Transcript Igneous
Types of Rock include Igneous, Sedimentary and Metamorphic Igneous Rocks Lecture 3 Marble demo, rock specimens, Petrographic Microscope, Olivine Porphyry or Gabbro Characteristics of magma • Igneous rocks form as molten rock cools and solidifies • Characteristics of magmas (molten rock) depend on parent material and where they crystallize • Where determines speed of crystallization • At surface, fast cooling makes small crystals Geothermal Gradient Cool Silica-rich rocks (with Quartz) melt at cooler temperatures. Melts are viscous Silica-poor rocks (with Olivine, Pyroxene) melt at higher temperatures Melts are very fluid Hot http://www.casdn.neu.edu/~geology/department/staff/colgan/iceland/usgs/ Characteristics of magma • General Characteristics of molten rock • Forms from partial melting of rocks inside the Earth • Rocks formed from lava at the surface are classified as extrusive, or volcanic rocks • Rocks formed from magma that crystallizes at depth are termed intrusive, or plutonic rocks Two Geologic Environments Where Igneous Rocks Form Igneous Rocks Formed in Rift Igneous Rocks Formed Above Sinking Plate Extrusive Igneous Rock - Lava (Hawaii) Intrusive Igneous Rock (Granite) – This granite cooled 30 kilometers under the surface Characteristics of magma • The nature of magma • Consists of three components: – A liquid portion, called melt, that is composed of mobile ions – Solids, if any, are silicate minerals that have already crystallized from the melt – Volatiles, which are gases dissolved in the melt, including water vapor (H2O), carbon dioxide (CO2), and sulfur dioxide (SO2) Characteristics of magma • Crystallization of magma • Cooling of magma results in the systematic arrangement of ions into orderly patterns • The silicate minerals resulting from crystallization form in a predictable order Rock-forming minerals crystallize with increasing complexity as the magma cools. The most complex 3-D minerals crystallize last. The hottest magmas can only crystallize Olivine (Independent Tetrahedra), but as the magma cools, more complex minerals can form. Crystallization of minerals in magma bodies • Bowen’s reaction series and the composition of igneous rock • Norman L. Bowen demonstrated that as a magma cools, minerals crystallize in a systematic fashion based on their melting points http://vgp.agu.org/bowen_paper/bowen_paper.html Bowen’s Reaction Series Molten- VERY Hot Molten- Not so hot 100% Solid Fine crystals Need a microscope Low silica, HOT, fluid Course crystals Easily seen Intermediate High silica, warm, viscous Bowens reactrion series says: as a granitic melt cools, Biotite Mica and Plagioclase Feldspar crystallize out before Quartz Granite Hand Sample Granite Thin Section Order of Crystallization We can see the order of crystallization under the microscope Crystals can react with the melt if they touch it If the first formed crystals of Calcium-rich (Ca) Plagioclase are allowed to react with the melt, they will become more sodium-rich on their outer rims Zoned feldspar (plagioclase) showing change in composition with time in magma chamber (calcium-rich in core to sodium-rich at rim) However, if early crystals are removed the melt becomes richer in Silica Remove Fe, Mg, Ca Some Si Left with K and Al Most of Si You can start with a Mafic (silica-poor) magma and end up with some Felsic (silica-rich) Granites. Marble Demo A rising melt may leave its mafic components behind, and the upper part may be granitic Characteristics of magma • Crystallization of magma • Texture in igneous rocks is determined by the size and arrangement of mineral grains • Igneous rocks are typically classified by both: – Texture – Mineral composition Igneous textures • Texture is used to describe the overall appearance of a rock based on the size, shape, and arrangement of interlocking minerals • Most important is crystal size • Factors affecting crystal size • Rate of cooling – Slow rate promotes the growth of fewer but larger crystals Igneous textures • Factors affecting crystal size • Rate of cooling (continued) – Fast rate forms many small crystals – Very fast rate forms glass • Amount of silica (SiO2) present • Amount of dissolved gases Types of Igneous textures • Types of igneous textures • Aphanitic (fine-grained) texture – Rapid rate of cooling of lava or magma – Microscopic crystals – May contain vesicles (holes from gas bubbles) • Phaneritic (coarse-grained) texture – Slow cooling – Crystals can be identified without a microscope Aphanitic texture Fine grained because it cooled quickly at the surface Phaneritic texture Coarse crystals cooled slowly at great depth Igneous textures • Types of igneous textures • Porphyritic texture – Minerals form at different temperatures as well as differing rates – Large crystals, called phenocrysts, are embedded in a matrix of smaller crystals, called the groundmass • Glassy texture – Very rapid cooling of molten rock – Resulting rock is called obsidian Porphyritic texture Granite Glassy texture Obsidian More types of Igneous textures • Types of igneous textures • Pyroclastic texture – Various fragments ejected during a violent volcanic eruption – Textures often appear to more similar to sedimentary rocks Pyroclastic Rock Superheated Flows Naming igneous rocks – pyroclastic rocks Composed of fragments ejected during a volcanic eruption Varieties Tuff – ash-sized fragments Volcanic breccia – particles larger than ash Ash and pumice layers Still more types of Igneous textures • Types of igneous textures • Pegmatitic texture –Exceptionally coarse grained crystals –Form in late stages of fractionation of magmas –This is often what prospectors are looking for A Pegmatite with Feldspar and Zircon Zircon is very good for obtaining radiometric ages Show tray of Mafic Minerals Igneous Compositions • Igneous rocks are composed primarily of silicate minerals that include: • dark (or ferromagnesian) colored silicates – Olivine – Pyroxene – Amphibole – Biotite mica – And … “MAFIC” Magnesium and Iron Show tray of Felsic Minerals Igneous Compositions • Igneous rocks also contain light colored silicate minerals that include: – Quartz – Muscovite mica – Feldspars “FELSIC” Feldspar and Silica Igneous Rock Classification- Bowen’s Reaction Series on its side Note Minerals in Felsic rocks crystallize from warm melts Note Minerals in Mafic from hot melts Igneous compositions Granitic composition – Composed of light-colored silicates – Designated as being felsic (feldspar and silica) in composition – Contains high amounts of silica (SiO2) – Major constituents of continental crust Igneous compositions • Naming igneous rocks – granitic (felsic) rocks • Granite – Phaneritic – Over 20 percent quartz, about 25 percent or more feldspar (usually much more feldspars). – Plagioclase is Sodium-rich – Abundant and often associated with mountain building – The term granite covers a wide range of mineral compositions Figure 3-7 continued Igneous compositions • Naming igneous rocks – granitic (felsic) rocks • Rhyolite – Extrusive equivalent of granite – May contain glass fragments and vesicles – Aphanitic texture (means fine grained minerals) – Less common and less voluminous than granite – Phenocrysts can include quartz and feldspar Rhyolite fine grained because extruded, so crystallized quickly Igneous compositions • Basaltic composition can be fine or coarse – Composed of dark Olivine and Pyroxene and grey calcium-rich plagioclase feldspar – No Potassium-rich feldspar (no K-spar ‘Microcline’) – Designated as being mafic (magnesium and ferrum, for iron) in composition – Much denser than granitic rocks - sinks – Comprises the ocean floor as well as many volcanic islands such as Hawaii. Also rift valley lavas Igneous compositions • Naming igneous rocks – basaltic (mafic) rocks: Fine-grained • Basalt – Volcanic origin – Aphanitic texture – Composed mainly of pyroxene, some olivine and also calcium-rich plagioclase feldspar – Most common extrusive igneous rock Scoria type Basalt note Gas Bubbles Igneous compositions • Naming igneous rocks – basaltic (mafic) rocks: Coarse Grained • Gabbro –Intrusive equivalent of basalt –Phaneritic texture consisting of pyroxene and calcium-rich plagioclase (med. grain “diabase”) –Makes up a significant percentage of the oceanic crust, beneath the basalt