Physical Geography - Version 1 Unit 14 Earths Dynamic Surface Tectonics Force

Figure Tilting of Material in Golden Canyon , Death Valley California . Image by Jeremy is used under a license . UNIT 14 EARTHS DYNAMIC SURFACE TECTONICS FORCE Goals Objectives of this unit Describe the causes of tectonic stress and how they influence earthquakes and volcanoes . Understand where earthquakes and volcanoes are located around the world and why . Explain geologic structures that lead to folding and faulting of the physical landscape . Determine how the composition of magma determines the explosiveness of the volcanic eruption . Describe the various types of fault lines and volcanoes and the they form . GEOGRAPHY

CAUSES TYPES OF TECTONIC STRESS Enormous slabs of lithosphere move unevenly over the planet spherical surface , resulting in earthquakes . This chapter deals with two types of geological activity that occur because of plate tectonics mountain building and earthquakes . First , we will consider what can happen to rocks when they are exposed to stress . Stress is the force applied to an object . In , stress is the force per unit area that is placed on a rock . Four types of stresses acting on materials . A deeply buried rock is pushed down by the weight of all the material above it . Since the rock can not move , it can not deform called confining stress . Compression squeezes rocks together , causing rocks to fold or fracture . Compression is the most common stress at convergent plate boundaries . Rocks that are pulled apart are under tension . Rocks under tension lengthen or break apart . Tension is the major type of stress at divergent plate boundaries . When forces are parallel but moving in opposite directions , the stress is called shear . Shear stress is the most common stress at transform plate boundaries . When stress causes a material to change shape , it has undergone strain or deformation . Deformed rocks are common in geologically active areas . A rock response to stress depends on the rock type , the surrounding temperature , and pressure conditions the rock is under the length of time the rock is under stress , and the type of stress . The rocks then have three possible responses to increasing stress elastic deformation , plastic deformation , or fracturing . Elastic deformation occurs when the rock returns to its original shape when the stress is removed . When rocks under stress do not return to its original shape when the stress is removed , it is called plastic deformation . Finally , when a rock under stress breaks , it called a fracture . Under what conditions do you think a rock is more likely to fracture ?

Is it more likely to break deep within Earth crust or at the surface ?

What if the stress applied is sharp rather than gradual ?

At the Earth surface , rocks usually break quite quickly , but deeper in the crust , where temperatures and pressures are higher , rocks are more likely to deform plastically . Sudden stress , such as a hit with a hammer , is more likely to make a rock break . Stress applied over time often leads to plastic deformation . GEOGRAPHY

GEOLOGIC STRUCTURES Sedimentary rocks are important for deciphering the geologic history of a region because they follow certain rules . First , sedimentary rocks are formed with the oldest layers on the bottom and the youngest on top . Second , sediments are deposited horizontally , so sedimentary rock layers are originally horizontal , as are some volcanic rocks , such as ash falls . Finally , sedimentary rock layers that are not horizontal are deformed in some manner . Often looking like they are tiling into the earth . You can trace the deformation a rock has experienced by seeing how it differs from its original horizontal , position . This deformation produces geologic structures such as folds , joints , and faults that are caused by stresses . Folds Rocks plastically under compressive stresses crumple into folds . They do not return to their original shape . Ifthe rocks experience more stress , they may undergo more folding or even fracture . There are three major types of rock folding , and . A monocline is a simple bend in the rock layers so that they are no longer horizontal . are folded rocks that arch upward and dip away from the center of the fold . The oldest rocks are at the center of an anticline and the youngest are draped over them . When rocks arch upward to form a rounded structure , that structure is called adobe . A syncline is a fold that bends downward , causing the youngest rocks to be at the center and the oldest is on the outside . When rocks bend downward in a rounded structure , that structure is called . Ifthe rocks are exposed at the surface , where are the oldest rocks located ?

ANTICLINE SYNCLINE Figure Example of Both a Syncline Anticline . Image is used under a license . GEOGRAPHY Faults A rock under enough stress will eventually fracture . is no movement on either side of a fracture , the fracture is called a joint . But ifthe blocks of rock on one or both sides of a fracture move , the fracture is called a fault . Sudden motions along faults cause rocks to break and move suddenly , releasing the stress energy to create an earthquake . A slip is the distance rocks move along a fault and can be up or down the fault plane . Slip is relative because there is usually no way to know whether both sides moved or only one . Faults lie at an angle to the horizontal surface of the Earth . That angle is called the dip . The dip defines which of two basic types a fault is . If the dip is inclined relative to the horizontal , the fault is a fault . There are two types of faults . In normal faults , the hanging wall drops down relative to the footwall . Normal faults can be huge and are often responsible for uplifting mountain ranges in regions experiencing tensional stress . With reverse faults , the footwall drops down relative to the hanging wall . A type of reverse fault is a thrust fault , in which the fault plane angle is nearly horizontal . Rocks can slip many miles along thrust faults . A fault is a fault in which the dip of the fault plane is vertical and result from shear stresses . California San Andreas Fault is the world most famous fault . It is a lateral fault . A ' Figure The or Faulting in Red Rock State Park , in California . Image by Jeremy is used under a license . GEOGRAPHY

