Landforms


Outline

1.      Age of Earth

o    Big Bang Theory

o    First Life on Earth

o    Relative to Humans

2.      Plate Tectonics

o    Theory

o    Components- Crust, Mantle, Core

o    Movement

o    Divergent Plate Boundaries

o    Convergent Plate Boundaries

o    Transform Plate Boundaries

o    Hot Spots

o    Related Phenomenon

3.      Gradation

o    Mechanical Weathering

o    Physical Weathering

o    Chemical Weathering

o    Rivers

o    Waves

o    Glaciation

o    Mass Wasting

4.      Deposition

o    Floodplain

o    Delta

o    Moraine

5.      References

6.      Review Questions      


Age of Earth

The earth was formed approximately 4.6 billion years ago.  According to the Big Bang Theory, there was a massive explosion estimated some 10 to 20 billion years ago, that sent forth, at great speed, a huge amount of heat and matter that consisted almost exclusively of hydrogen and helium. As this matter cooled, the stars and planets were formed. When the planet first formed, it was an extremely hot, molten ball of magma.  It wasn’t until 4.1 million years ago that earth cooled enough for a crust to solidify around the cooling magma. As the earth cooled, it separated into three layers. The crust is the outer layer on which we live. The next layer is the mantle which is made up of molten magma. The third layer is the core and it too is made up of liquid magma, but is much hotter than the mantle. During this period, around 3.8 billion years ago, the moon was formed from debris broken off from the earth by an enormous impact with another planet.

The earth had its first life forms around 3.5 billion years ago. The first life forms were simple, unicellular organisms in the oceans that captured sunlight for energy. As a by product of this photosynthesis, oxygen was released into the atmosphere, paving the way for today’s life. It took almost another 3 billion years after this for the first multi-cellular organisms to appear.

Humans evolved about 50,000-100,000 years ago (The exact date is still in dispute and may be older).  Compared to the age of the earth, human existence is but a fraction of a second on a 24-hour clock. While humans have not been around for long, they have had a greater impact on the earth than any other species. The earth has progressed through many stages in the development of the physical environment and humans are only a tiny part of its history.

Plate Tectonics

Text Box:  Plate tectonics (from the Greek word "tektonikos" meaning "builder") is the theory that the surface of the earth rests on plates that float on the magma core of the planet. Each continent is on a separate plate that is constantly in motion, although very slowly, which has caused the continents to drift across great distances.  There are around 20 rigid plates that are in slow, continuous motion.  Some continents move at a rate of 1/2 to 4 inches per year. This movement is caused by heat driven convection cells in the molten rock deep below the crust.  As they move, they carry the continents and ocean floor.  In the late 1800's, Alfred Wegener, a German physical geographer, used spatial analysis to propose the continental drift hypothesis.  Wegener studied the outlines of the continents, particularly South America and Africa, and suggested that the landmasses had been united at one point.  He called his theory "die Verschiebung der Kontinente" meaning "continental displacement.”  His idea stated that these seemingly stable and fixed continents were actually mobile.  In addition, with the large amounts of amphibian and reptile fossils that are spread over a wide area of separated continents, this only adds strength to his theory.  With further research, and after considerable controversy, his theory was accepted some 60 to 70 years after it was first proposed. 

The earth is made up of three layers: the crust, the mantle and the core.  The crust is a thin layer (15 miles on average) covering the outside part of the earth. The continental crust is less dense than the oceanic crust and "floats" above it.   The second layer is the mantle, which is 1,800 miles thick.  The crust and the upper mantle make up the lithosphere.  This lithosphere descends to 60-90 miles below the continents and 40-50 miles below the oceans.  The base of the lithosphere is called the asthenosphere.  Here is where the lithosphere is unattached from the mantle and moves around, mostly by gravity and thermal differences in the mantle. The core, the third layer, is 1,000 miles thick.  The core and the mantle are both composed of hot, molten rocks but the core is much hotter than the mantle.  Below the 15 mile crust there is an increased amount of heat.  This heat is 'left over' from the formation of the earth and new heat is added from decaying radioactive material in the core. 

How does this heat cause the plates to move?  The earth's crust is cold, the mantle is hot, and the core is even hotter.  In order to equalize these temperatures, the molten rock from the core moves outward, cools, and sinks back down. Convection cells form as a result of this circular movement. Propelled by convection cells, tectonic plates move very slowly, at a rate of one to four inches per year.  While a few inches per year does not sound like much, the earth's history is measured in millions of years.  225 million years ago, it is postulated that a giant continent called Pangaea ('all-earth') existed.  Pangaea was a giant land mass made up of all the separate continents that are around today. This giant continent remained until about 135 million years ago, when it began to break up during a geologic period called the Mesozoic.  The break up began with India detaching itself from Africa and drifting across what is now the Indian Ocean.  The Himalaya Mountains were formed as a result of the Indian Plate pushing into the heart of Asia.

