Start At The CenterWith the exception of a few meteorites, all of the rocks on the surface of the Earth started beneath the surface. Combinations of magma flowing beneath the crust, upheaval, and plate tectonics keep the rocks of the world circulating in the same way that water and air circulate. One big difference is that rocks cycle very very very slowly. A million years is a short period of time for rocks.
As a hot Earth cooled millions of years ago, the crust began to form. Those were the first rocks. With the movement of the surface, newer rocks have appeared over time. For our example we're going to follow the rocks near a volcano. Simply enough... The molten rock oozed out of the volcano and wound up near the surface.
Forced UpAs that magma moved towards the surface, our example is going to see a pool form just beneath the surface in a pluton. As this pool of igneous rock cools, it was close, but didn't break the surface. Since our example volcano was near a fault line, there was a lot of tectonic activity beneath the surface. That activity and movement applied pressure on the pluton and the rock was changed from an igneous form to metamorphic. Given a few million more years, tectonic activity forced that metamorphic rock to the surface and eventually to the top of a mountain.
Falling DownWhen you're at the top of a hill or a mountain there is only one direction you can go. Down. Let's bring in a little erosion and weathering that will break up the metamorphic rock. It starts simply enough by breaking the mountain into boulders and then smaller rocks. A few landslides help the process move a bit faster too. Remember we're working with thousands and millions of years. Those rocks slowly move downhill and are small enough to be carried away in the runoff water. The runoff carries the small pieces of stone in the form of sand to an area where the sediment collects.
These super small rock fragments can go in a few directions from here. There may not be any vegetation and the sediment continues to build up over time. Eventually, sedimentary rock types may be created. Some examples might be some nice limestone or sandstone near the coasts. The other direction would be to break down even more into the basic elements of the rock. The plants in the region can then use those elements as their roots reach out for nutrients.
From A Mountain To AtomsNow we're thinking on a microscopic level. Rocks have been broken down into sand and the sand into fine powder. These smaller particles are more easily subject to biological break down by basic organisms such as lichen, molds, or algae. They take a few atoms of nutrients at a time. Looking at one day it's not that impressive. But if you think of millions of microscopic organisms working over thousands of years, its starts to make a dent. Once those nutrients are available to organisms, the elements take a new road.
The chemistry of the world can also break down these small pieces of rock. Oxidation reactions are only one example of the way the chemical makeup of the rocks can be changed. The thing to remember is that rocks don't always wind up as little rocks. There is a final phase of their existence when they become a part of many biological cycles.
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Laser Mapping the Earth (NASA/GSFC Video)
Useful Reference MaterialsEncyclopedia.com (BGC Cycles):
Encyclopædia Britannica (Geochemical Cycles):