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 minerals, 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)
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3. 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.
*ENR* 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 with few welldeveloped crystals.
4. Classification Based on Where Rocks Cooled:
 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)
forms larger crystals
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5. Types of Magma – all are very hot solutions of
silicates > 1100C
 Felsic (High in silica) – thick, high % SiO2
Low density
More Aluminum
Light in color
Forms rocks composed of Quartz, Orthoclase,
Muscovite, and some Amphibole
Mafic (Low silica) – more fluid, less SiO2
High density
Hotter
More Ca, Fe, and Mg
Dark in color
Forms rocks composed of Hornblende,
Pyroxene, Biotite, and Olivine
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6. Textures of Igneous Rocks:
 Range from glassy (obsidian)
to coarse (granite)
 The size of the crystals depends on the
cooling rate: See Demo
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 gas bubbles. (Vesicular texture)
*ENR* Porphyrytic rocks have a fine-grained crystal
matrix with large crystals embedded throughout
(phenocrysts) This is the result of two cooling
periods, first __slow____, then _____fast_____. (Ex:
A once-dormant volcano erupts)
SEE ESRT CHART
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