Geol. 402 Igneous and Metamorphic Petrology Lecture 1 PDF

GY Course No. Geol. 402 (IGNEOUS AND METAMORPHIC PETROLOGY) Dr. Adnan Khan Assistant Professor Department of Geology University of Karachi Recommended Book Petrology: Igneous, Sedimentary, and Metamorphic By Harvey Blatt and Robe...

GY Course No. Geol. 402 (IGNEOUS AND METAMORPHIC PETROLOGY) Dr. Adnan Khan Assistant Professor Department of Geology University of Karachi Recommended Book Petrology: Igneous, Sedimentary, and Metamorphic By Harvey Blatt and Robert J. Tracy Selected chapters are available on Physics Photoshop  PETROLOGY Genetic Distribution METAMORP IGNEOUS BRANCHES HIC SEDIMENTA RY  Petrography: Rocks’ description and classification Petro-genesis: Conditions under which they were formed.  IGNEOUS Branch PETROLOGY of Geology that deals with description of igneous rocks and the processes involved in their formation. IGNEOUS ROCK Derived from the Latin word ignis meaning fire or magmatic rock. Igneous rock is formed by the cooling and solidification of magma or lava.  Magma Term first introduced into geologic literature in 1825 by Scope, who referred to it as: “compound liquid” consisting of solid particles suspended in a liquid, like mud.  NATURE OF MAGMA Natural fluid occurring in the earth. Generally very hot and found as molten substance at high temperature. It is called magma until it comes up on the surface (LAVA).  COMPOSITION Chemically complex and contains the molecular building blocks for minerals. It largely consists of solution of silicates with some oxides/sulphides of metals. It always contains water and other gases held in solution by pressure but heat is the main factor in its liquidity.  GENERATION Magma is produced when temperature at the depth in the earth exceeds the melting temperature of rocks. It may be produced by three distinguished processes or combination of them.  PROCESSES Radiation: involves emission of EM energy from the surface of hot body into the transparent cooler surroundings. Not important in cool rocks, but increasingly important at T’s >1200°C Conduction: transfer of kinetic energy by atomic vibration. Cannot occur in a vacuum. For a given volume, heat is conducted away faster if the enclosing surface area is larger. Convection: movement of material having contrasting T’s from one place to another. T differences give rise to density differences. In a gravitational field, lower density (generally colder) materials sink.  SOURCES OF HEAT FOR MELTING ROCKS Solar Radiation: 50,000 times greater than all other energy sources; primarily affects the atmosphere and oceans, but can cause changes in the solid earth through momentum transfer from the outer fluid envelope to the interior. Core Movement: Exponential movement of heat that is caused by the movement of mental around the core of Earth. Down Going Slab Crustal Melting  SOURCES OF HEAT FOR MELTING ROCKS Radioactive Decay: 238U, 235U, 232Th, 40K, and 87 Rb all have t1/2 that >109 years and thus continue to produce significant heat in the interior. This may equal 50 to 100% of the total heat production for the Earth. Extinct short-lived radioactive elements such as 26Al were important during the very early Earth.  Core Formation: Initial heating from short-lived radioisotopes and accretionary heat caused widespread interior melting (Magma Ocean). Additional heat was released when Fe sank toward the center and formed the core. FACTORS GENERATING MAGMA INCREASE OF HEAT: Heat increase in the rock causes melting temperature to melt, as a result of which, rocks start to melt and thus magma is formed” DECREASE OF PRESSURE: The decrease of pressure in the rock may causes the phase change in the rock and magma is generated” CHANGE OF COMPOSITION/ADDITION OF WATER: Addition of water decrease the melting temperature of the rock.  Crustal Geothermal Gradients Crustal Rocks Melt!  models Global Heat Flow convection in the mantle observed heat flow warm: near ridges cold: over cratons Bowen’s Reaction Series  EVOLUTION IN MAGMAS Differentiation Partial Melting Assimilation Mixing of Magmas Magmatic Differentiation: Crystal Settling PARTIAL MELTING ASSIMILATION MIXING OF MAGMA END