Igneous_Rocks

Igneous_Rocks - Up from the Inferno: Magma and Igneous...

Info iconThis preview shows page 1. Sign up to view the full content.

View Full Document Right Arrow Icon
This is the end of the preview. Sign up to access the rest of the document.

Unformatted text preview: Up from the Inferno: Magma and Igneous Rocks Igneous Rocks Intrusive /plutonic igneous rocks: Cool slowly underground igneous Intrusive Extrusive /volcanic igneous rocks: Cool quickly at the surface igneous Extrusive Lava – Cooled liquid Pyroclastic debris – Cooled fragments Volcanic ash Fragmented lava Many types of igneous rocks Oceanic crust (basalt) Most continental crust (granite) Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 4: Up from the Inferno: Igneous Rocks Igneous Rocks Solidified molten rock that freezes at high temp Earth is mostly igneous rock Earth 1,100°C to 650°C Temp depends on composition Magma – Subsurface melt; Magma Subsurface also contains crystals and gas Lava – Melt at the surface Lava Melt Volcanoes erupt lava Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 4: Up from the Inferno: Igneous Rocks Magma Formation Geothermal gradient – The Earth is hot inside Crustal temperature (T) averages 25°C / km of depth At the base of the lithosphere T ~ 1280°C The geothermal gradient varies from place to place Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 4: Up from the Inferno: Igneous Rocks Where does all that heat come from? Where Sources of heat for the early Earth: Planetesimal and meteorite accretion Gravitational compression Differentiation Differentiation Decay of radioactive minerals Tidal pull of the Sun and Moon The crust does NOT float on a sea of molten rock – sea Magma forms in special settings* Magma * hint: plate tectonics hint: Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 4: Up from the Inferno: Igneous Rocks Magma Formation & Plate Tectonics Partial melting occurs in the crust and upper mantle Melting is caused by: Pressure release Volatile addition Heat transfer Heat (result of P increase) (result Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 4: Up from the Inferno: Igneous Rocks Magma Formation & Plate Tectonics Pressure release Base of the crust is hot enough to melt mantle rock Base Due to high pressure, the rock does not melt Due A drop in pressure initiates “decompressional melting” Pressure drops when hot rock rises to shallower depths Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 4: Up from the Inferno: Igneous Rocks Addition of Volatiles Volatiles lower the melting T of a hot rock Volatiles by chemical bonds by Common volatiles include water and carbon dioxide Subduction introduces water to the mantle, melting rock Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 4: Up from the Inferno: Igneous Rocks Magma Formation Heat transfer (convection & conduction) Rising magma carries mantle heat with it This raises the T in nearby crustal rock, which then melts Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 4: Up from the Inferno: Igneous Rocks What is Magma Made of? What Magmas have three components (solid, liquid and gas) Solid – Solidified mineral crystals are borne by the melt Liquid – The melt itself is comprised of mobile ions Dominantly Si and O; also Al, Ca, Fe, Mg, Na, and K Other ions present to a lesser extent Other Different mixes of elements yield different magmas Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 4: Up from the Inferno: Igneous Rocks What is Magma Made of? What Gas – Magmas contain abundant dissolved volatile gas Dry magma – Scarce volatiles Wet magma – To 15% volatiles Water vapor (H2O) Carbon dioxide (CO2) Sulfur dioxide (SO2) Nitrogen (N2) Hydrogen (H2) Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 4: Up from the Inferno: Igneous Rocks Magma Compositions Four major magma types based on silica (SiO2) percentage Felsic (feldspar and silica) 66 to 76% SiO2 Intermediate 52 to 66% SiO2 Mafic (Mg and Fe-rich) 45 to 52% SiO2 Ultramafic Ultramafic 38 to 45% SiO2 38 Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 4: Up from the Inferno: Igneous Rocks Mafic & Felsic Minerals in Igneous Rocks Mafic MINERAL Quartz CHEMICAL COMPOSITION SiO2 Potassium Feldspar K Al Si3O8 Plagioclase Feldspar Ca Al2 Si2O8 - Na Al Si3O8 Muscovite Mica K Al2 (Al