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Geology – Chapter 3 – Igneous Rocks Magma, Igneous Rocks, and Intrusive Activity Magma, Igneous Rocks, and Intrusive Activity IgneousRocks

Solid rock Pressure Pressure 150 mi below earth’s surface 150 mi below earth’s surface partial melting of solid rock partial melting of solid rock Solid Rock Droplets rise through the mantle and collect to form magma pools Magma pool Surface Cooled solid rock Extrusive Ig-rocks Lava Intrusive Ig-rocks

Magma: The parent material for all rocks There are three distinct components of magma: Liquid portion referred to as “melt” “liquid rock” – low density “liquid rock” – low density composed of: Si, O, Al, Fe, Ca, Mg, Na, K composed of: Si, O, Al, Fe, Ca, Mg, Na, K Solid portion crystallized portions of the melt as magma cools – precipitating various silicate minerals. Gaseous portion “volatiles” created from vaporizing “volatiles” created from vaporizing materials at the Earth’s surface materials at the Earth’s surface (H 2 O, CO 2 and SO 2 ) (H 2 O, CO 2 and SO 2 )

Magma Typical temperature depending on chemical composition: C (1,292 0 F) – C (2,372 0 F) High in Si,O High in Fe,Mg

Melting solid rock into magma Rising temperatures within a solid (tightly packed atoms) creates “vibrating ions” eventually breaking the chemical bonds and causing ions to roll over each other producing a liquid. How would the process of crystallization work?

Intrusive Rocks Extrusive Rocks Intrusive Rocks Solidified below the Earth’s surface Plutonic rocks – “the God of lower world” magma cooled slowly allowing xl growth coarse-grained textures (can see grains) large granite mountains Sierra Nevada Mountains Mt Rushmore Extrusive Rocks Extrusive Rocks Solidified above the Earth’s surface Solidified above the Earth’s surface Volcanic rocks – “the roman God of fire” Volcanic rocks – “the roman God of fire” magma cooled fast allowing microscopic magma cooled fast allowing microscopic xl growth (can’t see grains) xl growth (can’t see grains) fine-grained textures (can’t see grains) fine-grained textures (can’t see grains) Hawaiian Volcanoes, Cascade Range Hawaiian Volcanoes, Cascade Range visiblegrains non-visible grains

When observing “frozen magma” (an igneous rock), how does a geologist know if the rock is an Extrusive or Intrusive igneous rock???????? When observing “frozen magma” (an igneous rock), how does a geologist know if the rock is an Extrusive or Intrusive igneous rock???????? Checking the crystal size - does the rock contain visible crystals or are crystals non-existent? Checking the crystal size - does the rock contain visible crystals or are crystals non-existent? (4) Factors that affect crystal size in an igneous rock The rate at which magma cools slow cooling – visible minerals rapid cooling – nonviable minerals The amount of silica present the more silica – more visible minerals The amount of dissolved gases abundant dissolved gases – larger crystals The amount of space for xl growth (4) Factors that affect crystal size in an igneous rock The rate at which magma cools slow cooling – visible minerals rapid cooling – nonviable minerals The amount of silica present the more silica – more visible minerals The amount of dissolved gases abundant dissolved gases – larger crystals The amount of space for xl growth Igneous Rock Textures

Slow cooling of magma Phaneritic – xl grains can be seen as an interlocking “mosaic” mass of xls coarse- grained texture Types of igneous rock textures Interlocking grains Interlocking grains

Pegmatitic – xl grains are greater than 1 cm results from late stage magma cooling- charged with volatiles Slow cooling of magma Large Grains

What do you observe ? No minerals visible xl grains are to small to see with an unaided eye No minerals visible xl grains are to small to see with an unaided eye Fast cooling magma

Glassy – xl grains are “frozen” before xlation. Produced from high silica viscous rich magmas that are rapidly quenched Fast cooling magma

Porphyritic – two distinct xl sizes Phenocrysts Groundmass (same mineral)

Fast cooling of magma Pyroclastic – fragmented texture, fragments incorporated into molten lava as it solidifies Tuffacous (tuff) – composed of fragments Fragments of other rock pieces (“different pieces other rock)

Igneous Rock Textures -

Igneous Rock Composition – What’s in your rock? Composition is based on the “mineral make-up” dominantly composed of silicates (Si and O) Al, Ca, Na, K, Mg and Fe Ferromagnesian composition (Mafic) high in Fe, Mg low in Si, O Dark colored rocks Pyroxene Amphibole Biotite Ca-plagioclase orthoclase MuscoviteQuartz Non-ferromagnesian (felsic) high in Si, O high in Si, O low in Fe, Mg low in Fe, Mg light colored rocks light colored rocks

Classification of Igneous Rocks Igneous rocks are classified based on TEXTURE and COMPOSTION TEXTURE phaneritic aphanitic COMPOSITION Felsic (light color) High silica Mafic (dark color) Low silica Intermediate color GraniteDioriteGabbro RhyoliteAndesite Basalt Ig rocks have the same chemistry but different textural characteristics due to the type of geologic environment the rock forms

