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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.

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Introduction

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. ...read more.

Middle

Igneous rocks form by direct crystallization of minerals from a magma melt; we see a surface expression of magmatic activity at sea-floor spreading ridges and other rift zones, volcanic arcs (subduction zones) and hot spots (intraplate volcanism).. Intrusive (plutonic) rocks crystallize at depth, whereas extrusive (volcanic and pyroclastic rocks) rocks crystallize after the magma reaches the earth's surface. In general, extrusive rocks have a finer grained texture than intrusive rocks. Igneous rocks are often classified according to the percentage of SiO2. The figure below is a general guide to igneous rock classification, showing the rock names and the differences in mineralogy. Sedimentary rocks form from pre-existing rock particles - igneous, metamorphic or sedimentary. The Parent rock undergoes WEATHERING by chemical and/or physical mechanisms into smaller particles. These particles are TRANSPORTED by ice, air or water to a region of lower energy called a sedimentary basin. DEPOSITION takes place as a result of a lowering of hydraulic energy, organic biochemical activity or chemical changes (e.g., solubility). Once deposited, the sediments are LITHIFIED (turned into rock) ...read more.

Conclusion

Think of exfoliation as flaking. Freeze-thaw weathering occurs when water gets into cracks in rock and then freezes (below 0 �C) in cold weather. Water expands when it freezes and turns to ice, and the expansion can cause the rock to split and fragment. In warmer weather (above 0 �C) the ice melts (called thawing), and new cracks are exposed allowing the process to repeat itself. Erosion is a slow process of wearing away weathered rock by action of the wind, rivers and waves. Metamorphic rocks Both slate and schist (pronounced "shist") are formed from the sedimentary rocks mudstone and shale. Mudstone and shale are formed from compacted mud or clay. They are made of very small flat particles like tiny plates, which have often become layered during the sedimentary rock formation. Metamorphic Rocks - Marble. Marble is formed from limestone. At high temperatures and pressures, the shells which formed limestone break down and recrystallize. The resultant rock is marble, which is much harder than limestone, with a smooth even texture. Marble is a valuable rock for construction and decoration. It can be polished to produce an attractive mottled surface. Marble is still calcium carbonate, and reacts chemically in the same way as limestone. Granite pics - Bsalt - ...read more.

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