Transcript chapt03_lecture Getis 13e

Introduction to
Geography
Arthur Getis, Judith Getis, &
Jerome D. Fellmann
Physical Geography:
Landforms
Chapter 3
Overview
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Introduction
Earth Materials
Geologic Time
Movements of the Continents
Tectonic Forces
Gradational Processes
Landform Regions
Introduction
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Geomorphology
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Study of the origin, characteristics and development of
landforms
Two types of forces interact to produce
landforms:
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Forces that push, move and raise the earth’s surface
Forces that scour, wash and wear down the surface
Earth Materials
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Rocks of Earth’s crust vary according to mineral
composition.
Although one can classify rocks according to
physical properties, the more common approach
is to classify them by the way they were formed.
Three main groups of rocks:
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Igneous Rocks
Sedimentary Rocks
Metamorphic Rocks
Earth Materials
Igneous Rocks
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Formed by the cooling and solidification of
molten rock
Magma: molten rock below ground
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Cooling forms intrusive igneous rocks
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Lava: molten rock above ground
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Cooling forms extrusive igneous rocks
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Granite
Basalt, pumice, obsidian
Composition of magma and lava plus cooling
rate determines the minerals that form
Earth Materials
Sedimentary Rocks
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Composed of eroded particles of gravel, sand,
silt, and clay
Rocks evolve in horizontal strata at bottom of
bodies of water
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Sediment is compressed by weight of additional
deposits and cemented by water and certain minerals
Type of sediment determines rock type
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Large, rounded particles form conglomerates
Sand forms sandstone
Silt and clay form shale or siltstone
Organic materials form limestone or coal
Earth Materials
Metamorphic Rocks
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Formed from igneous and sedimentary rocks by
earth forces that produce heat, pressure, or
chemical reactions
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Mineral structure is changed
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Shale (under great pressure) becomes slate
Limestone may become marble
Granite may become gneiss
Geologic Time
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Earth formed about 5 billion years ago
Theory of continental drift
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Based on early 20th-century work of Alfred Wegener
All land masses were once united in a supercontinent
called Pangaea; continents drifted apart over many
millions of years
Forerunner of today’s plate tectonics theory
Movements of the Continents
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Plate tectonics theory
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Asthenosphere
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Partially molten layer above the earth’s interior
Lithosphere
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Outermost layer of the earth (the crust and upper mantle)
Crust consists of one set of rocks below the oceans and
another set of rocks that makes up the continents
Consists of 12 large, plus numerous small, plates that slide
or drift slowly over the asthenosphere
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A single plate may contain oceanic and continental crust
Movement may be due to convection of molten material in
asthenosphere
Movements of the Continents
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Plate tectonics theory
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Divergent plate boundaries
 Plates move away from each other
 E.g., Mid-Atlantic Ridge
Transform boundaries
 One plate slides horizontally past another
 E.g. San Andreas Fault
Movements of the Continents
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Plate tectonics theory
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Convergent boundaries
 Plates move toward each other
 Results in formation of deep-sea trenches and
continental-scale mountain ranges
 Subduction may occur
 One plate is forced beneath another
Denser, thinner oceanic crust is forced below
lighter continental crust
Movements of the Continents
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Earthquakes and volcanic activity may occur in
the vicinity of plate boundaries
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Ring of Fire, parts of which are densely populated
Despite scientific knowledge about earthquake
zones, the general disregard for this danger is a
difficult cultural phenomenon with which to deal.
Tectonic Forces
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Diastrophism
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Earth force that folds, faults, twists, compresses rock
Volcanism
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Earth force that transports subsurface materials to or
toward the surface of the earth
Tectonic Forces
Diastrophism
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Broad warping
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Bowing of a large region of the earth’s surface, e.g.,
down-warping of eastern U.S.
Folding
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Layers of rock are forced to buckle from compression
caused by plate movements, e.g. Ridge and Valley
Region of the eastern U.S.
