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Earthquakes are the shaking, rolling or sudden shock of the earth’s surface. Earthquakes happen along fault lines in the earth’s crust. Earthquakes can be felt over large areas although they usually last less than one minute. Earthquakes cannot be predicted -- although scientists are working on it! • Most of the time, you will notice an earthquake by the gentle shaking of the ground. You may notice hanging plants swaying or objects wobbling on shelves. Sometimes you may hear a low rumbling noise or feel a sharp jolt. A survivor of the 1906 earthquake in San Francisco said the sensation was like riding a bicycle down a long flight of stairs. • The intensity of an earthquake can be measured. One measurement is called the Richter scale. Earthquakes below 4.0 on the Richter scale usually do not cause damage, and earthquakes below 2.0 usually can’t be felt. Earthquakes over 5.0 on the scale can cause damage. A magnitude 6.0 earthquake is considered strong and a magnitude 7.0 is a major earthquake. The Northridge Earthquake, which hit Southern California in 1994, was magnitude 6.7. Earthquakes often occur when tectonic plates collide What happens when plates collide? It depends how the plates are moving when they meet: When two plates collide head-on, they push each other up and form mountains. That's how the Himalayas and other great mountain ranges (including the Rockies, long ago) were created. When one plate dives below another plate, it creates a subduction zone as the diving plate is crushed and melted. This process often creates volcanoes as the magma (molten rock) rises up to the surface. When two plates slide past each other, they create a transform fault, like the San Andreas fault. Types of Faults • When a section of rock breaks, rocks on either side of the break might move because of elastic rebound. The surface of this break is called a fault • Elastic rebound is when the stress of the forces within the rocks becomes so great that the rocks separate and an earthquake results. • There are different kinds of faults Normal Faults • Normal Faults – When tension forces pull rocks apart Rock above the fault moves down compared to rock below the fault The linked image cannot be displayed. The file may have been moved, renamed, or deleted. Verify that the link points to the correct file and location. Reverse Faults • Reverse Faults – When compression forces squeeze rocks together Rock above the fault moves up compared to rock below the fault The linked image cannot be displayed. The file may have been moved, renamed, or deleted. Verify that the link points to the correct file and location. Strike-Slip Faults • Strike-slip Faults – When shear forces cause rocks to move past one another in opposite directions on Earth’s surface Rocks do not move up or down, but rather move side by side The linked image cannot be displayed. The file may have been moved, renamed, or deleted. Verify that the link points to the correct file and location. • Earthquakes can happen in any of these situations. Despite the powerful forces driving plate movement, the plates themselves spend much of the time locked in place by the friction of the plates rubbing against each other. Eventually, however, they build up so much pressure that the plates abruptly snap forward. Then the ground can shift a few feet—or a few dozen! Shock waves from that sudden motion shoot out in all directions, creating an earthquake. The linked image cannot be displayed. The file may have been moved, renamed, or deleted. Verify that the link points to the correct file and location. • Two great plates, the Pacific and the North American, meet in California. The Pacific Plate is moving north, creating a transform fault (the San Andreas and related faults) Over the last 20 million years the Pacific Plate has slid about 200 miles north. If it keeps moving as predicted, San Francisco will become neighbors with Seattle in 20 million years! Because the San Andreas fault curves around Los Angeles, and then again into the Pacific in northern California, the two plates cannot slide smoothly against each other. Instead, the complex stresses of plate movement have fractured the land and created dozens of smaller fault lines. • Earthquakes can also occur within plates, although plate-boundary earthquakes are much more common. Less than 10 percent of all earthquakes occur within plate interiors. The New Madrid earthquakes of 1811-1812 and the 1886 Charleston earthquake occurred within the North American plate. • Earthquakes: Facts and Fiction • Fiction: Earthquakes usually happen in the morning. Fact: Earthquakes happen in both the day and the night. There is no pattern. • Fiction: There is such a thing as "earthquake weather." Fact: There is no connection between earthquakes and weather. Remember, earthquakes