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

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

Magma: The parent material for all rocks There are three distinct components of magma: Liquid portion referred to as “melt” “liquid rock” – low density 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 materials at the Earth’s surface (H2O, CO2 and SO2)

Melting solid rock into magma How would the process of crystallization work? 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.

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

Typical temperature depending on chemical composition: Magma Typical temperature depending on chemical composition: 7000C (1,2920F) – 13000C (2,3720F) High in Si,O High in Fe,Mg

I Geology , especially igneous rocks! Describe how solid rock can covert from the solid phase to liquid rock Describe the three components to magma 3. What is the temperature range for high Si and high Fe type magmas?

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

Temperature inside the earth Geothermal gradient 500 1000 1500 2000 the rate at which temperature increases with depth Continent gradient In thicker crust, gradient increases. average 7oC/km rate temperature increases gently 100 5,000 Depth (km) 200 Pressure (mpa) 10,000 300 Oceanic gradient Below the ocean floor, temperature increases rapidly. average 130C/km 15,000 400

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 1000C/.1% water!!

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

I Geology , especially magma 4. Briefly describe the role of magma production from the geothermal gradient, decompression melting, and volatiles. 5. Why is the earth’s mantle solid --- when it should be “liquid”?

Evolving Magmas: A variety of igneous rocks = wide variety of magmas observations of many igneous compositions from volcanoes magma – the parent material of all igneous rocks investigated by N. L. Bowen (famous geologist) (1887-1956) --- Bowen’s Reaction Series

Magmatic Differentiation the formation of many kinds of igneous rocks from a single magma liquid Simple example SiO2 SiO2 SiO2 Mg SiO2 Fe SiO2 SiO2 Fe SiO2 Mg SiO2 Fe FeSiO2 MgSiO2 Mg FeSiO2 Fe MgSiO2 FeSiO2 Mg SiO2 Cooling SiO2 SiO2 FeSiO2 MgSiO2 solid SiO2 Liquid magma Part liquid/solid 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.

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

I Geology , especially magma 6. Explain why there are a verity of igneous rock compositions. 7. Distinguish between magmatic differentiation magma assimilation 8. What is Bowen’s reaction series?

Solidified above the Earth’s surface Intrusive 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 visible grains Extrusive Rocks Extrusive Rocks Solidified above the Earth’s surface Volcanic rocks – “the roman God of fire” magma cooled fast allowing microscopic xl growth (can’t see grains) fine-grained textures (can’t see grains) Hawaiian Volcanoes, Cascade Range 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???????? Checking the crystal size - does the rock contain visible crystals or are crystals non-existent? Igneous Rock Textures (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

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

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

I Geology, especially ig rocks! 9. Briefly describe the textural differences between intrusive and extrusive ig rocks. 10. What four factors influence igneous rock textures? 11. Why are pegmatites unique ?

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

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

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 -

I Geology, especially ig rocks 12. Distinguish the textural differences between porphyritic and fragmental (tuffaceous) – Draw a picture 13. What type of geological environments may be responsible for igneous rocks having a glassy, porphyritic or fragmental 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 Non-ferromagnesian (felsic) high in Si, O low in Fe, Mg light colored rocks Ca-plagioclase orthoclase Muscovite Quartz

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

I Geology, especially ig rocks 14. Explain how igneous rocks are classified? 15. Explain the compositional differences between light and dark colored igneous rocks. 16. Give examples (rock name) of the following: light colored intrusive rock dark colored extrusive rock intermediate extrusive rock light colored extrusive rock :

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

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 Tabular intrusive bodies forming below the earth’s surface 38

Intrusive Bodies Sill Loccolith Dike Stock Batholith

Intrusive Bodies: Batholith: intrusive body GREATER than 40 mi2 Stock: intrusive body LESS than 40 mi2 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 Melting magma rises and mixes with continental material (high SiO2) and solidifies beneath the surface. Sierra Nevada Batholith Granite/Diorite 41

I intrusive rocks. 17. Given the block diagram below, describe the following plutonic (intrusive) type features: