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Chapter 5

GLG110 Textbook Notes Exploring Geology 2nd Ed Chapter 5

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Earth Sciences
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[GLG Text Notes Chapter 5] 1 Chapter 5: Rocks and Rock-Forming Processes • Sandstone: a rock composed of rock grains • Important knowledge of rock-forming processes (at beach): o For some rocks, the mineral constituents are pieces of other rocks with the addition of some biologically produced mineral matter o An abrasion process—the grinding together of pieces together during a transport by moving water or wind—reshapes the fragments o Fragments of older rock consolidate into new rock by the addition of mineral cement • Important knowledge of rock-forming processes (at a spring): o Some rocks form by precipitation of minerals from water that cements loose sediment together, or by intergrowth of precipitated minerals • Important knowledge of rock-forming processes (at a volcano): o A major difference is the shape of the mineral grains in the sandstone compared to those in the volcanic rock; the mineral grains in the volcanic rock have sharp edges and flat surfaces representing the crystal faces whereas those in the sandstone are rounded and do not preserve crystal outlines; minerals in the volcanic rock crystallized in the place from the original molten material and were not tumbled and abraded during transport by moving wind and water o Intergrowth of newly formed minerals explains the solid nature of the rock formed at both the spring and the volcano; these rock types different, however, in that the rock at the spring formed by precipitation of minerals out of water that remained liquid; the volcanic rock formed by the complete crystallization of a molten-rock liquid • Most natural rocks are aggregates of mineral grains • Many rocks originate from observable processes that take place at Earth’s surface • The presence of rocks that are not related to observable surface processes suggests that they relate to processes active within the Earth • Descriptive classifications group items of similar appearance for their origins; genetic classifications group features or phenomena by noting similarities in the processes that cause or create them [GLG Text Notes Chapter 5] 2 • Sedimentary rocks: rocks formed from the products of the breakdown of preexisting rock; it contains the weathering product of other or rocks; o Clastic rocks: formed from the particles of preexisting rocks liberated by physical weathering or formed during chemical weathering; “broken” o Chemical and biogenic rocks: formed of mineral precipitates originating by dissolution of rock; includes biologic materials, mostly formed of mineral precipitates o Note: quartz is the only silicate mineral to form a common mineral precipitate from water o Lithification: compaction under the weight of accumulating sediment precipitation of cementing minerals between grains convert loose sediment into consolidated sedimentary rocks; “making of rock” o Bedding: sedimentary are most readily recognized from a distance by their distinctive layering; e.g. Grand Canyon; bedding originates from the sequential deposition of different sedimentary materials, one above the other, at Earth’s surface • Igneous rocks: rocks formed by the solidification of molten rock material (magma); “fiery” o Magma: molten material originating from melting rock from within the Earth o Volcanic rocks solidify at the surface whereas plutonic rocks solidify beneath the surface; in some cases, their synonyms extrusive (or volcanic) and intrusive (or plutonic) are applied and refer to, respectively, magma extruded onto the surface volcanoes and intruded into preexisting rocks below the surface o Volcanic rocks: formed by the solidification of magma (lava) at Earth’s surface o Plutonic rocks: formed by solidification of magma below Earth’s surface o Igneous rocks, therefore, consist mostly of silicate minerals, a composition that also helps to distinguish them from chemical sedimentary rocks, in most cases o Geologists use the term massive to describe rocks that lack layering [GLG Text Not
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