Introduction to Geography

Introduction to Geography
Arthur Getis, Judith Getis, & Jerome D. Fellmann

Physical Geography: Landforms
Chapter 3

Overview Introduction Earth Materials Geologic Time
Movements of the Continents Tectonic Forces Gradational Processes Landform Regions

Introduction Geomorphology
Study of the origin, characteristics and development of landforms Two types of forces interact to produce landforms: Forces that push, move and raise the earth’s surface Forces that scour, wash and wear down the surface

Earth Materials 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: Igneous Rocks Sedimentary Rocks Metamorphic Rocks

Earth Materials Igneous Rocks
Formed by the cooling and solidification of molten rock Magma: molten rock below ground Cooling forms intrusive igneous rocks Granite Lava: molten rock above ground Cooling forms extrusive igneous rocks Basalt, pumice, obsidian Composition of magma and lava plus cooling rate determines the minerals that form

Earth Materials Sedimentary Rocks
Composed of eroded particles of gravel, sand, silt, and clay Rocks evolve in horizontal strata at bottom of bodies of water Sediment is compressed by weight of additional deposits and cemented by water and certain minerals Type of sediment determines rock type 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
Formed from igneous and sedimentary rocks by earth forces that produce heat, pressure, or chemical reactions Mineral structure is changed Shale (under great pressure) becomes slate Limestone may become marble Granite may become gneiss

Geologic Time Earth formed about 5 billion years ago
Theory of continental drift 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
Plate tectonics theory Asthenosphere Partially molten layer above the earth’s interior Lithosphere 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 A single plate may contain oceanic and continental crust Movement may be due to convection of molten material in asthenosphere

Plate tectonics theory 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

Plate tectonics theory 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

Earthquakes and volcanic activity may occur in the vicinity of plate boundaries 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 Diastrophism Volcanism
Earth force that folds, faults, twists, compresses rock Volcanism Earth force that transports subsurface materials to or toward the surface of the earth

Tectonic Forces Diastrophism
Broad warping Bowing of a large region of the earth’s surface, e.g., down-warping of eastern U.S. Folding Layers of rock are forced to buckle from compression caused by plate movements, e.g. Ridge and Valley Region of the eastern U.S.

Faulting Rock is broken or fractured 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

Faulting Earthquakes 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

Faulting Tsunami 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
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 Explosive Steep sides Alternate layers of solidified lava and ash and cinders Shield volcano Non-explosive Gently sloping sides

Tectonic Forces Volcanism
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 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
Reduction of the land’s surface through: Weathering Mass movement Erosion

Gradational Processes Weathering
The breakdown and decomposition of rocks and minerals at or near the earth’s surface in response to atmospheric factors (water, air and temperature) Mechanical weathering Physical disintegration of earth materials Frost action Salt crystals Root action

Chemical weathering Decomposition of rock as a result of chemical reactions 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)

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
Also known as mass wasting Downslope movement of material due to gravity Avalanches Landslides Soil creep Mudflows Talus Conelike landform created by the accumulation of rock particles at the base of hills and mountains

Gradational Processes Erosional Agents and Deposition
Wind, water, and glaciers 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

Running Water 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

Running Water 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

Running Water Flooding may cause human and financial loss, e.g., Yellow River of China in 1887 – 900,000 lives lost

Arid areas Lack of vegetation increases erosional forces of running water Playas Alluvium Alluvial fans Arroyos Washes Buttes and mesas Stream Landscapes Humid areas Waterfalls V-shaped channels Rapids Floodplains Meandering streams Oxbow lakes Natural levees Effect of stream erosion is to round landforms

Groundwater 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

Groundwater 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

Groundwater Karst topography Limestone region marked by sinkholes, caverns, and underground streams E.g., East Central Florida, Mammoth Cave in Kentucky

Glaciers 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

Glaciers 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

Glaciers 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

Glacial Landforms Erosional Glacial troughs (U-shaped valley) Fiords Horns Cirques Arêtes

Glacial Landforms Depositional Moraines Eskers Drumlins Outwash plains

Waves, Currents, and Coastal Landforms 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

Waves, Currents, and Coastal Landforms 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

Waves, Currents, and Coastal Landforms 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

Waves, Currents, and Coastal Landforms Atolls Reefs formed in shallow water around a volcano that has since been covered or nearly covered by water Found in the South Pacific

Wind 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

Wind Dunes Produced by wind-driven sand Dunes move across desert Crescent-shaped barchan dune

Wind 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 Large section of the earth’s surface where a great deal of homogeneity occurs among the types of landforms that characterize it Mountains Plains Plateaus

Introduction to Geography

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