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Transcript
LESSON GOALS You will learn • how igneous rocks change. • how sedimentary rocks change. • how metamorphic rocks change. rock cycle, the ways that rocks change from one form to another. When the lava cools, it will harden into igneous rock. 254 4 How Do Rocks Change? The earth's crust is made of about twen plates-pieces-of rock that fit together like piec of a puzzle. These plates move slowly over t melted top of the mantle. The plates can bum together, slide past each other, or spread apart. When plates bump together, the edges of the plat slowly crumple and fold, forming mountains. Whe plates slide past each other, earthquakes happen. A plates move, melted rock from inside the earth c push up between the edges of the plates and form volcano like the one below. Sometimes these changes in the earth's cru happen quickly. However, most of these change happen slowly over millions of years. These change cause new rocks to form and cause old rocks t change. Like the earth's crust, rocks are alway changing. The changes that rocks go through mak up the rock cycle. gneous Rocks The moving plates that form the earth's crust are ostly made of igneous rocks. These rocks form irom magma, melted rock found deep inside the arth. Magma changes to igneous rocks as it cools. Some igneous rocks form inside the crust. These cks form when magma flows into cracks in the crust and cools slowly. You know that rocks are ade of minerals. When magma cools slowly, inerals in the magma often form large crystals. otice the large quartz crystals and the crystals in the ~ anite. Both of these rocks formed when magma cooled very slowly. Some igneous rocks form outside the crust. Pressure sometimes pushes magma out of the earth ough a volcano. Magma that reaches the earth's surface is called lava. The lava cools quickly on the surface. When magma cools very quickly, minerals the magma form very tiny crystals. How is the asalt rock different from the granite? _ igneous (ig/ne os) rock, rock formed from cooled magma. crystal (kris/tl), a substance made up of atoms arranged in a repeating pattern. All the crystals of a mineral have the same basic shape, but can be different in size. T Granite Quartz crystals 255 SCIENCE IN YOUR LIfE Sedimentary rocks such as sandstone and limestone are common building materials. These rocks can be cut into blocks which are used to build strong, beautiful walls, steps, fences, and other structures. sedimentary (sed/o men/tor e) rock, rock that forms from cemented or pressed sediments. 256 Sedimentary Rocks Weathering changes igneous and other rocks into sediments. Many sediments end up on the bottoms of oceans and lakes. The water and new sediments press down on old layers of sediments. Those sediments are pressed and cemented into sedimentary rocks. This cycle that changes rock into sediment and sediment into rock can take millions of years. As the plates forming the earth's crust move, land that was once at the bottom of an ocean can be slowly pushed upward. The sedimentary rocks shown below were formed at the bottom of an ocean and were slowly forced upward by the movement of the plates. This shale is a sedimentary rock made of mud and lay. As new sediments settled to the bottom of a lake or ocean, they pressed on mud and clay below. The veight of the new sediments pressed on the mud, slowly causing the mud to harden into shale. Notice the sediments in the sandstone. This rock is made mostly of the mineral quartz. Large grains of uartz sand were slowly cemented together by tiny uartz sediments in water. The tiny sediments filled spaces between the sand grains. How do you think e quartz sediments formed? What do you see in the piece of limestone? This ock is made from dead organisms. As these rganisms died, their bodies settled to the bottom of e ocean and were buried by sediments. Bones and shells became part of the sedimentary rock that slowly formed from these sediments. Sedimentary rocks can change back into - diments. How do you think this change happens? Shale Sandstone Limestone 257 metamorphic (met/a mor /fik) rock, rock formed when heat and pressure change the minerals in igneous or sedimentary rocks. . Layers of metamorphic 258 rock Metamorphic Rocks Heat and pressure inside the earth can change igneous and sedimentary rocks into. metamorphic rocks. How is the cliff below like the hills on page 256? How is it different? The layers in the cliff below were once granite. As plates of the earth' crust moved, the granite was pressed and bent. Thi pressure changed the granite's minerals. The granite changed into a metamorphic rock called gneiss (nis), Heat and pressure cause physical and chemical changes in many kinds of igneous and sedimentary rocks. Heat and pressure changed limestone into the marble on page 259. Because of its strength and beauty, people use marble in buildings. Heat and pressure changed shale into the slate on page 259. Slate breaks apart in smooth layers. Slate is often used for roofs and floors . Like igneous and sedimentary rocks, metamorphic rocks can change. Wind, water, and ice can weather metamorphic rocks into sediments that might later form sedimentary rocks. Heat inside the earth can melt metamorphic rocks into magma. What might this magma then form? Physical and chemical weathering, erosion, heat, and pressure constantly change all rocks. The changes in the rock cycle never stop. Lesson Review 1. How do igneous rocks form and change? 2. How do sedimentary rocks form and change? 3. How do metamorphic rocks form and change? 4. Challenge! Some mountains are made of all three kinds of rock-igneous, sedimentary, and metamorphic. How could all three kinds of rock be found in one mountain? Study on your own, pages 422-423. Slate Marble When magma cools, minerals in the magma often form crystals. These crystals have many shapes, but they can be divided into six main groups. Find out what the six main groups of crystals are. PHYSICAL SCIENCE FIND OUT ON YOUR OWN 259 1. The earth's crust is made of about iWI plates. 2. When the edges of plates bump together and crumple and fold, a %ft forms: 3. What causes an earthquake? 4. Melted rock from inside the earth that pushes up between the edges of the earth's plates forms a Ill!: . 5. The moving plates of the earth's crust are mostly made of LESSON 4 pages 254-259 6. What is magma? 7. When magma cools slowly, my: in the magma often form large crystals. 8. Magma that reaches the earth's surface is f::l . 9. MJ is a sedimentary rock made of mud and clay sediments. 10. How is sandstone formed? 11. The bones and shells from dead organisms form part of the sedimentary rock called %t. 12. Heat and pressure inside the earth can change igneous and sedimentary rocks intot:W • 13. Heat and pressure can change mil: to marble. 14. Which of the following igneous rocks was formed by magma cooling very slowly? A 8