Fault - The shape of the land, Forces and changes, Spotlight on famous forms, For More Information Fault Fault Photo by: Lee Prince Earth's crust, the surface layer of the planet, is not solid and unbroken. The forces that rage inside the planet have fractured this brittle layer. Some of these fractures, called faults, lie beneath the surface of the crust. Other faults, however, have ruptured the surface, cracking the crust into various-sized blocks of rock. These blocks dip and rise along faults in response to pressure underground. One block may move up while the other moves down. Sometimes the movement is enough to form valleys or mountains. Other times that movement is not vertical but horizontal, as one block slips along the fault relative to the block on the other side. Movement of crustal blocks along faults may be regular and slow or sporadic and sudden. When two blocks are forced to move against each other but are locked into position, stress builds up. When that stress becomes greater than the forces holding the blocks together, the blocks are forced to move suddenly and violently. The ground vibrations accompanying that release of energy are better known as an earthquake.

There are more than one million earthquakes a year on Earth, though more than 60 percent of those are too faint to be felt. Crustal movements along faults are occurring continuously across most of the planet's surface. The shape of the land A fault is defined as a crack or fracture in Earth's crust along which rock on one side has moved relative to rock on the other. (When no movement has occurred, the fracture is known as a joint). When a fault breaks the planet's surface, it may range in length from a few inches to thousands of miles. The line on Earth's surface defining the fault is known as the fault line or fault trace. Aerial view of the San Andreas Fault slicing through the Carrizo Plain just east of San Luis Obispo, California. The fault runs along most of the entire Pacific coast of California, extending for more than 800 miles . PHOTOGRAPH REPRODUCED BY PERMISSION OF THE U.S. GEOLOGICAL SURVEY . A fault may extend downward from the fault line at least several miles into the crust. The area where crustal blocks meet and move along a fault from the fault line down into the crust is known as the fault plane.

The fault plane may be vertical in relation to Earth's surface. If so, the fault is known as a vertical fault. If the fault plane is slanted, the fault is known as an inclined fault. The crustal block that lies beneath the fault plane in an inclined fault is referred to as the footwall. The block that is above or seems to rest on the fault plane is referred to as the hanging wall. These terms do not apply if the fault plane is vertical. The measure of the angle between Earth's surface and the fault plane is called the fault's dip. Faults are classified according to how steeply they dip and the relative movement of the crustal blocks on either side of a fault. The movement of crustal blocks along a fault has been measured from 0.4 inch (1 centimeter) to 50 feet (15 meters). As mentioned, that movement may be slow or rapid. Slow, continuous movement is known as fault creep. Some faults creep from 0.5 inch (1.3 centimeters) to 4 inches (10 inches) a year. Normal faults occur when underground pressure causes the crust to stretch or pull apart.

When this happens, the hanging-wall block (the one above the fault plane) drops down relative to the footwall block. Most normal faults have dips of about 60 degrees. The resulting steep-sided ledge or cliff created at the top of the footwall block is called a fault scarp. Fault: Words to Know Asthenosphere: Large sections of Earth's lithosphere that are separated by deep fault zones. Plate tectonics: The geologic theory that Earth's crust is composed of rigid plates that "float" toward or away from each other, either directly or indirectly, shifting continents, forming mountains and new ocean crust, and stimulating volcanic eruptions. Strike: The compass direction of a fault line. Normal faults usually occur in elevated regions such as plateaus. They are not often found singly in a landscape; typically, they occur in multiple arrangements, often in pairs of parallel faults. When movement Illustration of four types of faults. Faults are classified according to how steeply they dip and the relative movement of the crustal blocks on either side of a fault. Normal faults, for example, occur when underground pressure causes the crust to stretch or pull apart, whereas reverse faults occur when underground pressure causes the crust to compress .

When underground pressure causes the crust of normal faults to stretch or pull apart, the hanging-wall block drops down relative to the footwall block. The resulting steep-sided ledge or cliff created at the top of the footwall block is called a fault scarp . takes place between parallel normal faults whose fault planes are angled downward toward each other (such as \ /), a crustal block may drop down between them. This down-dropped block, which forms a valley between the opposing footwall blocks, is called a graben (pronounced GRAH-bin). If the fault planes of the parallel faults are angled downward away from each other (such as / \), a crustal block between them may be elevated. This uplifted block is called a horst. A large horst that is lifted high can form a fault-block mountain. (For further information on faultblock mountains, see the Mountain chapter.) A series of uplifted and down-dropped blocks across a landscape is called a horst and graben structure. This feature is common in the western United States and northern Mexico. In contrast, reverse faults occur when underground pressure causes the crust to compress, pushing blocks together.

