Lab 2 - Igneous Rocks

Definitions The Rock Cycle: A conceptual model of how the major rock groups are formed through natural processes and environmental change. Rock: An aggregate, composed primarily of mineral grains, that forms the solid Earth and most of the other solid bits of matter in the Universe. Igneous Rock: Forms as minerals crystallize during the cooling of magma, forming a mass of intergrown crystals. Texture: A rock's texture involves the size, shape, distribution, and boundary relationships among mineral grains within a rock (it does not directly involve how the surface of a particular rock 'feels', but they are related). Phaneritic: An igneous rock with a larger or coarser grain size which are 'visible' or that are large enough to been seen without magnification. Aphanitic: An igneous rock with an average grain size that is less than 0.25mm, or which small enough that they can not be clearly distinguished by eye, or without magnification Felsic: Is a term used to describe minerals such as quartz, feldspars, and muscovite micas, and the rocks which are dominated by them in their composition. They are often a lighter-toned rock with less than 35% dark minerals. Mafic: Is a term used to describe minerals which include some form of magnesium and/or iron, such as biotite mica, pyroxene, amphibole, olivine, and iron minerals. Mafic rocks are generally dark-toned and contain more than 35% mafic minerals, and rocks with more than 90% are referred to as ultramafic. The Characteristics of Igneous Rocks Igneous rocks are identified and classified according to their mineral compositions and their textures. As the name implies, igneous rocks are formed at high temperatures and are defined as those rocks formed by the solidification of molten material that originated within the earth. This molten matter, or magma, is forced upwards from depth within the crust and is highly charged with dissolved gasses. The gasses escape when the confining pressures on the magma are reduced. By the time that the magma has cooled down and solidified, almost all the gas has escaped. Magma which reaches the surface of the crust and is extruded as lava from a volcanic vent solidifies to form extrusive or volcanic rocks. Much magma never reaches the surface, but travels upwards from the region in the crust where it was generated, and is forced by pressure into the country rocks comprising the crust. The mass of magma intruded in this manner then cools down, at a rate depending upon the size of the mass and the distance from the surface of the earth and crystallizes as an igneous intrusion. The slower the rate of cooling, the coarser the crystal size of the resulting igneous rock. Such intrusive rocks later
become exposed to view by the removal of the overlying rocks by weathering processes. Minerals and Igneous Rocks Most igneous rocks consist of perhaps four or five minerals which make up the bulk of the rock and determine the major characteristics of the rock. Such minerals are called the essential minerals. These are the minerals which determine the identification of the rock. Other minerals may occur in igneous rocks as very minor constituents. Such minerals are termed the accessory minerals. They may be present as scattered crystals throughout the mass of the rock, often scattered unevenly, and sometimes concentrated in part of an intrusive or extrusive mass. Textures of Igneous Rocks The texture of a rock is the name for the general physical appearance of the rock as controlled by the sizes, shapes and arrangements of the crystals of the mineral components of the rock. Most igneous rocks are crystalline aggregates of minerals but a few consist of natural volcanic glass or a mixture of glass and crystals. The glass is a product of extremely rapid cooling of magma when it is ejected from a volcano as lava. The lava in such cases has probably been rapidly cooled because it flowed into a lake or the sea, or perhaps came into contact with an accumulation of snow on the flanks of the volcano. The size of crystals formed from a melt depends mainly on the rate of cooling of the melt. Slow steady cooling, as would be found in a pluton, encourages the growth of large crystals that can be readily seen with the unaided eye. The texture of these plutonic rocks is called phaneritic. Rapid cooling, as would be found in a volcanic extrusion, would result in fine-grained rocks with an aphanitic texture. Occasionally, a rock will form with two very different sizes of crystals: large phenocrysts in a finer grained groundmass or matrix. This texture is called porphyritic. Porphyritic rocks are classified on the basis of the grain size of their groundmass. The following are some of the terms to describe the textures of igneous rocks: Phaneritic: all crystals making up the rock can be distinguished with the naked eye. Aphanitic : the crystalline constituents are too small to be distinguished with the naked eye, through a microscopic examination shows that the rock is actually crystalline. Glassy or Vitreous : a rock composed of volcanic glass is described using these terms. Such rocks often show a characteristic conchoidal fracture. Pegmatitic : igneous rocks of extremely coarse grain-size, usually found as
dykes. Porphyritic : two different ranges of gran-size in an igneous rock, caused by a complex cooling history. Larger crystals called phenocrysts are set in a matrix or ground-mass composed of smaller-size crystals. Vesicular : spherical cavities seen in volcanic lavas, formed by the expansion of gas or steam bubbles during the solidification of lava. Equigranular : the 'opposite' of porphyritic, implying that all the crystals are about the same size. Pyroclastic : general term applied to the textures of fragmental volcanic rocks formed by the accumulation of volcanic debris. A fine to medium grained rock of this type is called tuff. Flow-banding : bands or streaks of a different colour seen in aphanitic, glassy or vesicular volcanic rocks, displaying evidence that the material was once in a molten state. The banding is the result of the streaking-out of masses of small crystals of a mineral which is of a different colour or shade from that of the matrix. Essential Minerals: Felsic (or light coloured minerals) Quartz Feldspars - orthoclase, plagioclases Muscovite mica Mafic (or dark coloured minerals) Biotite Amphiboles eg. Hornblende Pyroxenes e.g Augite Olivine Accessory Minerals: Magnetite Tourmaline Zircon Beryl Apatite Fluorite Pyrite Barite
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