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James Hutton (17271797), the eminent 18th century gentleman farmer and
founder of modern geosciences, authored the concept of the rock cycle, which
depicts the interrelationships between igneous, sedimentary, and metamorphic
rocks. The upper part of the earth (mantle, crust and surface) can be envisioned
as a giant recycling machine matter that makes up rocks is neither created nor
destroyed, but is redistributed and transformed from one rock type to another.
PETROLOGY, the study of rocks and their origins, is essentially the formal
process by which we resolve the interrelationships expressed in the rock cycle.
Liquid (molten) rock material solidifies at depth or at the earth's surface to form
IGNEOUS ROCKS. Uplift and exposure of rocks at the Earth's surface
destabilizes these mineral structures (c.f. Bowen's Reaction Series). The
minerals break down into smaller grains which are transported and deposited
(either from solution or by lowering the hydraulic energy regime) as sediments.
The sediments are lithified (compacted and cemented), and SEDIMENTARY
ROCKS are formed. Changes in temperature, pressure, and/or rock or fluid
chemistry can allow igneous and sedimentary rocks to change physically or
chemically to form METAMORPHIC ROCKS. At higher temperatures,
metamorphic (or any other rock type) rocks may be partially melted, and
crystallization of this melt will create igneous rocks. Uplift and erosion can
expose all rock types at the surface, reinitiating the cycle.
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THE ROCK CYCLE
If we examine the rock cycle in terms of plate tectonics, as depicted in the figure
above, we see that mafic (tholeiitic) igneous rocks form at sea floor spreading
ridges. Fluid intrusion of these rocks, both during and after formation, results in
some low grade metamorphism. As the rocks cool, and more magma is
introduced from below, the plate is forced away from the spreading ridge, and
acquires a sediment cover.…read more