Stress Mountain Building It is the shear power and strength of two or more converging continental plates smash upwards that create mountain ranges . Stresses from this uplift cause folds , reverse faults , and thrust faults , which allow the crust to rise upwards . The subduction of oceanic lithosphere at convergent plate boundaries also builds mountain ranges . When tensional stresses pull crust apart , it breaks into blocks that slide up and drop down along normal faults . The result is alternating mountains and valleys , known as a . Causes of Earthquakes An earthquake is a sudden ground movement caused by the sudden release of energy stored in rocks , called the elastic rebound theory . Earthquakes happen when so much stress builds up in the rocks that the rocks rupture . The energy is transmitted by seismic waves . Each year there are more than earthquakes strong enough to be felt by people and recorded by . In an earthquake , the initial point where the rocks rupture in the crust is called the focus ( sometimes called the ) The epicenter is the point on the land surface that is directly above the focus . In about 75 of earthquakes , the focus is in the top 10 to 15 kilometers ( to miles ) of the crust . Shallow earthquakes cause the most damage because the focus is near where people live . However , it is the epicenter of an earthquake that is reported by scientists and the media . EARTHQUAKES ZONES Nearly 95 of all earthquakes take place along one ofthe three types of tectonic plate boundaries , but earthquakes do occur along all three types of plate boundaries . About 80 of all earthquakes strike around the Pacific Ocean basin because it is lined with convergent and transform boundaries . Called the Ring of Fire , this is also the location of most volcanoes around the planet . About 15 take place in the Belt , where convergence is causing the Indian Plate to run into the Eurasian Plate creating the largest mountain ranges in the world . The remaining are scattered around other plate boundaries or are earthquakes . Transform Plate Boundary Zone Transform plate boundaries occur where two tectonic plates are grinding parallel to each other rather than colliding or . Deadly earthquakes occur at transform plate boundaries , creating faults because they tend to have shallow focuses where the rupture occurs . GEOGRAPHY

The faults along the San Andreas Fault zone produce around earthquakes a year . Most are tiny , but occasionally one is massive . In the San Francisco Bay Area , the Hayward Fault was the site of a magnitude earthquake in 1868 . The 1906 quake on the San Andreas Fault had a magnitude estimated at . During the 1989 World Series , a magnitude earthquake struck Loma , near Santa Cruz , California , killing 63 people , injuring , and cost billion . A few years later in , California , a magnitude earthquake killed 72 people , injured people , and caused billion in damage . This earthquake occurred on an unknown fault because it was a blind thrust fault near Los Angeles , California . Although California is prone to many natural hazards , including volcanic eruptions at . or . and landslides on coastal cliffs , the natural hazard the state is linked with is earthquakes . New Zealand also has earthquakes , about a year , but only a small percentage of those are large enough to be felt . fault Figure Made . Image is in the public domain . Convergent Plate Boundary Zone Earthquakes at convergent plate boundaries mark the motions of the lithosphere as it plunges through the mantle , creating reverse and thrust faults . Convergent plate boundaries produce earthquakes all around the Pacific Ocean basin . The Philippine Plate and the Pacific Plate subduct beneath Japan , creating a chain of volcanoes and produces as many as earthquakes annually . In March 2011 an enormous earthquake struck off of in northeastern Japan . This quake , called the 2011 earthquake , was the most powerful ever to strike Japan and one of the top five known in the world . Damage from the earthquake was nearly overshadowed by the tsunami it generated , which wiped out coastal cities and towns . Two months after the earthquake , about people were dead or missing , and buildings had been damaged or destroyed . some as large as major earthquakes , have continued to rock the region . The Pacific Northwest of the United States is at risk from a potentially massive earthquake that could strike any time . Subduction of the Juan de plate beneath North America produces GEOGRAPHY

active volcanoes , but large earthquakes only hit every 300 to 600 years . The last was in 1700 , with an estimated magnitude of around . Massive earthquakes are the hallmark of the thrust faulting and folding when two continental plates converge . The 2001 earthquake in India was responsible for about deaths , and many more people became injured or homeless . In Understanding Earthquakes From Research to Resilience , scientists try to understand the mechanisms that cause earthquakes and tsunamis and the ways that society can deal with them . I Reverse fault Figure Reverse Fault Model . Image is in the public domain . Divergent Plate Boundary Zone Earthquakes at ridges are small and shallow because the plates are young , thin , and hot . On land where continents split apart , earthquakes are larger and stronger . A classic example of normal faulting along divergent boundaries is the Front in Utah and the entire Basin and Range through Nevada . Normal fault Figure Normal Fault Model . Image is in the public domain . GEOGRAPHY

