Saturday, August 14, 2010

Earth Crust

The Earth’s crust only accounts for 0.6 percent of the planet’s volume, yet we have not penetrated right through even that. Oceanic crust is 5 to 10 Km thick and made of less dense rocks, including granite and thick accumulations of sediment. The continental crust resembles the scum on the surface of a big cauldron.
The oldest regions of continents, made of material that has been piling up for billions of years, have been “cooked” by heat and pressure and are made of crystalline metamorphic rocks. At the base of the crust is a boundary that reflects seismic waves, called the Mohorovicic discontinuity, or Moho. Beneath it are the rocky slabs of lithospheres mantle, composed mostly of iron and magnesium rich peridotite, on which the cruse floats.  The more the weight laden on the crust, the lower it sinks. Mountainous areas have “roots” within the mantle which are significantly greater than the height of the mountains above. The balance maintained is called Isostasy.
The crust is divided into relatively rigid plates, some made of ocean crust, some of continents. Over geological time the plates jostled around as the convection mantle moves beneath them triggering earthquakes. Ocean crust is created where molten rock wells up along mid ocean ridges, and is destroyed where it is subsumed under continental plates. Continental crust can be roasted, stretched and split by mantle plumes, or uplifted, warped and eroded at the surface, but ancient continental cores remain intact.
It is possible to trace the history of the movements of continents’ waltzes around the globe over hundreds of millions of years. Their edges fit together like the pieces of a jigsaw; the similarities between their fossils and rock strata, the past climates they experienced and the orientation of magnetic minerals, frozen in volcanic rocks like tiny compass needles, all record their travels. Precise laser measurements taken from satellites reveal the present rate of continental drift. It is roughly comparable to the rate at which finger nails grow- a mere 4 to 5 Cm in a year across the Atlantic, 12 to 14 Cm in a year across parts of the Pacific