NOTES: IGNEOUS ROCKS

AIM: What are the characteristics of igneous rocks?

  1. IGNEOUS ROCKS are formed by the cooling and hardening (freezing) of hot, molten rock (magma / lava) from within the Earth’s crust.
  2. Igneous rocks are composed of ______ the most important of which are:
  • Plagioclase Feldspar (Ex: Albite)
  • Potassium- Feldspar (Ex: Orthoclase)
  • Quartz
  • Amphibole (Ex: Hornblende)
  • Olivine
  • Pyroxene (Ex: Augite)
  • Mica (Ex: Muscovite, Biotite, Lepidolite)
  1. Crystallization of minerals from a magma occurs between 1200 – 600C.

Those minerals with the highest freezing point crystallize first and thus develop larger, well-formed crystal shapes. Those minerals that crystallize at lower temperatures are forced to grow in the spaces between the earlier formed crystals and are commonly irregular in shape and smaller.

  1. Classification Based on “Environment of Formation”:
  2. Extrusive (Volcanic) – formed when hot lava pours out of a volcano and solidifies.

Cooling is fast (seconds – weeks)

forms smaller crystals (demo)

  • Intrusive (Plutonic) - formed when magma solidifies beneath the surface.

Cooling is slow (up to 105 years)

and forms larger crystals

  1. Types of Magma – all are hot solutions of silicates > 1100C
  • Felsic(High in silica) – thick, high % SiO2

Low density

Lower melting point

More Aluminum

Light in color

Forms rocks composed of Quartz, Orthoclase, Muscovite, and some Amphibole

Mafic (Low % silica) – more fluid, less SiO2

High density

Higher melting point

More Ca, Fe, and Mg

Dark in color

Forms rocks composed of Hornblende, Pyroxene, Biotite, and Olivine

  1. Textures of Igneous Rocks:
  2. Range from glassy (obsidian)=Non-crystalline

to coarse (granite)

  • The size of the crystals depends on the cooling rate:

Slow cooling  large crystals (coarse)

Environment of formation______

Rapid cooling  small crystals (fine)

Environment of Formation______

Volcanic eruptions can cause lava to be churned into a froth (suds) of lava and gas bubbles. Once solidified, this process forms Pumice or Scoria. The bubbles of gas are called gasholes. (Vesicular texture)

Porphyrytic rocks have a mostly fine-grained crystal matrix with large (coarse) crystals embedded throughout (phenocrysts). This is the result of two cooling periods, first ______, then ______. (Ex: A once-dormant volcano erupts)

SEE ESRT CHART

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