PLATE TECTONICS

Alfred Wegener (1915) proposed that in the past there had been only one supercontinent, Pangaea. Three hundred million years ago (or so), Pangaea broke up and the pieces began to drift apart. Strong support for this hypothesis comes from fossils found in South America and Africa which indicate that the regions had similar lifeforms and from fossils which indicate that tropical lifeforms existed in Antarctica around 200 million years ago. Further evidence is provided by paleomagnetism [the study of the Earth's magnetic field.]

The continents currently drift apart at a rate of 2 to 4 cm per year.

The reason that this idea, which seems so compelling, was not initially accepted was that no one could imagine how things like continents could move around (or more precisely, what could push continents around). We believe that we have a reasonable idea for the mechanism underlying tectonic motions (however the details still elude us).

Recall that the interior of the Earth is hot and that it can be divided into the crust, mantle, and core based upon chemical differences but that from a mechanical standpoint, it was better to consider the crust and the outer part of the mantle as one unit, the lithosphere and the plastic layer underneath as one unit, the asthenosphere. Some properties of the lithosphere are as follows:

The lithospheric plates are thought to be moved around by convective motions (e.g., Oatmeal convection, horizontal motion ). Because the Earth is hottest at its center and cools as one moves outward an outward flow of heat is set up. Through the solid portions of the Earth, the heat is carried by conduction, but in the liquid and in the liquid and plastic portions of the Earth, the heat is carried by convection.

The lithospheric plates separate (new crustal material is produced) near rifts. There is a large rift in the mid-Atlantic stretching from Iceland to Antarctica, the mid-Atlantic Ridge. Overall, there are around 60,000 km of active rifts on the surface of the Earth. Most are in the oceans, but some are on land, e.g., the Great Rift Valley in Africa. New crust is thus continuously created. The oceanic crust is replaced roughly on a time scale of 200,000,000 years.

PLATE BOUNDARIES

Because the Earth is not growing in size while crust is created continuously implies that crustal material is also destroyed continuously. To see and where and how this occurs consider the interactions when plates collide. There are three types of interactions which occur near plate boundaries:

  • rift zones: or spreading centers occur where plates move apart (see the Mid-Atlantic Rdige). The upwelling convective motions in the asthenosphere push the plates apart. The upwelling lava cools forming basaltic rock. In addition to new crust, heat and minerals are also deposited.

  • subduction zones:

    Subduction zones occur where continental plates meet oceanic plates (note that the oceanic basin spreading from the mid-Atlantic ridge pushes the North American and South American plates, there is no subduction zone at the eastern coasts of North and South America). Because the oceanic plates are denser and thinner than are the continental plates, they are forced inward (into the Earth). The oceanic plates are forced downward to the regions of high temperature where the rocks are melted (around 200 - 300 km below the surface). Some of the released material is re-inputed to the surface via volcanoes while most is recycled into the mantle to be spewed out in rift zones. Near subduction zones you find oceanic trenches, mountain ranges, volcanism, and earthquakes. Of particular interest to us is the Juan de Fuca plate which forms a shallow angle subduction zone. Shallow angle subduction zones lead to violent activity such as earthquakes and volcanism. In Oregon, we get large earthquakes every 500 years or so. The last happened at 9 pm on January 26, 1700, the Cascadia earthquake, magnitude 8.7-9.2. This is large; a magnitude 6 earthquake has 1 Megaton of seismic energy. A magnitude 7 earthquake has 32 Megatons of seismic energy. A magnitude 8 earthquake has 1,000 Megatons of seismic energy. A magnitude 9 earthquake has 32,000 Megatons of seismic energy! We are roughly due for a large earthquake.

    Great Tohoku Earthquake, 11 March 2011

    When two continental plates collide, because there is no strong tendency for one plate to slide under the other, we get mountain range formation. An example of this is the Himalayan mountain range.

  • transform faults occur where two plates slide parallel to each other. An example of this is the San Andreas fault in California. The Pacific plate is forced northward at a rate of a few centimeters per year with respect to most of the North American plate (map). At this rate, Los Angeles will be next to San Francisco in about 20 million years. Earthquakes occur near fault lines because the plates do not slide smoothly. There is friction between the plates which causes the motion to go in fits and spurts. The San Andreas fault lets go every century or so. The last big earthquake occured in 1906 in San Francisco where the plates slipped by around 6 m (the 1989 earthquake relieved less than 3 m of stress). The southern section moves about 7 m in a large earthquake and hasn't really let go since the great Fort Tejon earthquake of 1857. This is a concern.

Comment:

    The Hawaiian Islands represent a different kind of volcanism (compared to the volcanos found, e.g., in the Cascades). They do not occur near plate boundaries. They are formed near a hot-spot in the mantle of the Earth. Through so-called plumes, magma rises to the surface of the Earth. The magma oozes to the surface forming what are known as shield volcanoes (as opposed to cinder cone volcanoes). Shield volcanoes have gradual slopes and are produced by lava flows building on each other. The Hawaiian islands are the largest volcanoes on the Earth having bases with diameters of ~ 200 km (120 mi) and heights of 9 km (the volcano on the Big Island, Mauna Loa, is the largest active volcano on the Earth). The reason that there is an island chain is due to the fact that the Pacific plate is moving (at a rate of a few cm per year). In 1 or 2 million years that plate moves a distance equal to the average separation between the islands. Thus the island chain represents a time sequence with the Big Island being the youngest. This process is continuing and ancient. There is another Hawaiian island forming, Loihi, about 20 km southeast of the Big Island. It is currently about a kilometer below the surface of the water. It is expected to make its appearance in 50,000 years or so.