Achtung Klass
Heuta, wir wollen ...
Weeeellllll, let me start over.
Long, long ago and not very far, far away in this galixy and, believe it or not, on this planet, there was very little water. What was de-gassed from the earth soon evaporated. This was not very much but was anyway held in what little atmosphere there was by gravity.
Imagine the earth the size of an onion. The solid part of the earth (at the scale of the onion) is the size of the dry skin that you shuck off before you use the onion. This part - the crust - is where the plates of solid earth are located. The hot inner part of the earth has been moving this solid crust around like the skim that forms on warm milk since it cooled to the point that the crust could form.
The oceans came later. Enough water falls to the earth every 20,000 years to add an inch of water to the earth. The earth itself is cooling slowly and since the earth is about 4.5 billion years old, it had cooled enough by about 3.8 billion years. Since this is probably anerobic bacteria water may not have been involved. BUT it is found in rocks that are some of the oldest rocks known on earth, on the west coast of Greenland. These rocks are already highly deformed and metamorphised (changed from their original state) by plate tectonics.
There is another 4.4 billion year old contender for oldest life from the middle of the Australia, but it too is in rocks that have been subjected to plate tectonics. Both are dependent on the ratio of C-12 to C-13 with the Greenland sandstone samples being a little more convincing.
A small note. The 4.5 Billion year date for the earth is from an earth that has been 'reset' because of a collision with a 1/4 earth size planitary body or asteroid that melted eveything, reseting the atomic clock for rocks and forming two celestial bodies - the earth and the moon. This was about 4.5 billion years ago.
Class dismissed.
C-12 and C- 13 are carbon isotropes.
An isotrope is:
One of two or more forms of a chemical
element having the same number of
protons, or the same atomic number, but
having different numbers of neutrons, or
different atomic weights.
In my geophysics lectures (about 6 years ago) the impression I got was that the mantle convection is largely independent of the motion of the plates - upwellings in the mantle cause particularly hot parts of the mantle which produce loads of lava - like in Hawaii or Iceland, and can cause one plate to split into two by melting the middle while it is in tension, but don't actually drive the motions themselves.Plates have formed as the earth has cooled ever since the earth started cooling. It is the very slow convection currents of mantle that drives the plate movements. (This is starting to be discussed again among earth scientists.)
Glad you like them, I hate describing things without diagrams.. which is why I wrote the diagram feature for the forum. One of these days I will get round to adding some features to it which will mean that they will be a little less scribbled ;)
| Dave said on 01/01 (with diagram, of course) In my geophysics lectures (about 6 years ago) the impression I got was that the mantle convection is largely independent of the motion of the plates - upwellings in the mantle cause particularly hot parts of the mantle which produce loads of lava - like in Hawaii or Iceland, and can cause one plate to split into two by melting the middle while it is in tension, but don't actually drive the motions themselves. Later: The motion is driven by a convection current involving the creation and destruction of crust. Edited By JB for this post. See above for original. |