GEY 101 - Introductory Geology: Exploring Planet Earth
Igneous Rocks

ROCKS -- There are three kinds of rocks, that are defined on the basis of how they formed.

Igneous Rocks are formed from the solidification of molten rock or magma.

Sedimentary Rocks form through when materials at the earth's surface (sediments) are buried and hardened (lithified).

Metamorphic Rocks are formed when older rocks are changed by heat and pressure without being melted.

Magma

• Parent material of igneous rocks
• Forms from partial melting of rocks inside the Earth
• Magme that reaches the surface is called lava

• Intrusive Rocks -- form when magma solidifies beneath the earth's surface
• Extrusive Rocks -- form when magma solidifies on the earth's surface as lava (at volcanoes).

Components of Magma

• Melt -- liquid portion that is composed of mobile ions
• Solids -- silicate minerals that have already cyrstallized from the melt
• Volatiles -- gases which are dissolved in the melt. This includes water vapor (H2O), carbon dioxide (CO2), and sulfur dioxide (SO2)

Crystallization

• Cooling of magma results in the systematic arrangement of ions into orderly patterns
• The silicate minerals resulting from crystallization form in a predictable order
• texture in igneous rocks is determined by the size and arrangment of mineral grains
• Igneous rocks are typically classified by --
  • texture
  • mineral composition

Texture -- describes the overall appearance of a rock based on the size, shape, and arrangement of interlocking minerals.

Factors affecting crystal size

• Rate of cooling
  • Slow rate promotes growth of fewer but larger cystals
  • fast rate forms many small crystals
  • very fast forms glass
• Amount of silica (SiO2) present
• Amount of dissolved gases

Types of Igneous Textures

• Phaneritic (or Coarse Grained) texture is when all the crystals are large enough to see.
• Aphanitic (or Fine Grained) texture when most crystals are too small to see.
• Porphyritic texture is when large crystals are floating in an aphanitic matrix.
• Fragmental or Pyroclastic textures form in explosive eruptions.
• Glassy textures forms when a melt cools too quickly for crystals to form.
• Pegmatitic texture forms exceptionally coarse grained rocks, late stage.

Mineralogy Igneous rocks are composed of primarily silicate minerals.

• Dark Silicates
  • Olivine
  • Pyroxene
  • Amphibole
  • Biotite
• Light Silicates
  • Quartz
  • Muscovite
  • Feldspars

Igneous Compositions

• Granitic
  • Composed of mainly light colored minerals
  • felsic - >65% SiO2, much Na, K
  • major constituents of continental crust
• Basaltic
  • composed mainly of dark colored minerals
  • mafic - < 52% SiO2, much Fe, Mg
  • more dense than granitic rocks
  • comprise the ocean floor as well as mant volcanic islands
• Intermediate (Andesitic)
  • 25% dark silicate minerals
  • associated with explosive volcanic activity
• Ultramafic
  • high in Mg and Fe, rare
  • composed entirely of dark silicate minerals

• A low of 45% in Ultramafic rocks
• over 70% in felsic rocks
• Granitic Magmas
  • High Silica
  • Extremely viscous
  • Liquid exisit at temperatures as low as 700 degrees C
• Basaltic Magmas
  • Much lower Silica
  • Fluid-like behavior
  • crystallizes at higher temperatures

Rock Names

• Granite
• Rhyolite
• Obsidian
• Pumice
• Andesite
• Diorite
• Basalt
• Gabbro
• Peridotite
• Scoria Tuff - Welded Tuff
• Pillow basalts Breccia

Origin of Magma -- Highly debated, Generating magme from solid rock

• Produced from parital melting of rocks in the crust and upper mantle
• Role of Heat
  • Geothermal gradient
  • Lower crust and upper mantle rocks near melting points
  • any additional heat may induce melting
• Role of Pressure
  • Increase confining pressure --> increase rock's melting temperature or vice versa
  • Confining pressure drops --> Decompression melting occurs
• Volatiles
  • Primarily water causes a lower melting temperature
  • Important where oceanic lithosphere descends into the mantle

Evolution of Magma

• A single volcanoe can exhibit different compositions
• Bowen's reaction series -- Bowen's reaction series shows the entire sequence of igneous minerals and explains the associations different minerals in different igneous rocks. As a magma cools different minerals crystallize out at different temperatures. Olivine and Calcium-Plagioclase crystals form first. These minerals are rich in Magnesium, Iron and Calcium if these crystals are removed by crystal settling, the remaining melt is enriched in Sodium, Potassium, Silicon and Aluminum. The left branch of the chart is the discontinuous branch. As the melt cools olivine reacts with the melt to form pyroxene. Then pyroxene with amphibole. You only find two adjacent minerals from this branch in a rock. The Plagioclases are constantly reacting and changing composition.

  

• Changing Composition -- There are three major igneous substances magma, pyroclastic materials and gasses. Magmas usually are 900 to 1300 °C, felsic magmas are cooler, mafic magmas are hotter. Felsic magmas are more viscous and more gaseous, mafic magmas are more fluid and less gaseous.
  • Differentiation of Igneous Magmas -- Separation of a melt from earlier formed cystals to form a different composition of magma.
  • Assimilation -- Changing a magma's compostion by the incorporation of foriegn matter (surrounding rock bodies) into a magma

Processes Responsible -- The different types of igneous rocks can be explained by magmatic differentiation. Differentiation is important because almost all of the igneous rocks can only have formed by repeated changes in their composition.

• partial melting -- where only some minerals melt to make a magma.
• assimilation -- the changing of magmatic composition by melting or partial melting of country rock, crystal settling, where certain minerals crystalize first and settle from the melt.
• magma mixing -- where two different magmas, with different compositions mix.

Self Test

We call a primitive magma a magma that has undergone little differentiations. What minerals would you expect to find in an igneous rock crystallized from a primative melt?

Draw Bowen's reaction series from memory.

Describe one possible history for an intrusive rock that contains large crystals of olivine.

Additional Study Guide

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