Earth Science : Unit 3 Review: Forces Within

1. Earth Science : Unit 3 Review: Forces Within Ch 8: Earthquakes and Earth’s Interior Ch 9: Plate Tectonics Ch 10: Volcanoes Ch 11: Mountain Building

2. Earth Science : Unit 3 Review: Forces Within • An earthquake is the vibration of Earth produced by the rapid release of energy within the lithosphere. • Earthquakes are produced by slippage along a break in the lithosphere called a fault. • The point within the Earth where the earthquake starts is called the focus. The focus of an earthquake is located along a fault underneath the surface. • The energy released by the earthquake travels in all directions from the focus in the form of seismic waves. • The epicenteris the place on the Earth’s surface where the earthquake was centered. The epicenter’s location is the location on the surface directly above the focus.

3. Earth Science : Unit 3 Review: Forces Within • If the crust moves vertically, scientists say it has been uplifted. Vertical movements create a sharp edge called a fault scarp. • If the crust moves horizontally we say it has been offset or displaced. • The San Andreas fault in California is one of the most studied fault systems in the world. This fault extends about 1300 kilometers through the state and into the Pacific ocean. • According to the elastic rebound theory,most earthquakes are produced by the rapid release of energy stored in rock that has been subjected to great forces. • When the strength of the rock is exceeded, it suddenly breaks, releasing some of it’s energy as seismic waves.

4. Earth Science : Unit 3 Review: Forces Within • The tendency for the rock under pressure to spring back after an earthquake is called elastic rebound. • An aftershockis an earthquake that occurs sometime after a major earthquake. Aftershocks may occur anywhere from hours to weeks after an earthquake. • Small earthquakes called foreshocks sometimes precede major earthquakes days or even a year before a large event. • Seismic waves carry the energy released from an earthquake hundreds of kilometers away. • Earthquakes produce two main types of seismic waves • Body waves • Surface waves • The waves that travel through the Earth’s interior are calledbody waves. • There are two types of body waves • P waves • S waves

5. Earth Science : Unit 3 Review: Forces Within • P waves are push-pull waves that push and pull particles as they move in a direction. P waves are also called compressional waves. P waves travel faster than S waves. P waves can travel through both liquids and solids. • S waves shake particles at right angles to the waves direction. Like taking one end of a rope and shaking it, the waves move in a curve. S waves are also called transverse waves. When body waves reach the surface, they produce surface waves. Surface waves travel more slowly than body waves do. • Surface waves move up and down as well as side-to-side. Surface waves are usually much larger than body waves, so surface waves are usually the most destructive seismic waves. • Scientists have developed an instrument to record seismic waves: the seismograph. A seismographproduces a time record of the ground motion during an earthquake.

6. Earth Science : Unit 3 Review: Forces Within • The Richter Scale is based on the height of the largest seismic wave (P,S, or surface wave) recorded on a seismograph. • Today scientists use the Moment Magnitude Scale to measure earthquakes. The moment magnitude scale is derived from the amount of displacement that occurs along a fault. The moment magnitude scale is the only scale that estimates the energy released by earthquakes. • The Mercalli Scalerates an earthquake’s intensity in terms of an earthquake’s effects at different locations (how much shaking it creates or damage it does). The scale has 12 steps expressed as Roman numerals . • Locating an Earthquake Epicenter: The difference in speeds between the P and the S waves provides a way to locate the epicenter. The P wave always travels faster, arriving before the S wave. The longer the distance away the epicenter, the greater the amount of time between the waves arriving. Triangulatemeans to use three or positions to determine an exact location.