Si3 O10) (OH)2 Biotite Mica K (Mg, Fe)3 (AlSi3 O10 (OH)2 Amphibole Family e.g., Ca2 (Mg, Fe)5 [Si8O22] (OH, F)2 Pyroxene Family e.g., Ca (Mg, Fe) Si2O6 Olivine (Mg, Fe)2 SiO4 Essentials of Geology, 3rd edition, by Stephen Marshak (Anorthite) (Albite) Chapter 4: Up from the Inferno: Igneous Rocks Magma Compositions (important table!) Magma (important Composition controls magma density, T, and viscosity The most important factor is silica (SiO2) content Silica-rich magmas are thick and viscous Silica-poor magmas are thin and “runny” These characteristics govern eruptive style Type Density Temperature Viscosity Felsic Very low Very low (600 to 850°C) Very High: Explosive eruptions Intermediate Low Low High: Explosive eruptions Mafic High High Low: thin, hot runny eruptions Ultramafic Very high Very high (up to 1300°C) Very low Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 4: Up from the Inferno: Igneous Rocks Magma Variation Why are there different magma compositions? Magmas vary chemically due to… Magmas 1. 1. 2. 3. 4. Initial source rock compositions Partial melting Assimilation Fractional crystallization Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 4: Up from the Inferno: Igneous Rocks 1. Magma Variation The source of the melt dictates the initial composition Mantle source – Ultramafic and mafic magmas Mantle Crustal source – Mafic, intermediate, and felsic magmas Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 4: Up from the Inferno: Igneous Rocks 2. Partial Melting Upon melting, rocks rarely dissolve completely Instead, only a portion of the rocks melts Instead, Silica-rich minerals melt first Silica-poor minerals melt last Partial melting, then, yields a silica-rich magma Removing a partial melt from its source creates… Felsic magma Mafic residue Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 4: Up from the Inferno: Igneous Rocks Let’s try an analogy: Fancy Chocolate Partial melting of chocolate with nuts and raisins Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 4: Up from the Inferno: Igneous Rocks Fancy Chocolate Melting Scenario … It’s a hot summer day You buy a bar of fancy chocolate for your You But then you forget it in your car Upon your return to the car you discover the mess … Fancy chocolate rarely dissolves completely Instead, only a portion of the chocolate bar melts: Instead, Chocolate melts first Nut and raisins stay behind (would melt later) Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 4: Up from the Inferno: Igneous Rocks Partial Melting of Fancy Chocolate … Partial melting, then, yields a chocolate-rich “magma” Removing the chocolate magma from its source Removing (it drips to the floor of your car) creates… (it Chocolate-rich magma mess Nuts & Raisin residue Nuts & Raisins Chocolate Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 4: Up from the Inferno: Igneous Rocks 3. Assimilation Magma melts the country rock it passes through Blocks of country rock (xenoliths) fall into magma Assimilation of these rocks alters magma composition Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 4: Up from the Inferno: Igneous Rocks Magma Mixing Different magmas may blend in a magma chamber Different The result combines the characteristics The of the two magmas Often magma mixing is incomplete, resulting in Often blobs of one rock type suspended within the other within Essentials of Geology, 3rd edition, by Stephen Marshak Hey, my name is Gabbro — I’m pretty hot ! Chapter 4: Up from the Inferno: Igneous Rocks Magma Migration Magma doesn’t stay put; it tends to rise upward … until it gets stuck within the crust – a pluton pluton … or breaches the surface (as lava) – a volcanic eruption or This transfers mass from deep to shallow parts of Earth A crucial process in the Earth System Provides the raw material for soil, atmosphere, and ocean Provides Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 4: Up from the Inferno: Igneous Rocks Magma Migration Magma moves by… Injection into cracks Melting overlying rocks Squeezed by overburden The lower the viscosity the easier the movement The viscosity the Lower viscosity with … Higher temperature Lower silica content Higher volatile content Higher Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 4: Up from the Inferno: Igneous Rocks