CSULA - Geology Igenous Rocks

Earth as a system: The rock cycle

The Rock Cycle
- Magma = Crystallization
- Igneous rock = Weathering, transportation, and deposition
- Sediment = Lithification
- Sedimentary rock = Metamorphism
- Metamorphic rock = Melting = Magma
Full cycle does not always take place due to "shortcuts" or interruptions
- e.g., sedimentary rock melts
- e.g., igneous rock is metamorphosed
- e.g., sedimentary rock is weathered
- e.g., metamorphic rock weathers
Igneous Rocks

Form as magma cools and crystallizes
Rocks formed inside Earth
Plutonic (intrusive) rocks
Rocks formed on the surface
Volcanic (extrusive) rocks

Magma

The nature of magma
- Consists of three components:
  - A liquid portion, called melt, that is composed of mobile ions
  - Solids, if any, are silicate minerals that have already crystallized from the melt
  - Volatiles, which are gases dissolved in the melt, including water vapor (H2O), carbon dioxide (CO2), and sulfur dioxide (SO2)
Crystallization of magma
- Ions are arranged into orderly patterns
- Crystal size is determined by the rate of cooling
  - Slow cooling = large crystals
  - Fast cooling = microscopic crystals
  - Very fast cooling = glass
- The silicate minerals resulting from crystallization form in a predictable order (Bowen's Reaction Series)
- Texture in igneous rocks is determined by the size and arrangement of mineral grains
  - Igneous rocks are typically classified by
    - Texture
    - Mineral composition

Igneous textures

Aphanitic / fine-grained texture
- Rapid rate of cooling of lava or magma
- Microscopic crystals
- May contain vesicles (holes from gas bubbles)
- Typically occurs in extrusive / volcanic rocks
Phaneritic / coarse-grained texture
- Slow cooling
- Crystals can be identified without a microscope
- Typically occurs in intrusive / plutonic rocks
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
Pyroclastic texture
- Various fragments ejected during a violent volcanic eruption
- Textures often appear to more similar to sedimentary rocks
Pegmatitic texture
- Exceptionally coarse grained
- Form in late stages of crystallization of granitic magmas

Igneous Compositions

Composed mainly of silicate minerals
- Dark silicates = rich in iron and/or magnesium
  - Olivine
  - Pyroxene
  - Amphibole
  - Biotite mica
- Light silicates = greater amounts of potassium, sodium, and calcium
  - Quartz
  - Muscovite mica
  - Feldspars
Igneous rocks are often classified by mineral (chemical) composition
- Ultramafic igneous rocks
  - Rare composition that is high in magnesium and iron
  - Composed entirely of ferromagnesian silicates
  - Common rock is peridotite (intrusive) or komatiite (extrusive – very rare)
- Mafic (or basaltic) igneous rocks
  - Mafic: magnesium and ferrum (iron)
  - Contain substantial dark silicate minerals and calcium-rich plagioclase feldspar
  - More dense than granitic rocks
  - Comprise the ocean floor as well as many volcanic islands
  - Common rock is gabbro (intrusive) or basalt (extrusive)
- Intermediate (or andesitic) igneous rocks
  - Contains at least 25% dark silicate minerals
  - Associated with explosive volcanic activity
  - Common rock is diorite (intrusive) or andesite (extrusive)
- Felsic (or granitic) igneous rocks
  - Felsic: feldspar and silica
  - Composed almost entirely of light-colored silicates—quartz and feldspar
  - High silica content (about 70%)
  - Major constituents of continental crust
  - Common rock is granite (intrusive) or rhyolite (extrusive)
- Bowen’s Reaction Series
  - Magma crystallizes over a temperature range of several hundred degrees
  - Therefore, minerals crystallize in a predictable order
  - Last minerals to crystallize are very different in composition from the earlier formed minerals
- Ways to change a magma’s composition:
  - Magmatic differentiation
    - The formation of one or more secondary magmas from a single parent magma
    - Crystal settling
    - Earlier-formed minerals are denser than the liquid portion and sink to the bottom of the magma chamber
  - Assimilation
    - Changing a magma’s composition by the incorporation of foreign matter (surrounding rock bodies) into a magma
  - Magma mixing
    - Involves two bodies of magma intruding one another
    - Two chemically distinct magmas may produce a composition quite different from either original magma

Igneous Rock Classification

Mafic Rocks
- Basalt
  - Volcanic / extrusive origin
  - Aphanitic texture
  - Composed mainly of pyroxene and calcium-rich plagioclase feldspar
  - Most common extrusive igneous rock
- Gabbro
  - Intrusive equivalent of basalt
  - Phaneritic texture consisting of pyroxene and calcium-rich plagioclase
  - Makes up a significant percentage of the oceanic crust
Intermediate Rocks

Andesite
- Volcanic / extrusive origin
- Aphanitic texture
- Often resembles rhyolite
Diorite
- Plutonic equivalent of andesite
- Coarse grained
- Intrusive
- Composed mainly of intermediate feldspar and amphibole
- “Black and white makes DIORITE”

Felsic Rocks

Rhyolite
- Volcanic / extrusive origin
- May contain glass fragments and vesicles
- Aphanitic texture
- Less common and less voluminous than granite
Granite
- Plutonic equivalent of rhyolite
- Over 25 percent quartz, about 65 percent or more feldspar
- Very abundant as it is often associated with mountain building
- The term “granite” covers a wide range of mineral compositions

Varied Compositions

Obsidian
- Volcanic
- Dark colored
- Glassy texture
Pumice
- Volcanic
- Glassy texture
- Frothy appearance with numerous voids

Pyroclastic rocks
- Composed of fragments ejected during a volcanic eruption
- Varieties

Tuff – ash-sized fragments
Volcanic breccia – particles larger than ash

Earth as a system: The rock cycle

Magma

Igneous textures

Igneous Compositions

Igneous Rock Classification

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