Tectonic plates interact in one of three ways; divergent, convergent, or transform boundaries.  Divergent boundaries occur where the plates are moving away from each other.  In the ocean, divergent boundaries cause mid oceanic ridges to develop. These ridges are higher than the surrounding ocean floor because molten magma flows outward to fill in the gap between the plates and the pressure from the magma below pushes it outward. This magma then cools, forming a new edge of the plate. Some common characteristics include high heat flow, mild volcanic activity, and shallow earthquakes.

The second boundary is called a convergent boundary because two plates are moving towards each other. This causes the plates to compress or to be pushed upward.  On continents, convergent boundaries cause mountain ranges to develop.  The Himalayas were formed when the plate carrying India collided with the plate carrying Eurasia.  In the oceans, convergent boundaries cause deep trenches to form as one plate is forced underneath the other into the mantle.  The deepest ocean trench, deeper than the highest mountain, is the Mariana Trench in the Pacific Ocean southwest of Guam.  Strong, deep earthquakes and volcanic activity are found along convergent plate boundaries.

The third type of plate boundary is a transform boundary and occurs when two plates slide past each other along a fault line. Earthquakes are common along the fault lines as the plates lock up from friction until enough tension builds up to suddenly break that tension. The San Andreas Fault is an example of a transform plate boundary. The San Andreas Fault lies between the North American plate and the Pacific plate on the western coast of the U.S.  This type of boundary has little volcanic activity, but earthquakes are common here and the earthquakes tend to outline the major plates.

A 'Hot spot' is an isolated area of volcanic activity generally found in the middle of large tectonic plates. A 'hot spot' is caused by the magma that rises or plumes from the mantle to the surface causing volcanoes by penetrating the crust.  As the plate moves, the hot spot stays in the same place.  Islands form in a "chain" as a result of plate movement. Hot spots account for the formation of islands in the middle of the Pacific such as the Hawaiian Islands, the Line Islands, and the Tuamotus.   The Hawaiian Islands are younger from northwest to southeast in the chain because the plate has moved to the northwest.  On the island of Hawaii, which is still over the hot spot, volcanoes remain very active. The Galapagos Islands, off the coast of Ecuador are similar in fashion, though not as aligned.

Other events related to plate movements include earthquakes, volcanoes, and geothermal activity.  Earthquakes are caused by abrupt easing of strains that have been built up along geologic faults and also by volcanic activity such as eruptions.  Most of this activity exists along the fault lines between the plates. For example, the Pacific Ocean is surrounded by a nearly continuous plate-collision zone called the 'Ring of Fire'.  Both earthquakes and volcanoes are the results of instability in this zone.  Japan lies near the colliding edges of three plates; hence, earthquakes and volcanoes are a particular threat there.

Gradation

Weathering is the physical disintegration and chemical decomposition of earth materials at or near the earth's surface. There are different types of weathering; mechanical, physical, and chemical. Water is the driving force behind mechanical weathering.  Water sculpts the earth's surface through the eroding power of rivers and waves.  One of the most striking examples of water as a weathering force is the Grand Canyon in the western U.S.  Over the last 5 million years, the Colorado River gouged a path through rock, picking up sediment and moving it downstream. Gradually, enough sediment was excavated to create the spectacular landscape it is today.

Physical weathering breaks rocks into smaller pieces.  Animals, insects, worms, and burrowing mammals all work to loosen the soil by aeration and by mixing loose materials.  Another source of physical weathering is the freeze/ thaw process.  In warm weather, water soaks into cracks of rocks.  Then the water freezes when the temperature cools enough.  The expansion of frozen water in the cracks pushes hard enough to split the rock. This is why concrete roads are covered in potholes every spring.

Chemical weathering is very complex, but basically, it is the reaction of earth materials with atmospheric components such as water, oxygen, and carbon dioxide.  The end products are new minerals and/or dissolved minerals.  Rain, streams, and seawater dissolve some substances from rock and may cause the remaining substances from the rock to crumble.  The main cause of chemical weathering is the dissolving action of water.  There are other factors that may cause weathering such as humidity, pollution, acid rain, and wind.  There are monuments in Luxor, Egypt with writing that has remained visible for over 3500 years, but when the same ruins are sent to New York as a gift from the government of Egypt, the writing is quickly is worn away and is no longer legible.  All in all, the rate of chemical weathering is controlled by the surface environment, grain size, and the climate.

Rivers, especially fast-moving rivers, erode the land by carrying sediment away from one location and depositing it in another.  The Missouri River Valley has been formed by this slow-moving erosional force.  Tributaries are bodies of water which flow into a larger river.  These form valleys within a larger valley, leaving hills on each side.  'Fluvial landscape' is the landscape formed by rivers.  Rivers, at a certain stage of development, move back and forth in a snake-like pattern causing erosion and deposition.  This may cause the river to become so tightly curved that the river takes a short-cut and create a loop or an oxbow lake.  Lake Manawa and Carter Lake are two examples of oxbow lakes in the Midwest.

Waves are formed when wind blows over calm waters and ripples are created.  These ripples enlarge with time and form larger waves.  These waves then crash into continents and are a very powerful gradational force.  They wear away the sides of continents.  There have been islands that were formed by volcanic action and were worn away by waves in a few short years.  Beaches are continually being changed by the waves and can be created and destroyed very quickly.

