Obsidian rock type1/28/2024 Mafic magmas have high melting temperatures and low viscosity. Two of the most important are melting temperature and viscosity (resistance to flow). Mafic and silicic rocks have important differences in their characteristics and properties because their composition controls these variables. ![]() For example, mafic volcanic rocks with high potassium and sodium are known as trachybasalts. Thornberry-Ehrlich (Colorado State University) following standard International Union of Geological Sciences (IUGS) nomenclature.Īdditional types of volcanic rocks are classified based on the amount of other elements such as potassium, sodium, or calcium present in the rock. The corners represent compositions very rich in the corresponding mineral and poor in the two other minerals, but not necessarily other possible component minerals. Numbers 5, 20, and 60 refer to the overall quartz component, whereas numbers 10, 35, 65, and 90 refer to the contribution of plagioclase to the overall composition. Numbers refer to the percentage of the mineral constituent (quartz, alkali feldspar, or plagioclase feldspar) in the overall composition of the rock. Orange area refers to compositions of units included in the GRI GIS data for Blue Ridge Parkway. Rocks that appear mapped in Blue Ridge Parkway are syenogranite, granite, monzogranite, granodiorite, quartz monzodiorite, quartz monzonite, monzonite, monzodiorite, diorite, and quartz gabbro. Lowest level with almost no quartz content, from alkali feldspar rich to plagioclase feldspar rich is alkali feldspar syenite, syenite, monzonite, monzodiorite, and diorite gabbro norite. Next row down is quartz alkali feldspar syenite, quartz syenite, quartz monzonite, quartz monzodiorite, quartz diorite and quartz gabbro (all 5-20% quartz). From top down (quartz-rich to quartz-poor): quartz-rich granitoids (60-100% quartz), then across the triangle from alkali feldspar granite, leucogranite and syenogranite, granite, monzogranite, granodiorite, and tonalite (all 20-60% quartz). Triangular shaped diagram shows where rock composition falls between end members (at each corner): quartz, alkali feldspar, and plagioclase feldspar. Because this diagram doesn’t include mafic minerals (olivine and pyroxene), it can’t be used to differentiate the intermediate diorite from the mafic gabbro.ĭiagram Showing the Classification of Intrusive Igneous Rocks Rock types are plotted to show the relative amounts of quartz, alkali feldspar, and plagioclase felspar they contain. The diagram below shows the classification of intrusive igneous rocks in Blue Ridge Parkway and the some of the different types of intrusive rocks beyond the three main types. Gabbro consists mostly of pyroxene, olivine, and plagioclase feldspar.Diorite consists mostly of plagioclase feldspar, amphibole, and pyroxene.Granite consists mostly of quartz and alkali-feldspar, with relatively minor plagioclase feldspar and mafic minerals (biotite, muscovite and/or amphibole ).The minerals found in the three main types of plutonic rocks are: The different types of mafic intrusive rocks are classified on their relative amounts of olivine, pyroxene, and plagioclase. Mafic and intermediate intrusive rocks also have a significant proportion of minerals that contain iron and magnesium such as olivine, pyroxene, amphibole, and biotite. Intrusive rocks are classified on the proportion of major minerals in them, particularly the relative amounts of quartz, alkali-feldspar, and plagioclase. ► Intrusive (Plutonic) Rock Classification Silicic - high in silica (and low in magnesium and iron) Mafic - high in magnesium and iron (and low in silica) The two major divisions of igneous rocks based on composition are: In turn, high silica rocks have less iron and magnesium, and more sodium and potassium. Low silica rocks have higher concentrations of iron and magnesium. ![]() Composition of igneous rocks is expressed as weight percent silica, which ranges from about 45 to 78 (wt % SiO2). In descending order, they are:Īlmost all igneous rocks are silicates meaning that silicon and oxygen are the two most common elements in them. Igneous rocks are mostly made of the most common elements found in the Earth’s crust. Texture and structure refer to the relationships between crystals, groundmass, vesicles, and other components of a rock. Igneous rocks are classified both by their chemical composition, mineral composition, texture, and structure.
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