From: The Bandar Log
The the layer of granitic, sedimentary, and metamorphic rocks known as continental shelves (sial) due to its [aluminium based] granitic rock, contrasts the oceanic crust (sima) basaltic (mafic) rock. Due to the change in velocity of
seismic waves it is believed sial
becomes close in its physical properties to sima and the dividing line
is called Conrad discontinuity.
Granitt
has a density of 2.7g/cm3, less dense than mafic rock.
oceanic crust. About 40% of the
Earth's surface is now underlain by continental crust.
Because
continental crust mostly lies above sealevel, its existence allowed land
life to evolve from marine life. Its existence also provide broad
expanses of shallow water known as epeiric seas and continental shelves where complex metazoan life could become established during early Paleozoic time.
In
contrast to the relative permanence of continental crust, the size,
shape, and number of continents is constantly changing, as different
tracts rift apart, collide and recoalesce as part of a grand
supercontinent cycle. There are ~7 billion cubic kilometers of
continental crust. Because of its relative low density,
continental crust is rarely subducted or re-cycled.
The height of mountain ranges is related to the thickness of crust. The isostasy of orogeny (mountain formation). The crust is thickened by the compressive forces related to
subduction or continental collision. The buoyancy of the crust forces
it upwards, the forces of the collisional stress balanced by gravity and
erosion. This forms a keel or mountain root beneath the mountain range,
which is where the thickest crust is found.
The thinnest continental
crust is found in rift zones, where the crust is thinned by geology)">detachment faulting and eventually severed, replaced by oceanic crust. The edges of continental fragments formed this way (both sides of the Atlantic Ocean, for example) are termed passive margins.
The
high temperatures and pressures at depth, often combined with a long
history of complex distortion, mean that much of the lower continental
crust is metamorphic - the main exception to this being recent igneous
intrusions. Igneous rock may also be "underplated" to the underside of
the crust, i.e. adding to the crust by forming a layer immediately
beneath it.
[These igneous intrusions are from the volcanic process. the monkeys are failing to explain so use big word to confuse you and help you ell good about it.]
Today there are ~7 billion cubic kilometers of
continental crust, but in the past there may have been less or more.
[More or less what you would expect a monkey to say.
I am removing all the miracles I can find in the passage because monkeys don't believe in miracles. I have to leave some of the assertions in of course or I would have nothing to show you.]
Today plate tectonics adds new material at
convergent plate boundaries by the partial melting of oceanic crust at subduction zones,
causing the lighter material to rise as magma, forming volcanoes. Volcanic island arcs,
seamounts or similar structures collide with the side of the continent
as a result of plate tectonic movements. Continental crust is lost,
due to erosion and sediment subduction, tectonic erosion of forearcs,
delamination, and deep subduction of continental crust in collision
zones. [Odd how it builds up on the windward side of the shelves isn't it?]
Many aspects of crustal growth are controversial, including rates
of crustal growth and recycling, whether the lower crust is recycled
differently than the upper crust and over how much of Earth history
plate tectonics has operated and so could be the dominant mode of
continental crust formation and destruction. It is a matter of debate
whether the amount of continental crust has been increasing, decreasing,
or remaining constant over geological time.
The growth of continental
crust appears to have occurred in spurts of increased activity corresponding to five episodes of increased production through geologic time (see graphic at Butler).
There you have it, or not as the case may be. If you have an hour to spend thinking about creation, take a look at the site linked above. And remember it is designed to hypnotise children.
What I wrote in the preceding thread still holds good, plus it is full of Froud and Reynold numbers and the Navier-Stokes Millennial Prize solution that a better mathematician that I can put together from the relationship I point out in there.
So if you are a mathematician: WTF are you waiting for?
