Tuesday, 24 May 2011
CASE 287 - Tectonic plates
Earth's outer shell, the lithosphere, long thought to be a continuous, unbroken, crust is actually a fluid mosaic of many irregular rigid segments, or plates. Comprised primarily of cool, solid rock 4 to 40 miles thick,* these enormous blocks of Earth’s crust vary in size and shape, and have definite borders that cut through continents and oceans alike. *[Oceanic crust is much thinner and more dense than continental, or terrestrial crust].
There are nine large plates and a number of smaller plates. While most plates are comprised of both continental and oceanic crust the giant Pacific Plate is almost entirely oceanic, and the tiny Turkish-Aegean Plate is entirely land. Of the nine major plates, six are named for the continents embedded in them: the North American, South American, Eurasian, African, Indo-Australian, and Antarctic. The other three are oceanic plates: the Pacific, Nazca, and Cocos.
Basically, three types of plate boundaries exist, with a fourth, mixed type, characterized by the way the plates move relative to each other. They are associated with different types of surface phenomena. The different types of plate boundaries are:
Transform boundaries (Conservative) occur where plates slide or, perhaps more accurately, grind past each other along transform faults. The relative motion of the two plates is either sinistral (left side toward the observer) or dextral (right side toward the observer). The San Andreas Fault in California is an example of a transform boundary exhibiting dextral motion.
Divergent boundaries (Constructive) occur where two plates slide apart from each other. Mid-ocean ridges (e.g., Mid-Atlantic Ridge) and active zones of rifting (such as Africa's Great Rift Valley) are both examples of divergent boundaries.
Convergent boundaries (Destructive) (or active margins) occur where two plates slide towards each other commonly forming either a subduction zone (if one plate moves underneath the other) or a continental collision (if the two plates contain continental crust). Deep marine trenches are typically associated with subduction zones, and the basins that develop along the active boundary are often called "foreland basins". The subducting slab contains many hydrous minerals, which release their water on heating; this water then causes the mantle to melt, producing volcanism. Examples of this are the Andes mountain range in South America and the Japanese island arc.
Plate boundary zones occur where the effects of the interactions are unclear and the boundaries, usually occurring along a broad belt, are not well defined, and may show various types of movements in different episodes.
The relative small size of the numerous other plates neither diminishes their significance, nor their impact on the surface activity of the planet. The jostling of the tiny Juan de Fuca Plate, for example, sandwiched between the Pacific and North American Plate near the state of Washington, is largely responsible for the frequent tremors and periodic volcanic eruptions in that region of the country.