pp.10, Fowler: It is probable that if the thick, relatively low density continental material (the continental crustal density is approximately 2.8 10**3 kg m**-3) reaches a subduction zone, it may descend a short way, but, because the mantle density is so much greater (approximately 3.3 10**3 kg m**-3), the downwards motion does not continue. Instead, the subduction zone ceases to operate at that place and moves to a more favorable location.
Pp 6, Fowler: Along divergent boundaries, which are called accreting or constructive, plates are moving from each other. At such boundaries new plate material, derived from the mantle, is added to the lithosphere. The divergent plate boundary is represented by the mid-ocean ridge system, along the axis of which new material is produced.
Along convergent boundaries, which are also called consuming or destructive, plates approach each other. Most such boundaries are represented by the oceanic-trench, island-arc systems of subduction zones where one of the colliding plates descends into the mantle and is destroyed.
pp.6. Fowler: Along conservative boundaries, lithosphere is neither created nor destroyed. The plates move laterally relative to each other. These boundaries are represented by transform faults, of which the San Andreas Fault in California, U.S.A. S a famous example. Transform faults can be grouped into six basic classes. By far the most common transform fault is the ridge-ridge fault, which can range from a few kilometers to hundreds of kilometers in length. Some very long ridge-ridge faults occur in the Pacific, equatorial Atlantic and southern oceans. Adjacent plate move relative to each other at rates up to about 15 cm yr-1.
Solid Earth Geophysics Ali Oncel [email_address] Department of Earth Sciences KFUPM Today’s class: Plate Motions Reading: Fowler Chapters 2-3 http://neic.usgs.gov/neis/plate_tectonics/plates.html
OVERVIEW: PLATE TECTONICS Basic concept: The outermost layer ( LITHOSPHERE ) is divided in a small number of “rigid” plates in relative motion one respect to the other. It FLOATS on the weak ASTHONOSPHERE .
As a result of DISCOVERY, the plates are made up of both oceanic and continental material, whereas the ONLY oceanic parts of plate are created or destroyed. However, it is hard to understand why continental material usually is not destroyed at convergent plate boundaries.
Sea floor spreading at the mid ocean ridges produces only oceanic lithosphere.
From: Fowler, the solid Earth
How many plates? What is the largest plate and smallest plate?