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Unit 3: Earth Materials Lesson 3: Classifying Rocks (Heath Earth Science pg. 62-78) Today’s Objectives Differentiate between rocks and minerals, including: Classify rocks as igneous, sedimentary, or metamorphic using texture and composition Describe the relationship between crystal size and cooling rate in igneous rocks Classify igneous rocks as volcanic (extrusive) or plutonic (intrusive) on the basis of texture Review Last day we learned the 3 main types of rocks, and how these rocks are formed in the rock cycle The 3 rock types are: Igneous Sedimentary Metamorphic Today, we will look more closely at these rock types and how to identify them The Rock Cycle Igneous Rocks Igneous rocks are formed by the cooling and hardening of hot molten rock from inside of the earth This hot molten rock is called magma When magma reaches the surface of the earth during a volcanic eruption it is called lava When magma cools, elements in it form distinct, interlocking mineral grains, or crystals There are two general types of magma: Felsic (granitic) magma: high % silica (SiO2); thick, slow flowing; forms light-coloured minerals/rocks Mafic (basaltic) magma: low % silica; hotter, thinner, fast flowing; forms dark-coloured minerals/rocks Igneous Rocks Rocks that form underground from cooled magma are called plutonic, or intrusive, igneous rocks These rocks are only seen at the surface after the rock that covers them is worn away These rocks tend to have large mineral grain (crystal) size because the magma cools more slowly beneath the surface Rocks that form on the surface from cooled lava are called volcanic, or extrusive, igneous rocks These rocks tend to lack, or have very small mineral grains because they cool very rapidly Igneous Rock Textures A rock’s texture depends on the size, shape, and arrangement of it’s mineral grains, or crystals Glassy Texture: produced by very rapid cooling No visible grains Fine-grained texture: Produced when lava cooled quickly on or near Earth’s surface Small grains Porphyritic texture: Produced by slow then rapid cooling Has both fine and coarse grains Coarse-grained texture: Produced when magma cools slowly at depth Large (coarse) grains Grain size and Cooling Rate Igneous Rocks Glassy Textures Igneous Rocks Classifying Igneous Rocks Practice: Topic Questions: Page 68, #3-8 Sedimentary Rocks Sedimentary Rocks Sedimentary rocks form when sediments harden into rocks There are three main kinds of sedimentary rocks: Clastic (Detrital): Formed from fragments of other rocks Ex.) shale, sandstone, conglomerate Chemical: Formed from mineral grains that fall out of a solution (precipitate) by evaporation or chemical action Ex.) rock salt, limestone Organic: Formed from the remains of plants and animals Ex.) coal, limestone made of shell fragments (coquina) Sedimentary Rocks Clastic Sedimentary Rocks: Conglomerate (top left), Breccia (bottom left), Shale (right) Sedimentary Rocks Clastic Sedimentary Rock: Sandstone Organic Sedimentary Rock: Coquina Sedimentary Rocks Chemical Sedimentary Rocks Limestone Rock Salt Sedimentary Rock Features The origin of some sedimentary rock can be discovered by observing certain features Ripple marks indicate the rock was formed under water moving water or on a beach Mud cracks indicate the rock was formed from mud that dried and shrunk, such as on a tidal plain or desert Sedimentary Rock Features Cross bedding may develop when beds are deposited by wind on sand dunes or deposited by rivers on deltas or sandbars Sedimentary rocks with visible layers are said to be stratified Classifying Sedimentary Rocks Practice: Topic Questions: page 74-75, #9-17 Metamorphic Rocks Metamorphic Rocks Metamorphic rocks are formed when igneous rocks, sedimentary rocks, or even other metamorphic rocks are altered due to intense heat and/or pressure There are two kinds of metamorphism: Regional metamorphism occurs when large areas of rock are under intense heat and pressure that causes them to change form Contact metamorphism occurs when hot magma forces its way into overlying rock and bakes the rocks that are in contact with it Much smaller scale than regional metamorphism Metamorphism Results of Metamorphism Under the pressure of metamorphism, some mineral grains in the parent rock become reoriented and aligned at right angles to the stress The resulting orientation of mineral grains gives the rocks a foliated (layered) texture Some rocks are under pressure from more than one direction (differential stress) that they may produce folds (such as the rocks on the first slide of metamorphic rocks) Foliation Foliated and Non-Foliated Rock Classifying Metamorphic Rocks Practice: Topic Questions: page 79, #19-23 Homework Create a rock cycle diagram/rock description according to the provided rubric One page diagram One page describing 3 examples of each rock type with all the classification information Due date: Monday, December 29 Mineral Identification Quiz next class!