7. Earth Science : Unit 3 Review: Forces Within Earthquake hazards: Seismic Shaking:The ground vibrations caused by an earthquake, called seismic shaking, are the most obvious hazard of an earthquake. Liquefaction: Where sediment layers and rock are saturated with water, earthquakes can cause a process called liquefaction. • When liquefactionoccurs, what had been stable soil suddenly turns into liquid. • In areas where the water content of the soil is high, an earthquake can start a mudflow. During a mudflow, a mixture of soil and water slides downhill rapidly burying everything beneath. • Earthquakes often cause loose rock and soil on slopes to move. These movements are called landslides. • A tsunamis is a wave formed when the ocean floor shifts suddenly during an earthquake. A tsunami wave in open ocean is only a meter high but moves very fast; at hundreds of miles per hour. As the wave enters shallower water near a shore, the water slows down and the waves begin to pile up. When the waves do hit shore, they are much bigger due to the compression and hit with devastating force sweeping inland for miles.

8. Earth Science : Unit 3 Review: Forces Within Earth can be divided into layers based on the physical properties of each layer: • Lithosphere: Earth’s outermost layer consists of the crust and the uppermost mantle and forms a cool, rigid shell called the lithosphere. This layer averages about 100 kilometers in thickness. • Asthenosphere:Beneath the lithosphere lies a soft, comparatively weak layer known as the asthenosphere. Within the asthenosphere, the rocks are close enough to their melting point that they are easily deformed. • Lower Mantle: From a depth of about 660 kilometers down to near the base of mantle lies a more rigid layer called the lower mantle. • Inner and Outer Core: The core, which is composed of an iron-nickel alloy, is divided into two regions with different physical properties. The outer core is a liquid layer 2260 kilometers thick. The flow of metallic iron within this zone generates the Earth’s magnetic fields.

9. Earth Science : Unit 3 Review: Forces Within • In 1915, German scientist Alfred Wegner proposed his theory of continental drift; that the continents had once been joined to form a single landmass; a super-continent we called Pangaea. • Wegener also hypothesized that about 200 million years ago, Pangaea began breaking into smaller continents. The continents than slowly drifted to their present positions. • The theory of plate tectonics provided explanation for the continental drifting as well as many other geological processes. • By the late 1950s, scientists had constructed a more complete map of the earth’s ocean floor. This map showed a mid-Atlantic ridge; a long mountain chain that ran the length of the Atlantic ocean. Earth’s mid-ocean ridge system forms the longest feature on Earth’s surface. • In the process of sea-floor spreading, new ocean floor forms along Earth’s mid-ocean ridges, slowly moves outward across ocean basins, and finally sinks back into the mantle beneath deep-ocean trenches. • To accommodate newly created lithosphere, older portions of the ocean floor return to the mantle in a process we call subduction.

10. Earth Science : Unit 3 Review: Forces Within • As certain rocks form, they acquire the polarity of the Earth’s magnetic field at the time they formed. These rocks posses paleomagnetism.Once the rock is formed, it’s polarity remains frozen unless the rock is heated above a certain level. • In the theory of plate tectonics, Earth’s lithospheric plates move slowly relative to one another, driven by convection currents in the mantle. Hot material deep within the mantle moves upward by convection. At the same time, cooler, denser slabs of oceanic lithosphere sink into the mantle. • Three types of plate boundaries exist: Divergent boundaries are found when two of Earth’s plates move apart. Oceanic lithosphere is created where divergent boundaries occur and sea-floor spreading happens. • Convergent boundaries happen when two plates move together towards each other. Lithosphere can be destroyed at convergent boundaries when oceanic lithosphere sinks into the mantle during subduction. • Transform boundaries occur when two plates grind past each other. Along transform boundaries, lithosphere is neither created nor is it lost.

11. Earth Science : Unit 3 Review: Forces Within • Alongdivergent boundaries, plates move apart. Because they are the areas where sea-floor spreading begins, divergent boundaries are called spreading centers. We think of these plate boundaries as constructive plate margins because this is where new oceanic lithosphere is produced. • As two plates slowly converge, the leading edge of one plate is bent downwards, allowing it to slide beneath the other plate. We call this sliding under the other plate subduction. • At convergent boundaries, plates collide and interact, producing features including trenches, volcanoes and mountain ranges. Because lithosphere is destroyed at convergent boundaries, they are also called “destructive plate margins”. • When two oceanic slabs converge, one descends beneath the other. This causes volcanic activity similar to what happens in oceanic-continental. The volcanoes form on the ocean floor instead of on land, however. If this activity continues, it will build a chain of volcanic structures that become islands. This newly formed land we call a volcanic island arc.

