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Rocks Activator: Look at your sample of marble and conglomerate using the hand lens. Describe each rock. What is the color and texture of each? Try scratching the surface of each rock with the edge of a penny. Which rock seems harder? Hold each rock in your hand. Allowing for the fact that the samples aren’t exactly the same size, which rock seems denser? • Marble • Conglomerate Classification Rocks are classified by their texture, mineral content and how they form (origin). Geologists group rocks into three different categories. Those categories are: – Igneous – Metamorphic – Sedimentary When studying a rock sample, geologists observe the rock’s color, texture and mineral composition to classify it. • Texture (look and feel) – Color alone does not provide enough information to identify a rock but texture is an important characteristic. Examples - chalky, smooth, glassy, rough • Most rocks are made up of minerals and other rocks called grains. To describe a rock’s texture, geologists use a number of terms based on the size, shape, and pattern of the rock’s grains. Texture (cont.) • Grain size: fine-grained (hard to see) like slate and coarsegrained (easy to see) like diorite • Grain Shape: smooth-and-round or jagged • Grain Pattern: banded or non-banded, wavy, stacked layers • No Visible Grain: no crystal grains when they form because may have cooled very quickly, very smooth and shiny or particles are extremely small Why does grain shape matter? • Rocks with round grains are more likely to absorb water (porous) than those rocks with interlocking grains. This allows porous rocks to become more crumbly and softer than rocks with interlocking grains. granite sandstone Mineral Composition • Geologists must look very closely to determine the minerals that make up a rock. • They can do this by cutting a small sliver of the rock and look at it under a microscope. • They can also perform test on the properties of the minerals making up the rock such as how it reacts to an acid test or does it possess any magnetic properties. Igneous Rocks Text pgs. 150-153 Igneous rocks are classified by their origin, texture, and mineral composition. Scientists classify igneous rock according to how they are formed. If igneous rocks form from lava on the surface of the Earth they are known as extrusive igneous rocks. If igneous rocks form from magma beneath the Earth’s surface they are known as intrusive igneous rocks. Granite is the most abundant intrusive rock in the Earth’s crust. Igneous Rocks Classification Igneous rocks are classified by texture. Textures depend on how fast the magma or lava cools. The four texture categories include: Fine Grained – Crystals not visible. Cools quickly. Example: Rhyolite Coarse Grained – Large crystals. Cools slowly. Example: Granite Glassy – No crystals. Cools quickly Example: Obsidian Porphyritic – Two different-sized crystals. Large crystals scattered on the background of smaller ones. Cools slowly then quickly. • Granite is an Igneous Rock. – Quartz and feldspar minerals, and mica. Copyright © 2010 Ryan P. Murphy Mineral Composition • Remember that the silica content of magma and lava affect how easily the magma or lava will flow. Lava low in silica will usually form darkcolored rocks such as basalt. Basalt contains feldspar as well as certain dark colored minerals, but does not contain quartz • Magma that is high in silica usually forms light-colored rocks such as granite. However, granite can be many colors, Granite’s color changes with its mineral composition. Granite rich in feldspar will be pinkish, while granite rich in hornblende will be grayish. • Pumice Copyright © 2010 Ryan P. Murphy • Obsidian – Glassy Copyright © 2010 Ryan P. Murphy • Rhyolite • Which is Rhyolite and which is Granite? • Which is Rhyolite and which is Granite? • Which is Rhyolite and which is Granite? • Which rock is Obsidian and which is Pumice? Copyright © 2010 Ryan P. Murphy • Which rock is Obsidian and which is Pumice? Copyright © 2010 Ryan P. Murphy • Which rock is Rhyolite, and which is Pumice? Copyright © 2010 Ryan P. Murphy • Which rock is Rhyolite, and which is Pumice? Copyright © 2010 Ryan P. Murphy • Gabbro – Same composition as basalt but cooled slower (Intrusive) Copyright © 2010 Ryan P. Murphy • Gabbro – Same composition as basalt but cooled slower (Intrusive) Copyright © 2010 Ryan P. Murphy • Gabbro – Same composition as basalt but cooled slower (Intrusive) Copyright © 2010 Ryan P. Murphy • Which of the rocks below cooled quickly (Extrusive) and which cooled slowly (Intrusive)? A B • Which of the rocks below cooled quickly (Extrusive) and which cooled slowly (Intrusive)? • Can rocks float? • Picture of floating Pumice Island. Copyright © 