Fundamentals of geology I. (lithosphere) 1 1. The formation of the Earth 1
Fig. 1.7. The places of igneous rock formation (
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- Fig. 1.8. Cristallization model of Bowen
Fig. 1.7. The places of igneous rock formation ( geo-team-tc.blogspot.com) A schematic description of the order in which minerals form during the cooling and solidification of magma and of the way the newly formed minerals react with the remaining magma to form yet another series of minerals. The series is named after American geologist Norman L. Bowen (1887-1956), who first described the scheme. Bowen determined that specific minerals form at specific temperatures as a magma cools (Fig.1.8.). 
Fig. 1.8. Cristallization model of Bowen Igneous rocks are classified according to mode of occurrence, texture, mineralogy, chemical composition, and the geometry of the igneous body. The classification of the igneous rocks can be taken on the base of their silica-content. Ultramafic rocks have a lowest content of silicon (SiO2 < 45%), with more than 90% of mafic minerals (e.g., dunite) (Fig.1.9.). Mafic rocks content less silicon relative to felsic rocks (SiO2 < 50%), with predominance of mafic minerals pyroxenes, olivines and calcic plagioclase; these rocks (example, basalt, gabbro) are usually dark coloured, and have a higher density than felsic rocks (Fig. 1.10.). Intermediate rock’s silicon content is between 50-70%, with predominantly feldspars and plagioclases. Quartz doesn’t occur in these rocks. They are usually dark coloured: grey, reddish or brownish (example andesite, diorite) (Fig. 1.11.). Felsic rocks have a highest content of silicon(SiO2> 70%), with predominance of quartz, alkali feldspars and plagioclases: the felsic minerals; these rocks (e.g., granite, rhyolite) are usually light coloured, and have low density (Fig. 1.12.). 
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