The Origin of Magma How can magma form from “solid rock ?”How can magma form from “solid rock ?” Why is one magma mafic and others felsic?Why is one magma mafic and others felsic? Why do volcanoes erupt basaltic lava, andWhy do volcanoes erupt basaltic lava, and continent volcanoes erupt andesitic/rhyolitic lava ? continent volcanoes erupt andesitic/rhyolitic lava ? most magma originates in the upper mantle greatest quantities form at divergent boundaries small amounts of magma at subduction zones Questions:

Increasing Temperature: increasing the temperature within the earth increasing the temperature within the earth begins to melt solid rock begins to melt solid rock Temperatures within the Earth increase as a function of depth ---- Geothermal Gradient

Temperature inside the earth Depth (km) ,000 10,000 15,000 Pressure (mpa) Geothermalgradient the rate at which the rate at which temperature increases temperature increases with depth with depth Continent gradient In thicker crust, In thicker crust, gradient increases. gradient increases. average 7 o C/km rate average 7 o C/km rate temperature increases temperature increases gently gently Oceanic gradient Below the ocean floor, Below the ocean floor, temperature increases temperature increases rapidly. rapidly. average 13 0 C/km average 13 0 C/km

So, why is the mantle still considered solid? Pressure increases with depth – raising the rocks melting points

Decompression Melting: ascending mantle rock moves into lower pressure zones which lowers rock melting points generating voluminous magma Remember: most magma occurs along spreading ridges.

Addition of volatiles (water and gases) addition of water lowers the rocks melting point “wet” rock drives the melting points downward mantle rock (peridotite) melting points lower by C/.1% water!!

Magma is generated in three ways: Increase in temperature causing rocks to exceed Increase in temperature causing rocks to exceed their melting points ---- melting occurs their melting points ---- melting occurs Decrease pressure (decompression melting), rocks Decrease pressure (decompression melting), rocks ascend to low pressure zones ascend to low pressure zones Introduction of volatiles (principally water) lowers Introduction of volatiles (principally water) lowers rock melting point. rock melting point.

Evolving Magmas: A variety of igneous rocks = wide variety of magmas A variety of igneous rocks = wide variety of magmas observations of many igneous compositions from observations of many igneous compositions from volcanoes volcanoes magma – the parent material of all igneous rocks magma – the parent material of all igneous rocks investigated by N. L. Bowen (famous geologist) investigated by N. L. Bowen (famous geologist) ( ) --- Bowen’s Reaction Series ( ) --- Bowen’s Reaction Series

Magmatic Differentiation the formation of many kinds of igneous rocks from the formation of many kinds of igneous rocks from a single magma a single magmaFe Simple example Fe Fe Fe Mg Mg Mg Mg SiO 2 Liquid magma Part liquid/solid FeSiO 2 MgSiO 2 FeSiO 2 MgSiO 2 FeSiO 2 MgSiO 2 solid liquid SiO 2 Cooling How has the liquid magma changed composition? As the liquid magma begins to cool, various minerals precipitate as solids and become separated from the liquid melt. This separation of various chemistries changes the composition of the original magma. 33

Changing the composition of magmas: Magmatic Differentiation: separating the xlized minerals from the melt changing the overall magma composition Assimilation and Mixing Magma: mixing magmas with various compositions Magmatic Differentiation Mixing Magmas

Changing the magma from a basaltic composition to a granitic composition

Composition of magmas – explained by P.T. The Earth- One big chemistry set! Basaltic composition Granitic composition Basalt magma: Assimilating oceanic plate material (basalt + basalt = basalt magma) Rocks concentrated in mafic minerals – formed high on BRS Low in Silica Granitic magma: Assimilating oceanic plate material (basalt + continental = granitic magma) Rocks concentrated in felsic minerals – formed low on BRS High in Silica

What types of features are formed when magma cools below the surface? 37

Intrusive Igneous Rock Activity Igneous rocks solidifying below the surface can be classified as: tabular or massive : (“table-tops” or irregular”) discordant: cuts across other rock bodies concordant : parallels other rock bodies

Tabular intrusive bodies forming below the earth’s surface Dikes Sill Tabular Batholith 38

BatholithBatholith LoccolithLoccolith DikeDike SillSill StockStock Intrusive Bodies 40

Intrusive Bodies: Batholith: intrusive body GREATER than 40 mi 2 Stock: intrusive body LESS than 40 mi 2 Dike: intrusive body cutting across strata (discordant) Sill: intrusive/extrusive body parallel to strata (concordant) Laccolith: “mushroom-shaped” intrusive body forming a dome-like structure

Sierra Nevada Batholith Granite/Diorite Melting magma rises and mixes with continental material (high SiO 2 ) and solidifies beneath the surface. 41

II intrusiveintrusive 1.Given the block diagram below, describe the following plutonic (intrusive) type the following plutonic (intrusive) type features: features: rocks.