pillow lavas. Gabbro - a mafic igneous rock. A largegrain version of Basalt Igneous compositions • Other compositional groups • Intermediate (or andesitic) composition – Contain at least 25 percent dark silicate minerals – Associated with explosive volcanic activity – Often gray Igneous compositions • Intermediate rocks • Andesite – Volcanic origin – Aphanitic texture – Often resembles rhyolite – Intermediate silica content – Frequent composition in volcanoes above subduction zones, e.g. in Andes Mountains Igneous compositions • Extrusive products can include: • Pumice – Volcanic – Glassy texture, very light weight, mostly air – Frothy appearance with numerous voids (extrusive foam) – Forms when lavas have a lot of water and other volatiles Common with intermediate compositions Igneous compositions • Intermediate rocks • Diorite – Plutonic equivalent of andesite – Coarse grained – Intrusive – Composed mainly of intermediate feldspar and amphibole Igneous compositions • Silica content influences a magma’s behavior • Granitic magma Plutonic “Granite” – High silica content – Extremely viscous – Liquid exists at temperatures as low as 700oC – Huge explosion if it erupts (Yellowstone, Toba) Volcanic “Rhyolite” Igneous compositions • Silica content influences a magma’s behavior • Basaltic magma – Much lower silica content – Fluid-like behavior – Crystallizes at higher temperatures – Gurgles when it erupts (Hawaii) Origin of Magma • Generating magma from solid rock • Produced from partial melting of rocks in the crust and upper mantle • Role of heat – Temperature increases within Earth’s upper crust with increasing depth. The geothermal gradient averages between 20oC to 30oC per kilometer Origin of Magma • Role of heat –Rocks in the lower crust and upper mantle are near their melting points –Any additional heat (from rocks descending into the mantle or rising heat from the mantle) may induce melting Origin of Magma • Role of Pressure – Reducing the pressure lowers the melting temperature – the rock probably melts – RIDGE: When confining pressures drop, decompression melting occurs Decompression melting At a Mid-Ocean Ridge Origin of Magma • Role of volatiles - WATER –Volatiles (primarily water) cause rocks to melt at lower temperatures –This is particularly important where oceanic lithosphere descends into the mantle in a subduction zone The role of water Pillow Lavas Water promotes melting despite high pressures Evolution of magmas • Processes responsible for changing a magma’s composition • Assimilation – Changing a magma’s composition by the incorporation of foreign matter (surrounding rock bodies) into a magma – The rising magma melts near surface rock, changing the composition of the melt. • Magmatic differentiation = Fractionation – Separation of a melt from earlier formed crystals causes a different composition in the remaining magma Evolution of magmas • Processes responsible for changing a magma’s composition • Magma mixing –Involves two bodies of magma intruding one another –Two chemically distinct magmas may produce a composition quite different from either original magma Assimilation and magmatic differentiation Show Samples Evolution of magmas • Partial melting and magma formation • Incomplete melting of rocks is known as partial melting • Formation of basaltic magmas – Most originate from partial melting of ultramafic rock in the mantle – Basaltic magmas form at mid-ocean ridges by decompression melting of rising mantle material – also by partial melting of ocean crust + sediments+ water in subduction zones Basalts forming in rifts Decompression Melting: Magma under lithosphere heats and cracks it. Mantle rock is exposed to low pressures – it partially melts Origin of Andesite & Diorite: intermediate silica content Good diagram for the Andes Mountains Small blobs, not much heat in them Assimilate some crust, fractionate Plate Tectonics- Andesite Line Andes 62 Andesites form above the deep portions of a subduction zone Origin of Granitic Rocks Huge blobs w/ low temps but lots of magma, fractionation & assimilation => Granite Batholiths Can also get granites from deep rock burial End of Lecture 3