Tectonic Forces
Diastrophism
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Faulting
Rock is broken or fractured
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Escarpments may form where one side of fault is
uplifted or downthrust
Rift valleys form where separation away from the fault
causes sinking of land
Many fractures are merely cracks called joints
Tectonic Forces
Diastrophism
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Faulting
Earthquakes
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Movement along a fault or point of weakness
The greater the movement, the greater the magnitude
of the earthquake
Occur daily in hundreds of places throughout the
world
Most are slight and only noticeable on seismographs
May be catastrophic, e.g., China, 242,000 deaths in
1976
Tectonic Forces
Diastrophism
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Faulting
Tsunami
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Sea waves generated by an earthquake, volcanic
eruption, or underwater landslide
As waves enter shallower water, friction with the
ocean floor causes the waves to slow down producing
a buildup of water that can reach 15 m (50 ft)
Tectonic Forces
Volcanism
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Usually at or near plate intersections
Also occur at hot spots (breaks in earth’s crust
where a rising plume of molten material reaches
the surface ), e.g., Hawaiian islands
Strato or composite volcano
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Explosive
Steep sides
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Alternate layers of solidified lava and ash and cinders
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Shield volcano
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Non-explosive
Gently sloping sides
Tectonic Forces
Volcanism
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Magma may not reach the surface, may solidify
underground into a variety of underground
formations, subsequent erosion may reveal the
formations, e.g., The Palisades facing New York
City and Stone Mountain near Atlanta, GA
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Lava may flow through fissures or fractures
without forming a volcano, e.g., Deccan Plateau
of India and the Columbia Plateau of the Pacific
Northwest of the U.S.
Gradational Processes
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Reduction of the land’s surface through:
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Weathering
Mass movement
Erosion
Gradational Processes
Weathering
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The breakdown and decomposition of rocks and
minerals at or near the earth’s surface in
response to atmospheric factors (water, air and
temperature)
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Mechanical weathering
Physical disintegration of earth materials
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Frost action
Salt crystals
Root action
Gradational Processes
Weathering
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Chemical weathering
Decomposition of rock as a result of chemical
reactions
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Oxidation (oxygen combines with mineral
components, such as iron, to form oxides)
Hydrolysis (water reacts with rock minerals)
Carbonation (carbon dioxide gas from atmosphere
dissolves in water forming a weak carbonic acid which
decomposes rock)
Gradational Processes
Weathering
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Weathering processes create soil.
After weathering processes decompose rock,
the force of gravity and the erosional agents of
running water, wind and moving ice carry the
weathered material to new locations.
Gradational Processes
Mass Movement
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Also known as mass wasting
Downslope movement of material due to gravity
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Avalanches
Landslides
Soil creep
Mudflows
Talus
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Conelike landform created by the accumulation of
rock particles at the base of hills and mountains
Gradational Processes
Erosional Agents and Deposition
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Wind, water, and glaciers
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Carve, wear away, and remove rock and soil particles
Material is deposited in new places
 New landforms are created
Each erosional agent is associated with a distinctive
set of landforms
Gradational Processes
Erosional Agents and Deposition
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Running Water
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Powerful erosional agent
Ability to erode depends upon:
 Amount of precipitation
 Length and steepness of the slope
 Kind of rock and vegetation cover
Force of water and the particles in the stream are
agents of erosion
 Abrasion
Gradational Processes
Erosional Agents and Deposition
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Running Water
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Load of a stream
 Materials, suspended and dissolved, transported by
a stream
 Decline in velocity results in deposition
 Deltas: where streams meet bays, oceans, and
lakes
 May be deposited in adjacent plains (floodplain)
 May be beneficial to farmers, e.g., Nile River
historically
 May have negative effect if deposition is
composed of sterile sands and boulders
Gradational Processes
Erosional Agents and Deposition
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Running Water
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Flooding may cause human and financial loss, e.g.,
Yellow River of China in 1887 – 900,000 lives lost
Gradational Processes
Erosional Agents and Deposition
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Stream Landscapes
 Humid areas
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Waterfalls
V-shaped channels
Rapids
Floodplains
Meandering streams
Oxbow lakes
Natural levees
Effect of stream
erosion is to round
landforms
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Arid areas
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Lack of vegetation
increases erosional
forces of running water
Playas
Alluvium
Alluvial fans
Arroyos
Washes
Buttes and mesas
Gradational Processes