happen deep in the earth, far away from the weather! • Fiction: Earthquakes are on the increase. Fact: It may seem like we’re having more earthquakes because there are more reporting stations, but the truth is we’re not. • Fiction: We can prevent earthquakes from happening. Fact: No. You can protect yourself by doing things to secure buildings, like your home, but earthquakes can’t be prevented -- or predicted. The point beneath the Earth's surface where the rocks break and move is called the focus of the earthquake. The focus is the underground point of origin of an earthquake. Directly above the focus, on the Earth's surface, is the epicenter. • Earthquake waves are known as seismic waves. There are three main types of seismic waves. • Each type of wave has a characteristic speed and manner of travel. • Primary Waves • Seismic waves that travel the fastest are called primary waves, or P waves. P waves arrive at a given point before any other type of seismic wave. P waves travel through solids, liquids and gases. • P waves are push-pull waves. As P waves travel, they push rock particles into the particles ahead of them, thus compressing the particles. The rock particles then bounce back. They hit the particles behind them that are being pushed forward. The particles move back and forth in the direction the waves are moving. • Secondary Waves • Seismic waves that do not travel through the Earth as fast as P waves do are secondary waves, or S waves. S waves arrive at a given point after P waves do. S waves travel through solids but not through liquids and gases. Body P waves S waves Primary (1st to arrive) Secondary (2nd to arrive larger) Transverse, Shear waves AKA Longitudinal, Compression Moves through all states of matter (solid, liquid, gas) Movement of rock back and forth movement of rock • push/pull or compression/stretch out • Like slinky down stairs Vibration is same as the direction of travel Can go through solids only Move sideways • perpendicular to direction of wave travel • Like snake Lets test your understanding!! Is this a P or an S wave? S Wave P wave! The linked image cannot be displayed. The file may have been moved, renamed, or deleted. Verify that the link points to the correct file and location. • This is an image of a seismograph, an instrument used to record the energy released by an earthquake. When the needle is moved by the motion of the earth, it leaves a wavy line. HOW TO READ SEISMOGRAMS P & S (body waves) move through earth & arrive first • P & S waves used to calculate magnitude of earthquake • Amplitude = height of wave (how much the rock moves; size of vibration) Ways to Predict Earthquakes • Scientists cannot predict earthquakes, but they can study changes in the Earth that lead up to them, such as: -movement along fault lines -changes in groundwater levels -electrical properties of rocks under stress -rock layers that have been affected by ancient earthquakes Tsunamis • Tsunamis – Seismic sea waves resulting from an earthquake on the ocean floor – These waves can travel outward from the earthquake thousands of kilometers in all directions More Tsunamis Why are they so dangerous? At sea their wave heights are less than a meter in deep water, ships would ride right over them When they approach the shore, the waves slow down and their heights increase as the waves drag along the bottom of the seafloor Even More Tsunamis This can create waves 30 meters tall! The linked image cannot be displayed. The file may have been moved, renamed, or deleted. Verify that the link points to the correct file and location. Chapter 15, section 2 Volcanoes! The linked image cannot be displayed. The file may have been moved, renamed, or deleted. Verify that the link points to the correct file and location. • Lava magma that has come through to the surface of the Earth • Crater circular holes at the top of volcanoes where lava and other matter is shot out • Tephra bits of rock and solidified lava dropped from the air during a volcanic eruption (can be very small like ash and cinders to very large rocks called bombs or blocks) Convergent boundaries can result in volcanic islands When subduction occurs, the sinking plate melts and forms a magma chamber This magma is what caused the volcanic eruptions that formed the Caribbean Islands Pyroclastic Flows Huge avalanches of hot, glowing rock flowing on a cushion of extremely hot gases (it flows very quickly, up to 200 km/hr) https://www.youtube.com/watch? feature=player_detailpage&v=95bYATFIO xs Volcanoes can destroy but they also add new rock to Earth’s crust Different types of eruptions form different types of volcanoes Why are some volcanoes violent and others calmer? Violent vs. Non-violent Eruptions The makeup of the magma/ lava! Lava that contains more silicon is thicker and flows more slowly Lava that has more iron and magnesium and