Chapter 9Earth’s Changing Surface

Discover:

What earth’s layers are.

What causes earthquakes and volcanoes.

How weathering and erosion change Earth’s surface.

How minerals and rocks are identified.

Lesson 1What is the structure of Earth?

• The earth’s crust is its outermost and thinnest layer.

• There are two kinds of crust, the continental crust and the oceanic crust.

• The top part of the mantle and Earth’s crust make up the lithosphere.

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• Earth’s inner core is solid, and the outer core is liquid.

• Scientists learn about the mantle and core by studying material pushed up through the crust, by measuring vibrations from earthquakes, and by performing laboratory experiments.

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• Earthquake vibrations are used to study Earth’s layers because vibrations change speed and direction as they come in contact with various layers. Scientists can then infer characteristics of each layer. These vibrations are measured with a seismograph.

Lesson 2What causes earthquakes and volcanoes?

• The lithosphere is composed of sections called plates that shift in relation to one another.

• There are three basic kinds of plate boundaries.

• Converging boundary – Two plates collide, and mountains are made when the crust folds, tilts, and lifts as a result.

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• Spreading plate boundary – Two plates pull apart from each other, and a rift valley will form.

• Sliding plate boundary – Two plates move past each other in opposite directions. If these two plates were stuck together and then suddenly moved, an earthquake might occur.

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• Constructive forces build new features on Earth’s surface. A volcano is an example.

• Destructive forces wear away or tear down features. Weathering is an example.

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• Earthquakes most often occur at faults along plate boundaries. Faults are cracks in Earth’s crust where the surrounding rock has moved or shifted.

• The epicenter is the position on Earth’s surface above an earthquake’s origin, or focus.

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• Earthquakes can cause rapid destruction. They can cause landslides, the downhill movement of large masses of rock and soil. Also, they can cause tsunamis, enormous waves.

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• Most volcanoes form near colliding plate boundaries. As the plates move, rock partially melts and forms magma, which is forced to the surface through a weak spot in the crust.

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• When a volcano builds from the ocean floor and reaches the water’s surface, a volcanic island forms.

Lesson 3What is weathering?

• Weathering is a slow process that reduces rocks into smaller pieces of rock.

• There are two types of weathering: mechanical and chemical.

• Mechanical weathering physically breaks rock into smaller pieces by forces due to gravity, ice, plant roots, or other forces. Chemical

Weathering

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• Chemical weathering is the changing of materials in a rock by chemical processes. An example would be carbonic acid (made from rain and carbon dioxide in the air) dissolving some types of rock.

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• Mechanical and chemical weathering both contribute to the formation of soil.

• The layers of soil consist of topsoil, subsoil, and bedrock.

• Topsoil is the top layer of soil. It has a large amount of decayed material that makes the soil fertile.

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• Subsoil is the second layer of soil. It has many minerals but less decayed matter than topsoil.

• Bedrock is nearly solid rock that lies under the surface. It also has less decayed matter than topsoil.

Lesson 4What is erosion?

• Erosion is the movement of materials away from one place.

• Deposition is the placing of materials in a new place.

• Gravity is the main force that causes erosion. Gravity causes landslides which pull rock and dirt downhill and rivers and ocean currents to flow

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• which wear away or erode the land.

• Gravity also pulls glaciers down along a valley and cause erosion. Rainwater can also wear away the land.

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• Waves are a major agent of erosion and deposition along coastlines.

• Harbors and inlets are formed because some areas of a shoreline erode more quickly than others. Harbors are areas of deep water protected from wind and currents forming a place of shelter for ships.

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• Inlets are narrow strips of water running from a larger body of water into the land or between islands.

• Waves usually hit a beach at an angle, creating a current that shapes beach features.

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• Some of these features are:

• spit – A sandy peninsula (land surrounded by water on 3 sides such as Florida)

• baymouth bar – a sandy peninsula that completely closes access to a bay, sealing it off from the main body of water

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• lagoon – shallow water separated from the sea by low ridges of sand

• bay – area of water bordered by land on 3 sides

• sandbars – ridges of sand that are either above or just below water level

• tidal marshes – swampy areas along the ocean

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• Wind erosion is caused by wind blowing dust, soil, or sand from one place to another. When sand and dust blow against a rock, tiny bits of the rock might break off.

• Sand dunes are large loose deposits of sand. The size and shape of a sand dune depends on wind, amount of sand, and number of plants.

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• Field erosion caused by wind can be a serious problem on farms. To prevent wind erosion of topsoil, they surround fields with natural barriers such as trees.

Lesson 5How are minerals identified?

• A mineral is a naturally made solid that has a regular arrangement of particles.

• Every mineral has properties, such as hardness, magnetism, luster, shape, streak, and texture, that can be used to identify it.

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• Hardness is determined by the Moh’s Scale which rates the hardness 1 to 10.

• A few minerals are magnetic such as pyrrhotite and magnetite.

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• Luster describes the way light is reflected by a mineral’s surface.

• Streak is the color of a mineral in its powdered form. To see a mineral’s streak, you rub it on a hard, rough, white surface.

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• Some minerals have a definite shape that is helpful for identification.

• Minerals can also be identified by their texture, or the way it feels.

Lesson 6How are rocks classified?

• Rocks are classified as igneous, sedimentary, or metamorphic, according to how they formed.

• Igneous rocks form when hot, melted rock cools and hardens.

• They can be identified by its visible crystals. If igneous rock cools quickly, it has small crystals, and if cooled slowly, it has large crystals.

• Examples are granite, basalt, and pumice.

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• Sedimentary rocks form when layers of minerals and rock particles settle on top of each other, are pressed together, and harden.

• Some examples are sandstone and conglomerate.

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• Metamorphic rocks result from great heat and pressure changing existing rocks.

• Example are slate transforming into gneiss (pronounced nice) and limestone becoming marble.

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• The rock cycle is a slow process during which rocks change from one type to another. Some rocks may change in thousands of years while others may take millions of years to change. An igneous rock forms after a volcanic eruption. After thousands of years of being exposed to high

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• temperatures and being squeezed, the igneous rock turns into metamorphic rock. Gradually, the metamorphic rock weathers into tiny bits of sediment. These pieces of sediment eventually form sedimentary rock after many layers of sediment are pressed together.

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• Fossils and rocks in upper layers of the earth are considered to be younger than fossils found in lower layers.