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Chapter 6:. 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.

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what are igneous rocks
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 Earth’s surface.
  • Igneous rocks may be either extrusive if they form at the surface (ex. basalt) or intrusive if magma solidifies underground (ex. granite).

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igneous rock textures
Igneous Rock Textures
  • Texture refers to a rock’s 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

igneous rock textures1
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
how are igneous rocks classified
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.

what happens when magma cools underground
What happens when magma cools underground?
  • Cooling magma may form intrusive structures, or bodies of magma that crystallize/harden after flooding surrounding rock layers
examples of intrusive structures
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

Shiprock, New Mexico

Volcanic neck


Shiprock Video (4:20 mark)

examples of intrusive structures cont d
Examples of Intrusive Structures (cont’d)
  • 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
examples of extrusive structures
Examples of Extrusive Structures
  • The most obvious structures made of extrusive igneous rock (or igneous rock at the surface) are: