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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.
Volcanoes and Their Products
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
As discussed in Chapter 5, after eruption, lava will cool quickly, and there is little time for mineral crystals to grow. Consequently, crystals, if present, may not be visible with the naked eye, or even with a microscope, and most volcanic rocks are aphanitic. Figure 7.40 shows a typical felsic volcanic rock, rhyolite. The minerals it contains (K-feldspar and quartz) are so fine grained that they cannot be seen in this photo. The reddish color of the rock is because this specimen contains K-feldspar, a mineral that often has a salmon or reddish color.
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Published in Eric W. Harmsen, Megh R. Goyal, Flood Assessment, 2017
Volcanic rocks are usually fine-grained or aphanitic to glassy in texture and are named according to both their chemical composition and texture. Basalt is a very common volcanic rock with low silica content. For Basalt rock we assumed a total 0.17 (reported range of 0.03 to 0.35); effective porosity 0.1 and saturated hydraulic conductivity 570 cm/h for fractured basalt (10 to 105 m/year).
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.