12. Earth Science : Unit 3 Review: Forces Within • When oceanic lithosphere is subducted beneath continental lithosphere, a continental volcanic arc develops along the margin of the continent. However, if the subduction plate also contains continental lithosphere, the subduction eventually brings two continents together. The result is a collision between the two continental plates. Since neither sinks below the other, collision results and mountainsform. • This kind of collision occurred when India rammed into Asia and produced the Himalayas. Mountain systems such as the Alps, the Appalachians, and the Urals were formed by such a process. • The third type of plate boundary system is the transform fault boundary. Pieces of lithosphere move past each other horizontally along a transform fault boundary. At a transform fault boundary, plates grind against each other without destroying or creating lithosphere.

13. Earth Science : Unit 3 Review: Forces Within • 800 active volcanoes on average occur on Earth each year sending magma from below the Earth’s crust upward to the surface. Recall that magma is molten rock from beneath Earth’s surface. • Fortunately for us, hot magma only reaches the surface in certain areas. What determines where volcanoes form? Most volcanoes form along divergent and convergent plate boundaries. Some volcanoes form far from plate boundaries above “hot spots” in the crust. • We have learned that pressure increases with increased depth inside the Earth. Increasing pressure raises the melting point of rock deep inside the Earth. Decreasing pressure, decreases rock’s melting point. When pressure drops enough; decompression meltingoccurs.

14. Earth Science : Unit 3 Review: Forces Within • Volcanoes form at convergent boundaries where two oceanic plates meet and oceanic lithosphere is subducted beneath another oceanic plate. This process results in a chain of volcanoes being formed on the ocean floor. Eventually these volcanic mountains grow large enough to rise above the surface and are calledvolcanic islands. • Several volcanic island arcs, such as the Tonga Islands and the Mariana island arcs, lie on the eastern side of the Pacific ocean. Together with other volcanoes bordering the pacific, they form the Ring of Fire. • The Ring of Fire is the long belt of volcanoes that circles much of the Pacific Ocean.

15. Earth Science : Unit 3 Review: Forces Within • Volcanism may also occur at convergent plate boundaries where a continental plate meets an oceanic plate and slabs of oceanic lithosphere are subducted under continental lithosphere. The result is a continental volcanic arc. The process is basically the same as for an island arc. Kilauea volcano in Hawaii is Earth’s most active volcano. But Kilauea is in the middle of the pacific plate, thousands of kilometers away from any plate boundary. Kilauea is an example of intraplate volcanism; volcanic activity that occurs within a plate. Most intraplate volcanism occurs where a mass of hotter-than-normal mantle material, called a mantle plume, rises toward the surface. Magma that has reached the surface is called lava. Lava cools and hardens to form solid rock.

16. Earth Science : Unit 3 Review: Forces Within • Magma’s viscosity , the thickness of the fluid, affects the type of eruption that occurs. Viscosityis a substance’s resistance to flow. For example; maple syrup is more viscous than water; it flows more slowly when you pour it. The temperature and chemical composition determine the magma’s viscosity. • In general, the more silica in magma, the greater it’s viscosity (the thicker it stays when heated). Because of their high silica content; rhyolitic lavas are very viscous and erupt explosively. • During explosive eruptions, the gasses trapped in magma provide the force to propel molten rock out of the vent, an opening to the surface. These gases are mostly water vapor and carbon-dioxide. • Lava may appear to be the main material produced by a volcano but this is not always the case. Just as often, explosive eruptions eject huge clouds of broken rock, lava bombs, fine ash, and dust. Depending on the type of eruption, volcanoes may produce lava flows or eject “pyroclastic” materials or both. Particles produced in volcanic eruptions are called pyroclastic materials. When basaltic lava is extruded, dissolved gases propel chunks of lava to great heights.

