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Transcript
Deformation is the bending & breaking of the crust! Balance of Earth’s lithosphere (crust) as though they were floating on the denser underlying layer of the athensphere (upper mantle composed of weak, plastic rock that is about 110 km below the surface. Isostasy controls the regional elevations of continents and ocean floors because of the density of their underlying rocks. ISOSTATIC ADJUSTMENT CAUSE EFFECT Wind, Water, & Ice erode River deposit sediment Glaciers form from snow Glaciers melt with temp. Uplift occurs due to lighter mountain from erosion. The ocean floor becomes heavier so goes down (subsidence) The glacier’s weight causes sinking (subsidence) The lessening of glacier’s weight causes sinking (subsidence) in ocean. Most earthquakes occur when rocks fracture, or break, deep within Earth. Fractures form when stress exceeds the strength of the rocks involved. Stress is the forces per unit/area. Strain is the deformation of materials in response to stress. (Temperature, pressure, & rock composition matter.) • not always permanent if slow • Permanent a) Brittle b) ductile There are three kinds of stress that act on Earth’s rocks: – Compression (squeezing) is stress that decreases the volume of a material. – Tension (stretching)is stress that pulls a material apart. – Shear (break or twist) is stress that causes a material to twist. There is a relationship between stress and strain that can be plotted as a stress-strain curve. – A stress-strain curve has two segments: a straight segment and a curved segment. – Low stresses produce the straight segment, which represents the elastic strain of a material. – If the elastic strain is reduced to zero, the deformation disappears. Ductile Deformation – When stress exceeds the strength of a material, the material breaks, or fails, as designated by the X on the graph. It produces permanent deformation, which means that the material stays deformed even if the stress is reduced to zero. – Most rocks, though brittle on the surface, become ductile at the higher temperatures present at greater depths. Folding & Fault Block Mountains Folding is a bending of the rock layer due to compression. Rocks will folded symmetrically or asymmetrically (most). Folds can very in shape & size (>1,000’s km). Folds have sloping sides called limbs & the bend is the hinge (meeting place). Anticline - Oldest rocks in the middle of the fold. Syncline – oldest rocks on the outside. Monocline limbs are horizontal. –2 Mountain Ranges- groups of mountains with similar sizes, shapes, & ages. Ex. Himalaya & Cascade Mtns. http://www.youtube.com/watch?v=kSFur_X5moo http://www.cascadeloop.com/index.php?page_id=473 Mountain System – made up of mtn. ranges . Ex. The Appalachian Mtns. Consists of the Smoky, Blue Ridge, Green, White, & Cumberland. http://www.visitnc.com/journeys/highlights/outdoor-playground http://www.visitnc.com/journeys/highlights/appalachiantrail http://www.visitnc.com/journeys/highlights/blue-ridge Mountain Belts – largest systems located at convergent boundaries. The circum – Pacific & the EurasianMelanesian belts. Continental – Continental produces thrust faults. Ex. Himalaya Mtns. Oceanic- Continental produce terranes Ex. Andes Mtns. Oceanic-Oceanic –produce island arc (volcanic mtn. on the ocean floor). Ex. Mariana Islands These different types of mountain have distinguishing physical characteristics &also different formations. Volcanic Mountain Volcanoes form volcanic mountains and these mountains are then shaped by further eruptions, lava flows, and collapses forever altering the view of this mountain. So explosive was this eruption, that it can alter the height of the mountain by nearly 1,300 feet. other major Volcanic Mountains in North America include Mt. Ranier, Washington, with an elevation of 14,410 feet, making it the highest peak of the Cascade Mountain Range. Mount Shasta, California, with an elevation of 14, 162. Also in the USA is Mount Hood, Oregon, with an elevation of 11, 235 feet, and Mount Spur, Alaska, with an elevation of 11, 067 feet. Dome Mountain As their name states, Dome Mountains have a characteristic ‘dome’ top. In the USA, the Black Hills of South Dakota offer excellent examples of dome topped mountains. Erosion is believed to be a major factor in the shaping of most dome formations. Fold Mountain Again, the name tells a lot. and through time with great force, pushed pieces of earth upward and folded them over onto themselves. Example of Fold Mountains includes the Appalachian Mountains. Fault-Block Mountain As in ‘Fold’ mountains, great force is behind the Fault-Block Mountains. What differs is that instead of the earth folding over, the earth fractures and