1. ROCK FORMATION A rock is a group of minerals with common characteristics. Igneous rocks: formed by cooling magma or lava

2. Sedimentary Rocks: Formed by layers of materials cementing together. Metamorphic rocks:Formed when heat and pressure transform other rocks.

3. Igneous rocks are grouped in two ways. How they cool: Intrusive (plutonic) rocks cool slowly under the Earth’s surface. This causes the rock to look speckled. Extrusive (volcanic) rocks cool quickly on top of the Earth’s surface. This causes the rock to have the same color throughout.

4. What they’re made of Felsic rocks contain mostly Na, Al, or Si minerals. These are light colored (gray, white, or pink) and low density. Mafic rocks contain mostly Mg or Fe minerals. These are dark colored (black or dark green) and high density.

5. ROCK TYPES Felsic—cool thick magma Mafic—hot runny magma Plutonic— cools slowly within the crust GraniteGabbro Volcanic— cools quickly on the surface RhyoliteBasalt

6. CRYSTALS IN IGNEOUS ROCK Large crystals mean the magma cooled very slowly (under plutonic conditions). This causes large crystals to form, giving the mineral a speckled appearance. Small crystals mean the magma cooled very quickly (under volcanic conditions). This causes the mineral to have a uniform color.

7. OTHER IGNEOUS ODDITIES Obsidian: lava cools so quickly that no crystals form. Obsidian is also called “volcanic glass” because it looks like glass. Porphyry: large crystals are surrounded by small crystals (looks like a fruitcake). This happens when a rock cools in two different places.

8. The large crystals in porphyry form deep in the Earth, then the small crystals form close to the surface. Porous rocks: large amounts of gas trapped in magma create large holes in the rock. Porous rocks are usually less dense than water, so they float. Porous felsic rock is pumice Porous mafic rock is scoria