Boundary Zone earthquakes are the result of stresses caused by plate motions acting in solid slabs of lithosphere . In 1812 , a magnitude earthquake struck near New Madrid , Missouri . The earthquake was strongly felt over approximately square miles and altered the course of the Mississippi River . Because very few people lived there at the time , only 20 people died . Many more people live there today . A similar earthquake today would undoubtedly kill many people and cause a great deal of property damage . Measuring Earthquakes People have always tried to quantify the size of , and damage done by earthquakes . Since early in the century , there have been three methods . The oldest ofthe scales is called the Intensity scale . Earthquakes are described in terms of what nearby residents felt and the damage that was done to nearby structures . This scale is more qualitative in information because it based on visual damage and not the actual energy released by the earthquake . Pin It ! Shake Maps Did you feel a quake ?

Check out this Earthquake to see the most recent earthquakes ! lot Earthquake Ian . I . a ) my , 17 mar um um . an am my awn . I run am , Figure Example of a Shake Map of the 1994 Earthquake . Image used with permission GEOGRAPHY With the invention of the seismograph station , the Richter magnitude scale was created . Developed in 1935 by Charles Richter , this scale uses a seismometer to measure the magnitude of the largest jolt of energy released by an earthquake . Today , the moment magnitude scale has replaced the Richter scale . The moment magnitude scale measures the total energy released by an earthquake . The moment magnitude is calculated from the area of the fault that is ruptured and the distance the ground moved along the fault . The Richter scale and the moment magnitude scale are logarithmic . The amplitude of the largest wave increases ten times from one integer to the next . An increase in one integer means that thirty times more energy was released . These two scales often give very similar measurements . How does the amplitude of the largest seismic wave of a magnitude earthquake compare with the largest wave of a magnitude earthquake ?

How does it compare with a magnitude quake ?

The amplitude of the largest seismic wave of a magnitude quake is 10 times that of a magnitude quake and 100 times that of a magnitude quake . MAGNITUDE Table Richter Scale DESCRIPTION WHAT IT FEELS LIKE FREQUENCY LESS THAN OR OVER Micro Minor Minor Light Moderate Strong Major Great Great Massive GEOGRAPHY Normally only recorded by seismographs . Most people can not feel them . A few people feel them . No building damages . Some people feel them . Objects inside can be seen shaking . Most people feel it . Indoor objects shake or fall to the floor . Can damage or destroy buildings not designed to withstand earthquakes . Everyone feels it . Widespread shaking far from the epicenter . Damages buildings . Widespread damage in most areas . Widespread damage in large areas . Severe damage to most buildings . Never recorded . Millions per year . Over Million per year . Over per year . to per year . to per year . 100 to 150 per year . 10 to 20 per year . About per year . per years . Never recorded .

Each scale has its own benefits . As mentioned above , the Intensity scale is based on how much damage someone would see , or instrumental intensity . This is relative though because some places have strong building codes , and the rock material underneath will impact ground shaking without changing the energy released at the focus . With the Richter scale , a single sharp jolt measures higher than a very long intense earthquake that releases more energy . The moment magnitude scale more accurately reflects the energy released and the damage caused . Today , most seismologists now use the moment magnitude scale . Earthquake Prediction Scientists are along way from being able to predict earthquakes . A good prediction must be accurate as to where an earthquake will occur , when it will occur , and at what magnitude it will be so that people can evacuate . An unnecessary evacuation is expensive and causes people not to believe authorities the next time an evacuation is ordered . Where an earthquake will occur is the easiest feature to predict . Scientists know that earthquakes take place at plate boundaries and tend to happen where they occurred before . communities should always be prepared for an earthquake . These communities can implement building codes to make structures earthquake safe . When an earthquake will occur is much more difficult to predict . Since stress on a fault builds up at the same rate over time , earthquakes should occur at regular intervals . But so far scientists can not predict when quakes will occur even to within a few years . Signs sometimes come before a large earthquake . Small quakes , called , sometimes occur a few seconds to a few weeks before a major quake . However , many earthquakes do not have and small earthquakes are not necessarily followed by a large earthquake . Often , the rocks around a fault will dilate as microfractures from ground tilting , caused by the buildup of stress in the rocks , may precede a large earthquake , but not always . Water levels in wells fluctuate as water moves into or out of fractures before an earthquake . This is also an uncertain predictor of large earthquakes . The relative arrival times of and also decreases just before an earthquake occurs . Folklore tells of animals behaving before an earthquake . Mostly these anecdotes are told after the earthquake . If indeed animals sense danger from earthquakes or tsunami , scientists do not know what it is they could be sensing , but they would like to find out . Damage from Earthquakes Earthquakes are natural disasters that cause enormous amounts of damage , second only to hurricanes . construction techniques , securing heavy objects , and preparing an emergency kit are among the precautions people can take to minimize damage . Earthquakes GEOGRAPHY