less silicon flows more easily The amount of water vapor and other gasses trapped within the lava also plays a part in how the volcano erupts Steam builds pressure in magma the pressure is released as the volcano erupts The more gas/ water vapor is trapped within the lava, the more violent the eruption will be (lava with more silicon traps more water/ gas) Shield Volcanoes • Made from basaltic lava (iron and magnesium-rich, low in silicon) • Flows in broad, flat layers • Forms a broad volcano with gently sloping sides • These are the largest types of volcanoes • Formed where magma is forced up from very deep within the Earth or at divergent boundaries Shield Volcano The linked image cannot be displayed. The file may have been moved, renamed, or deleted. Verify that the link points to the correct file and location. Cinder Cone Volcanoes • Rising magma accumulates gases on it’s way to the surface • The gasses build up pressure that finally causes an eruption • The eruptions are moderate to violent and spew ash, cinders and lava high into the air • The tephra forms a small coned volcano • Eruptions are usually quick because once the gas is released the force behind the eruption is gone Cinder Cone Volcano The linked image cannot be displayed. The file may have been moved, renamed, or deleted. Verify that the link points to the correct file and location. Composite Volcanoes • • • • • Steep-sided Alternating layers of lava and tephra Some violent eruptions (release tephra) Some quieter eruptions (release lava) Form because of subduction Composite Volcano The linked image cannot be displayed. The file may have been moved, renamed, or deleted. Verify that the link points to the correct file and location. Fissure Eruptions • Very fluid magma seeps through cracks in Earth’s surface • Lava is not very viscous (not thick, flows easily) • Lava flows freely across land and forms flood basalts • Flood basalts erode and over millions of years become lava plateaus (flat landforms) Fissure Eruptions The linked image cannot be displayed. The file may have been moved, renamed, or deleted. Verify that the link points to the correct file and location. The linked image cannot be displayed. The file may have been moved, renamed, or deleted. Verify that the link points to the correct file and location. Chapter 15 section 3 Earthquakes, Volcanoes, and Plate Tectonics Where Volcanoes Form The linked image cannot be displayed. The file may have been moved, renamed, or deleted. Verify that the link points to the correct file and location. Most volcanoes form along plate boundaries! Divergent plate boundaries: • When plates separate rifts form between them • The rifts have cracks that conduct magma from the mantle • Fissure eruptions occur along rifts (form lava that cools and solidifies into basalt • Most rocks in the Earth’s crust are basalt The linked image cannot be displayed. The file may have been moved, renamed, or deleted. Verify that the link points to the correct file and location. Convergent Plate Boundaries: • when one plate subducts, basalt and sediment move down into the mantle • Water that moves in lowers the melting point of the surrounding rock • Heat in the mantle causes pat of the subducted plate to melt • This molten material gets forced upward Hot Spots: Large rising bodies of magma Hot spots can force their way up through the mantle and crust creating volcanoes within plates rather than at plate boundaries The linked image cannot be displayed. The file may have been moved, renamed, or deleted. Verify that the link points to the correct file and location. There is a hot spot under Hawaii Volcanoes usually form along rift zones, subduction zones, or over hot spots Most (~80%) earthquakes occur in the Pacific Ring of Fire, the same area where most of Earth’s volcanoes occur The linked image cannot be displayed. The file may have been moved, renamed, or deleted. Verify that the link points to the correct file and location. Movement of the plates produces forces that generate the energy that cause earthquakes How seismic waves pass through a material depends on the properties of that material Scientists can study seismic waves to determine what kind of material is inside the Earth Waves will travel differently through solids, liquids and gases and through liquids with different viscosity Remember that p waves travel through solids, liquids, and gases, while s waves only travel through solids Convection Currents Convection currents inside the Earth cause plate movement which then creates conditions that cause earthquakes and volcanoes Hot, less dense magma is forced towards the surface where it eventually cools and sinks back into the mantle towards Earth’s core The linked image cannot be displayed. The file may have been moved, renamed, or deleted. Verify that the link points to the correct file and location.