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Igneous Petrology and the Nature of Magmas
Published in Dexter Perkins, Kevin R. Henke, Adam C. Simon, Lance D. Yarbrough, Earth Materials, 2019
Dexter Perkins, Kevin R. Henke, Adam C. Simon, Lance D. Yarbrough
Cooling rate directly affects grain size, so intrusive rocks are overall coarser grained than extrusive rocks are. Many volcanic rocks contain fine grains that we can only see with the aid of a microscope, and some volcanic rocks may be partly or entirely made of volcanic glass. Igneous rocks containing crystals large enough to be seen with the naked eye are termed phaneritic; those containing crystals too small to see are aphanitic. Overall, due to contrasting cooling rates, most phaneritic rocks have an intrusive origin and most aphanitic rocks have an extrusive origin. There are exceptions, however. For example, fine-grained intrusive rocks can be found in narrow dikes that cool quickly after intrusion, and some coarse-grained extrusive rocks can be found in the bottom layers of thick lava flows.
Igneous Rocks
Published in F.G.H. Blyth, M. H. de Freitas, A Geology for Engineers, 2017
F.G.H. Blyth, M. H. de Freitas
Plutonic rocks, which have cooled slowly under a cover perhaps several kilometres thick, are coarsely crystalline or phaneritic; their component crystals are large (2 to 5 mm or more) and can easily be distinguished by the naked eye (Fig. 5.16a and b). Rocks of medium grain often have crystals between about 1 and 2 mm, and in fine-grained rocks crystals may be considerably less than 1 mm across. When the texture is so fine that individual crystals cannot be distinguished without the aid of a microscope it is called aphanitic, or microcrystalline. For extremely fine-grained rocks, when their crystalline character is only revealed by viewing a rock slice through crossed polars, which enables the birefringent colours of each embryo crystal to be displayed, the term cryptocrystalline is used. These textures are all even-grained, or equigranular, i.e. having crystals of much the same size (Fig. 5.16a); but some rocks show the porphyritic texture (Fig. 5.16b), in which a number of larger crystals are set in a uniformly finer base (or groundmass). The large, conspicuous crystals are called porphyritic crystals or megacrysts. Porphyritic feldspars in some granites, for instance, may be 5 to 10 cm long.
Igneous rocks
Published in W.S. MacKenzie, A.E. Adams, K.H. Brodie, Rocks and Minerals in Thin Section, 2017
W.S. MacKenzie, A.E. Adams, K.H. Brodie
We shall see that there are considerable differences in igneous rocks and it is the aim of the petrologist to try to understand what causes this diversity, to determine their relationships to each other and to the geological environments in which they occur. To describe a rock it is desirable to have a system of classification and to give names to each class: we have already made a beginning by considering the circumstances under which the rocks are formed. There are special names for plutonic, hypabyssal and volcanic rocks but the names for the hypabyssal rocks are now rarely used except for the term dolerite (in America, diabase) for a dyke rock formed from a basaltic magma. Nowadays the plutonic rock name tends to be used for a rock which is coarse grained, i.e., grain size greater than 4 mm, for medium grained rocks (1–4 mm) the prefix ‘micro’ is attached to the plutonic rock name, e.g. microgranite, and for fine grained rocks (0.1–1 mm) the volcanic rock name is used. Thus the fine-grained chilled margin of a gabbro mass could be described as basaltic. The term aphanitic is used if the crystals are too small to be seen by the naked eye.
Spectral indices derived, non-parametric Decision Tree Classification approach to lithological mapping in the Lake Magadi area, Kenya
Published in International Journal of Digital Earth, 2018
Gayantha R. L. Kodikara, Tsehaie Woldai
Trachyte series rocks collected from the study area were grouped based on their texture and petrology. Basaltic rocks (e.g. P020) containing pyroxene and plagioclase phenocrysts.Alkali trachytes (e.g. P033, Figure 3) characterized by fine-grain matrix in greenish or brownish color.Rhyolite (e.g. P005, Figure 3). Volcanic rock, mainly composed of alkali feldspar and free silica with a minor amount of mafic minerals. The rocks are characterized by light brown color with an aphanitic texture.Scoriaceous basalt (e.g. P015, Figure 3). Basaltic rocks with empty cavities. It is heavier, darker and more crystalline than pumice.Vesicular basalt (e.g. P001) is a basaltic rock with small ellipsoidal cavities which are formed by bubbles of gas trapped during the solidification of rocks.