17. Earth Science : Unit 3 Review: Forces Within • The fragments ejected during eruptions range in size from very fine dust and volcanic ash to pieces that weigh several tons.Particles that range in size from small beads to walnuts (2-64 millimeters) are called lapilli, or cinders. Particles larger than lapilli are called blockswhen they are made of hardened lava and bombs when they are ejected as glowing lava. ___________________________________________________ • Volcanic landforms come in a wide variety of shapes and sizes. The three main types of volcanoes are • Shield volcanoes • Cinder cones • Composite cones (also called stratovolcanoes) • Volcanic activity begins when a fissure, or crack, develops in the crust as magma is forced toward the surface. The gas-rich magma rises from the magma chamber, travels through a circular pipe, and reaches the surface at a vent. Repeated eruptions of lava or pyroclastic material eventually build a mountain called a volcano. Located at the summit of many steep walled volcanoes is a depression called a crater.

18. Earth Science : Unit 3 Review: Forces Within • The form of avolcanois largely determined by the composition of the magma.Shield volcanoes are produced by the accumulation of fluid basaltic lavas. Shield volcanoes have the shape of a broad , slightly domed structure that resembles a warrior’s shield. • Cinder Cone Volcanoes: Ejected lava fragments that harden in the air build a cinder cone volcano. The fragments range in size from fine ash to bombs but consist of lapilli, or cinders. Cinder cones are usually a product of relatively gas-rich basaltic or rhyolitic magma. • Composite Cone Volcanoes: Earth’s most beautiful and potentially most dangerous volcanoes are composite cones or stratovolcanoes. A composite cone is a large, nearly symmetrical volcanic mountain composed of layers of both lava and pyroclastic deposits. • Most composite cones are located in a relatively narrow zone that rims the Pacific ocean that we call the “Ring of Fire”. The Ring of Fire includes the large cones of the Andes in South America and the Cascade Range of the western United States and Canada. The most active regions in the Ring of Fire are located along volcanic island arcs next to deep ocean trenches.

19. Earth Science : Unit 3 Review: Forces Within Volcanic mountains are not the only landform to result from volcanic activity. Calderas, volcanic necks, lava plateaus also are byproducts. • A caldera is a depression in a volcanic mountain. Most calderas form in one of two ways. By the collapse of the top of a composite volcano after an explosive eruption. Or, from the collapse of the top of a shield volcano after the magma chamber is drained. • Another volcanic landform that provides evidence of past volcanic activity is the volcanic neck. A volcanic neck is a landform made of magma that hardened in a volcano’s pipe and later was exposed when the volcano eroded away. When a volcano’s activity ends, the magma remaining in the pipe hardens to form igneous rock. • A lava plateau is a volcanic landform produced by repeated eruptions of very fluid, basaltic lava. The lava that forms a lava plateau erupts through long cracks called fissures. Instead of building a cone, the lava spreads out over a large broad area.

20. Earth Science : Unit 3 Review: Forces Within • Lava flows are a major volcanic hazard. Frequent lava flows from Mount Kilauea in Hawaii destroy anything in it’s path. • A composite volcano can eject huge quantities of volcanic ash, burying widespread areas under thick ash deposits. • An explosive eruption can also release a pyroclastic flow; a scorching mixture of glowing volcanic particles and gases that sweeps rapidly down the sides of a volcano. • Composite volcanoes may also produce mudflows called lahars. Alaharoccurs when water-soaked volcanic ash and rock slide rapidly downhill.

