1. Igneous Rock Notes

2. I. Composition of the Earth’s Crust
The earth’s crust is composed of rocks.
A rock is defined as two or more minerals, found in the earth’s crust bonded together in a solid form
98% of the earth’s crust is composed of the following 8 elements:
Silicon (Si), Oxygen (O2), Aluminum (Al), Magnesium (Mg), Calcium (Ca), Sodium (Na), Potassium (K) and Iron (Fe)

3. II. Rock Types Sedimentary rocks -
formed by the hardening and cementing of layers of sediment.
Metamorphic rocks -
formed when rocks that already exist are changed by heat and pressure into new kinds of rocks.
Igneous rocks -
formed by cooling and hardening of hot molten rock from inside the Earth.

4. III. The Rock Cycle

6. IV. Studying Igneous Rocks
Color:
Indicates differences in chemical composition.
Crystal size: (mm)
Indicates the rate of cooling of a rock, and depth of formation
Rocks with larger crystals cooled more slowly, and deeper in the crust than rocks with smaller crystals.
C. Density: mass (g) ÷ volume (ml or cm3)
Indicates differences in depth of formation and chemical composition.
More dense rocks form from magma that originates deeper in the mantle, and has a higher % if iron.

7. V. Igneous Rock Formation
A. Three major factors affect the chemical composition, crystal size and density of Igneous Rock:
Source of parent magma.
Interaction between the parent magma and the rocks of the crust.
Rate of cooling.

8. VI. Types of Igneous Rocks
Intrusive Igneous Rock
Definition: rock formed underground when magma cools.
Magma: molten (liquid) rock found beneath the earth’s surface.
1. From Magma (Intrusive ) Cooled slowly
Deep Shallow
Large/Coarse Small
Crystals Crystals

9. B. Extrusive Igneous Rock
Definition: rock formed on the surface when lava cools.
Lava: molten (liquid) rock that emerges from the crust through cracks or fissures.
1. From Lava (Extrusive)
Cooled quickly
Surface
Fine Crystals
Glassy Rocks

10. VII. Igneous Rock Characteristics
A. Felsic Rocks
Chemical Composition: High percent of silicates (Si) and water. Low percent of iron (Fe), calcium (Ca), & magnesium (Mg)
Color: Light colored minerals
Viscosity: The degree to which a substance resists flow.
Relative Level: High (Low temperatures and slow flowing)
Trapped gases: High level: H2O, O2, CO2, H2, CO, H2S, SO2
Relative Density Level: Low (high gas content)
Volcanic source: Explosive volcanoes and cinder cones (ex. Mt. Pinatubo)
Location: Continental Crust

11. B. Mafic Rocks
Chemical Compostion: Low percent of silicates (Si). High in Calcium (Ca), Iron (Fe) and Magnesium (Mg).
Color: Dark colored minerals
Relative Viscosity Level: Low (high temperature and more fluid)
Trapped gases: Low level
Relative Density Level: High (low gas content)
Volcanic source: “Gentle” eruptions from broad shield volcanoes (ex. Hawaii)
Location: Oceanic Crust and Hot Spots

12. C. Andesitic (Intermediate) Rocks
Chemical Composition: Intermediate percent of silicates (Si). Intermediate in Ca, Fe, Mg.
Color: Intermediate colored minerals
Relative Viscosity Level: Intermediate
Trapped gases: Intermediate amounts present
Relative Density Level: Intermediate
Volcanic source: Intermediate eruption volcanoes
Location: Continental Crust

13. VIII. Review Mostly light Mostly dark
A. Felsic B. Mafic
Mostly light Mostly dark
colored minerals colored minerals
[ Si] [Ca, Fe, Mg] [ Si] [Ca, Fe, Mg]
C. Andesitic
Contain light and dark minerals
Intermediate levels of [Si, Ca, Fe, Mg]

14. Note: Dissolved Gases Include H2O, S, CO2, H2, CO, H2S, SO2
A. Felsic Dissolved B. Mafic Dissolved Gases Gases
Thick & Slow Thin & Fast
Hard For Gases Gases Escape
To Escape Easily
Explosive Eruptions “Gentle” Eruptions
Note: Dissolved Gases Include H2O, S, CO2, H2, CO, H2S, SO2

15. IX. Summary
Lighter colored rocks
Darker colored rocks
Large crystals
Intrusive
Small crystals
Extrusive
No crystals

16. Felsic
Fine crystals
Andesitic
Mafic
Small crystals
Large crystals

17. X. Plate Tectonics and Igneous Rocks