The Plate Tectonics Information Page

The Plate Tectonics Info Page will discuss how Wegener's continental drift theory developed after technology allowed scientists to map ocean floors with sonar and record continental movements from space with satellites. Technological advances, combined with Alfred Wegener's early rock and fossil evidence, led scientists to form the concept of Plate Tectonics.  Plate Tectonics combines the ideas of continental drift and crust recycling to explain crust formation, movement, and destruction. 

More evidence for Continental Drift:  (click Continental Drift INFO Page to see Alfred Wegener's early evidence)  Despite Alfred Wegener's wonderful evidence in support of his continental drift hypothesis (remember rocks and fossils) , he was lacking the mechanism to explain how continents actually move.  This information did not surface until after Wegener's death. 

Wegener originally believed that the continents plowed through the ocean floor as a result of either centrifugal forces caused by the rotation of the earth, or because of the gravitational forces caused by the sun and moon (Weil, 1997).  These ideas were rejected by geologists of the time, but after Wegener's death more research was done into one of his hypotheses... that the mantle undergoes thermal convection.  The resulting convection currents are caused by the very hot magma at the deepest part of the mantle rising, then cooling and sinking, and then heating, rising and repeating the cycle over again.  Arthur Holmes suggested that the thermal convection currents were like a conveyor belt and that the upwelling pressure could break a continent apart, forcing the broken continent in opposite directions (Weil, 1997).  This idea received very little attention at the time,    convection currents are accepted as the mechanism that drives continental drift!! 

Technological support for continental drift - satellites and sonar:  Question - What do sonar used to track subs in WWII and the Global Positioning Systems (GPS) running by satellites high above have in common?  Answer - The two systems have both supplied technological evidence supporting the continental drift theory!! 

GPS satellites have measured continental movement ranging from 2.5 cm per year to more than 15 cm per year (USGS, 1999).  Perhaps even more interesting than satellite evidence is the sonar techniques that mapped the depth and terrain of the ocean floor.  Sonar works by sending signals to the bottom of the ocean and recording the amount of time it takes to return to the source - this amount of time gives an indication of depth.  What was once thought to be flat and boring was found to be rugged, mountainous, and at times very hot evidence in favor of continental drift.

Crust Recycling - How lithospheric plates interact with each other...

1) Divergent Boundaries & Mid-Ocean Ridges:  Sonar systems improved throughout the 20th century, allowing scientists to create topographical maps of the ocean floor.  What was discovered in the middle of the Atlantic Ocean (and later in other oceans) was the longest continually running mountain range in the world - 46,000 km according to Dr. Sean Chamberlin (2002).  These mid-ocean ridges, or underwater mountains, are volcanically active.  The Mid-Atlantic ridge runs the length of the Atlantic Ocean and is the most studied and understood mid-ocean ridge.  Because the plates at mid-ocean ridges move away from each other, this type of plate boundary is called a divergent boundary (Maton, 1999).  To "diverge" means to split, so the boundary where two plates are moving away from each other is appropriately named (see Figure 7.1).

The ocean floor spreading at divergent boundaries and mid-ocean ridges helps to explain continental drift because as the oceanic plates spread apart, they can take a continent with it (Maton, 1999).  Mid-Ocean ridges are also the site where new crust is formed.  Because mid-ocean ridges are volcanically active, magma from the mantle erupts as lava into a rift valley running the length of the ridges.  A rift valley is the gap created from two diverging plates.  As convection currents move the oceanic plates away from these mid-ocean ridges, the magma that erupts as lava into the rift valley fills in the gap, cooling to form new oceanic crust (the youngest oceanic crust on earth). 

2)Convergent Boundaries & Subduction Zones:  With new crust being formed at mid-ocean ridges, you would think that the size of Earth would increase.  However, sonar technology and detailed mapping of the world's earthquakes have found areas where crust is destroyed.  These locations, called subduction zones, are where an oceanic plate is bent and driven down into the mantle by a continental or oceanic plate.  Because the plates at subduction zones move toward each other, this type of boundary is called a convergent boundary (Maton, 1999).  To "converge" means to come together or collide, so the boundary where two plates are colliding with each other is also appropriately named (see Figure 7.2).

Deep ocean trenches, earthquakes, mountains and volcanoes are usually associated with subduction zones.  Subduction zones give us an explanation of not only how crust is destroyed, but also how mountains and volcanoes are formed.  The subduction of the oceanic plate into the hot mantle melts and destroys the oceanic crust.  The melted crust, now magma, can get forced up through cracks in the continental crust because pressure is greater in the mantle.  If the magma erupts back onto the earth as lava, it can create mountains, volcanoes, and new continental crust.  Earthquakes are common at this type of boundary as the rocks of the oceanic and continental plates "catch" each other and then release with force the build up pressure. 

3) Transform Boundaries and Earthquakes: Another interaction of the earth's plates is called a transform boundary.  Transform boundaries are when two plates slide past each other.  A famous example of this is the San Andreas Fault of California, but even more common are the transform faults that occur within the mid-ocean ridges, offsetting the diverging plates creating a zig-zag margin (see Figure 7.3) (USGS, 1999). 

Earthquakes are very common at transform boundaries.  As plates try to slide past each other, large pieces of crust catch, building pressure and preventing the slide.  When those crustal rocks break and the plates continue their movement, the release of the built up pressure is sent out as earthquake, or seismic, waves throughout the crust!!  Tsunamis, sometimes incorrectly called tidal waves, are the result of underwater earthquakes that cause oceanic crust to raise or drop, thus displacing large amounts of ocean water above.  The tsunamis are created when the displaced water seeks to level.  We will explore the devastating 2004 Tsunami in the Indian Ocean through videos and articles. 

Work Cited

Chamberlin, Sean. "Continental Drift." Geological Oceanography. 27 Aug. 2002.  05 Nov. 2002 <http://www.oceansonline.com/continen.htm>. 

Maton, Anthea; Hopkins, Jean; Johnson, Susan; LaHart, David; Quon Warner, Maryanna; Wright, Jill D. Exploring Earth Science. Upper Saddle River, New Jersey: Prentice Hall, 1999.

USGS. "Understanding Plate Motions." This Dynamic Earth: the Story of Plate Tectonics. 05 May 1999. United States Geological Society. 05 Nov 2002 <http://pubs.usgs.gov/publications/text/understanding.html>.

Weil, Anne. "Plate Tectonics: The Rocky History of an Idea." Geology. 27 Aug. 1997. University of California, Berkeley, Museum of Paleontology. 05 Nov. 2002 <http://www.ucmp.berkeley.edu/geology/techist.html>.

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