blocks are stacked. Plateau Mountain To appreciate theses High levels of flat land one only has to look at the Catskills of New York. More Mountain Facts * The Himalayas are the world’s overall tallest mountains. * The Andes, which runs more than 4,900 miles, is the longest mountain range in the world. * Mauna Kea, with an elevation of 13,796 feet, is actually 32,000 feet tall from its start on the sea floor, making it the world’s highest island peak from base to tip. * Mount Everest, part of the Himalayas, is the highest point on earth, with a height of 29,023 feet. What many don’t realize is that Mt. Everest is only one in this range of over 30 peaks that rise to over 24,000 feet. Also in this range is Kanchenjunga at 28,208 feet, Makalu at 27,766 feet, and Dhaulagiri at 26,810 feet. * Mount Mitchell is the Appalachians highest peak at 6,684 feet. * High Knob, in the Pocono Mountains is only 2,162 feet tall. * Pikes Peak Mountain, part of the Great Rocky Mountains, is an astounding 14,110 feet high. Various Mountain Chains - 4 different ways •Volcanic Ring of Fire: Pacific Rim •Dome (Erosional) Rocky Mountains : Western US •Fault-Block Sierra Nevada Mountains : Nevada Basin and Range Province : Arizona •Folded Himalaya Range : China/Tibet/India/Nepal Appalachian Mountains : Eastern US Ural Mountains : Spain/France Types of Faults There are 3 types of faults: Reverse faults are fractures that form as a result of horizontal compression. Normal faults are fractures caused by horizontal tension. Strike-slip faults are fractures caused by horizontal shear. Earthquakes are natural vibrations of the ground caused by movement along fractures in Earth’s crust, or by volcanic eruptions. In some instances a single earthquake have killed more than 100,000 people and destroyed entire cities. More than 1million earthquakes occur each year. More than 90 % of earthquakes are not felt and cause little damage. A fault is the fracture or system of fractures along which movement occurs called the fault plane. MOST EARTHQUAKES ARE CAUSED BY MOVEMENTS ALONG FAULTS. • Irregular surfaces in rocks can snag and lock, causing stress to build in the rocks. • When the rocks reach their elastic limit they break, and this produces an earthquake. HAITIAN EARTHQUAKE Types of Seismic Waves The vibrations of the ground during an earthquake are seismic waves. Every earthquake generates 3 types of seismic waves. Primary, Surface, & Secondary Waves Primary waves, or Pwaves, squeeze and pull rocks in the same direction along which the waves are traveling. Types of Seismic Waves Secondary waves or S-waves, cause rocks to move at right angles to the direction of the waves. Surface waves travel along Earth’s surface, moving in two directions. Types of Seismic Waves – P-waves and S-waves (body waves) pass through Earth’s interior. – The focus of an earthquake is the point of failure of rocks at the depth where an earthquake originates. – The epicenter of an earthquake is the point on Earth’s surface directly above the focus. Seismology is the study of earthquake waves. • The seismic waves that shake the ground during a quake also penetrate Earth’s interior. • This has provided information that has enabled Earth scientists to construct models of Earth’s internal structure. Seismometers, or seismographs, are sensitive instruments that detect and record the vibrations sent out by earthquakes. • All seismometers include a frame that is anchored to the ground and a mass that is suspended from a spring or wire. • The relative motion of the mass in relation to the frame is recorded during an earthquake. A seismogram is the record produced by a seismometer. Travel-Time Curves – The time separation between the P-waves and S-waves increases with travel distance. – From this separation, the distance from the epicenter of a quake to the seismic facility that recorded the seismogram can be determined. – The P-waves always arrive first at a seismic facility. Seismic waves change speed and direction when they encounter different materials in Earth’s interior. http://science.howstuffworks.com/nature/naturaldisasters/earthquake4.htm http://www.youtube.com/watch?v=yOGoKCK17a4 – P-waves and S-waves traveling through the mantle follow fairly direct paths. – P-waves that strike the core are refracted, or bent, causing P-wave shadow zones . P-waves appear on seismograms. – S-waves do not enter Earth’s core because they cannot travel through liquids and do not reappear beyond the P-Wave shadow zone. Seismic Waves and Earth’s Interior Earth’s Internal Structure – This disappearance of Swaves has allowed seismologists to reason that Earth’s outer core must be liquid – Detailed studies of how other seismic waves reflect deep within Earth show that