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How Geologists Classify Rocks

How Geologists Classify Rocks

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How Geologists Classify Rocks

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  1. How Geologists Classify Rocks • When studying a rock sample, geologists observe three things: • Origin • texture • mineral composition.

  2. Texture • Texture is the look and feel of the rock’s surface. • Texture can be smooth, glassy, rough or chalky. •

  3. Grains • Particles of minerals or other rocks that make up a rock • Grains are large and easy to see = coarse-grained • Grains so small they can only be seen with microscope • = fine-grained

  4. Grain Shape • Grain shapes can be round or jagged • Also result from crystal shape

  5. Grain Pattern • Can be dotted throughout (non-banded) • Can be organized in horizontal rows (banded)

  6. No Visible Grain • No grain visible even under microscope • This occurs when lava cools very quickly • Extremely small pieces of silica make up the rock

  7. Mineral Composition • Using a microscope to identify minerals by cutting a thin slice • Use acid to detect carbon (Calcium Carbonate) • Also test with magnet to detect iron or nickel (magnetic properties

  8. - Classifying Rocks How Rocks Form • Geologists classify rocks into three major groups: igneous rock, sedimentary rock, and metamorphic rock.

  9. Graphic Organizer Metamorphic Igneous Organic Foliated Extrusive Chemical

  10. Igneous Rock • Forms when molten rock (magma/lava) cools and hardens • Cools slowly beneath Earth’s surface = intrusive • Cools rapidly on the surface = extrusive • Intrusive rocks usually have large, visible grains • Extrusive rocks usually have small to no visible grains • Color is based on the amount of silica in the magma

  11. Sedimentary Rocks • Forms from the compaction and cementation of rock pieces, mineral grains, or organic matter called sediments • Can contain fossils • Sediments are formed through the process of weathering and erosion of exposed rocks.

  12. 3 kinds: • Clastic= made of mineral or rock pieces (shale; conglomerate; sandstone; Breccia) • Organic = made from plant or animal remains (coal; coquina; some limestones) • Chemical = made from dissolved minerals in water (halite; other limestones)

  13. Metamorphic Rock • Forms when rocks are changed into different kinds of rocks by great heat and pressure • They are heated, squeezed, folded, or changed by contact with hot magma. • 2 kinds: • foliated = grains are in bands • nonfoliated = no banding

  14. - Igneous Rocks Classifying Igneous Rocks • Igneous rocks are classified according to their origin, texture, and mineral composition.

  15. - Igneous Rocks Identifying Main Ideas • As you read the section “Classifying Igneous Rocks,” write the main idea in a graphic organizer like the one below. Then write three supporting details. The supporting details further explain the main idea. Main Idea Igneous rocks are classified by origin, texture, and composition. Detail Detail Detail Extrusive rock forms from lava on the surface; intrusive rock forms from magma from beneath the surface. Intrusive rocks have larger crystals than extrusive rocks because they cool more slowly. High-silica rocks are light colored; low-silica rocks are dark colored.

  16. - Sedimentary Rocks From Sediment to Rock • Most sedimentary rocks are formed through a series of processes: erosion, deposition, compaction, and cementation.

  17. - Sedimentary Rocks Outlining Sedimentary Rocks • As you read, make an outline about sedimentary rocks. Use the red headings for the main topics and the blue headings for the subtopics. • From Sediment to Rock • Erosion • Deposition • Compaction • Cementation • Types of Sedimentary Rock • Clastic Rocks • Organic Rocks • Chemical Rocks • Uses of Sedimentary Rocks • Building Materials • Tools

  18. - Rocks From Reefs Using Prior Knowledge • Before you read, look at the section headings and visuals to see what this section is about. Then write what you know about coral reefs in a graphic organizer like the one below. As you read, write what you learn. What You Know Coral reefs grow in the oceans. Florida has coral reefs. Oceans used to be where there is dry land now. What You Learned Coral animals cannot live below 40 meters. In the United States, only the coasts of Florida and Hawaii have coral reefs. Some limestone deposits on land formed from ancient reefs.

  19. - Metamorphic Rocks Previewing Visuals • Before you read, preview Figure 17. Then write two questions that you have about metamorphic rocks in a graphic organizer like the one below. As you read, answer your questions. Previewing Q. Why do the crystals in gneiss line up in bands? A. Gneiss is a type of metamorphic rock that is foliated—the crystals are flattened to form parallel lines. Q. How does quartzite form from sandstone? A. High temperature and pressure on the minerals in sandstone cause them to be changed into minerals that make up quartzite.

  20. - The Rock Cycle A Cycle of Many Pathways • Forces deep inside Earth and at the surface produce a slow cycle that builds, destroys, and changes the rocks in the crust.