Types of Rock include Igneous, Sedimentary and Metamorphic Igneous Rocks Lecture 3 Marble demo, rock specimens, Petrographic Microscope, Olivine Porphyry or Gabbro Trays of mafic and felsic minerals

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 Hot Cool Silica-rich rocks (with Quartz, K-feldspar) melt at cooler temperatures. Melts are viscous Silica-poor rocks (with Olivine, Pyroxene, Ca-feldspar) melt at higher temperatures Melts are very fluid Hot

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 Formed in Rift Igneous Rocks Formed Above Sinking Plate Both melts are "Basaltic" i.e. Olivine, Pyroxene and Ca-Feldspars

Extrusive Igneous Rock - Lava (Hawaii)

Intrusive Igneous Rock (Granite) – This granite cooled 30 kilometers under the surface Plagioclase Feldspar Quartz Amphibole K Feldspar

Characteristics of magma Three parts: Liquid portion, called melt, that is mobile ions Solids, if any, are silicate minerals 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.

Bowen’s Reaction Series Molten- VERY Hot No solids First mineral to crystallize out Molten- Not so hot 100% Solid

Fine crystals Need a microscope Low silica, HOT, fluid Intermediate High silica, warm, viscous Course crystals Easily seen

Granite Hand Sample Order of Crystallization Granite Thin Section Microscope Demo Bowens reaction series says: as a granitic melt cools, Biotite Mica and Plagioclase Feldspar crystallize out before Quartz Granite Hand Sample Order of Crystallization We can see the order of crystallization under the microscope Granite Thin Section

Crystals can react with the melt if they touch it If the first formed crystals of Calcium-rich (Ca) Plagioclase touch the melt they will react with it, and 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 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 melt will crystallize its mafic components first, and the remaining melt may be granitic

Characteristics of magma Igneous rocks are typically classified by both: Texture Mineral composition Texture in igneous rocks is determined by the size and arrangement of mineral grains

Igneous textures Most important is crystal size Factors affecting crystal size Rate of cooling Slow rate promotes the growth of fewer but larger crystals Fast rate forms many small crystals Very fast rate forms glass

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

Fine grained because it cooled quickly at the surface Aphanitic texture Fine grained because it cooled quickly at the surface

Coarse crystals cooled slowly at great depth 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 Two-stage cooling?

Glassy texture Obsidian Fast cooling

More 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 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 versus … “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 Note Minerals in Felsic rocks crystallize from warm melts Mafic from hot melts

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

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 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 Basalt 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 Bubble Pits

Igneous compositions Gabbro Naming igneous rocks – basaltic (mafic) rocks: Coarse Grained Gabbro Intrusive equivalent of basalt Phaneritic texture consisting of pyroxene and calcium-rich plagioclase Makes up a significant percentage of the oceanic crust, beneath the basalt pillow lavas.

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

Silica Content Silica content influences a magma’s behavior Granitic magma High silica content Extremely viscous Liquid exists at temperatures as low as 700oC Huge explosion if it erupts (Yellowstone, Toba) Plutonic “Granite” When Yellowstone explodes, half of Wyoming will perish Volcanic “Rhyolite”

Silica Content 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 Role of Pressure Reducing the pressure lowers the melting temperature – the rock probably melts RIDGE: When confining pressures drop, decompression melting occurs

Role of volatiles - WATER 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

Assimilation and magmatic differentiation Show Samples

Basalts forming in rifts and MORs 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 Basaltic here Good diagram for the Andes Mountains Small blobs, not much heat in them Assimilate some crust, fractionate

Plate Tectonics- Andesite Line Andesites form above the deep portions of a subduction zone

Origin of Granitic Rocks Huge blobs under thick part of continent w/ low temps but lots of magma, fractionation & assimilation => Granite Batholiths Can also get small amounts of granites from deep felsic rock passed by ascending magma

Some intrusive igneous structures