PLATE TECTONICS

Alfred Wegner (1924) proposed that in the past there had been only one supercontinent, Pangaea. Hundreds of million years ago, 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 - 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 up 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 lithosphere is less than ~ 100 km in thickness

• The lithosphere is composed of two types of material distinct in composition and structure; the oceanic crust (55 %) and the continental crust (45 %).

  The oceanic crust is thin (< 6 km) and is composed of basalts, similar to those found in lunar maria. The oceanic crust is denser than the continental crust.

  The continental crust is thicker (~ 20 - 70 km) and is composed of granites. Granites (as are the basalts) are igneuous rocks, however, granites were formed under high pressure below the surface of the Earth. The continental crust is less dense than the oceanic crust.

• The lithosphere is not one unit, it is segmented. It is composed of around 7 major plates and around two dozen smaller ones. The places where the plates come together are well-marked as regions of enhanced geological activity.

The lithospheric plates are thought to be moved around by convective motions (atmospheric; mpeg video of numerical simulation by Andrea Malagoli, The University of Chicago). 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 large scale circulations (motions) in the asthenosphere moves the overlying lithospheric plates on the surface of the Earth leading to the continental drift observed today.

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. 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. 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 occur where continental plates meet oceanic plates. 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 fast subduction zone. Fast subduction zones lead to very violent earthquakes. In Oregon, we get large earthquakes every 300 years or so. We are currently due for a large earthquake.

• fault zones occur where two plates slide parallel to each other. An example of this is the San Andreas fault in Northern 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. At this rate, Los Angeles will be next to San Francisco avter about 20 million years. Earthquakes near fault lines because the plates do not slide smoothly. There is friction between the slips which causes the motion to go in fits and spurts. The San Andreas fault lets go every century or so. The last real 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. They do not occur near plate boundaries. They are formed near a hot-spot in the mantle of the Earth. Through these so-called plumes, magma rises to the surface. 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 up upon each other. The Hawaiian islands are the largest volcanoes on the Earth having bases with diameters of 100 km and heights of 9 km. 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 still continuing. There is another Hawaiian island forming, Lohi, about 20 km south 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.