IGNEOUS ROCK Felsic rich igneous lunar rock collected from the lunar highlands of the moon by Apollo 16 astronauts. Rock Classification ROCKS IGNEOUS -most abundant -primary rocks -source is magma or lava SEDIMENTARY -thin veener in Oceanic and Continental Crusts -secondary rocks METAMORPHIC -proportion is similar to that of Igneous rocks -change of forms of Ig. And Sed. Due to Temperature, Pressure and Chemical Fluids Rock Cycle Origin: Igneous Rock “Fire-formed rocks“ Crystallize from molten material: IGNEOUS ROCKS: The rocks formed through volcanic action OR The rocks which are derived from a molten mass “magma or lava” Magma – molten rock below the Earth's surface Lava – molten rock that erupts onto the Earth's surface through a volcano or crack (fissure) Origin: Igneous Rock VOLCANO: A gap in the Earth’s Crust where molten rocks and other material escape onto the Earth’s surface SOURCE OF IGNEOUS ROCKS: Magma: Molten mass comprising most abundant elements in earth – Si, Al, Fe, Ca, Mg, K, H & O. Where the SiO2 is most abundant amongst all. Temperature of Magma- 10400 to 12000 C Formation of Magma How are rocks melted? 1. Heating 2. Depressurization 3. Increase water content 4. Increased silica content Where do rocks melt? Subduction zones (Felsic and Intermediate) Mantle Plumes (“Hot Spots”) not only at Divergent Boundaries Mafic Hot and High Pressure Hot and Low Pressure Cooling Rates of Magma Cooling rates influence the texture if the igneous rock: Quick cooling = fine grains Slow cooling = coarse grains Magma Different magmas have different viscosities. Viscosity is the property of a substance to internally resist flow. Example: Pine Sap or Molasses (High Viscosity) vs. Water (Low Viscosity) The HIGHER the viscosity, the more resistance to flow!! Igneous rocks are classified on their texture and their composition. Igneous textures: Glassy Aphanitic Phaneritic Porphyritic Vesicular Pyroclastic Igneous rocks are classified on their texture and their composition. Igneous textures: Glassy instantaneous cooling Obsidian = volcanic glass Igneous rocks are classified on their texture and their composition. Aphanitic - fine grain size (< 1 mm); result of quick cooling Basalt Rhyolite Andesite Igneous rocks are classified on their texture and their composition. Phaneritic coarse grain size; visible grains (1-10 mm); result of slow cooling Granite Diorite Gabbro Igneous rocks are classified on their texture and their composition. PorphyriticMixture of grain sizes caused by mixed cooling history; slow cooling first, followed by a period of somewhat faster cooling. Igneous rocks are classified on their texture and their composition. Vesicular contains tiny holes called vesicles which formed due to gas bubbles in the lava or magma. Very porous. May resemble a sponge. Commonly low density; may float on water. Igneous rocks are classified on their texture and their composition. Pyroclastic or Fragmental pieces of rock and ash come out of a volcano and get welded together by heat. Tuff - made of volcanic ash Volcanic breccia - contains fragments of finegrained igneous rocks that are larger than ash. BROAD CLASSIFICATION OF IGNEOUS ROCKS Intermediate rocks (Hypabyssal rocks) Volcanic rocks (Extrusive rocks) -Lava or Magma flows -Pyroclastic flows Plutonic rocks (Intrusive rocks) -dykes, sills, batholiths, laccoliths etc. Igneous Rocks Are Subdivided Into Two Classes •Volcanic (Extrusive) Volcanic igneous rocks form at the earth's surface as lava cools. •Plutonic (Intrusive) Plutonic igneous rocks form deep underground where magma cools slowly. Mineral Percentage Igneous Rock Classification Intrusive (Plutonic) Extrusive (Volcanic) Continental Crust Oceanic Crust Mantle Intrusive Igneous Rock E.g., Felsic -Granite (Phaneritic): Quartz Biotite Na Plagioclase Crystallized (Solidified) Felsic Magma Poor in: Fe, Mg, Ca, (<20%) Rich in: Silica (>70%) Extrusive vs. Intrusive (Mafic Rocks) Mafic Magmas Hot (>1000oC) Non-Viscous (runny, flows “Dry” (no H2O or C02) easily) Mafic Rocks Usually Extrusive, Fine-grained, Mafic (Basalt) rock forms oceanic crust, Shield Volcanoes and Basalt Floods If Intrusive, course-grained mafic rocks are formed Gabbro. If intrusive, Dikes and Sills more common Igneous Rock Classification Intermediate Mafic Granite Rhyolite Diorite Andesite Gabbro Basalt Extrusive Intrusive Felsic (Porphyritic) Bowen’s Reaction Series Minerals form as molten rock crystallizes. A generalized order of crystallization of minerals is shown in the Bowen's Reaction Series. As the magma cools, first olivine, then pyroxene, then amphobole, etc. will be crystallized (assuming the proper elements are available). Bowen's Reaction Series has two branches. They are: 1.Discontinuous reaction series, from olivine to biotite,. 