Tuesday, November 23, 1999

By: Alan Morgan

rock cycle

When we look at rocks exposed on the surface of the Earth we do not appreciate that nature is the ultimate recycler. Our present generation of rocks has, to use a cliché, "been through the mill" many times. What is this mill and how can rocks be recycled? The diagram above shows a simplified version of where rocks originate and how they move from one state to another.

We recognise that rocks fall into one of three great families. These are:

  1. the Igneous or "fire-formed" rocks
  2. Sedimentary rocks, that have been created by the lithification (hardening) of pre-existing sediments and
  3. Metamorphic, or "changed" rocks.

Each of these have special characteristics that are explained later, but how do they fit into the "rock cycle"? Starting at 1 in the diagram, igneous rocks are exposed at the Earth's surface. They can get there by one of two processes, Either they reach the surface as extrusive igneous rocks, that is they are poured onto the surface from volcanoes as explosive products (like ash), or rapidly-cooled liquid products (like lavas), or they are exposed by erosion. This second category of igneous rocks are called "intrusive"; - they were created within the subsurface and "intruded" into the crust and cooled at depth. Either way these igneous rocks are now at the surface.

Once there they are subject to different types of weathering. They break down by heating and cooling (especially frost action) and by chemical attack and biological activity. Individual grains from the igneous rock are liberated and transported under gravity by various forms of erosion. Erosion is the process by which rock surfaces are abraded and worn down. Erosion and transportation includes a variety of agents such as running water (in rivers, lakes and seas), glacial ice, and wind. Transportation is generally downslope under gravity and will involve mass-movements of materials. These materials may accumulate in depositional areas on land (deserts and in intermontane basins) or they may be transported to the oceans of the world.

All transported sediments will ultimately reach areas where they are no longer moved any substantial distance. These are areas of deposition. Note that in the diagram there is also an input from space as our planet encounters clouds of space debris such as meteorite swarms, and fortunately, very occasionally, much larger individual objects such as small asteroids or comets. Most of the space material arrives as microscopic particles (see elsewhere in this issue of WAT ON EARTH) or as larger chunks known as meteorites. Usually these end up in the oceans of our world where they join other sediments from terrestrial materials worn away from the continental mass. These sediments undergo lithification to form the second great family of Sedimentary rocks.

Sedimentary rocks can suffer two different fates. They may accumulate in basins that because of plate tectonic forces are uplifted in a process known as "mountain building" or orogenesis. If they are uplifted they are again subject to weathering and erosion as described above. The loop has been "short-circuited" and these rocks are destined to be recycled back to sediments again. The time frame involved here is perhaps in the tens of millions of years range, and the rocks have only been subjected to heating of generally less than 200 degrees Celsius.

The second fate suffered by sedimentary rocks is to be trapped into a far longer cycle that involves deep burial, heating and stress, and recrystallisation. Individual minerals in the sedimentary rock become changed by these processes (also with a very important role played by water), and emerge as the third great category of rocks - the Metamorphic rocks.

Understandably the metamorphic rocks have questionable pedigrees. Different rock types pre-metamorphism, may produce very similar metamorphic rocks and certain quite similar rocks pre-metamorphism might produce quite different rock types after metamorphism. Perhaps we can explore these relationships in a later issue of WAT ON EARTH. Either way all we need to know in this discussion is that the metamorphic rocks can also suffer two fates. They too can be involved in uplift (orogenesis) in which case they too will be subjected to weathering, erosion, transportation, and re-deposition. The time frames involved here are from tens to hundred(s) of millions of years and the heating involved can see the rocks subjected to temperatures of 200 degrees Celsius to ca. 450 degrees Celsius.

The second route is for them to be trapped within the earth and heated to a point where they start to melt (temperatures up to ca. 500+ degrees Celsius). Beyond this the increasing thermal gradients will cause the rock to melt converting it to an igneous state. Igneous rocks are formed at high temperatures ca. 600 degrees Celsius to 1000+ degrees Celsius in near-surface conditions. Here the cycle will begin again.

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