all continental and oceanic
crust.
Common minerals, part 1
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There are seven principal categories of rockforming minerals (on basis of chemical
properties and internal structure).
– Silicates are a category of minerals composed of
silicon and oxygen combined with another element or
elements.
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They are the largest and most important group; most are hard
and durable.
– Oxides are a category of minerals composed of
oxygen combined with another element.
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Quartz has chemical composition of oxide, but classified as
silicate because of its internal structure.
Common minerals, part 2
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Seven principal categories (continued)
– Sulfides are a category of minerals composed of
sulfur, combined with another element or elements.
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They include many of the most important ore minerals.
– Sulfates are a category of minerals composed of sulfur
and oxygen, combined with another element or
elements.
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Calcium is the principal combining element.
– Carbonates are minerals that are a carbonate
compound of calcium or magnesium.
Common minerals, part 3
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Seven principal categories (continued)
– Halides are a category of minerals that are notably
salty.
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They are the least widespread.
– Native elements are those minerals that aren’t
combined chemically with others, but appear as
discrete elements (e.g., gold and silver).
Composition of the crust, part 2
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Magma is a molten material in Earth’s interior.
– The quantity is unknown.
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Rock is a solid material composed of aggregated
mineral particles (in lithosphere).
An outcrop is the surface exposure of bedrock.
Bedrock is a buried layer of residual rock that
has not experienced erosion.
Composition of the crust, part 3
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Igneous rocks
– Igneous rock is rock formed by solidification of
molten magma.
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There are many kinds, but the principal shared trait is
crystalline structure.
– Extrusive rock is molten rock ejected onto Earth’s
surface, solidifying quickly in the open air.
– Intrusive rock is rock that cools and solidifies beneath
Earth’s surface (it may be pushed up to surface or
exposed through erosion).
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Granite is the most common and well known.
Composition of the crust, part 4
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Sedimentary rocks
– Sediment is small particles of rock debris or organic
material deposited by water, wind, or ice.
– Sedimentary rock is rock formed of sediment that is
consolidated by the combination of pressure and
cementation.
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During sedimentation, materials are sorted roughly by size
(the finer particles are carried farther than heavier particles).
Strata (plural; stratum, singular) are distinct layers of
sediment which result in parallel structure (stratification),
with layers varying in thickness and composition.
Composition of the crust, part 5
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Sedimentary rocks (continued)
– Sedimentary rock formation and consolidation
(continued)
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Sedimentary rocks are categorized by how they are formed:
mechanically, chemically, or organically.
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Mechanically accumulated: fragments of preexisting rocks (for
example, shale and sandstone).
Chemically accumulated: precipitation of soluble materials or
chemical reactions (for example, calcium carbonate and
limestone).
Organically accumulated: remains of dead plants or animals
(for example, coal and limestone).
Composition of the crust, part 6
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Metamorphic rocks
– Metamorphic rock is rock that was originally
something else (igneous or sedimentary) but has been
drastically changed by massive forces of heat and/or
pressure working on it from within Earth.
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Process recrystallizes and rearranges mineral components.
The processes are somewhat predictable, such as the fact that
limestone tends to metamorphose into marble.
Sometimes the metamorphosis is so great, the nature of the
original rock cannot be determined.
The most common metamorphic rocks are schist and gneiss.
Metamorphic Rock Foliation
Composition of the crust, part 7
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Relative frequency of rock classes
– The lithosphere has a very uneven distribution of
sedimentary, igneous, and metamorphic rocks.
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Sedimentary rocks are dominant on the surface of the Earth.
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This dominance, however, is only on the surface, as
sedimentary cover is not thick.
The average thickness of sedimentary rock is less than 2.4 km.
Assume that igneous rocks make up the bulk of the crust, but
metamorphic rocks might because of the enormous pressures
at play beneath the crustal surface.
Critical concepts, part 1
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Basic terms
– Topography is the surface configuration of the Earth.
