Assignment Igneous Rock Questions with Answers

Introduction to Geology 1. What is magma? How does magma differ from lava? Magma is material formed by partial melting that occurs at depth within Earth. Lava is molten rock that reaches Earth’s surface. 2. Describe the process of crystallization. Crystallization occurs as the temperature of a magma drops. As the temperature cools, mobile ions pack closely together as their movement slows. When they are cooled to their melting point, the ions begin to combine into orderly crystalline arrangements with chemical bonds. 3. Compare and contrast extrusive and intrusive igneous rocks. Extrusive igneous rocks are those rocks that form from the solidification of lava at Earth’s surface. Intrusive igneous rocks are those rocks that form at depth in the Earth as a magma body cools and crystallizes. We see intrusive igneous rocks where uplift and erosion expose them at the surface. 4. Igneous rocks are composed mainly of which group of minerals? Igneous rocks are composed mainly of silicate minerals. 5. How do light-colored igneous rocks differ in composition from dark-colored igneous rocks? Light-colored igneous rocks are composed primarily of quartz and feldspar with lesser amounts (about 10 percent) of dark silicate minerals. Dark-colored igneous rocks are composed primarily of dark-colored ferromagnesian silicates and calcium-rich plagioclase feldspar. 6. List the four basic compositional groups of igneous rocks, in order from the group with the highest silica content to the group with the lowest silica content. Felsic, Intermediate, mafic, ultramafic. Felsic (granitic) rocks are composed primarily of quartz and feldspar with lesser amounts (about 10 percent) of dark silicate minerals. Intermediate (Andesitic) rocks contain at least 25 percent dark-colored silicates along with primarily plagioclase feldspar and lesser amounts other light colored silicates. Mafic (basaltic) rocks are composed of primarily of dark-colored ferromagnesian silicates and calcium-rich plagioclase feldspar. And ultramafic rocks are composed primarily of dark-colored olivine and pyroxene with lesser amounts of calcium-rich plagioclase feldspar. 7. Define texture. Texture is the size, shape, and arrangements of mineral grains in a rock. 8. How does the rate of cooling influence crystal size? What other factors influence the texture of igneous rocks? Magmas that cool over long time periods create rocks with large crystals. In magmas that cool quickly, the crystals are very small as they do not have time to grow large. Other factors that influence igneous rock texture are the amount of dissolved gases in the magma and the amount of silica present in the magma. 9. List the two criteria by which igneous rocks are classified. Classification of igneous rocks is based on mineral composition and texture. Texture is defined as the size, shape, and arrangement of mineral grains. 10. How are granite and rhyolite different? In what ways are they similar? Granite and rhyolite are similar in that they both have a felsic composition—they are composed of mineral grains of quartz and potassium feldspar with lesser amounts of biotite, amphibole, and muscovite. Granite is an intrusive igneous rock—its grains are visible with the naked eye because it formed slowly deep within the Earth. Rhyolite is extrusive —it formed on the surface of the earth and is fine-grained, often with glass fragments and voids from rapid cooling. 11. Classify each of the following rocks by their mineral composition (felsic, intermediate, or mafic): gabbro, obsidian, granite, and andesite. Gabbro is mafic; obsidian has a felsic composition although it is dark in color; granite is felsic; and andesite is intermediate. 12. Describe each of the following in terms of composition and texture: diorite, rhyolite, and basalt porphyry. Diorite is phaneritic (coarse-grained) and is composed of relatively equal amounts of plagioclase feldspar and amphibole. Rhyolite is aphanitic (fine-grained) and is composed of quartz and plagioclase feldspar. Basalt porphyry is porphyritic (coarse grains in a fine-grain matrix) and is composed of a groundmass of pyroxene and plagioclase feldspar and phenocrysts of amphibole or olivine. 13. What role do water and other volatiles play in the formation of magma? Water and other volatiles lower the melting temperature of rock, and can cause partial melting in the mantle where water-rich plates are subducted. 14. Name two plate tectonic settings in which you would expect magma to be generated. Magma is generated at divergent plate boundaries where reductions in confining pressure allow decompression melting of mantle rocks. Magma is also generated at convergent margins where water driven from subducting plates causes partial melting of the upper mantle, and where ascending magma triggers melting of surrounding crustal rocks. 15. Define Bowen’s reaction series. Bowen’s reaction series demonstrates the sequence in which minerals will crystallize from a mafic magma and how that magma changes composition (differentiates) as it gradually cools. The reaction series predicts that calcium rich plagioclase, olivine, and pyroxene will crystallize first (basalt or gabbro), followed by hornblende and plagioclase with relatively equal amounts of Na and Ca (diorite and andesite). At lower temperatures, quartz and potassium feldspar (granite and rhyolite) crystallize from magmas enriched in silica and potassium. 16. How does the crystallization and settling of the earliest formed minerals affect the composition of the remaining magma? Crystal settling occurs as early-formed minerals settle out of the liquid portion of a magma and sink towards the bottom of the magma chamber. This leaves a melt with a different mineralogy of the original parent magma (magmatic differentiation), resulting in solidification of rock with a different mineralogy. As crystallization progresses, a magma becomes more silica-rich. This is due to the early crystallization of ferromagnesian minerals early in the cooling process. 17. Compare the processes of assimilation and magma mixing. Assimilation and magma mixing both result in magmas of different composition than the original parent magma. Assimilation occurs through the incorporation of foreign material into a magma body. For example, the surrounding rock may be incorporated into the melt, creating a new melt with a different mineralogy than the parent melt. When two magmas meet, magma mixing may occur, resulting in a composition intermediate of the two parent magmas. 18. Briefly describe why partial melting results in a magma having a composition different from the rock from which it was derived. Partial melting is the incomplete melting of rock; this is caused by the differing melting temperatures of minerals as depicted in Bowen’s reaction series. Rocks that undergo partial melting form melts enriched in ions from minerals with the lowest melting temperatures (quartz and potassium feldspar). The un-melted portion is composed of minerals with higher melting temperatures. In general, the compositions of partial-melt fractions are more felsic than the solid parent and the residual solid rocks are more mafic. 19. Distinguish among batholiths, stocks, and laccoliths in terms of size and shape. Batholiths are massive structures, typically felsic to intermediate igneous rocks, that represent large, solidified magma chambers later exposed by erosion and weathering. Stocks are those massive structures less than 100 square kilometers in surface exposure. Laccoliths are massive, semi-concordant structures that arch sedimentary beds above them, while leaving beds below them flat. Laccoliths represent magmas injected forcibly between sedimentary strata.