Igneous Rock Notes

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 (O), Aluminum (Al), Magnesium (Mg), Calcium (Ca), Sodium (Na), Potassium (K) and Iron (Fe)

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.

III. The Rock Cycle

IV. Studying Igneous Rocks Color: Indicates differences in chemical composition. Crystal size: (mm) Indicates the rate of cooling of a rock. Rocks with larger crystals cooled more slowly 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.

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.

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

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

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

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

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

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]

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

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

Felsic Fine crystals Andesitic Mafic Small crystals Large crystals

X. Plate Tectonics and Igneous Rocks