Chapter 6: Igneous Rocks

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Chapter 6
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What are igneous rocks?

• Igneous rocks gets its name from the Latin word
  for fire, because it is formed from hot molten
  rock (magma) inside the Earth. When the molten
  rock solidifies, it forms solid igneous rock.
• The way the magma cools determines the kind of
  igneous rock that is formed.
• Magma is molten rock, while lava is magma on the
  Earths surface.
• Igneous rocks may be either extrusive if they
  form at the surface (ex. basalt) or intrusive if
  magma solidifies underground (ex. granite).

Watch Video
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From Minerals to Rocks
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Igneous Rock Textures

• Texture refers to a rocks appearance with
  respect to the size, shape and arrangement of its
  grains.
• Extrusive rocks are typically fine-grained
  (grains smaller than 1mm). The grains are small
  because the magma cooled rapidly at the surface,
  and large crystals did not have time to form.
  Some intrusive rocks are fine grained, but these
  would have formed near the surface in cold
  rock.

Extrusive rock called obsidian. This is a
glass-like rock that forms at the surface
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Igneous Rock Textures

• Intrusive rocks usually form at considerable
  depth (more than a few km) and are called
  plutonic rocks (after Pluto, the Roman god of the
  underworld).
• Intrusive rocks are typically coarse-grained
  (bigger than 1mm) because of the slow cooling and
  solidification of magma

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How are igneous rocks classified?

• Igneous rocks names are based on texture
  (basically grain-size) and mineral composition.
• So, each type of igneous rock will have
• 1. a specific combination of minerals.
• 2. coarse or fine grains.
• Plutonic rocks are easier to identify because of
  the larger crystals.

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Common Igneous RocksGroup 1
Granite Rhyolite
Formed inside Earth Formed at surface
Coarse-grained Fine-grained
Quartz crystals visible Crystals too small
Feldspars Feldspars
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Common Igneous RocksGroup 2
Diorite Andesite
Formed inside Earth Formed at surface
Coarse-grained Fine-grained
No quartz Crystals too small to see
Feldspars 30-50 ferromagnesian minerals Feldspars 30-50 ferromagnesian minerals
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Common Igneous RocksGroup 3
Gabbro Basalt
Formed inside Earth Formed at surface
Coarse-grained Fine-grained
Mostly ferromagnesian minerals Mostly ferromagnesian minerals
Some feldspars Some feldspars
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Identification of Igneous Rocks
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Occurrences of Igneous Rock
Occurrence
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What happens when magma cools underground?

• Cooling magma may form intrusive structures, or
  bodies of magma that crystallize/harden after
  flooding surrounding rock layers

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Examples of Intrusive Structures

• 1. Volcanic neck (magma is solidified in the neck
  of a volcano)
• 2. Dike Sills (magma solidifies into a wall
  after being squeezed through two rock layers)
• Ex. Ship Rock in New Mexico, USA

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Shiprock, New Mexico
Volcanic neck
Dike
Shiprock Video (420 mark)
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Dikes Sills
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Examples of Intrusive Structures (contd)

• Plutons are blobs of magma that crystallize
  within the crust. If these plutons make it to
  the surface, large areas of rock (usually
  granite) form huge outcrops of rock at the
  surface. Batholiths are areas of plutons greater
  than 100km2. Batholiths are present in the Sierra
  Nevada Mountains of California and in the
  Appalachian Mountains

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Halifax Pluton, South Mountain Batholith, Nova
Scotia
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Batholith Formation
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Examples of Extrusive Structures

• The most obvious structures made of extrusive
  igneous rock (or igneous rock at the surface)
  are