Landforms


 

OUTLINE

  1. Age of Earth
  2. Tectonic Forces
  3. Gradational Forces



 

Age of Earth

The earth has an absolute age of approximately 4.6 billion years old. According to the Big Bang Theory there was a massive explosion that sent forth, at great speed, a huge amount of heat and matter that consisted almost exclusively of hydrogen and helium. The essential result of this explosion was our solar system's development.

The earth had its first life forms some 4.0 billion years ago. The oldest rock discovered is 4.1 billion years old. Scientist learn the age of the rock by measuring the amount of radioactive substance in them. Comparitive studies have been done to meteorite parts which have impacted on earth they to date to the time of the planets first forming about 4.3 billion years.

Humans evolved about 50,000-100,000 years ago. The exact date has still not been determined. Our human existence is but a fraction of a second on a 24-hour clock in geologic time. The earth has gone through many stages and mankind is only a small part of this history.

 

Tectonic Forces

Plate tectonic is the idea that plates carry the continents and are great slabs of solid material that make up the ocean floor. Plate tectonics comes from the Greek word,"tektonikos" meaning "builder." It has been determined that there are about 20 rigid plates that are in slow, continuous motion. Some continents move at a rate of 1/2 to 4 inches per year which is directed 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 and suggested that the existing land masses had been united at one point in the earths early history . He called his theory"die Verschiebung der Continent" meaning "continental displacement." His idea stated that these stable, immovable continents were mobile with the help of the tectonic plates. With further research his theory was accepted, but not until 60 to 70 years later.

The Earth is made up of three layers: the crust, the mantle and the core. The crust is a thin (15 mile) layer covering the outside part of the earth. The second layer is the mantle which is 1,800 miles thick. The crust and the upper mantle make up what is called the lithosphere. This lithosphere is 60-90 miles below the continents and 40-50 miles below the oceans. The plates in the plate tectonic theory are the lithosphere. The continental crust is less dense or lighter than the oceanic crust and "floats" above it. 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 made 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. It is believed that this heat is 'left over' from the formation of the earth and decaying radioactive material is fueling the fire. In fact, it is a possibility we could use the energy from this heat to fuel our lives if we were to run out of oil. 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, thus providing us with the explanation. In order to equalize these temperatures, convection cells are formed. Two types of rotation are produced by these convection cells.

Propelled by these heat convection cells, these plates move very slowly; one to four inches per year. (A couple of inches per year isn't much, since 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. This giant continent remained until about 135 million years ago, when it began to break up during the Mesozoic time. The break up consisted of India detaching itself from Africa and Antarctica and headed into the Indian Ocean. A giant mountain range is formed where the Australian-Indian Plate is pushing into the heart of Asia. Additional evidence supporting the continental drift theory is supplied by the amphibian and reptile fossils that are spread out among the widely separated continents. And the evidence of polar wandering and evidence of magnetic field reversals locked into oceanic basalt samples.

There are only three ways plates can interact while moving. This, in turn, causes there to be only three types of boundaries which are produced by different stress fields. The first boundary is called divergent. This is a tension or a stress that pulls the plates apart. Divergent boundaries cause mid oceanic ridges. Some other common characteristics are high heat flow, mild volcanic activity, and shallow earthquakes.

The second boundary is called convergent and this is a compression, or a stress that can shorten or compresses the plates. Convergent boundaries cause mountain ranges to develop. The Himalayas for example, were formed when the plate carrying India collided with the plate carrying Eurasia. This continental collision is still active and moving at a rate of 3 to 4 inches per year as the India plate is pushed under the Asian plate and the mountain continue to grow. Strong earthquake activity is very common with areas of convergent boundaries as well.

