Geology for Engineers Homework 4 lgneous Rocks 1) What is the source of heat in
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Geology for Engineers Homework 4 lgneous Rocks 1) What is the source of heat in the Earth? How did the first igneous rocks on the planet form? 2) Describe the three processes that are responsible for the formation of magmas. 3) What factors control the viscosity of a melt? 4) What is the difference between a sill and a dike and how do both differ from a pluton? 5) How does grain size reflect the cooling time of a magma? 6) What is the difference between a felsic and a mafic magma? 7) What is a large igneous province (LIP)? How might the formation of LIPs have affected the Earth's System? 8) Describe the three different kinds of material that can erupt from a volcano? 9) Contrast a pyroclastic flow with a lahar 10) Describe the differences among shield volcanoes, strato-volcanoes, and cinder cones. How are these differences explained by the composition of their lavas and other factors? 11) How do geologists predict volcanic eruptionsExplanation / Answer
1) The Earth is heated by planetismal accretion, gravitational compression, differentiation into the crust, mantle, & core, and radioactive decay.
The first igneous rocks formed from freezing magma.
2) Decrease in pressure: a decrease in pressure permits melting because atoms are able to break free of solid mineral crystals. This occurs when hot mantle rock rises to shallower depths in the Earth.
Addition of volatiles: when volatiles mix with mantle rock, they help break chemical bonds, volatiles also decrease a rock's melting temperature.
Heat transfer from rising magma: surrounding crustal rock is melted.
3) Temperature: hotter magma is less viscous, volatile content: volatile atoms tend to break apart bonds and may accumulate to form bubbles, and silica content: mafic magmas are less viscous than felsic magmas.
4) A dike is a sheet of rock that formed in a fracture in a pre-existing rock body. Dikes can be either magmatic or sedimentary in origin. Magmatic dikes form when magma intrudes into a crack then crystallizes as a sheet intrusion, either cutting across layers of rock or through an unlayered mass of rock. Clastic dikes are formed when sediment fills a pre-existing crack.
In geology, a sill is a tabular sheet intrusion that has intruded between older layers of sedimentary rock, beds of volcanic lava or tuff, or even along the direction of foliation in metamorphic rock. The term sill is synonymous with concordant intrusive sheet. This means that the sill does not cut across preexisting rocks, in contrast to dikes, discordant intrusive sheets which do cut across older rocks. Sills are fed by dikes, except in unusual locations where they form in nearly vertical beds attached directly to a magma source. The rocks must be brittle and fracture to create the planes along which the magma intrudes the parent rock bodies, whether this occurs along preexisting planes between sedimentary or volcanic beds or weakened planes related to foliation in metamorphic rock. These planes or weakened areas allow the intrusion of a thin sheet-like body of magma paralleling the existing bedding planes, concordant fracture zone, or foliations.
5) Extrusive rocks cool rapidly and have small grains. Intrusive rocks cool much slower and have more time to grow larger grains.
6) Mafic is used for silicate minerals, magmas, and rocks which are relatively high in the heavier elements. The term is derived from using the MA from magnesium and the FIC from the Latin word for iron, but mafic magmas also are relatively enriched in calcium and sodium. Mafic minerals are usually dark in color and have relatively high specific gravities (greater than 3.0). Common rock-forming mafic minerals include olivine, pyroxene, amphibole, biotite mica, and the plagioclase feldspars. Mafic magmas are usually produced at spreading centers, and represent material which is newly differentiated from the upper mantle. Common mafic rocks include basalt and gabbro.
Felsic, on the other hand, is used for silicate minerals, magmas, and rocks which have a lower percentage of the heavier elements, and are correspondingly enriched in the lighter elements, such as silicon and oxygen, aluminum, and potassium. The term comes from FEL for feldspar (in this case the potassium-rich variety) and SIC, which indicates the higher percentage of silica. Felsic minerals are usually light in color and have specific gravities less than 3.0. Common felsic minerals include quartz, muscovite mica, and the orthoclase feldspars. The most common felsic rock is granite, which represents the purified end product of the earth's internal differentiation process.
7) In geology, a large igneous province (LIP) is an extremely large accumulation of igneous rocks, including plutonic rocks (intrusive) or volcanic rock formations (extrusive), arising when hot magma extrudes from inside the Earth and flows out.
Mass extinction events are sometimes portrayed in illustrations of volcanic eruptions causing widespread destruction. The basaltic lava flowing from ancient volcanoes and the portion of magma (liquid rock) emplaced underground can create geologic conditions linked with climate change and, subsequently, extinction events. This climatic effect is particularly true for LIPs, in which mainly basaltic magma up to millions of cubic kilometers can be emplaced in a geologically short time of less than a few million years. LIP is not the only factor. Also contributing to climatic/extinction effects are the abundance of LIP-produced pyroclastic material and volatile fluxes that reach the stratosphere, and in particular the role of super-eruptions.
8) Basaltic lava: low viscosity, very hot, flows quickly, creates pahoehoe and a'a, can flow long distances.
Andesitic lava: flow is too viscous to flow far, tends to break up as it flows.
Rhyolitic/Felsic lava: so viscous that it may pile up in a dome shaped mass, this can cause pressure to build up and eventually lead to explosive eruptions.
9) Pyroclastic flow is a fast moving avalanche that occurs when hot volcanic ash and debris mix with air and flow down the side of a volcano. Lahar is a thick slurry formed when volcanic ash and debris mix with water, either in rivers or from rain or melting snow and ice on the flank of a volcano.
10) Shield volcanoes are broad domes, generally form from low viscosity basaltic lava flows. Effusive eruptions only.
Cinder cones are cone-shaped piles of tephra, they are typically symmetrical and have deep craters at their summit. Pyroclastic eruptions only.
Stratovolcanoes are also known as composite volcanoes, they are large, cone shaped and consist of alternating layer soft lava, tephra, and debris. Ex. Mt. Fuji. Alternate between effusive and pyroclastic eruptions.
11) Danger assessment maps, evacuation, and diverting flows.
10)
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