1. Formation and Characteristics
Igneous Rocks
Formation and Characteristics

2. Igneous Rocks
Igneous rocks are formed from the crystallization of magma.
Molten rock can occur in three different ways:
Magma: molten rock below Earth’s surface
Lava: magma on the Earth’s surface
Pyroclastic material: (pyro=fire, clastic=debris) Airborne lava that cools at it falls.

3. Magma
When we talked about Earth’s crust we said that the most common elements were:
Oxygen
Silicon
Aluminum
Iron
Magnesium
Calcium
Potassium
Sodium
We said that the crust was mostly Silicate (SiO2) rocks, magma is classified by its Silica content.

4. Magma Mafic magma (Basaltic) Intermediate magma (Andesitic)
Thin and runny
LOW silica content (<50%)
LOW gas content
HIGH in Ca, Fe, and Mg
Intermediate magma (Andesitic)
Medium Consistancy
MODERATE silica content (~60%)
MODERATE gas content
MODERATE amount of all crustal elements

5. magma Felsic magma (Rhyolitic) Thick and blocky HIGH silica content
HIGH gas content
LOW in Ca, Fe, and Mg
HIGH in K and Al

6. Viscosity of Magma/ Lava
Viscosity: the resistance of a liquid to flow.
Very important for volcanic activity
MAFIC- low silica, low viscosity, runny, less violent eruptions
FELSIC- high silica, high viscosity, thick, more violent eruptions

7. Igneous rock composition
98.5% of earths crust is made of only 8 elements!
Oxygen and Silicon make up 75%
Aluminum=8.1%
Iron=5%
Calcium=3.6%
Sodium=2.8%
Potassium=2.6%
Magnesium=2.1%
Sooooooo, the type of rock created is mainly determined by:
What elements are present during cooling
How rapidly does the rock cool
Neatly, illustrated by Bowen’s reaction series.

8. Bowen’s reaction series

9. Igneous Textures Igneous rocks are classified in two ways
Composition
Textures (How big are the mineral grains)
Phaneritic: can be seen with naked eye different colored and interlocking shapes
Aphanitic: mineral grains too small to be seen, microscopic
Porphyritic: two distinct sizes of mineral

10. Phaneritic Texture Intrusive rock Cooled beneath Earth’s surface
Cooled very slowly
Higher temperature and pressure

11. Aphanitic texture Extrusive Igneous Rock Cooled on Earth’s surface
Cooled relatively fast
Lower temperature and pressure

12. Porphyritic Texture Complex cooling
Partially cools below and above Earth’s surface
Distinct crystal sizes

13. Composition
Igneous rocks are named by texture and mineral composition

14. Summary Magmas have three main types
Mafic
Intermediate
Felsic
Composition and cooling rate determine what minerals will form
Three types of igneous textures
Phaneritic Texture
Aphanitic Texture
Porphyritic Texture
Put it all together and you can identify your igneous rock