2010 Ryan P. Murphy Many igneous rocks are hard, dense, and durable Sedimentary Rocks Text pgs. 154-158 Most Sedimentary rocks are made from particles deposited by water and wind and ice over time. These particles are known as sediments. Sediment is small, solid pieces of material that come from rocks and living things. Destructive forces are constantly breaking up and wearing away the rocks on the Earth’s surface. Erosion and deposition are two of the processes that help build sedimentary rocks. After sediment has been deposited, the processes of Compaction and Cementation Cementation While compaction is taking place, the minerals in the rock slowly dissolve in the water. The dissolved minerals seep into the spaces between particles of sediment. Cementation is the process in which dissolved minerals crystallize and glue sediments together. Example: sandstone Compaction • At first sediments fit together loosely. But gradually over millions of years, thick layers of sediment build up. These layers become heavy and press down on the layers beneath them, compacting them together. • Compaction is the process that presses sediments together. These layers often remain visible. • Example: shale Sedimentary Rocks Most Sedimentary rocks are made from bits and pieces of other rocks that cement together over time. There are three classifications of Sedimentary rocks. They are Clastic, Chemical, and Organic. Sedimentary rocks are usually found in layers. Most fossils are found in Sedimentary rocks. Sedimentary Rocks Clastic Sedimentary Rocks – Clastic rocks are made of different rock particles that are squeezed together. Clastic Sedimentary rocks are classified by particle size. Examples: • Conglomerate • Sandstone • Shale Sedimentary Rocks Organic Sedimentary Rocks – Not all sedimentary rocks are made from particles of other rocks. Organic rocks form from the remains of dead plants and animals. Examples: Limestone Coal Sedimentary Rocks Chemical Sedimentary Rocks – Chemical rocks form when oceans, lakes, and rivers evaporate and leave behind sediment particles. They are sometimes called evaporites. Minerals dissolve and crystallize. Examples: Gypsum Calcite Halite Can you determine the order of these rocks? Metamorphic Rock is a rock that has changed its form Text pgs. 162-165 Metamorphic rocks form from heat and/ or pressure deep beneath the Earth’s surface. Any rock can be changed into a metamorphic rock. Changes this rock goes through appearance texture crystal structure mineral content Metamorphic rocks can be formed by Contact Metamorphism or Regional Metamorphism. Contact Metamorphic Rocks – Form when existing rock comes in contact with magma. The changes that happen are because of great amounts of heat. Regional Metamorphic Rocks – Form when pressure from magmas change rocks over a large area. Metamorphic Rocks Minerals in the original rock determine the mineral composition of the metamorphosed rock. Metamorphic rocks are classified by arrangement of the grains that makeup the rock. The two metamorphic textures are Foliated and Nonfoliated. • Foliated Texture – Minerals line up in bands or parallel layers. Example – Gneiss • Nonfoliated Texture – Minerals do not line up in bands. Example - Marble Examples of Metamorphic Rocks • The sedimentary rock shale can change into the metamorphic rock slate. • The sedimentary rock sandstone can change into the metamorphic rock quartzite. The igneous rock granite can be melted and changed into the metamorphic rock, gneiss Changing Rocks Limestone changes to marble Sandstone changes to quartzite Shale changes to slate The Rock Cycle Text pgs. 166-169 • Which came first? granite sandstone quartzite A Cycle of Many Pathways • Earth’s rocks are not as unchanging as they seem. Forces inside and at the surface produce a rock cycle that builds, destroys, and changes the rocks in the crust. The rock cycle is a series of processes on Earth’s surface and inside the planet that slowly change rocks from one kind to another. What drives the rock cycle? Earth’s constructive and destructive forces – including plate tectonics- move the rocks through the rock cycle. • New Area of Focus. The Rock Cycle Copyright © 2010 Ryan P. Murphy Quiz Answer the following questions. 1. What are the three major classes of rocks? 2. How are intrusive rocks different from extrusive rocks? 3. What are the four igneous rock textures? 4. What are two conditions that must be present in order to form metamorphic rocks? 5. How is Contact Metamorphism different from Regional Metamorphism? 6. What are three classifications of Sedimentary Rocks? 7. How are Clastic Sedimentary rocks classified? 8. What is another name for Chemical Rocks? 9. List examples of Igneous Rocks. 10. List examples of Sedimentary Rocks.