Rock Identification INTRODUCTION A rock is a coherent, naturally occurring solid con- sisting of an aggregate of mineral

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Rock Identification Introduction A Rock Is A Coherent Naturally Occurring Solid Con Sisting Of An Aggregate Of Mineral 1 (1.26 MiB) Viewed 49 times

Rock Identification INTRODUCTION A rock is a coherent, naturally occurring solid con- sisting of an aggregate of minerals, glass, or organic material. The aggregate is held together by the inter- locking nature of the materials comprising the aggre- gate or by a cement. Rocks reflect their environment of formation so they provide an historical record of geologic events. Geologists split all rocks into three inter-related groups: igneous, sedimentary, and metamorphic. This classification system is based upon the processes involved in the formation of the rock. IGNEOUS ROCKS Igneous rocks form by the cooling and crystalliza- tion of molten rock (magma). If magma cools slowly deep within Earth over thousands of years, the atoms have time to arrange themselves into crystals, so the resulting rock will be 100% crystals that are large enough to see with the unaided eye. These rocks are referred to as intrusive (cooled inside Earth) or plutonic (ancient Greek and Roman god of the underworld) rocks. If magma is extruded out on the surface of Earth, it cools quickly, so it doesn't have time to grow all crystals. These rocks are referred to as extrusive or volcanic rocks. Volcanic rocks may have some crystals that formed while the magma was deep within Earth, but they will always have a fine-grained matrix in which no crystals are distinguishable with the unaided eye. The classification of igneous rocks is based upon the chemical composition of the magma and the texture (the size, shape, and arrangement of the grains). The texture is phaneritic if there are 100% visible crystals, aphanitic if there are no visible crystals, Chapter 4 porphyritic if there are large crystals in a finer grained matrix, and vitreous if the texture is like glass. FIGURE 4.1 Phaneritic is the texture of 100% crys- tals, all visible with the unaided eye. Phaneritic texture is the common texture of plutonic rocks. FIGURE 4.2 Aphanitic is the texture where no crystals are visible with the unaided eye. Aphanitic texture is the common texture of volcanic rocks. Magma compositions are commonly grouped into four subdivisions: silicic, intermediate, mafic, and ultramafic. Silicic magmas (also know as felsic or 29
FIGURE 4.3 Phorphyritic is a texture where there are larger crystals, called phenocrysts, in a finer-grained groundmass (matrix). The groundmass will be smaller vis- ible crystals if the rock is plutonic and aphanitic or glassy if the rock is volcanic. FIGURE 4.4 Vitreous is the texture where the rock is glassy in appearance. If the glass is dense the rock is called obsidian and if the glass is frothy the rock is called pumice. Pumice will float in water. granitic magma) have a SiO₂ content of about 70% with Na, K, and Al being common cations bonding 30 Chapter 4 Rock Identification with the silicon-oxygen tetrahedrons. Na, K, and Al tend to make white, light gray, colorless, or pink minerals so rocks originating from silicic magma will tend to be light colored. The most common minerals are feldspars and quartz. The silicic plutonic rock, granite, generally has some dark min- erals, usually biotite or hornblende, but the percent of dark-colored minerals is much less than the percent of light-colored minerals. Magmas of intermediate composition have a SiO₂ content of about 60% and have a mix of the minerals common in both silicic and mafic magmas. Plutonic rocks of intermediate composition tend to contain about 50% light color minerals and 50% dark colored minerals. Volcanic rocks of intermediate composition tend to have a porphyritic texture with some visible crystals (commonly plagioclase) in an aphanitic dark matrix. FIGURE 4.5 Basalt with numerous vesicles is called scoria. Mafic magmas have a SiO₂ content of about 50% with Mg and Fe being common cations bonding with the silicon-oxygen tetrahedrons. Mg and Fe tend to make black, dark gray, or green minerals so rocks originating from mafic magma will tend to be dark colored. The most common minerals will be olivine, pyroxene, amphibole, and Ca-rich plagioclase. Quartz is never present. The mafic volcanic rock, basalt, tends to be very dark gray to black colored and have few, if any, visible crystals. If the basalt has numerous vesicles (air bubbles), it is called scoria. Ultramafic magmas have a SiO₂ content of about 35%. The ultramafic plutonic rock, peridotite, consists mostly of olivine with some pyroxene so the rock is generally green. Komatiite is a rare ultramafic volcanic rock. Komatiite lava existed prior to about 2.5 Ga
FIGURE 4.6 Spinifex is a texture of long thin blades of olivine and shorter blades of pyroxene in an aphanitic matrix. when the Earth was much hotter and still capable of erupting lava at temperatures in excess of 1600°C. The diagnostic feature of komatiites is spinifex texture, characterized by long thin blades of olivine and shorter blades of pyroxene in an aphanitic matrix. SEDIMENTARY ROCKS Sedimentary rocks are classified by the processes that created them. Sedimentary rocks are subdivided into four major groups: clastic, biochemical, chemi- cal, and organic sedimentary rocks. Texture (grain size) Coarse (over 2 mm) Medium (1/16 to 2 mm) Fine (1/16 to 1/256 mm) Detrital Sedimentary Rocks Very fine (less than 1/256 mm) FIGURE 4.7 mentary rocks. Sediment Name Gravel (Rounded fragments) Gravel (Angular fragments) Sand (If abundant feldspar is present the rock is called Arkose) Mud Mud Rock Name Conglomerate Breccia Sandstone Siltstone Shale Classification of clastic (detrital) sedi- Clastic (detrital) sedimentary rocks form from the fragments of other pre-existing rocks. The pre-existing rocks were exposed at the Earth's surface where they were weathered, transported, deposited, and then buried beneath other sediment, eventually lithifying into solid rock. Clastic sedimentary rocks are classified on the grain size of the component materials. Grains larger than 2 mm are called gravel and the rock is called a conglomerate if the clasts (rock fragments) are rounded and breccia if the clasts are angular. Grains between 2 mm and 1/16 mm are called sand, and the rock is called a sandstone. Grains between 1/16 mm and 1/256 mm are called silt, and the rock is called a siltstone. Rocks with grains smaller than 1/256 mm are called shale, if the beds have fine laminations and claystone, if laminations are not present. The individual grains may or may not be clay minerals but they are too small to see without a microscope. Chemical and biochemical sedimentary rocks are classified on the minerals present, irrespective of grain size. If the dominant mineral is calcite the rock is called limestone. However, if the limestone consists of poorly cemented shell fragments it is called coquina; and chalk, if the limestone consists Chemical, Biochemical and Organic Sedimentary Rocks Composition Calcite, CaCO, Quartz, SiO₂ Gypsum CaSO, 2H₂O Halite, NaCl Altered plant fragments Texture (grain size) Fine to coarse crystalline Visible shells and shell fragments loosely cemented Various size shells and shell fragments cemented with calcite cement Microscopic shells and clay Very fine crystalline Fine to coarse crystalline Fine to coarse crystalline Fine-grained organic matter Rock Name FIGURE 4.8 and organic sedimentary rocks. Crystalline Limestone Travertine Coquina Fossiliferous Limestone Chalk Chert BIOCHIMICO- Rock Gypsum Rock Salt LIEOSTONO Bituminous Coal Classification of chemical, biochemical, Chapter 4 Rock Identification 31
Name the silicic plutonic rock. Name the clastic sedimentary rock with a grain size of 1/16-2 millimeters. What is metamorphosed limestone called?