-Stage 1.2.2-

Igneous Rocks

Definition - Igneous rocks form from the solidification of molten rock (AKA lava or magma)

Lava - molten rock above the ground surface

Magma - molten rock below the ground surface

Igneous Rocks Page Layout

• Websites

• Types of Igneous Rocks

• Identifying Igneous Rocks

• Petrogenesis (Creating Rocks)

• Bowen's Reaction Series

• Definitions Used for Identification

Websites

Types of Igneous Rocks

• There are 2 types of Igneous Rocks

  • Intrusive - (Plutonic) Rocks formed from molten rock Inside the Earth's crust. They cool slower allowing the crystals in the rock to form larger. The slower the rock cools the larger the crystals.

   

  • Extrusive - (Volcanic) Rocks formed from molten rock on the External surface of the Earth. They cool quickly which prevents large crystals from forming. The faster it cools the smaller the crystals.

Identifying Igneous Rocks

The basic types of Igneous Rocks are listed below

Petrogenesis (Creating Rocks)

Bowen's Reaction Series

Understanding Bowen's Reaction Series

• Minerals at the top crystallize first at the magma cools

• These minerals then fall to the bottom of the magma chamber

• The remaining magma is then more rich in lower temperature minerals (i.e. magma that will stay liquid for longer)

• Minerals on the bottom crystallize last within the spaces left between already crystallized minerals

• This generally applies to Intrusive igneous rocks only because Extrusive cools too quickly

• Discontinuous - Means that when the temperature of the magma cools enough the cooler temp mineral starts to form

• Continuous - Means that the minerals that form have a variety of chemical combinations that form depending on the temperature all with similar chemical properties (i.e. all are plagioclase but they mainly range from dark (Ca) to light (Na))

Definitions Used for Identification

Textures

     Visible Crystals

• Pegmatitic - This is when the crystals in the rock are larger than 1 cm across. This is the most common type of rock where gem stones are found due to the large crystal size.

Picture from Wikipedia

• Phaneritic - This is when crystals are between 1 mm and 1 cm

     Not Visible Crystals

• Aphanitic - This is when there are no visible crystals in the rock

• Porpharitic - This is when the background are not visible but there are large crystals "floating" in the mass

• Glassy - This is when there are no visible crystals and the surface is reflective (i.e. like glass)

• Vesicular - This is when the rock contains lots of air holes, causing the rock to be lighter then usual

• Pyroclastic - This is when the rock forms from angular pieces of other rocks in a aphanitic matrix (similar to porpharitic but larger crystals)

Picture from www.pitt.edu

Compositions

• Felsic - "light" rocks, contain light colored minerals (Also low temp minerals See Bowen's Below) (quartz, Potassium feldspar, micas)

• Intermediate - A mixture of felsic and mafic, "salt and pepper" "light and dark" (Potassium feldspar, Sodium Plagioclase, Biotite, Amphibole)

• Mafic - "dark" rocks, contain dark colored minerals (Also high temp minerals See Bowen's Below) (Pyroxene, Amphibole, Calcium Plagioclase)

• Utramafic - "green" rocks, contain green colored minerals (The highest temp on Bowen's) (Olivene)

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