The third type of boundary is a transform boundary and this is when the plates slide past each other along faults causing mid-oceanic ridges and trenches. One plate may be forced down into the mantle under the other plate. When this occurs, a deep oceans trench forms. The largest ocean form is the Mariana Trench in the Pacific Ocean southwest of Guam. Another example is the San Andreas Fault with is between the North American plate and the Pacific plate. Here, earthquakes are common but not volcanoes, and the earthquakes tend to outline the major plates.

The earth is constantly being shaped by the dynamics of the tectonicactivity and plates motion. Some of the present day's biggest mountains and ocean trenches are examples of the great power of these plates in motion.

'Hot Spots' is a generally accepted term used to explain the formation of islands in the middle of the Pacific such as the Hawaiian Islands, the Line Islands, and the Tuamotus. The Galapagos Islands are similar in fashion, though not as aligned, but are located off the coast of Ecuador. A Hot Spot is caused by the magma that rises or plumes from the core to the surface causing volcanoes by penetrating the mantle. As the plate moves, it carries along the volcano that was formed. In it's place, a new one begins to form from the sea floor, while the hot spot stays in one place. Islands form in a "chain" as a result. The Hawaiian Islands get younger from east to west in the chain. On the island of Hawaii, which is still over the hot spot, volcanoes remain very active.

Other events related to this activity include earthquakes, volcanoes, and geothermic activity. Earthquakes are caused by abrupt easing of strains that have been built up along geologic faults and by volcanic action. The result of this is movement in the earth's surface. These vibrations can be felt in the locally affected areas and measured by scientific devices around the world. These plate cycles may also form volcanic activity. For example, the Pacific Ocean is surrounded by a nearly continuous plate-collision zone, called the 'Ring of Fire', here Volcanoes are the results of the instability of this zone. Japan lies near the colliding edges of three plates, hence, earthquakes and volcanoes are a constant threat to the islands population.

 


 

Gradational Forces

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 a mechanical weathering force or erosional force (gradational force). Evidence shows that water molds the earth's surface through chemical weathering, rivers, waves, glaciation, and deposition. One of the largest examples of water as a weathering force is the Grand Canyon. Mainly the work of one river, the Colorado River, has molded and shaped the canyon over the last 5 million years and continues to do so today.

Physical weathering breaks rocks into pieces. Animals, insects, worms, and burrowing mammals all work to loosen the soil by aeration and by mixing loose materials. Another main causes of physical weathering is the formation of ice in rocks. First, the water soaks into the cracks. Then, when the temperature falls low enough, the water in the cracks freeze. The expansion of the water in the cracks pushes hard enough to split the rock.

Chemical weathering is very complex, but basically it is the reactions between earth materials and 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 causes 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 that have remained visible for over 3500 years, but when the same ruins are sent to New York as a gift from the government in Egypt, the monument 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) erodes the land by carrying sediment away from one location and depositing it in another. Alluvium is all the sediment that is deposited by running water. Deltas are formed where running water moves into standing water. 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. "Bird's foot" deltas get their name from their appearance on a map. Rivers 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 may take a short cut across the corners and create a loop or an oxbow lake. An oxbow lake is the explanation for the creation of Lake Manawa and Carter Lake.

Deposition is the process of eroding materials into sediments then depositing them elsewhere. Deltas are areas of built up from soil dumped when a river empties into a lake or ocean. Sand bars are offshore shoals of sand deposited with slower moving water.

Waves are formed when wind blows over calm waters and ripples are created. These ripples enlarge with time and form larger wind waves. These Ocean waves then crash into continents and are considered to be 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 erosion in a few short years. Beaches and coastal land fronts are continually being changed by the wave action. These environment are semi fragile and can be created and destroyed quite simply by long shore drift, waves action and storms as these landmasses under go continuous change.

