Understanding Formation of Igneous Rocks: Textures & Compositions, Assignments of Geology

Download Understanding Formation of Igneous Rocks: Textures & Compositions and more Assignments Geology in PDF only on Docsity! Optional GEOL 103 Writing Assignment: Igneous Rocks and Processes 1) Assuming the original magma compositions are the same, explain how an intrusive and an extrusive rock from a magma would differ. Use the rate of cooling in your explanation. Explain how the terms phaneritic, aphanitic, and porphyritic, and how a rock with each of these compositions would form. Intrusive rocks would have larger crystals (large enough to see w/ naked eye = phaneritic texture) because the crystals have time to grow. Extrusive rocks would have no crystals or small crystals (too small to see w/ naked eye = aphanitic texture) because the crystals lack time to grow. A porphyritic texture means a rock has two distinct crystal sizes. With a porphyritic/phaneritic texture, the smaller crystals would still be visible, and such rock would have cooled slowly for a while, then more rapidly, but all still underground. With a porphyritic/aphanitic texture, the smaller crystals would not be visible, and such rock would have cooled slowly for a while, then the lava would probably have been extruded and cooled quickly. 2) Igneous rocks are mainly classified by mineralogical composition and texture. 3) Explain how vesicles form in igneous rocks. Do they form in some intrusive and some extrusive rocks? Why? Vesicles are semi-round holes that form when gases escape from lavas as pressure is released, so they form in extrusive rocks. 4) Why do some igneous rocks have glassy textures? Glassy textures usually form when pyroclastic material is ejected from a volcano into the air. The extrusive rocks and particles such as pumice, ash, and obsidian are formed during very rapid cooling, allowing no crystal growth. 5) What is the main difference between the rock pairs: basalt/gabbro, andesite/diorite, rhyolite/granite? These rock pairs have the same range of chemical compositions, but cooled fast/slow and thus the extrusive/intrusive equivalents of each other. 6) Explain the meaning of the terms felsic, intermediate and mafic. How do these terms relate to the color of igneous rocks (be careful, color is completely diagnostic for these terms)? Instead of thinking in terms of rock color, how should we best think of these terms? These terms really relate to chemical composition, not color, although dark minerals tend to be more mafic – there are exceptions; e.g., the olivine in a peridotite is an ultramafic mineral although it can be rather light green. Also, K-spar can be fairly deep pink/red, but it is felsic. Felsic = high Si, Na, K. Mafic = high Fe, Mg, Ca. Intermediate rocks have compositions in between felsic and mafic. 7) Explain the diagram below in your own words. Relate it to Bowen’s Reaction Series. Note that this diagram is similar to Fig 4.4 in your text. Diagram shows the for a given rock x-axis: relative silica (SiO2 – not the mineral quartz) concentration, and therefore whether rocks are felsic to ultramafic y-axis: percent minerals found in intrusive rocks going into the page: grain size, which is a function of cooling rate and determines whether we interpret a rock as intrusive or extrusive E.g., we would expect an diorite to be an intermediate rock with little or no quartz or pyroxene, no K-spar or olivine, lots of plag containing both Na and Ca, some amphibole and lesser amounts of muscovite and biotite. An andesite would be the extrusive equivalent, but some are all of the minerals might not be visible to the naked eye. 8. Explain Table 4.3 in your own words. Table 5.3 list factors that affect melting. T & P increase with depth. Increasing T favors melting; Increasing P favors the solid state, so these factors compete. Most of the lithosphere (not near igneous intrusions) is not hot enough to melt. The asthenosphere (immediately below the lithosphere) partially melts because it is warm enough and the pressure is not so great to keep it completely solid. The mesosphere (the part of mantle below the asthenosphere) has a similar composition to the asthenosphere, but the pressure is great enough to prevent partial melting in most of it. Rocks that contain water (which can be tied up in minerals such as micas and amphiboles [hornblende is an example]) melt at lower T than rocks without water. Felsic composition minerals and rocks melt at lower T than more mafic minerals and rocks.