– A landform is an individual topographic feature, of
whatever size.
– Geomorphology is the study of the characteristics,
origin, and development of landforms.
– Relief is the difference in elevation between the
highest and lowest points in an area (e.g., the vertical
variation from mountaintop to valley bottom).
Critical concepts, part 2
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Uniformitarianism
– Uniformitarianism is the concept that “the present is
the key to the past” in geomorphic processes. The
processes now in operation have also operated in the
same way in the past and should also operate in the
future.
Critical concepts, part 3
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Geologic time
– Geologic time enumerates temporal expanses of
almost unfathomable scope.
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Geologic time encompasses millions and hundreds of
millions of years.
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Four eras, three most recent being subdivided into 12 periods.
Two most recent periods divided into seven epochs.
Critical concepts, part 4
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Geologic time (continued)
– Unfathomable scope (continued)
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The following table offers a chart of geologic time expressed
in an equivalent one-year scale.
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For the first four months, the planet was lifeless.
Single-celled life appeared in early May.
Multicellular organisms began evolving in early November.
Antediluvian fishes, the first vertebrates, appeared about
November 21.
Amphibians appeared late November.
Vascular plants appeared about November 27.
Reptiles began era of dominance about December 7.
Critical concepts, part 5
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Geologic time (continued)
– Unfathomable scope (continued)
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One-year scale (continued)
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Mammals arrived about December 14; birds arrived December
15.
Flowering plants arrived December 21.
On December 24, the first grasses and first primates appeared.
The first hominids came New Year’s Eve.
Homo Sapiens arrived one hour before midnight.
The age of written history equals the last minute of the year.
If geologic time was examined in the context of cliff one km
high, an individual’s existence would equal less than the
thickness of the finest hair.
Study of landforms, part 1
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Land surfaces alone encompass 150 million
square km scattered over seven continents and
innumerable islands.
– Such study is a complex endeavor that requires an
organized approach, including examination of the
following four elements.
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Structure includes the nature, arrangement, and orientation of
the materials in feature being studied.
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It is the geological underpinning of landforms.
Study of landforms, part 2
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150 million square km (continued)
– Four elements (continued)
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Process includes the actions that have combined to produce
the landform.
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Process encompasses the interaction of forces such as geologic,
hydrologic, atmospheric and biotic.
Slope is a fundamental aspect of shape for any landform.
Drainage refers to movement of water (from rainfall and
snowmelt).
After identifying the above four basic elements,
geographer can analyze topography.
Study of landforms, part 3
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Fundamental questions of geographic inquiry:
– What? (the form of features)
– Where? (the distribution and pattern of landform
assemblage)
– Why? (an explanation of origin and development)
– So what? (the significance of the topography in
relationship to other elements of environment and to
human life and activities)
Geomorphic processes, part 1
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A variety of topography reflects the complexity
of interactions between process and structure.
Internal and external processes
– Internal processes operate within the Earth, drawing
energy from heat.
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In general, they are building forces, increasing relief of land
surface.
Geomorphic processes, part 2
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Internal and external processes (continued)
– External processes operate at the base of atmosphere
(subaerial), drawing energy from sources above t/he
lithosphere (atmosphere or oceans).
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External processes are better understood than internal
processes; their behavior is predictable.
In general, they are wearing-down or destructive forces,
diminishing relief of land surface.
Question of scale
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Scale of observation affects how one perceives
and understands a landscape.
The following images offer an example of
different perspectives of scale, from the largest
scale of ordinary human experience, walking
though a landscape; to driving thorough it; to
flying over it; to a satellite’s viewpoint; then to
the smallest scale, from perspective of a
spacecraft.
Pursuit of pattern
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Orderly patterns of distribution are much more
difficult to discern in geomorphology than in
other geographic elements, such as climate.
Overall, global distribution of topography is very
disordered and irregular.
Comprehending the process (dynamics of
topographic development) is more important than
the study of landform distribution.