Tectonics

Tectonic Plates, fossil correlations in Pangea and notional diagram of plate motions

Plate tectonics is a scientific theory describing how continents move around on the mantle and how sea floor is produced and destroyed. Plate tectonics is able to account for many major geological features: mountain building, volcanoes, earthquakes, the world-wide distribution of fossils and the ages of rocks on continents and the sea floor.

What we know as solid ground (rocks, soil, etc) is the top of the Earth's continental crust. For the most part, the crust is composed of old rocks whose density is relatively low, causing them to float on the much denser mantle. The solid crust and upper mantle are together a more-or-less rigid body called the lithosphere. The lithosphere is composed of approximately fourteen tectonic plates, whose thickness varies between about 25 and 65 km. The crust under mountains is thicker than it is under low plains, much in the way that tall icebergs extend deeper underwater than short ones do. Oceanic crust is much thinner, being about 5 km thick, and is denser than continental crust.

Tectonic plates move around on the Earth's surface ("continental drift") at a rate of a few cm/yr. Why they do this is not well understood. It is almost certainly more complicated than just simple convection, as indicated by many illustrations, including the upper right hand one on this page.

Two main tectonic structures are spreading centers and subduction zones. Spreading centers occur at the boundary between two plates that are moving apart, called divergent plate boundaries. Here the plate motion opens a gap between the plates and magma from the mantle rises up through it. When the magma reaches water at the ocean floor (most spreading centers are in the ocean), it cools and hardens, and becomes new oceanic crust. As the plates continue to move apart, more and more new basaltic crust is created.

The most famous spreading center is the Mid-Atlantic Ridge, but there are many others. California has a small spreading center located in the Brawley Seismic Zone of Imperial County, the northern-most spreading center of the East Pacific Rise.

If new crust is created, then it must be destroyed some where, otherwise the Earth would grow, which it cannot. Crust is consumed in subduction zones. When two plate move together, one of them must slide under the other one. This is called subduction. When an oceanic plate and a continental plate meet, the denser oceanic plate subducts under the less dense continental plate. The subducting plate slips down into the mantle where it is heated, melts and is recycled. Thus spreading centers and subduction zones act like a conveyor belt, manufacturing crust at one end and consuming it at the other. The deepest parts of the ocean occur in trenches along subduction zones. The largest earthquakes are associated with subduction zones.

Because of subduction, the sea floor is relatively young, no where more than about 200 million years old. The continents, on the other hand, are much older. They can last a very long time while spreading and subduction take place. Some continental rocks are as much as 4 billion years old. Because California is largely the product of tectonic activity during the last 230 million years, most of its rocks are relatively young, those older than 230 million years being rare.

In California we have the Cascadia subduction zone. It extends from offshore Cape Mendocino northward to Vancouver, and is responsible for volcanoes of the Cascades Range, including Mt. Shasta and Lassen Peak.

When two continents collide, it gets messy. Instead of subducting, some of the sea floor gets scraped off by the over-riding continent and remains stuck, usually mixed with continental sediments carried into the ocean by rivers. Such pieces of sea floor in California are known as the Franciscan Formation, or melange. Much of the west coast of California between San Luis Obispo and Humboldt counties are composed of Franciscan rocks. Complete sections of the oceanic crust that have been uplifted to the surface are called ophiolites. Ophiolites are produced at spreading centers.

California's Josephine, Elder Creek, Jarbo Gap, Coast Range and Cuesta Ridge ophiolites are good examples.

More About Plate Tectonics

Plate tectonics videos

USGS This Dynamic Earth
CSU Pomona tectonics website
SDSU tectonics website
U. Maryland tectonics website
Dispersal of Gondwanaland

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