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Chapter 6 – Igneous rock Magma and igneous rocks. • Igneous rocks are the most abundant rocks on earth and make up all of the mantle, the oceanic crust, and much of the continental crust. • Melt that solidifies beneath the earth’s crust is called magma. • Melt that erupts at the surface is called lava. • Intrusive igneous rock forms underground • Extrusive igneous rock forms above-ground. Extrusive igneous rock can form from lava and pyroclastic debris, particles from lava that exploded into the atmosphere. • Intrusive igneous rocks (magma that hardens belowground) include such common, coarse-grained rocks as granite, diorite, and gabbro. • When large blobs of magma harden underground, it can create different structures such as plutons (i.e. Enchanted Rock) and tabular intrusions. • Extrusive igneous rocks spill out on the surface as lava, which hardens to produce rocks such as rhyolite, andesite, basalt, obsidian, pumice, and different types of pyroclastic rocks. • Extrusive lava produce volcanoes and lava flows. • The Earth’s interior is not composed mostly of molten rock. Molten rock only occurs in special areas within the lithosphere and crust under special conditions. • Decreased pressure • Addition of volatiles • Heat transfer Decompression melting results when hot mantle rock rises to shallower depths in the earth. Normally, even though it is very hot at depths in the upper mantle, the tremendous pressure of the overlying rock keeps the atoms together and prevents melting. If that pressure is released, melting occurs. The geothermal gradient is the temperature change with depth. The rate (i.e. slope of line) of change decreases with depth. Decompression and heat transfer melting Heat transfer melting caused by the hot basaltic magma melting the surrounding rock. An example of melting as a result of the addition of volatiles. For example, when water and/or carbon dioxide percolate into a solid hot rock. The addition of volatile substances trigger melting because it decreases the melting temperature. • The major types of magma are: • silicic magmas (> 70% silica with little Mg or Fe); • intermediate magmas (55% silica); • mafic magmas (< 50% silica and rich in Mg and Fe; • ultramafic magmas (< 40% silica and lots of Mg and Fe). Mafic magma is hotter than silicic magma. • Magmas and rocks are frequently defined by the amount of silica they contain. Remember though, all magma’s usually contain some silica. • Magma’s are different because of: • source rock composition; • partial melting of the source rock; • contamination; and • fractional crystallization • When rocks melt, only some of the minerals that make up the rock melt because each mineral has a different melting temperature. Minerals with lower melting points melt first, minerals with higher melting points remain solid. • Because of differential melting, magma’s do not have the same composition as the source rock. • Silicic minerals tend to have a lower melting point that mafic rocks so magma’s tend to be enriched in silica compared to their source rock. • Contamination occurs when magma beneath the crust starts to melt the surrounding rock. • Similarly, the composition of the magma can change when blocks of unmelted crust fall into the magma and dissolve, a process called called assimilation. • Fractional crystallization. When magma starts to cool and, just as different minerals have different melting points, different minerals have different “freezing” points. For example, mafic minerals solidify at a higher temperature than silicic minerals, leaving the magma progressively more silicic as it cools. • Bowen’s Reaction Series was developed to show the different crystallization rates of different common minerals. •Magma is less dense than surrounding rock and heated rock expands as it melts so magma tends to move upward due to buoyancy forces. •The pressure of surrounding rocks squeeze magma in an upward direction. •Magma containing volatiles can move upward faster because the presence of gas bubbles decrease its density. •Vesicles form when magma is extruded and freezes before the gas bubbles can escape. • Viscosity is the resistance to flow. Low viscosity magmas flow more easily than high viscosity magmas. • Viscous lava is silica-rich and tends to form a mound-like dome on top of the volcano • Mafic lava is thinner, can fountain upward, and can travel far from the vent. How fast magma travels depends on the viscosity. • Viscosity is also affected by presence or absence of volatiles and also temperature. The presence of volatiles make lava less viscous. Hot lavas are less viscous than cooler lavas. Summary of the two major kinds of magma: (1) A chemical composition high in silica, low in iron and magnesium (i.e. silicic); light-colored, light-weight rocks, lowtemperature, high-viscosity melts (2) A chemical composition high in iron and magnesium, low in silica (mafic), dark-colored, heavy rocks, high-temperature, lowviscosity melts. • When extruded lava freezes, it forms a sheet of igneous rock known as a lava flow. Volatile-rich lavas tend to erupt explosively. • Volcanic ash can form an ash flow, ash cloud or ash fall. Volcanic ash consists of tiny, glass shards that form during an explosive eruption. A layer of lava buried by subsequent layers of ash fall that has then eroded away. The lava flow has broken into fractures called columnar joints. • When magma intrudes belowground into country or wall rock, the boundary between the two is called an intrusive boundary. • Stoping is the process of magma breaking off blocks of wall rock • Sometimes these broken off blocks don’t melt. Then they are called xenoliths. • Picture at right shows a xenolith in granite. • However, most intruded magma forms tabular intrusions called dikes and sills. • Dikes cut across existing rock layers • Sills are horizontal. A basalt dike within granite. The dike extends, wall-like into the granite rock. An example of a basalt sill in sandstone country or wall rock. Some sills dome upward and are called laccoliths. Plutons are blob-shaped intrusions and, if large and numerous, can create a batholiths. At time 2, the magma chamber itself freezes into a pluton. At time 3, most of the volanic rocks have eroded exposing the pluton or, if there are large and numerous plutons, a batholith. Mt. Rushmore is a granite pluton. • Magma cools at different rates depending upon depth, shape and size of a magma body and the presence or absence of circulating groundwater. The large interlocking crystals of granite. A thin section of fine-grained, glassy volcanic rock • Coarse-grained igneous rocks are called phaneritic. • Fine-grained igneous rocks are aphanitic. Igneous rocks can be either coarse or fine-grained, dependent on cooling rate. Horizontal lines represent proportions of different minerals in a particular rock. Low density rocks Basalt High density rocks Gabbro • Crystalline igneous rocks are classified based on silica content and texture and composition. Mafic rocks tend to be black or dark gray, rocks with an intermediate silica content tend towards lighter gray or greenish gray, and rocks with the greatest amount of silica tend to be light tan to pink or maroon. • Cooling rate affects grain size and therefore rock types: silicic magma that cools quickly at the surface (lava) can become rhyolite or, if cooled slowly underground, granite. Mafic lavas can form basalt if cooled quickly but gabbro if a mafic magma is cooled slowly underground. • Glassy (non-crystalline) igneous rocks can include obsidian, pumice (many open pores), scoria (air-filled pores < 50% of the rock) and tuff (a pyroclastic rock formed from composed of ash and/or pieces of lava and pumice). • Igneous activity occurs along volcanic arcs, in isolated hot spots, within continental rifts, and along mid-ocean ridges. Molten rock or melt develops when a plume of hot rock rises to the base of the lithosphere; in the asthenosphere above subducting plates; in the asthenosphere above a mid-ocean ridge; along a continental rift. Ex: Hawaii Ex: Japan Ex: Iceland Ex: East African Rift zone The world-wide distribution of volcanos. Flood basalts occur when huge amounts of basalt (mafic lava) erupt at the surface and spread out into sheets covering tens to hundreds of kilometers across the landscape. End of Chapter 6