What are Igneous Rocks?

From the Latin word for “fire” - ignis Thus, rocks that are “fire-formed” Molten rock (magma) cools to form a solid rock

Magma is molten rock, and contains:

• Melt - Liquid portion of the magma, composed of mobile ions
• Solids - Silicate minerals that have already crystallized from the melt
• Volatiles - gases dissolved in the melt, including water vapor (H₂O), carbon dioxide (CO₂), and sulfur dioxide (SO₂)

 

Identifying Igneous Rocks

 

Magma Compositions

Magma is composed mainly of silicate minerals.

Dark minerals iron and/or magnesium rich	Light minerals potassium, sodium, & calcium rich
Olivine Pyroxene Amphibole Biotite mica	Quartz Muscovite mica Feldspars

 

Bowen's Reaction Series

Bowen proposed (1922) that any one magma can potentially crystallize into rocks of different compositions because of two processes

 

Recall that the minerals at the top of the series . . .

• Have the highest melting temperatures
• Are dark in color
• Are rich in Ca (anorthite) Fe and Mg (e.g. olivine)
• And have the highest specific gravity.

Minerals at the bottom of the series. . .

•  Are light colored.
•  Are rich in Na (albite), K (orthoclase) and Si (quartz).
•  And are of low specific gravity.