Subject: essay1
Posted by: Katie Crooks on 6/11/99:
WHAT IS PLATE TECTONICS?
The theory of plate tectonics contends that the earth's surface is comprised of several large, rigid slabs of rock, called plates. The precursor of plate tectonics was the theory of "continental drift". Initially, most scientists believed that the earth was shaped by one catastrophic event, such as the Biblical forty-day flood. However, scientists began realizing that the earth itself, just as the life that inhabits it, is in a constant state of flux. In 1912, a German meteorologist name Alfred Lothar Wegener published two articles outlining his theory of continental drift. Wegener suggested that around 200 million years ago, the "supercontinent" Pangaea began to drift apart, forming smaller continents that eventually broke away from each other. The breaking away of the smaller continents formed the continental structure of today. Though Wegener never explained how such a large landmass could separate, his theory was key in creating the concept of plate tectonics. As scientists began studying the ocean floor and documenting patterns in the occurrences of earthquakes, continental drift began to become an even more viable theory, eventually upheld by the idea of plate tectonics.
WHAT EVIDENCE SUPPORTS THE THEORY OF PLATE TECTONICS?
Four different areas of research support the idea of plate tectonics.
1) Coastal Shapes
Some of the coastlines of today's continents seem to fit together as pieces of a jigsaw puzzle. When examining the shape of the east coast of North and South America and the shape of the west coast of Africa and Europe, it is obvious that the two landmasses could have easily been conjoined long ago.
2) Fossil Evidence
Comparisons of fossils indicate that, around the same period of time, the landmasses of Africa and Europe and North and South America were inhabited by similar species. Since it is highly improbable that these species could have somehow migrated across the Atlantic Ocean, the only logical conclusion that can be reached is that the land masses were once joined, allowing animals to travel freely across the continents.
3) Seismic Activity
Because scientists have postulated that earthquakes, volcanoes, tsunamis, and other geothermal activities are caused by plates colliding and/or separating, the areas in which most geological phenomena would result would be along the borders of the tectonic plates. Large amounts of seismic, volcanic, and geothermal activity have occurred and are occurring along the areas that scientists have delineated as possible plate boundaries.
4) Changing Landscape
In some areas, such as in the Mid-Atlantic Ridge, plates are separating and lava is welling up between the plates. Additionally, new mountain ranges are being formed due to the plates colliding; the Himalayas are still growing because of the constant collisions between plates.
WHAT IS THE "DRIVING FORCE"?
Unanswered questions behind the theory of plate tectonics still remain. The main point of debate regarding the theory is what exactly propels the plates. Tectonic plates basically float along molten rock that exists at an unknown depth beneath the earth's surface. The molten rock engages in a cycle that scientists term "convective flow". Molten rock is heated at the lowest depths and rises to the top, where it cools and spreads. Then the cooled rock sinks down again, and is reheated at the bottom. The convective flow causes the plates to shift against and away from one another, but the primary drive--the heat source--has not been determined yet. A convective flow cannot exist without a heat source, and scientists have not discovered how the convective flow originated. Additionally, while scientists agree that some type of convection occurs in the mantle of the earth, they are unsure if the convection flow is the primary cause of plate movement. No experiments or tests can be done to decipher what exactly drives plate movement because the mechanisms are so deeply buried within the earth.
HOW DO PLATE TECTONICS AFFECT HUMANS?
Humans are influenced by plate tectonics in three different ways.
1) Climate
Recent studies conclude that the process of convection actually alters the climate in certain areas.
Release of Volatile Chemicals
The process of erosion due to plates colliding influences the amount of volatile chemicals in the atmosphere. Free oxygen is released, as well as carbon, which reacts with chemicals in the atmosphere, and the reactions influence plant and animal life on the earth. For example, during the Proterozoic Era when there was a great deal of plate activity, ocean water became such that deposits of organic sediments could accumulate. The accumulation cause a reduced amount of carbon, and thus, and increased amount of oxygen, to enter the atmosphere. The newly oxidized atmosphere led to biological innovations and the evolution of animal life. While, of course, humans were not influenced through that period of time, the evidence does indicate that similar geological activity will occur in the future, and, consequently, similar atmospheric activity.