2.Continuous reaction series, from Ca plagioclase to Na plagioclase. Felsic Intermid Mafic KINDS OF IGNEOUS ROCK Bowen’s Reaction Series The discontinuous reaction series involves the darkcolored ferromagnesian minerals: 1.olivine 2.pyroxene 3.amphibole 4.biotite. The continuous reaction series involves the plagioclase feldspars. Plagioclase feldspars are an example of a "solid solution series", exhibiting gradations in chemical and physical properties. Chemically, this series consists of two "end members": 1. Na plagioclase (NaAlSi3O8), the sodium "end member", and 2. Ca plagioclase (CaAlSi2O8), the calcium "end member". BOWEN'S REACTION SERIES Shows a generalized crystallization order of minerals as a magma cools. GENERALITIES down the reaction series (from top to bottom): Easier to melt minerals The more complex the silicate mineral Higher the viscosity of the magma More resistant the mineral is to chemical weathering Lower the rock's final density FLUIDITY OF MAGMA Fluidity or Viscosity of magma depends on content (%) of Silica Silica Rich (felsic) -known as Acidic magma -More viscous, so do not spreads and piles up at one place (trap gas easily) Silica poor (mafic) -Known as Basic magma -Less viscous, moves faster and occupies larger area (release gas easily) However, the viscosity of magma is considerably influenced by temperature too. When temperature is less- magma is more viscous (thicker) and when temperature is high - less viscous (thinner). Volcanic Eruption of Magma When magmas reach the surface of the Earth they erupt from a vent. They may erupt explosively or non-explosively. Non-explosive eruptions are favored by low gas content and low viscosity magmas (basaltic to andesitic magmas). Usually begin with fire fountains due to release of dissolved gases Produce lava flows on surface Produce Pillow lavas if erupted beneath water Eruption of Magma Explosive eruptions are favored by high gas content and high viscosity (andesitic to rhyolitic magmas). Expansion of gas bubbles is resisted by high viscosity of magma - results in building of pressure High pressure in gas bubbles causes the bubbles to burst when reaching the low pressure at the Earth's surface. Bursting of bubbles fragments the magma into pyroclasts and tephra (ash). Cloud of gas and tephra rises above volcano to produce an eruption column that can rise up to 45 km into the atmosphere. Eruption of Magma Tephra that falls from the eruption column produces a tephra fall deposit. Eruption of Magma If eruption column collapses a pyroclastic flow may occur, wherein gas and tephra rush down the flanks of the volcano at high speed. This is the most dangerous type of volcanic eruption. The deposits that are produced are called ignimbrites. Eruption of Magma Lateral blasts and debris avalanches occur when gas is released suddenly by a large landslide or debris avalanche taking out part of the volcano Plutons Plutons = Igneous rocks cooled at depth. Name comes from Greek god of the underworld - Pluto. Pluton features Dikes are small (<20 m wide) shallow intrusions that show a discordant relationship to the rocks in which they intrude. Discordant means that they cut across preexisting structures. They may occur as isolated bodies or may occur as swarms of dikes emanating from a large intrusive body at depth. Pluton features Sills are also small (<50 m thick) shallow intrusions that show a concordant relationship with the rocks that they intrude. Sills usually are fed by dikes, but these may not be exposed in the field. Pluton features Laccoliths are somewhat large intrusions that result in uplift and folding of the preexisting rocks above the intrusion. They are also concordant types of intrusions. Pluton features Batholiths are very large intrusive bodies, usually so large that there bottoms are rarely exposed. Sometimes they are composed of several smaller intrusions. Stocks are smaller bodies that are likely fed from deeper level batholiths. Stocks may have been feeders for volcanic eruptions, but because large amounts of erosion are required to expose a stock or batholith, the associated volcanic rocks are rarely exposed. Methods of Igneous Intrusion Methods of intrusion Melting - crystallization Stoping - xenoliths Injection Igneous Intrusion