02/14/2006

The area of interest with regard to the Earth’s plate motion is the outer 350 km (220 miles or so).  This area includes:  the solid rocky lithosphere (“rock layer”) composed of the Earth’s crust and its upper mantle;  and, the soft, flowable asthenosphere (“weak layers”) that is partially melted. This zone is called the partial-melting zone or the low-velocity zone (because both P and S waves slow down when they enter it).

Tectonically, the lithosphere (which equals the plates) moves around on the underlying flowing asthenosphere.

Divergent Plate Boundaries

65,000 km of the Earth's surface (40,000 miles) are devoted to divergent boundaries.

These are places were new plate material is created and where the Earth's ocean basins originate.

The process begins with rifting ("tearing") of an older continental plate into two or more pieces. Heat-induced instability in the deep Earth interior starts the process. A rising plume of hot flowing material pushes the overlying older continental plate into a bump that eventually bends and tears. Flowing hot material carries the pieces away from the divergent zone.

The best place to see this happening today -- Eastern Africa. There, the Arabian plate is tearing away from the African plate creating the East African rift valley (with earthquakes and volcanoes), the Red Sea and the Gulf of Aden.

A good to see the effects of an ancient rift – go to the Hudson River’s Palisades of New York-New Jersey border. This rift split part of Pangaea about 200 million years ago.

  Divergent Plate Boundaries

  Divergent zones produce oceanic lithosphere (new ocean plates)

   If you were to visit an active divergent zone, you would find the following:

·    Higher-than-normal heat flow – as much as eight times the average heat flow for the Earth.

·    Earthquakes - which tend to be relatively small (2 to 4 in magnitude) and shallow (restricted to the 60 mile thickness of the plate)

·    normal faulting

·    an undersea mountain range

·    the youngest rocks on Earth

   Geologists can sample and study oceanic lithosphere by:

1. dredging the seafloor (scraping off the sediments)

2. coring the seafloor (to sample all the layers)

3. using P and S waves to identify changes in composition of the layers

4. visit places where continents have collided – and a slice of oceanic lithosphere has gotten “caught in the squeeze” and thus has become part of a continent.

 

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