Altering Terrain/Affecting Erosion
Plate movement changes the very terrain of certain areas, definitely altering the climate in those areas. Obviously, the climate at the peak of a mountain is radically different than the climate at the bottom of a valley. The climatic conditions due to plate shifts also influence the rate and type of erosion that affects the landscape over time. Erosion releases organic sediments, which are absorbed into the ocean, affecting climate, as during the Proterozoic Era.
2) Natural Hazards
Earthquakes
Earthquakes occur along plate boundaries, or fault zones. Some of the greatest damage has been caused by faults called "strike-slip faults", in which the plates grind against each other in a horizontal fashion. The San Andreas Fault, which separates the Pacific and North American Plates, is a strike-slip fault. The movement along the fault occurs in sudden jolts or slow, steady motion called "creep". Fault segments that are creeping usually do not cause major earthquakes, but if areas of the fault are locked into place, energy builds up over time. The energy from locked segments can be released suddenly, causing incredibly damaging earthquakes. Another type of fault is called a "thrust fault". In a thrust fault, the plane is inclined to the surface of the earth, and moves upward over the other plane. The Santa Monica Mountains Thrust Fault is an example of this type of fault; however, it is a specific type of thrust fault, called a blind thrust fault. With blind thrust faults, the movement never breaks the earth's surface, but is concealed underneath, making detection of the fault very difficult.
Tsunamis
Tsunamis, also known as tidal waves, result from earthquakes, submarine landslides, and eruptions of island volcanoes. When an earthquake occurs on the ocean floor, a large amount of water is set into motion. While the deeper parts of the ocean are not very affected by tsunami waves, when the waves reach shallower water they increase in height, forming huge walls of water traveling at incredible speeds. The affect is similar when submarine landslides and volcanic island eruptions affect the ocean.
Volcanoes
The most deadly type of volcano is called a "composite cone" and erupts with explosive force; because the magma within the volcano is too stiff to allow gases to escape, when the gases begin to ascend and expand, extreme pressure builds up, which is suddenly, violently released. Mount St. Helens is an example of a composite cone. Volcanoes, obviously, harm human beings because they cause mudslides and avalanches, but they also release gases during eruption that completely alter the atmosphere. When Mount Pinatubo erupted in 1991, a large amount of sulfur dioxide was release into the air. The sulfur dioxide combined with water, causing global cooling by blocking some sunlight. Additionally, the combination of sulfur dioxide and water forms sulfuric acid, which is believed to be responsible for "acid rain" which corrodes buildings and harms plant life.
3) Natural Resources
Fertile Soil
The physical and chemical weathering of volcanic rocks leads to the existence of lush vegetation. Early civilizations sometimes settled on the fertile land created by volcanoes in the Mediterranean-Aegean region. Areas of Hawaii and the western United States posses fertile soil of volcanic origin.
Ore Deposits
Most metals mined on Earth are formed from magma from extinct volcanoes. Often, rising magma does not reach the surface of the earth, but instead will cool and form different types of rocks, including copper, gold, silver, lead, and zinc.
Fossil Fuels
Fossil fuels are formed when accumulated organic materials are subjected to intense heat and pressure. Plate tectonics are largely responsible for the burial of organic materials and the changes in sea levels, which led to the creation of coal, oil, and natural gas.
Geothermal Energy
Energy released by inactive volcanoes still giving off heat can be utilized to generate electrical power. Additionally, steam given off during geological activity can be harnessed to drive turbines, again, generating electrical power. Finally, human beings can also utilize the differences in temperature and pressure between rocks within the earth to create steam, allowing for another method in energy production.
Sites:
"This Dynamic Earth"
http://pbs.usgs.gov/publications/text/dynamic.html
"Plate Tectonics and the Evolution of Climate"
http://earth.agu.org/regeophys/sleep00/sleep00.html
"Plate Tectonics"
http://www.ocean.washington.edu/education/magic/1page/background/platetec
"Evidence for Plate Tectonics"
http://csep10.phys.utk.edu/asta161/lect/earth/evidence.html