Rocks/Plate Tectonics

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1. The rock cycle

Rocks are naturally occurring aggregates of one or more minerals. James Hutton (1727-1797), the eminent 18th century gentleman farmer and founder of modern geoscience, 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, re-initiating the 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. As shown in the figure, in this case, the plate is eventually subducted under a continental plate. In the trench of the subduction zone, at relatively shallow depths, high pressure - low-high temperature metamorphism of the plate and its sediment cover occur. As the plate travels deeper, high temperature conditions cause partial melting of the crustal slab. Fluid intrusion plays a key role in partial melting. As the partial melt rises, and intrudes into the continental plate, the surrounding country rock is contact metamorphosed at high temperature conditions. This melt is either driven to the surface as volcanic eruptions, or crystallizes at depth to form plutonic igneous rocks. Sedimentary rocks form from the weathering, erosion, transport and deposition of arc material onto the continental platform and shelf.

Below is a brief schematic of the rock cycle.

2. Igneous, sedimentary, and metamorphic rock ...