Magmas, Igneous Rocks, and Intrusive Activity Magmas, Igneous Rocks, and Intrusive Activity 2

Pyroclastics Bombs Cinder Pumice Ash • Volcanic bomb – a steamlined pyroclastic fragment ejected a the volcano while still semi-molten • Cinder – ejected lava that forms pea- to walnutsized fragments 14 Vesicular Texture • Vesicular texture describes a rock with numerous vesicles • vesicles are small holes resulting from the magma hardening around bubbles of escaping gas • Vesicles only form in extrusive volcanic rocks because the rapid pressure decrease upon extrusion allows the volatiles to escape • Vesicles result in an extremely lightweight (low density) rocks that in some cases can float in water. • Examples of extrusive volcanic rocks with vesicular texture – vesicular basalt, pumice, scoria 15 Igneous rock compositions • Igneous rocks are composed of silicate minerals • For describing ingenious rocks we separate the silicate minerals into two groups. – Dark or ferromagnesian silicate minerals • Have the dominant cations: Fe-Mg rich • Examples: olivine, pyroxene, hornblende, biotite mica • These are referred to as mafic minerals – Light or nonferromagnesian silicate minerals • • • • Have the dominant cations: Na-Ca-K rich (compared to Fe-mg) Examples: quartz, muscovite mica, and feldspars These are referred to as felsic minerals (note: light in this case means light in color, not in weight) 16 Mafic/felsic minerals and rocks • Mafic mineral is a dark colored silicate mineral where Fe and Mg dominate – Mafic rock (or basaltic rock) is composed of predominantly mafic minerals (although there will be some felsic minerals in it) • Felsic mineral is a light colored silicate mineral where Na, K and Ca dominate – Felsic rock (or granitic rock) is composed of predominantly felsic minerals (although there will be some mafic minerals in it. 17 Igneous Rock types (classified on whether intrusive or extrusive and on composition) Composition Intrusive (Plutonic) Extrusive (Volcanic) Granitic (felsic; rhyolitic) Andesitic (intermediate) Basaltic (mafic) Ultramafic Granite Diorite Gabbro Peridotite Rhyolite Andesite Basalt 18 Granite • Granitic/felsic composition • Minerals – Quartz, feldspar, hornblende (or biotite) • Predominantly light-colored nonferromagnesian silicate minerals – felsic stands for feldspar and silica rich – High silica (SiO2) content • Major constituent of the continental crust 19 Basalt Basaltic/mafic composition • Minerals – Predominantly dark ferromagnesian silicates minerals – The termed mafic is for magnesium and ferrum, for iron – Higher density than granitic rocks • Comprise the ocean floor and many volcanic islands 20 21 22 23 Igneous compositions • Other compositional groups • Intermediate (or andesitic) composition – Contain 25% or more dark silicate minerals – Associated with explosive volcanic activity • Ultramafic composition – Rare composition that is high in magnesium and iron – Composed entirely of ferromagnesian silicates – Peridotite of the mantle is ultramafic 24 (Hornblende) 25