Silicate Minerals Silica tetrahedra link together by sharing oxygens More shared oxygen = lower Si:O ratio; governs… Melting temperature Mineral structure and cations present Mineral Susceptibility to chemical weathering Susceptibility Type of Silicate Structure Formula Si:O Ratio Independent Tetrahedra SiO4 0.25 Double Tetrahedra Si2O7 0.29 Ring Silicates Si6O18 0.33 Single Chains SiO3 0.33 Double Chains Si4O11 0.36 Sheet Silicates Si2O5 0.40 Framework Silicates SiO2 0.50 Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 4: Up from the Inferno: Igneous Rocks Viscosity, again Temperature: Temperature: Volatile content: Volatile Hotter – Lower viscosity Cooler – Higher viscosity More volatiles – Lower viscosity Fewer volatiles – Higher viscosity Silica (SiO2) content: Less SiO2 (Mafic) – Lower viscosity viscosity More SiO2 (Felsic) – Higher viscosity viscosity Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 4: Up from the Inferno: Igneous Rocks Cooling Rates Cooling rate – How fast does magma cool? Cooling Depth: Deep is hot; shallow is cool Deep plutons lose heat very slooowly and cool slooowly Shallow flows lose heat rapidly and cool quickly Shape: Spherical bodies cool slowly; tabular bodies cool faster Ground water: Ground water removes heat Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 4: Up from the Inferno: Igneous Rocks 4. Fractional Crystallization As magma cools, early crystals settle by gravity Melt composition changes as a result Fe, Mg, and Ca are removed in early settled solids Si, Al, Na, and K remain in melt and increase in abundance The original melt is mafic. Essentials of Geology, 3rd edition, by Stephen Marshak As early-formed minerals settle, the melt becomes more felsic. Chapter 4: Up from the Inferno: Igneous Rocks Fractional Crystallization Felsic magma can evolve from mafic magma Progressive removal of mafics depletes Fe, Mg, and Ca Remaining melt becomes enriched in Na, K, Al, and Si Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 4: Up from the Inferno: Igneous Rocks Bowen’s Reaction Series N. L. Bowen, in the 1920s, ran experiments with melts >> With cooling, early-forming crystals settled out >> Settling removed elements from the remaining melt He discovered that minerals solidify in two specific series: He Continuous – Plagioclase changed from Ca-rich to Na-rich Discontinuous – Minerals that solidify in a narrow T range Olivine Pyroxene Pyroxene Amphibole Biotite Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 4: Up from the Inferno: Igneous Rocks Igneous Rocks – Extrusive Settings Volcano – A vent where molten rock comes out of Earth Example: Kilauea Volcano, Hawaii Hot (~1,200oC) lava pools around the volcanic vent Hot, syrupy lava runs downhill as a lava flow The lava flow slows, loses heat and crusts over Finally, the flow stops and cools, forming an igneous rock Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 4: Up from the Inferno: Igneous Rocks Extrusive Settings Lava flows – Cool as blankets that often stack vertically Lava flows exit volcanic vents and spread outward Low-viscosity lava (basalt) can flow long distances (More Details on extrusive igneous rocks in Chapter 5) Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 4: Up from the Inferno: Igneous Rocks Extrusive Settings Explosive ash eruptions High-viscosity felsic magma builds volcanic pressure Violent eruptions yield huge volumes of volcanic ash Violent Ash can cover large regions Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 4: Up from the Inferno: Igneous Rocks Intrusive Settings Intrusive rocks cool at depth; they don’t surface Intrusive Magma invading colder country rock initiates… Thermal (heat) metamorphism and melting Thermal Inflation of fractures which wedges the country rock apart Incorporation of country rock fragments (xenoliths) Hydrothermal (hot water) alteration Hydrothermal Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 4: Up from the Inferno: Igneous Rocks Intrusive Settings Intrusive contacts preserve evidence of high heat Baked zone – Rim of heat-altered country rock Chill margin – Quenched magma at contact = tiny crystals Xenolith – Altered country rock fragment in magma Magma cooled before xenolith could be assimilated Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 4: Up