Earth’s inner core is solid Seismic Waves and Earth’s Interior Earth’s Internal Structure – Lithosphere - Igneous rocks granite, basalt, and peridotite. – Asthenosphere - peridotite. – Lower mantle -solid and is probably composed of simple oxides containing iron, silicon, and magnesium. – The core -a mixture of iron and nickel supported by studies of meteorites. Meteorites of iron, nickel, and chunks of rock similar to peridotite in the same proportions as the rocks thought to make up Earth’s core and mantle. Magnitude is the measurement of the amount of energy released during an earthquake. The Richter scale is a numerical scale based on the size of the largest seismic waves generated by a quake that is used to describe its magnitude. – Each successive number in the scale represents an increase in seismic-wave size, or amplitude, of a factor of 10. – Each increase in magnitude corresponds to about a 32-fold increase in seismic energy. Moment Magnitude Scale – The moment magnitude scale, used by seismologists to measure earthquake magnitude. Size of the fault rupture, The amount of movement along the fault Rocks’ stiffness. – Moment magnitude values are estimated from the size of several types of seismic waves produced by an earthquake. Modified Mercalli Scale Measures the amount of damage done to the structures. This scale uses the I to XII to designate the degree of intensity. The higher the # the worse the damage! Modified Mercalli Scale – Earthquake intensity depends on the amplitude of the surface waves generated. – Maximum intensity values near the epicenter; Mercalli values decrease to I at distances very far from the epicenter. – Modified Mercalli scale intensity values can be compiled to make a seismicintensity map. – Depth of Focus Earthquake intensity is related to earthquake magnitude. – The depth of the quake’s focus determines the intensity of an earthquake. – An earthquake’s focus is classified as shallow, intermediate, or deep. – A deep-focus earthquake produces smaller vibrations at the epicenter. Distance to an Earthquake – All epicenter locations & times of occurrence can be easily determined using seismograms and travel-time curves. – The P-S separation determines the epicentral distance (distance to a quake’s epicenter from the seismic station that recorded the waves). – The travel time of either wave at the epicentral distance of that station can be read from the traveltime graph. – The time of occurrence of the earthquake is determined by subtracting the travel time from the arrival time. Distance to an Earthquake – The earthquake could have occurred anywhere on a circle around the seismic station. – The radius of the circle is equal to the epicentral distance. – If 3 or more epicentral distances known - the exact location of the epicenter can be determined. The majority of the world’s earthquakes occur in relatively narrow seismic belts that are associated with tectonic plate boundaries. – 80 % of all earthquakes in the Circum-Pacific Belt. – 15 % take place across southern Europe and Asia. – 4% in narrow bands that run along the crests of ocean ridges. – 1% happen far from tectonic plate boundaries and are random. – http://www.digitalgeology.net/page9.html Structural Failure In many earthquake-prone areas, buildings are destroyed as the ground beneath them shakes. The most severe damage stone, concrete, or other brittle building materials. Wooden structures & modern high-rise, steel-frame buildings sustain little damage . “Pancaking” - the supporting walls of the ground floor fail, causing the upper floors to fall and collapse as they hit lower floors. When a quake has the same period of vibration as the natural sway of a building, they will sway violently. The natural sway of a building is related to height; Longer waves - taller buildings . Fault Scarps Fault movements associated with earthquakes can produce fault scarps. Fault scarps are areas of great vertical offset (slope) where the fault intersects the ground surface. Tsunami A tsunami is a large ocean wave generated by vertical motions of the seafloor during an earthquake. – Displaces the entire column of water overlying the fault, creating bulges and depressions in the water. – Spreads out from the epicenter in the form of extremely long waves. (Speeds 500 - 800 km/h) – May form huge breakers (Heights may > 30 m) – Earthquakes may trigger massive landslides in sloping areas. – In fluid-saturated sand, seismic vibrations may cause subsurface materials to liquefy act like quicksand. – Wave size and earthquake intensity are amplified in soft, unconsolidated sediments and are slowed in hard, resistant rocks such as granite. The past seismic activity is a reliable indicator of future earthquakes and can be used