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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 ___________
family
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. 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.
4. Classification Based on “Environment of Formation”:
 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
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5. 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
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6. Textures of Igneous Rocks:
 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 gas holes. (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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