Erosional Agents and Deposition
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Groundwater
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Precipitation sinks underground into cracks and pores
in rocks and soils
Aquifer
 Porous underground structure bearing water
 Zone of saturation
Water table
 Upper level of the water within an aquifer
 Below water table, soils and rocks are saturated
with water
 Ponds, lakes, marshes, and streams form when
land surface dips below the water table
Gradational Processes
Erosional Agents and Deposition
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Groundwater
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Groundwater moves slowly seeking lowest level
Water may find its way to the surface by capillary
action in the ground or in vegetation
Solution
 Chemical process by which groundwater
(particularly when combined with CO2) dissolves
soluble materials
 Groundwater decomposes many types of rocks
 Significant effect on limestone
 Underground caverns, stalactites, stalagmites,
sinkholes
Gradational Processes
Erosional Agents and Deposition
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Groundwater
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Karst topography
 Limestone region marked by sinkholes, caverns,
and underground streams
 E.g., East Central Florida, Mammoth Cave in
Kentucky
Gradational Processes
Erosional Agents and Deposition
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Glaciers
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Agent of erosion and deposition
Huge mass of slowly moving land ice
Covered a large part of the earth as recently as
10,000-15,000 years ago
Form only where annual snowfall exceeds annual
snowmelt and evaporation
 The weight of the snow causes it to compact at the
base and form ice
 Ice at the bottom becomes like toothpaste and
moves slowly
Gradational Processes
Erosional Agents and Deposition
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Glaciers
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Continental glaciers
 E.g., Antarctica, Greenland and Baffin Island in
Canada
Mountain glaciers
 Found in many parts of the world
About 10% of the earth’s land is under ice
Permafrost
Weight of glaciers breaks up underlying rock preparing
it for transportation by moving ice
Gradational Processes
Erosional Agents and Deposition
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Glaciers
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Glaciers change landforms by erosion
 Scour the land as they move (striations)
Glaciers create landforms when they deposit debris
they have transported
 Till consists of rocks, pebbles, silt
Gradational Processes
Erosional Agents and Deposition
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Glacial Landforms
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Erosional
 Glacial troughs (U-shaped valley)
 Fiords
 Horns
 Cirques
 Arêtes
Gradational Processes
Erosional Agents and Deposition
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Glacial Landforms
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Depositional
 Moraines
 Eskers
 Drumlins
 Outwash plains
Gradational Processes
Erosional Agents and Deposition
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Waves, Currents, and Coastal Landforms
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Waves
 Carry sand for deposition
 Erode landforms at coast
 Backwash carries eroded material away, results in
different kinds of landforms, depending on
conditions
Cliffs
 Formed by wave action when land at the coast is
well above sea level
Gradational Processes
Erosional Agents and Deposition
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Waves, Currents, and Coastal Landforms
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Beaches and spits
 Formed by the deposition of sand grains
 Longshore currents transport sand
Sandbars
 Formed by sand deposited by the backwash of
waves
 May expand to enclose lagoons or inlets
 Salt marshes may develop, e.g. Outer Banks of
North Carolina
Gradational Processes
Erosional Agents and Deposition
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Waves, Currents, and Coastal Landforms
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Coral reefs
 Composed of coral organisms growing in shallow
tropical water
 Formed by secretion of calcium carbonate in the
presence of warm water and sunlight
 Develop short distances offshore
 E.g. Great Barrier Reef, Australia
Gradational Processes
Erosional Agents and Deposition
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Waves, Currents, and Coastal Landforms
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Atolls
 Reefs formed in shallow water around a volcano
that has since been covered or nearly covered by
water
 Found in the South Pacific
Gradational Processes
Erosional Agents and Deposition
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Wind
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Powerful agent of erosion and deposition in dry
climates
 Limited vegetation leaves exposed particles subject
to movement by wind
 Creates various kinds of landforms
Sculptured features from abrasive action of sand and
dust particles
Desert pavement
 Found in Sahara, Gobi and western U.S. deserts
Gradational Processes
Erosional Agents and Deposition
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Wind
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Dunes
 Produced by wind-driven sand
 Dunes move across desert
 Crescent-shaped barchan dune
Gradational Processes
Erosional Agents and Deposition
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Wind
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Loess
 Deposit of windblown silt
 Found in midlatitude westerly wind belts of the U.S.,
central Europe, central Asia and Argentina
 Greatest development in northern China
 Covers hundreds of thousands of square miles
 Up to depths of 100 feet
 Rich soils usually form from loess deposits
 These areas are among most productive agricultural
lands in the world
Landform Regions
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Large section of the earth’s surface where a
great deal of homogeneity occurs among the
types of landforms that characterize it
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Mountains
Plains
Plateaus