to generate seismic-risk maps. Earthquake Prediction 2 factors: http://www.youtube.com/watch?v=xQXDt4VdS0E http://www.youtube.com/watch?v=o0tVbjrbkp8&feature=related – The history of earthquakes in an area – The rate at which strain builds up in the rocks Probability (ratio) based on Seismic gaps (active faults that haven’t experienced significant earthquakes for a long period of time.) To predict scientists make several measurements. • Amount of strain • Amount of time passed since an earthquake has struck that section of the fault Why might only buildings that are between 5 and 10 stories tall be seriously affected during an earthquake? If the shaking caused by the quake had the same period of vibration as the natural sway of buildings at that height, they would be more violently affected than either taller or shorter buildings. What is a seismic gap, and how would it possibly affect the prediction of earthquakes for the area? A seismic gap is a section of an active fault that hasn’t experienced a significant earthquake for a log period of time. A seismic gap would generally have a higher probability of a future earthquake. Identify whether the following statements are true or false. ______ false All high seismic risk areas in the United States are located on the Pacific coast. ______ false Fault scarps are areas of horizontal offset. ______ true Wooden structures generally fair better in an earthquake than do stone structures. ______ true San Francisco sits above a seismic gap. Match the following terms with their definitions. A. deformation of materials in E stress ___ A strain ___ D fault ___ ___ C focus ___ B epicenter response to forces acting upon them B. surface point directly above an earthquake’s point of origination C. actual point where an earthquake originates D. a fracture or system of fractures in Earth’s crust along which movement occurs E. force per unit area acting on a material Which type of fault best describes the San Andreas Fault? Why? The San Andreas Fault is a strike-slip fault. The movement in this fault is primarily horizontal and is caused by two plates that are sliding past each other rather than by subduction. Identify whether the following statements are true or false. ______ true A P-wave causes rocks to move back and forth. ______ true S-waves pass through Earth’s interior. ______ false A material that undergoes ductile deformation will return to its original state if stress is reduced to zero. ______ true The focus of an earthquake is usually several kilometers below Earth’s surface. What happens to P-waves when they enter Earth’s core? P-waves are refracted when they pass through Earth’s core, which causes a shadow zone where no direct P-waves can be detected by seismometers. How can the distance to an earthquake’s epicenter be determined? Seismologists have been able to construct global time-travel curves for the initial P-waves and S-waves of an earthquake. The difference in time between the arrival of the initial P-waves and S-waves indicates the distance from the seismometer to the earthquake’s epicenter. Identify whether the following statements are true or false. ______ false Direct S-waves are present in a P-wave shadow zone. ______ false Earth’s inner core is probably liquid. ______ true A 4-minute, 30-second separation between the arrival of P-waves and S-waves would indicate that the epicenter was about 3000 km away. ______ true Meteorites are believed to similar in composition to Earth’s core. Match the following terms with their definitions. A. rates intensity through the type ___ D magnitude of damage and other effects of an earthquake ___ Richter scale C ___ moment magnitude B. takes into account the fault B scale rupture, the amount of ___ modified Mercalli movement along the fault, and scale A the rock’s stiffness C. describes a quake based on its largest seismic waves D. the amount of energy released during an earthquake How can a high magnitude earthquake rank relatively low on the modified Mercalli scale? The modified Mercalli scale measures the intensity of an earthquake against specific damage that it causes. A high magnitude, deepfocus quake would probably have a lower Mercalli rating than a moderate, shallow-focus quake. The Mercalli rating will generally go down with distance from the epicenter, regardless of the magnitude. Identify whether the following statements are true or false. ______ true A Mercalli rating of VI would indicate less damage than a rating of VIII. ______ false A quake with a magnitude of 7 on the Richter scale would have 10 times the amount of seismic energy as magnitude 6 quake. ______ true Most earthquakes occur along plate boundaries. ______ false Less than 10 000 earthquakes occur in an average year.