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Rocks Igneous Rocks Sedimentary Rocks Metamorphic Rocks The Rock Cycle When classifying a rock sample geologists observe the rock’s color and texture and determine its mineral composition. Texture: the size, shape, and pattern of the rock’s grain. Color: the apparent color of the rock, on the inside and the outside. Mineral composition: The minerals that make up the different parts of a rock. Texture: Grain Size Often, the grains in a rock are large and easy to see. Such rocks are said to be coarse-grained. In other rocks, the grains are so small that they can only be seen with a microscope. These rocks are said to be fine-grained. Notice the difference in texture between the finegrained slate and the coarse-grained diorite to the right. Igneous Rocks Igneous means ‘of fire’ Igneous rocks are formed when magma or lava cools and hardens into rocks Intrusive Igneous Rocks Intrusive or plutonic rocks form deep within the Earth’s crust. They cool slowly so crystals can form. Sometimes they cool as a large mass of rock called a batholith or laccolith. Over time these rock formations can be pushed up to the surface to form mountains. Extrusive Igneous Rocks Extrusive or volcanic rocks form at or near the Earth’s surface. The lava or magma cools quickly so crystals don’t have time to form. Sometimes gas bubbles are trapped inside the cooling lava making a rock like pumice, or sometimes it hardens like black glass called obsidian. Sedimentary Rocks Sedimentary rocks form when there is a build up of deposited material called sediments. As new layers are added the ones on the bottom become compacted together by the heavy weight from above. Water is often involved in forming sedimentary rocks, as sediments fall to the bottom of seas, rivers and lakes. Sedimentary Rocks As sediments settle slowly into layers, the smallest particles squeeze in between the larger pebbles and rocks, cementing them together. Often these rocks are rich in fossils as the remains of plants and animals get trapped in the sediments over time. Three types of Sedimentary Rocks Clastic Rocks: these rocks are made up of rock fragments mixed with sand and clay which cements them together. These rocks are called conglomerates, sandstones, shale, or siltstone depending on the size of the fragments found in them. Three Types of Sedimentary Rocks Organic Rocks: these rocks contain within them the remains of once living things. Shells and fossils are a common identifying features. Examples are limestone and coal. Three types of Sedimentary Rocks Chemical rocks: these are formed when water evaporates leaving behind mineral deposits. This can happen to lakes or seas, and within underground caves. Examples are rock salt and gypsum. Metamorphic Rocks Metamorphic rocks form when existing rocks are changed in some way into a new type of rock. These rocks usually form deep within the Earth’s crust at depths of more than 12km. Here they have a lot of pressure on them, and temperatures can be 100 to 800 degrees Celcius. Metamorphic Rocks At these high pressures and temperatures rocks can soften and change shape. They are squeezed and flattened into more compact shapes. Chemical reactions inside the rocks can change the internal structure and composition also. Metamorphic rocks are usually harder and denser than other rocks. Metamorphism Contact metamorphism occurs in just a small area as rocks are contacted by heat from magma or pressure in a specific area, such as near volcanic activity. Regional metamorphism occurs when rocks are changed by the Earth’s internal heat and pressure over a huge region. Two Types of Metamorphic Rocks Foliated rocks have easy to see layers or bands of minerals and colors. They will break on these lines. Examples are slate and gneiss. Unfoliated rocks do not have any visible layers or bands. Examples are marble and quartzite. Metamorphic Rocks The Rock Cycle These three types of rocks, igneous, sedimentary and metamorphic are always changing. Each type of rock can be changed over time into one of the other types. The processes involved in the rock cycle are weathering, erosion, deposition, heating, high pressure, cooling, solidification, compaction and cementation.