GEOL 3015__LAB MANUAL__2016W_Igneous rocks.pdf

GEOL 3015__LAB MANUAL__2016W_Igneous rocks.pdf - GEOL 3015...

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G EOL 3015 : Introductory Geology For Engineers Igneous Rocks E XERCISE 2: Igneous Rocks INTRODUCTION Characteristics of Igneous Rocks Igneous rocks are those formed by cooling molten rock material called magma . The size of crystals which form during cooling depends to some extent on the rate at which cooling took place. However, all magmas are not alike in their composition, and if we are to make up a systematic classification of igneous rocks, we must take into account both the chemical composition of the magma and the conditions under which it cooled. The chemical composition will be reflected, to a very large extent, in the minerals that are formed. By identifying minerals, and measuring their relative proportions, we can estimate the chemical composition. As we can observe igneous rocks in place in the field, we know a good deal about their environments, and once we know what environments there are, we can learn what effects they produce on the textures of the rocks themselves. From their mode of origin we divide igneous rocks into two types: 1. EXTRUSIVE = poured out on the surface or VOLCANIC = derived from volcanoes 2. INTRUSIVE or PLUTONIC = injected or “intruded” among the solid rocks below the surface The intrusive rock are further subdivided into: HYPABYSSAL – small intrusions nearer the surface PLUTONIC (or Abyssal) – large intrusions at great depth. The volcanic rocks include all the volcanic lavas and ejecta (ash, etc.)
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G EOL 3015 : Introductory Geology For Engineers Igneous Rocks The intrusive rocks small intrusions called dykes, sills, sheets, laccoliths, and lopoliths, and large intrusions such as stocks and batholiths. The diagram below illustrates some of the principal occurrences igneous rocks. Consult your textbook for further information about them. The threefold division is reflected in the rocks themselves by the development of various features which we term “structures”, meaning large scale features easily seen in the field, but not always shown by small hand specimens (whereas “textures” are the mutual relations between the component grains). Some of the standard terms will be elaborated in the section dealing with the volcanic rocks, but they are discussed briefly here. The textures of igneous rocks take into account the following features: (a) The degree of crystallization (b) The sizes of the crystals (c) The shapes of the crystals (d) The mutual relations of the crystals (or crystals and glass).
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G EOL 3015 : Introductory Geology For Engineers Igneous Rocks (A) The degree of crystallization HOLOCRYSTALLINE – composed entirely of crystalline material HYPOCRYSTALLINE – composed partly of crystalline material and partly glass HOLOHYALINE or VITREOUS – composed entirely of glass In hand samples it is normally only possible to differentiate between holocrystalline and holohyaline.
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  • Fall '17
  • Dr. Conly
  • Petrology, Igneous rocks, Basalt

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