The Earth periodically undergoes periods of glaciation.  This is a time during in which global temperatures drop causing significant environmental changes.  Glaciers are large masses of ice that act as a significant erosional force. During the Ice Age, ten thousand years ago, glaciers covered large areas of land, wore away the surface, and disrupted the drainage system.  Alpine glaciers create large U-shaped glacial valleys and cut through large mountains.  Large boulders have been moved and deposited hundreds of miles from where they were absorbed into the glacier.  A glacial landscape, as seen in northern Wisconsin, Minnesota, and large parts of Canada, is one in which the natural drainage system has been disrupted. This causes many lakes to form where glaciers once existed. Currently, we are in an Interglacial Period with glaciers covering all of Antarctica and most of Greenland. Greatly receded as they are, glaciers still act as a huge reservoir for the earth’s water. The volume of the Antarctic ice contains 2% of the earth's water and about 90% of the world's fresh water.  If global warming continues, the glaciers will begin to melt and causing a rise in the levels of the oceans. If the global temperature increases just three degrees by the year 2050, the sea levels will rise 8 inches from the alpine glaciers alone.

Another gradational force is known as Mass Wasting.  This is the movement of loosened rock or soil down a slope due to gravity. Mass wasting can occur as quickly as a few seconds or can as a process of hundreds of years. The steeper the elevation, the quicker mass wasting occurs. Mass Wasting can take the form of rock falls, mudflows, and landslides. 

Deposition

Deposition is the process of eroding sediment deposited elsewhere.  Erosion is a natural process that moves fragmented rock and soil over long distances using the wind or water. When rivers flood over their banks, large amounts of silt are deposited onto the flooded land.  Due to the repeated flooding of some rivers, the land surrounding the river flattens out, creating a flood plain.  This floodplain has a rich soil, excellent for agriculture, because of the flooding and deposition of nutrients.

When the river reaches a large body of water, such as a lake or ocean, the water slows and is not able to support its load of sediments. A delta is where the river spreads out over a large area, depositing the sediments, often spread out in a triangular shape.  As more sediments are deposited, the shoreline grows outward in an arc.  Sandbars are offshore shoals of sand deposited by slower moving water.  Sandbars often form in the curves of rivers as the water slows to go around the curve. Sandbars also form in areas where the river spreads out over a larger area, which also slows the water flow.

A moraine is the sediment left behind by the melting of a glacier. As glaciers move, large amounts of the ground underneath is picked up and carried along with it. In a time of equilibrium, when the glacier is neither growing nor receding, a terminal moraine is formed along the leading edge. When the ice melts it leaves behind its load of suspended boulders, soil, and other sediment. The melted ice is replaced by the slow moving ice behind it. This new ice melts and deposits the sediments inside on top of the other sediments. This forms large mounds of rocks and boulders in glacial areas.


References

1.  Pulsipher, Lydia Milelic. (1999).  World Regional Geography.  New York: W.H. Freeman and Company, p. 21-22.

2.  Bradshaw, Michael.  A World Regional Geography.  The McGraw-Hill Companies, Inc., p. 32-36)

3.  Gabler, Robert, Robert Sager, Daniel Wise. (1994).  Essentials of Physical Geography.  Harcourt College Publishers, p. 339-345.

4. Dalrymple, G. Brent. (1991).  The Age of the Earth. Stanford University Press, p. 1-11.

5. Appenzeller, Tim. (2006). Earth in the Beginning. National Geographic 210 no6 , p. 58-67.

 


Review Questions

1.  The theory of Plate Tectonics was not as widely accepted until the 1970's.  Today, it is believed that tectonic forces are responsible for the development of: A. mountains; B. metamorphic rocks; C. volcanoes; D. hydrothermal fields; E. all of the above.

2.  Of the following countries, which has the least probability of earthquake activity?  A. United States; B. Peru; C. South Africa; D. Japan.

3.  All of the following are associated with the location of plate boundaries, except: A. volcanoes; B. earthquakes; C. geothermal activity (for example, in Iceland); D. friction between plates; E. localized changes in climate.

4.  Gradational forces include all of the following, except: A. river deltas; B. glaciers; C. water; D. wind; E. freezing and thawing of water

5.  How many miles on average is the earth’s crust?  A. 10 miles; B. 25 miles; C. 30 miles; D. 15 miles.

6.  This area around the edge of the Pacific Ocean is known as the_________________, because earthquakes and volcano’s are particularly common.  A. Bermuda Triangle; B. Magma; C. Quake Center; D. Ring of Fire; E. None of the above.

7.  Mountain ranges are developed by _________________.  A. Divergent boundaries; B. Transformation boundaries; C. Convergent boundaries; D. Volcanic boundaries; E. None of the above.


Submitted by Tim Blair on April 12, 2001. Updates submitted by Stephanie Bohnenkamp Sept. 26, 1996, Jayson Bisbee on May 5, 1997, Amy Garofolo on May 27, 1997, Mike Sundermeier April 9, 2007.