kill people and cause property damage . However , the ground shaking seldom kills people , and the ground does not swallow someone up . The damage depends somewhat on the earthquake size but mostly on the quality of structures . Structures falling on people injure and kill them . More damage is done , and more people are killed by the fires that follow an earthquake than the earthquake itself . Figure Photos Above Show Two Low Lying Areas in San Francisco After the 1906 Earthquake , Much Damage Was Contributed to Liquefaction . Image is in the public domain . There are a variety of ways earthquakes become deadly . Probably the greatest influence is population density . The magnitude Great Alaska Earthquake , near Anchorage , of 1964 resulted in only 131 deaths . At the time few people lived in the area . Oddly enough , size does matter . Only about people died in the 1960 Great Chilean earthquake , the largest earthquake ever recorded . The Indian Ocean earthquake of 2004 was one of the largest ever , but most of the fatalities were caused by the tsunami , not the earthquake itself . Ground type is an important factor in earthquake damage . Solid bedrock vibrates less than soft sediments so there is less damage on bedrock . Sediments that are saturated with water undergo liquefaction and become like quicksand . Pin It ! Liquefaction Read this to dig deeper . GEOGRAPHY

Figure Sand Bails Forms Along the Bear River in Utah . Liquefaction is in the public domain . This will have dangerous implications for Salt Lake City and the Front in Utah . Much of the Front is loose soil , leftover from the remnants of Lake . Along the middle of the two mountain ranges , between the Mountains and the Mountains , is the Jordan River which flows from Utah Lake northward to the Great Salt Lake . Near the river and surrounding , the water table is near the surface . When the ground shakes , the water near the surface shifts upward and the soil making it extremely unstable . Earthquakes Can humans create earthquakes ?

Maybe not intentionally , but the answer is why . If a water reservoir is built on top of an active fault line , the water may lubricate the fault and weaken the stress built up within it . This may either create a series of small earthquakes or potentially create a large earthquake . Also , the shear weight of the reservoir water can weaken the bedrock causing it to fracture . Then the obvious concern is if the dam fails . Earthquakes can also be generated if humans inject other fluids into a fault such as sewage or chemical waste . Finally , nuclear explosions can trigger earthquakes . One way to determine if a nation has tested a nuclear bomb is by monitoring the earthquakes and energy released by the explosion . GEOGRAPHY

UNIT 14 SUMMARY An earthquake is the shaking that results when a body of rock that has been deformed breaks and the two sides quickly slide past each other . The rupture is initiated at a point but quickly spreads across an area of a fault , via a series of initiated by stress transfer . Episodic tremor and slip is a periodic slow movement , accompanied by harmonic tremors , along the middle part ofa subduction zone boundary . Most earthquakes take place at or near plate boundaries , especially at transform boundaries ( where most quakes are at less than 30 depth ) and at convergent boundaries ( where they can be at well over 100 depth ) The largest earthquakes happen at subduction zones , typically in the upper section where the rock is relatively cool . Magnitude is a measure ofthe amount of energy released by an earthquake , and it is proportional to the area of the rupture surface and the amount of displacement . Although any earthquake has only one magnitude value , it can be estimated in various ways , mostly involving seismic data . Intensity is a measure of the amount of shaking experienced and damage done at a particular location around the earthquake . Intensity will vary depending on the distance to the epicenter , the depth of the earthquake , and the geological nature of the material below the surface . Damage to buildings is the most serious consequence of most large earthquakes . The amount of damage is related to the type and size of buildings , how they are constructed , and the nature of the material on which they are built . Other important consequences are fires , damage to bridges and highways , slope failures , liquefaction , and tsunami . Tsunami , which is almost all related to large subduction earthquakes , can be devastating . GEOGRAPHY