The Earth periodically under goes the event of glaciation.This is a time during which global temperatures drop and environments change. Glaciers,or large masses of ice, are gigantic erosional forces. There are many different types of glaciers and most contain a large portion of the earth's fresh water resources. During the Ice Age, ten-thousand years ago, glaciers covered large areas of land, wore away the surface, and disrupted the drainage system. The glaciers can create large U-shaped glacial valleys and cut through large mountains. The Missouri Valley was created by a glacier years and years ago, as well as the Kern Canyon in Sequoia-Kings Canyon National Park, California. Large boulders have been moved and deposited in the most unusual locations and the only explanation for these displacements are the glaciers. Glacial landscapes vary widely and are distinguished in forms as they carve mountains and valleys. One such feature are areas in which a lake may been formed without a river to carry water to or from the existing lakes. Moraines are another feature of this glacial sculpting, formed by the upward force of materiels in front of the glaciers path. Why did glaciers occur? The earth had a cooling period again before the ice which had formed years before had a chance to melt. This is viewed as a cyclical event. We are presently in an Interglacial Period. Today, glaciers cover all of Antarctica and most of Greenland. 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. This would cause flooded beaches every where. Aside from the sea level rising, the global temperatures and weather patterns would also change.


Questions: from the study notes,

1. The theory of Plate Tectonic was not widely accepted until the 1970's. Today,the science of tectonics is believed to have caused the development of: A. mountains; B. earthquakes; C. volcanoes; D. hydro-thermal fields; E. all of the above.

2. Which of the following countries has the least probability of suffering from 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 area 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. This Map shows all of the following information - except: A. Plate Boundaries and movement directions, B. Likely Earth Quake Zones, C. Plate Movement Rate, D. Area of Volcanic Activity. E. All of the Above.


 

6. This Illustration of the Hydrologic ("water") Cycle, give examples of all the following gradiational forces at work except: A. Glaciers "freeze -thaw", B. erosion by rivers carrying sediments. C. Oceans - wave action along coastal regions. D. Chemical weathering and erosion features.


 

7. This Fault Map of California is used to show: A. Fault Zone locations. B. Possible Earth Quake Zones. C. Areas we should not build Nuclear Power Plants. D. Cities that should do hazard map assessments. E. All of the above.


8. Geographic Maps can be used to: A. add visualization to land forms being studied. B. Aid in determining how urban construction can effect the current environment. C. Show effects of mass wasting events or tectonic activity. D. All of the above.

9. Lithosphere, inner and outer core, mantel,convection cell all describe: A. Tectonic mechanism, B. Volcanism, C. The Biosphere, D. How the inside of the earth is constructed.

10. All of the following terms or events describe Fluvial features or events except: A. Colorado River and Grand Canyon, B. Mississippi delta region. C. Karst and caves in limestone terrain, D. Long Shore Drift and wave action on beaches, E. Moraines and mountain lakes created by glaciers.

11. The Oldest dated rocks on earth have been dated to about: A. 5.7 Million years, B. 4.1 Million years, C. 4.5 Million years, D. Radio metric dating techniques can not give an exact date to the age of the earth.

12. Polar wandering and magnetic reversals were used to help prove: A the theory that the earth is spherical. B. Plate tectonic and Continental Drift theory, C. Effects of Astrogravitational influences on the earth and tides, D. All of the above.

13. Geography has 5 themes the study of: temporal (changes as related to time),and spatial-Place (distribution and form over the Earth's surface, human-Earth relationships (Anthropologic-human-,physical interactions good or bad), Movement-(where thing end up, plus how) and, A. Regions. B. National. C. County. D.Sector E.Industrial vs Rural.

14. Glaciers are formed because: A. it snows more in the arctic than other parts of the world. B. more snow is added than melts away. C.cooler temperatures make it harder for air to retain moisture, so it snows more. D. Volcanic dust creates more droplet nuclei, for precipitation.


1st Notes Correction Submitted by: Keri Bloes-January,29,96. Next Revisions;Stephanie Bohnenkamp-Sept.26,96. Jayson Bisbee-May,5,97. Amy Garofolo-May,27,97 & WEB-Graphics/Links/Edited/Design-William Guy {1/98-10/98}