Petrology Lecture 1 PDF

# PETROLOGY ## Course No. Geol. 402 (IGNEOUS AND METAMORPHIC PETROLOGY) Dr. Adnan Khan Assistant Professor Department of Geology ## Recommended Book Petrology: Igneous, Sedimentary, and Metamorphic By Harvey Blatt and Robert J. Tracy Selected chapters are available on ## Genetic Distribution A di...

# PETROLOGY ## Course No. Geol. 402 (IGNEOUS AND METAMORPHIC PETROLOGY) Dr. Adnan Khan Assistant Professor Department of Geology ## Recommended Book Petrology: Igneous, Sedimentary, and Metamorphic By Harvey Blatt and Robert J. Tracy Selected chapters are available on ## Genetic Distribution A diagram shows the following: - A central blue circle labeled **PETROLOGY** - 4 blue circles connected to the central circle via arrows - **IGNEOUS** - **SEDIMENTARY** - **METAMORPHIC** ## IGNEOUS PETROLOGY Scientific study of igneous rocks ### PETROGENESIS Processes of rock formation ### PETROGRAPHY Description of rocks - **Petrography:** Rocks' description and classification - **Petro-genesis:** Conditions under which they were formed. ## IGNEOUS - Branch of PETROLOGY 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 ## 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 a combination of them. ## PROCESSES - **Radiation:** involves emission of EM energy from the surface of a 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 ## 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 A diagram shows the following: - A subduction zone with oceanic and continental lithosphere. - A portion of the oceanic lithosphere has sunk into the asthenosphere. - Arrows show the movement of the plates and point toward the sinking oceanic lithosphere - An Andesite Volcano is shown above the sinking oceanic lithosphere. ## SOURCES OF HEAT FOR MELTING ROCKS 238 235 232 40 - **Radioactive Decay:** U, U, Th, K, and 87Rb all have t1/2 that >10^9 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. ## 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. ## Crustal Geothermal Gradients The diagram shows the following: - A cross section of Earth showing the mantle, an outer core and inner core - A convection cell in the Earth's mantle that moves hot material to the surface and cold material to the core - A mid-ocean ridge where new crust is formed - A trench where crust is subducted - The text **Crustal Rocks Melt!** shows the area where melting is likely to happen near the mid-ocean ridge ## Global Heat Flow A diagram shows the following: - 3 cross sections of Earth showing the movement of heat within the mantle. - The top of the mantle is warmer near mid-ocean ridges than over cratons. ## Bowen's Reaction Series A diagram shows the following: - 3 cross sections of Earth showing the crystallization process of magma in the mantle. - Green represents minerals which are enriched near the surface of the Earth. ## EVOLUTION IN MAGMAS - Differentiation - Partial Melting - Assimilation - Mixing of Magmas ## Magmatic Differentiation: Crystal Settling A diagram shows the following: - 3 cross sections of Earth showing the process of magmatic differentiation in the mantle. - The top of the mantle shows an accumulation of ferromagnesian minerals that have crystallized out of magma. ## PARTIAL MELTING A diagram shows the following: - 3 cross sections of Earth showing the process of partial melting in the mantle. - Magma is generated near the surface of the Earth because it melted at a lower temperature than the surrounding rock ## ASSIMILATION A diagram shows the following: - 3 cross sections of Earth showing the process of assimilation in the mantle. - As magma ascends it melts the surrounding rock, changing its composition. ## MIXING OF MAGMA A diagram shows the following: - 3 cross sections of Earth showing the process of magma mixing in the mantle - 2 different magmas with different compositions mix together ## Conclusions Magma generation on Earth is complex because we live on a dynamic planet. When we look up at the night sky we see an enormous volume of flood basalts (on the Moon). How different are magma generation processes in the Earth and in the Moon? ## END