Our Changing Earth From tall mountains to flat plains, Earth’s surface has extremes The physical features of Earth that we observe today did not always exist Instead, they formed over millions of years Those long, slow changes still take place, even if we can’t see them happening But there are other physical changes on Earth that we can witness These changes happen much more quickly Some are sudden changes, while others are more gradual Earth is a restless planet As it changes, so those who live on it Reading Content Science Reading Strategy Synthesize What this paragraph and the photograph suggest about some of Earth’s changes? ✓ Reading Check Recall details What are two extreme features on Earth’s surface? View of Earth from the moon’s surface Predict What is one change on Earth that we can see happen? restless not able to rest Unit • Chapter 305 Earthquakes Earth’s crust is the layer of rock closest to Earth’s surface An earthquake is a sudden, violent shaking in Earth’s crust The crust is divided into a number of blocks of rock, called plates, which cover the planet An earthquake happens when two of these plates, under great pressure, move past each other along a fault The violent shaking is caused by seismic waves that travel through the planet These waves can knock down buildings and bridges The place inside Earth where an earthquake starts is the focus The epicenter is the place on Earth’s surface directly above the focus Tsunami Plates Plates Fault Seismic Waves Reading Strategy Earthquake Focus Synthesize What clear idea the pictures give you about earthquakes? violent very strong pressure force 306 Unit • Chapter Epicenter The San Andreas Fault in California How Do Earthquakes Happen? Most earthquakes result when plates move over, under, or past each other Plate boundaries are places where two plates meet Earthquakes happen when the pressure on the rocks at plate boundaries builds up This happens in different ways Plates can move apart, or diverge As the rocks pull apart, magma rises from below Plates can come together, or converge The rocks are pushed from two different directions This makes them bend and break Earthquakes can also happen when plates move past one another without much motion up or down Surface Waves and Tsunamis Most waves on a lake or an ocean can occur when wind energy moves over the surface The energy that causes a tsunami comes from an earthquake on the sea floor All of the water above the epicenter is affected A wave forms It does not seem large at first, but the energy is great When the water hits the shore, it crashes over the land Reading Strategy Synthesize How is plate movement related to faults, like the San Andreas Fault pictured above? ✓ Reading Check Recall details What builds up at plate boundaries? Infer Where are you most likely to notice a tsunami wave? magma hot liquid rock motion movement Unit • Chapter 307 Reading Strategy Synthesize What effect does a volcano have on Earth’s surface? Connect the text and pictures on this page Volcanoes A volcano forms at an opening, or vent, in the crust near the edges of plates A volcanic eruption occurs when the plates move apart or when one plate is pushed under the other Water vapor, particles, gases, and magma pour out onto the surface Volcanoes form new land Gases and particles from the volcano may affect the weather Eruption Volcano Lava Vent Crater particles very small pieces 308 Unit • Chapter Magma Lava from Kilauea Volcano in Hawaii flows into the ocean How does Earth benefit from volcanic eruptions? How Do Volcanoes Form? Volcanoes often form at the boundaries between plates They occur when the plates pull apart and magma flows up from below through cracks Volcanoes can also form when plates come together One plate moves under the other The rock gets very hot and becomes magma The magma rises, forming a volcano The mantle is the layer of earth under the crust Some areas of mantle are hotter than others Volcanoes can form at these hot spots As the plates move, the volcanoes form an island chain The Hawaiian Islands are on a hot spot Hot Spot Forming Islands Reading Strategy Synthesize What creates a hot spot? Pull together information from the text and diagram ✓ Crust Reading Check R ecall details Where can you find volcanoes? Mantle I nfer How might gases and particles affect the weather? Unit • Chapter 309 Changes on Earth’s Surface 10 Earthquakes and volcanoes can change Earth’s surface These changes happen quickly 11 Earth’s surface can change slowly, too Weathering, such as abrasion and ice wedging, breaks rock into tiny pieces Abrasion happens when tiny pieces of rock wear down larger pieces of rock Erosion happens when water, glaciers, or wind carries sediment to new places Sediment is tiny pieces of rock and soil Deposition happens when sediment builds up in a place Deposition builds new landforms such as deltas, moraines, and sand dunes Glaciers, water, and wind can all cause deposition Reading Strategy Synthesize What changes to the earth’s surface are represented by the pictures? How are they different from each other? Erosion Weathering Glacier A glacier is a slowly moving mass of ice A glacier can cut into rock and carry the pieces away Abrasion Tiny bits of rocks moved by air, water, or a glacier can cause abrasion Ice wedging is the result of ice forming in small spaces in rocks The ice breaks up the rock Deposition Sand Dune Wind can cause a sand dune to build up 310 Unit • Chapter Weathering 12 Mechanical weathering happens when rocks are broken into small pieces Abrasion occurs when pieces of rock are rubbed off by tiny bits of rock moved by water, wind, or ice Ice and plants also cause weathering A seed may fall into a crack in a rock When the seed sprouts and grows, the plant’s roots make the crack bigger Ice wedging also affects cracks in rock Water gets into a crack When the water freezes, it Plant roots push apart rock and cause mechanical weathering expands, and the crack gets bigger The cracked rock finally breaks in two 13 In chemical weathering, chemicals in water, such as oxygen and carbon dioxide, react with chemicals in rock This breaks down the rock over time Glaciers 14 Over thousands of years, ice and snow can build up on mountains They form glaciers As a glacier moves, sand and rock inside the glacier wear down mountains and change their shape A mountain’s shape can tell us where glaciers formed in the past Erosion and Deposition 15 16 Weathering breaks rock into smaller pieces Wind, water, or ice moves these small pieces, called sediment, to new places This process is erosion The Grand Canyon was formed by water erosion Over millions of years, water cut into the rock and carried pieces away Sand is eroded from beaches by waves Glaciers carry rocks and soil as they move Wind carries dust and sand for miles When the wind or water slows down, the sediment drops out This is deposition Deposition forms river deltas and sand dunes When a glacier deposits sediment, it can form a moraine, a type of hill Reading Strategy Synthesize What kind of weathering is the Grand Canyon an example of? ✓ Reading Check R ecall details What kinds of changes are caused by wind? Water erosion formed this canyon F ind main idea What is the main idea of page 310? Unit • Chapter 311 ...Earthquakes Earth? ??s crust is the layer of rock closest to Earth? ??s surface An earthquake is a sudden, violent shaking in Earth? ??s crust The crust is divided... inside Earth where an earthquake starts is the focus The epicenter is the place on Earth? ??s surface directly above the focus Tsunami Plates Plates Fault Seismic Waves Reading Strategy Earthquake... the weather? Unit • Chapter 309 Changes on Earth? ??s Surface 10 Earthquakes and volcanoes can change Earth? ??s surface These changes happen quickly 11 Earth? ??s surface can change slowly, too Weathering,