from the Inferno: Igneous Rocks Intrusive Activity Magma intrudes into other rocks in two ways As planar, tabular bodies (dikes and sills) As As balloon-shaped blobs (plutons) Size varies widely Plutons can be massive Dikes and sills tend to Dikes be smaller be Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 4: Up from the Inferno: Igneous Rocks Plutonic Activity Most magma is emplaced at depth in the Earth. A large, deep, igneous body is called a pluton large, Plutonic intrusions modify the crust Plutonic Push aside preexisting rock Add new material Add Source of ore minerals Source Add heat May trigger May metamorphism metamorphism Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 4: Up from the Inferno: Igneous Rocks Plutonic Activity Plutons sometimes coalesce to form a larger batholith Plutons are created above subduction zones Magma generation may occur over tens of millions of years A long subduction history is linked long to the genesis of large batholiths to Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 4: Up from the Inferno: Igneous Rocks Tabular Intrusions Tend to have a uniform thickness Tend Can be traced laterally Two major subdivisions Sill – Parallels existing rock fabric Sill Dike – Crosscuts existing rock fabric Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 4: Up from the Inferno: Igneous Rocks Tabular Intrusions Dikes and sills modify invaded country rock They cause the rock to expand and inflate They thermally alter the country rock Dikes… Dikes… Cut across preexisting layering (bedding or foliation) Spread rocks sideways Dominate in extensional settings Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 4: Up from the Inferno: Igneous Rocks Tabular Intrusions Sills… Are injected parallel to preexisting layering Are usually intruded close to the surface Both dikes and sills exhibit wide variability in... Size Thickness (or width) Lateral continuity Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 4: Up from the Inferno: Igneous Rocks Tabular Intrusions Sills This basalt sill (dark band) was intruded into sandstones This (light colored rock) in Antarctica (light When intruded, When it lifted the entire landscape above it landscape Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 4: Up from the Inferno: Igneous Rocks Intrusive and Extrusive Intrusive and extrusive rocks commonly co-occur Intrusive Magma chambers feed overlying volcanoes Magma chambers may cool to become plutons Many igneous geometries are possible Many Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 4: Up from the Inferno: Igneous Rocks Intrusive and Extrusive With erosion, progressively deeper features are exposed Dikes Sills Laccoliths – Mushroom-shaped intrusions Chocoliths – nah, I made that up Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 4: Up from the Inferno: Igneous Rocks Influence on Landscape Continued uplift and erosion exposes the pluton Intrusive rocks are commonly more resistant to erosion Thus, intrusive rocks often stand high on the landscape Thus, “Unroofing” takes long periods of geologic time Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 4: Up from the Inferno: Igneous Rocks One more time: Textures … Small Scale, i.e., hand specimen or thin section Grain Size Grain Shape Arrangement / Alignment of Grains Large Scale, i.e., outcrop Layering, bedding, foliation Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 4: Up from the Inferno: Igneous Rocks Igneous Textures Igneous The size, shape, and arrangement of the minerals The Interlocking – Mineral crystals fit like jigsaw puzzle pieces Fragmental – Pieces of preexisting rocks, often shattered Fragmental Glassy – Made of solid glass or glass shards Texture directly reflects magma history! Interlocking or crystalline texture Fragmental texture Glassy texture Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 4: Up from the Inferno: Igneous Rocks Crystalline Classification COMPOSITION & TEXTURE A specific composition may specific occur as different textures occur Example: The finely crystalline equivalent Example: of a coarse granite is a rhyolite of A specific texture may be specific found in varying compositions found Example: Finely crystalline mafics are basalts; finely crystalline felsics are rhyolites Better to analyze texture first Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 4: Up from the Inferno: Igneous Rocks Crystalline Igneous Textures Texture reveals cooling history Aphanitic (finely crystalline) Rapid cooling Crystals do not have time to grow Extrusive Phaneritic – (coarsely crystalline) Slow cooling Crystals have a long time to grow Intrusive Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 4: Up from the Inferno: Igneous Rocks Crystalline Textures Texture reveals cooling history Porphyritic texture – A mixture of coarse and fine crystals Indicates a two-stage cooling history Initial slow cooling creates large phenocrysts Subsequent eruption cools remaining magma more rapidly Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 4: Up from the Inferno: Igneous Rocks XXL - Pegmatites Coarse mineral crystals found in dikes Here, large crystals are not due to slow cooling Instead, pegmatites form from water-rich melts Many unusual minerals are found in pegmatites Many Some pegmatites are rich in prized minerals Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 4: Up from the Inferno: Igneous Rocks Glassy Textures Form by very rapid cooling of lava in water or air Glassy textures are more common in felsic magmas They often preserve gas bubbles (vesicles) Underwater, basalt lava quenches into “pillows” Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 4: Up from the Inferno: Igneous Rocks Pillow Lava Formation Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 4: Up from the Inferno: Igneous Rocks Glassy Igneous Rocks Classification Obsidian – Volcanic glass from rapidly cooled lava Quenching – Lava flowing into water High silica lavas – These can make glass without quenching Pumice – Frothy felsic/intermediate volcanic rock full of Pumice vesicles; it floats! vesicles; Scoria – Glassy, vesicular, mafic rock; volcanic Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 4: Up from the Inferno: Igneous Rocks Fragmental Textures Preexisting rocks that were shattered by eruption After fragmentation, the pieces fall to the ground After and are cemented or welded together Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 4: Up from the Inferno: Igneous Rocks Fragmental Classification AKA Pyroclastic – Fragments of violent eruptions AKA Pyroclastic Tuff – Volcanic ash that has fallen on land and solidified Volcanic breccia – Made of larger volcanic fragments Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 4: Up from the Inferno: Igneous Rocks Classification by Composition If crystals are present Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 4: Up from the Inferno: Igneous Rocks Igneous Classification Classification is based upon composition and texture Composition – Felsic, intermediate, mafic, and ultramafic Texture – Fine (aphanitic); coarse (phaneritic) etc. C1 A Type Aphanitic (fine) Felsic Rhyolite Intermediate Phaneritic (coarse) Andesite Mafic Ultramafic Essentials of Geology, 3rd edition, by Stephen Marshak C2 B Basalt Very high A Granite B Diorite C2 Gabbro C1 Very high (up to 1300°C) Chapter 4: Up from the Inferno: Igneous Rocks Igneous Activity Distribution Igneous activity tracks tectonic plate boundaries Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 4: Up from the Inferno: Igneous Rocks Igneous Environments - Recap Two major categories – Based on place of cooling Extrusive settings – Cool at or near the surface Cool rapidly Chill too fast to grow big crystals Intrusive settings – Cool at depth Intrusive Lose heat slowly Crystals grow large Most mafic magmas extrude Most Most felsic magmas don’t WHY? Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 4: Up from the Inferno: Igneous Rocks QUESTION OF THE DAY Most mafic magmas extrude. Most Most felsic magmas don’t. Why on Earth do we have Why more basalts than gabbros, more but more granites than rhyolites? Half to one page; Deadline Thursday July 14, 11:59 PM. Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 4: Up from the Inferno: Igneous Rocks And remember: IGNEOUS IS BLISS! Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 4: Up from the Inferno: Igneous Rocks ...
View Full Document

This note was uploaded on 02/25/2012 for the course EARTH 2 taught by Professor Thomas during the Summer '11 term at UCSB.

Ask a homework question - tutors are online