21. Earth Science : Unit 3 Review: Forces Within • The structures that result from the cooling and hardening of magma beneath Earth’s surface are called plutons. There are several types of plutons: Sills, Laccoliths, Dikes, and batholiths. • A sill is a pluton that forms when magma flows between parallel layers of sedimentary rock. Horizontal sills are the most common to be found. • A laccolith is a lens-shaped pluton that has pushed the overlying rock layers upward. Like sills, laccoliths form when magma intrudes between sedimentary rock layers close to the surface. • A dike is a pluton that forms when magma moves into fractures that cut across rock layers. • Batholithsare very large bodies of intrusive igneous rock. A batholith is a body of intrusive igneous rock that has a surface exposure of more than 100 square kilometers.

22. Earth Science : Unit 3 Review: Forces Within • Deformation is any change in the original shape and/or size of a rock body. In Earth’s crust, most deformation takes place along plate boundaries. • Deformation occurs because of stress in a body of rock. Stress is the force “per unit area” acting on a mountain. ( pressure per square inch, foot, or meter for example) • The change in shape or volume of a body of rock is called strain. When stress is gradually applied, rocks first respond by responding elastically. A change that results from elastic deformation can be reversed. Like a rubber band; the rock will return to it’s original size and shape once the force upon it is removed. Once the elastic limit or strength for a rock is surpassed, the rock either flows or fractures. • Ductile deformation is a type of solid-state flow that produces a change in the size and shape of an object without fracturing the object. Ductile materials buckle or bend. • Rocks like granite and basalt that have a strong internal molecular bonds usually fail by brittle fracture.

23. Earth Science : Unit 3 Review: Forces Within • Forces that are unable to deform rock when first applied, may cause rock to flow if the force is maintained over a long period of time. The three types of stress that cause deformation of rocks are • Tensional stress • Compressional stress • Shear stress • When rocks are squeezed the stress is compressional stress. • When rocks are pulled in opposite directions, the force is tensional stress. • Shear stress causes a body of rock to be distorted. • Earth’s crust floats on top of the denser more flexible rocks of the mantle. The concept of a floating crust in gravitational balance is called isostosy.

24. Earth Science : Unit 3 Review: Forces Within • During mountain building, compressional stresses often bend flat-lying sedimentary rocks into wavelike ripples called folds. Folds of sedimentary strata come in three main types: Anticlines / Synclines / Monoclines • An anticline is usually formed by the upfolding, or arching of rock layers. Often found in association with anticlines are downfolds, or troughs, called synclines. Monoclinesare large step-like folds in otherwise horizontal sedimentary layers. • Recall thatfaultsare fractures in the Earth’s crust along which movement has taken place. The rock surface immediately above the fault is called the hanging wall. The rock surface below the fault is called the footwall. • The major types of faults are Normal faults, Reverse faults, Thrust faults, Strike-slip faults • A normal fault occurs when the hanging wall block moves down relative to the footwall block. A reverse fault is a fault in which the hanging block moves up (instead of down) relative to the footwall block. Thrust faults are reverse faults with dips of less than 45 degrees. Faults in which the movement is horizontal and parallel to the line of the fault is called a strike-slip fault.

25. Earth Science : Unit 3 Review: Forces Within • The major types of mountain types include Volcanic mountains, Folded mountains, Fault-block mountains, Dome mountains. • Earth’s mountains do not occur at random. Several mountains of similar shape, age, size and structure form a group called a mountain range. A group of different mountain ranges in the same region form a mountain system. • Recall from the previous chapters that volcanic mountains form along plate boundaries and at hot spots. • Mountains that are formed primarily by folding are called folded mountains. Thrust faulting is also important in the formation of folded mountains, which are often called fold-and-thrust belts. • Large scale normal faults are associated with fault-block mountains. Fault-block mountainsform as large blocks of crust are uplifted and tilted along normal faults. • Grabens and Horsts: Normal faulting occurs where tensional stresses cause the crust to be stretched or extended. As the crust is stretched, a block called a graben, which is bounded by normal faults, drops down. Grabens produce an elongated valley bordered by relatively uplifted structures called horsts.