Igneous Rocks Melting Within the Earth Magma - molten silicate-rich liquid that may contain solids and gases Geothermal Gradient - rate of increase in temperature with increasing depth Near the surface temperature rises at a rate of about 30 degrees Centigrade per kilometer - cannot continue to rise at this rate with increasing depth Pressure increases at a rate of about 333 bars per kilometer - one bar equals one atmosphere - about 14.7 pounds per square inch. Lithostatic pressure - due to mass of overlying material - assumed to be equal in all directions Partial Melting Rocks are mixtures of two or more minerals The melting behavior of mixtures differs from that of single substances Mixtures have a melting range - that is, a rock does not have a unique melting point but a melting range. Melting Behavior of H2O Melting of a Mixture Partial Melting 1000 C Liquid (100%) 900 C Solid + Liquid (75%) 800 C Solid + Liquid (25%) 700 C Solid + Liquid ( 5%) 650 C Solid (100%) 600 C Solid (100%) Magmas and Lavas Magmas migrate upwards driven by the density contrast between solids and liquids Magmas INTRUDE into older rocks as they migrate Magmas that cool at depth form INTRUSIVE rocks A Magma is called LAVA when it reaches the surface - forms EXTRUSIVE rocks Cooling Rate Heat flows from bodies at high temperature into cooler bodies - until the temperature is identical in both. The temperature contrast (DT) strongly influences the rate of heat flow. Lavas are in contact with the atmosphere and DT is large....cooling is rapid and solids tend to be small -APHANATIC TEXTURE For Magmas DT may be much smaller - function of the temperature of the COUNTRY ROCK. With a low cooling rate crystals may grow quite large - PHANERITIC TEXTURE. Mixed sizes - POPRHYRITIC TEXTURE .. slow cooling to develop some large crystals (the PHENOCRYSTS). Cooling rate increases (extrusion to the surface?) and remaining liquid cools more quickly. Mineralogy - Variation in Temperature In general, minerals with abundant Fe, Mg and Ca crystallize at higher temperatures. As the temperature declines, minerals with a higher percentage of Si and O begin to crystallize. The relationship between temperature and mineral assemblage is depicted in BOWEN¹s REACTION SERIES DISCONTINUOUS - Olivine begins to crystallize at high temperatures At some lower temperature olivine reacts with the liquid and a pyroxene forms CONTINUOUS - there is a continuous variation in composition within the Plagioclase Solid Solution Series At much lower temperatures Alkali Feldspar and Quartz crystallize from the melt VISCOSITY - the resistance a liquid offers to flowing : high viscosity - very sticky and liquid flows with difficulty Mineral Assemblages Olivine & Pyroxene : high temperatures - mantle/asthenosphere Olivine, Pyroxene and Ca-rich Plagioclase : high temperatures - oceanic crust - spreading centers Amphibole and Na-Rich Plagioclase : intermediate temperatures - subduction zones Mineral Assemblages Quartz and Alkali Feldspar : low temperatures - continental crust - continent/continent collision zones Consider a high temperature liquid that will crystallize Olivine and Ca-Plagioclase. Rapid cooling produces Aphanatic texture Slow cooling produces Phaneritic texture Classification of Igneous Rocks TEXTURE - size, shape and arrangement of grains - interpreted as measuring Rate Of Cooling MINERALOGY - minerals present - interpreted as measuring the Temperature of the liquid : the type of Feldspar is a good index for estimating temperature Names of Igneous Rocks Texture Alkali Na-rich Ca-rich Phaneritic Granite Diorite Gabbro Aphanatic Rhyolite Andesite Basalt CONTINENTAL CRUST - GRANITES OCEANIC CRUST - BASALTS SUBDUCTION - ANDESITE Evolution of Magmas Migration of melts upwards Crystal settling or floating Mixing of two magmas Assimilation of country rock(s) SHAPES OF INTRUSIVE BODIES Tabular - flows along cracks - low viscosity Recall that the higher temperature melts tend to have lower viscosities Massive - melt does not easily flow .. produces ³blobs² - stocks or batholiths Recall that lower temperature melts tend to have high viscosities The presence of water will reduce viscosity SHAPES OF EXTRUSIVE FlOWS AA - blocky flow - high viscosity Pahoehoe - smooth, fluid - low viscosity The higher the viscosity, the more likely that the flow will be explosive - Strato Volcanoes The lower the viscosity, the more likely that the flow will be ³gentle² - Shield Volcanoes Volcanic Deposits Flood Basalts - low viscosity Pahoehoe (smooth, ropy) Aa (blocky) Pillow Lavas Pyroclastics - ³fire broken² Eruptive Sytles and Landforms Central Euptions Shield Volcanoes Volcanic Domes Cinder Cones Composite Vlocanoes Craters Calderas