c. Classify rocks by their process of formation. d. Describe processes that change rocks and the surface of the earth. Identified by origin, texture, and mineral composition. From the Latin word “ignis,” which means fire. Tend to have average densities. Igneous Rocks form from the cooling of molten rock (lava and magma). LAVA is molten material found on the surface of the earth. MAGMA is molten material found beneath the surface of the earth. Intrusive rock Extrusive Rock At or near surface Inside Earth Magma Lava Cools slowly Cools fast Coarse-grained Fine Grained Large Crystals Small Crystals Classified by Formation: Extrusive Rock is formed from lava. Magma Exits the Earth to form lava at the surface. (Basalt) Classified by Formation: Intrusive Rock is formed when magma hardens beneath Earth’s surface. Inside the Earth (Granite) TEXTURE TEXTURE reflects cooling rate of magma or lava. RAPID COOLING = SMALL(fine) Crystals RAPID COOLING = VOLCANIC SLOW COOLING = LARGE (coarse) Crystals SLOW COOLING = PLUTONIC Classified by Texture: Fast Cooling Lava (Extrusive) Fine-Grained, small crystals, smooth Slow Cooling Magma (Intrusive) Coarse-Grained, large crystals, rough Aphanitic Phaneritic Glassy Grain Size (Texture) Aphanitic (fine grained) = grains are very fine -such rocks are formed when the rock solidifies at or near the surface (extrusive) Small crystals < 2 mm Fast cooling rates Rhyolite Felsite Basalt Scoria TEXTURE Types of Textures Phaneritic (coarse grained) = large, identifiable crystals -are formed when rocks solidified far below the surface (intrusive) Large crystals > 2 mm Slow cooling rate = Plutonic (formed in a pluton – a cave of magma within the volcano) TEXTURE Types of Textures Glassy = crystal formation does not occur -rocks are formed when molten rock is suddenly ejected from a volcano No crystals, lava cools too fast Extremely fast cooling rate = Obsidian An intrusive igneous rock would have what type of texture? Why? Coarse-Grained, large crystals. Smooth & rough. It is hotter in the earth than on the outside of the earth so it takes longer for magma to cool down. An extrusive igneous rock would have what type of texture? Why? Fine-Grained or glassy texture It is cooler on the surface of the Earth so the rocks can cool faster. MINERAL COMPOSITION You can determine basic igneous rock composition just by the COLOR of the rock. DARK COLORED igneous rocks = MAFIC LIGHT COLORED igneous rocks = FELSIC GRAY COLORED igneous rocks = INTERMEDIATE Lighter colored igneous rocks will have lots of silica. They tend to be less dense than the darker versions. Less silica means these rocks will be darker than their felsic counterpart. They will be denser also.