As two blocks are pushed together at a fault, the hanging-wall block is pushed up and over the footwall block. A fault scarp is created, but it takes the form of an overhanging ledge. A special type of reverse fault is called a thrust fault (sometimes also called an overthrust fault). The fault plane of a thrust fault lies at a low angle to Earth's surface. This angle is less than 30 degrees. Because the angle is so low, the hanging-wall block is thrust up and over the footwall block. The movement is mainly horizontal as the hanging-wall block travels over the footwall block, sometimes for thousands of feet. These previous faults are all categorized as dip-slip faults because the movement of the fault blocks is up or down along the fault plane. If the When fault planes are angled downward toward each other, a down-dropped block, called a graben, is formed. If the fault planes of the parallel faults are angled downward away from each other, a crustal block between them may be elevated. This uplifted block is called a horst .

movement is horizontal, with the two fault blocks scraping along side-by-side, the fault is known as a strike-slip fault (sometimes also known as a transcurrent or transverse fault). Strike is the compass direction—north, south, east, west—of the fault line or trace. The fault plane in a strike-slip fault is vertical or nearly vertical. There is little or no fault scarp created along this type of fault. Strike-slip faults are classified according to the direction of motion of the blocks on either side of the fault. If the block on the opposite side of a strike-slip fault has moved to the left, it is a left-lateral strike-slip fault. If it has moved to the right, it is a right-lateral strike-slip fault. The relative motion, left or right, is the same regardless on which block an observer stands. The famous San Andreas Fault in California is a right-lateral strike-slip fault. Land west of the fault is edging northwest; land east is edging southeast. In some faults, the movement is neither purely vertical nor horizontal, but a combination of the two.

In instances where a fault has both normal and strike-slip movement or reverse and strike-slip movement, it is known as an oblique-slip fault. Although this type of fault is not unusual, it is far less common than normal, reverse, or strike-slip faults. Forces and changes: Construction and destruction Any rock subjected to intense stress or pressure over time will deform. At higher temperatures and pressures, rock will soften and bend. Geologists call this ductile deformation. At lower temperatures and pressures, however, rock will break or fracture instead of bending. This type of deformation, called brittle deformation, happens to rock in the upper part of Earth's crust. Faults are a clear example of brittle deformation. The stress that is continually acting on and deforming Earth's surface may be in different forms: tensional stress, which stretches or pulls rock; compressional stress, which squeezes and squashes rock; and shear stress, which changes the shape of rock by causing adjacent parts to slide past one another. All of these stresses are directly related to events occurring deep within the planet. Earth's internal processes, from the core to the crust, have put the surface of the planet in motion, constantly changing its shape.

From the core to the crust Geologists divide the surface and the interior of Earth into layers. At the very center of the planet is the core. It begins at a depth of about 1,800 miles (2.900 kilometers) beneath the surface and extends to a depth of 3,960 miles (6,370 kilometers). The core is composed of the metal elements iron and nickel, and it has a solid inner portion and a liquid outer portion. Temperatures in the core are estimated to exceed 9,900°F (5,482°C). Blindly Faulting Los Angeles In 1999, geologists mapped for the first time a major fault located directly beneath the city of Los Angeles, California. Named the Puente Hills Fault, it is a blind thrust fault, a type of thrust fault that does not break Earth's surface. This makes it difficult to identify. Comprised of three sections, it runs for nearly 25 miles (40 kilometers) under downtown Los Angeles, through Santa Fe Springs, and into the Coyote Hills of northern Orange County. The fault is approximately 12.5 miles (20 kilometers) wide.

It is located about 2 miles (3.2 kilometers) beneath the surface in the center of Los Angeles, then dips to 10 miles (16 kilometers) underground as it continues northward. The entire fault system covers an area of roughly 250 square miles (650 square kilometers). In the spring of 2003, geologists released a study reporting that the fault is capable of generating major earthquakes. They estimated that the fault has been responsible for some of the most severe earthquakes to strike southern California in the last 11,000 years. They also believed that the fault was responsible for the 1987 earthquake that hit near the city of Whittier, located about 12 miles (19 kilometers) from the center of downtown Los Angeles. Although a major earthquake along the fault will probably occur only once every 2,000 years, geologists believe many of the buildings in downtown Los Angeles will not be able to withstand such an earthquake when it comes. Above the core lies the thick mantle, which forms the main bulk of the planet's interior. Above the mantle lies the brittle crust, the thin shell of rock that covers Earth. The upper portion of the mantle is rigid.

Geologists call the combination of this section of the mantle the overlying crust and the lithosphere (pronounced LITH-uh-sfeer). The lithosphere measures roughly 60 miles (100 kilometers) thick. The part of the mantle immediately beneath the lithosphere is called the asthenosphere (pronounced as-THEN-uh-sfeer). This layer is composed of partially melted rock that has the consistency of putty and extends to a depth of about 155 miles (250 kilometers). The driving force in Earth's interior The heat energy generated in the core is extreme. Earth's interior would melt if this energy were not released in some manner. Much like the circulating currents produced in a pot of boiling liquid on a hot stove, this energy is transported to the surface of the planet through currents called convection currents. When a liquid in a pot begins to boil, it turns over and over. Liquid heated at the bottom of the pot rises to the surface because heating has caused it to expand and become less dense (lighter). Once at the surface, the heated liquid cools and becomes dense (heavier) once more. It then sinks back down to the bottom to become reheated.

This continuous motion of heated material rising, cooling, and sinking forms the circular currents known as convection currents. Similarly, when mantle rocks near the core are heated, they become less dense than the cooler, upper mantle rocks. These heated rocks then slowly rise through the mantle. When they reach the lithosphere, the heated rocks move along the base of the lithosphere, losing heat. Cooling and becoming denser, they then sink back toward the core, only to be heated once again. Scientists estimate that convection currents move mantle rock only an inch or two a year. It takes about 200 million years for heated rock to make a circular trip from the core to the lithosphere and back again. Plate tectonics and faults The slowly moving convection currents are able to release their heat energy near the surface of the planet because the lithosphere is broken into many pieces called tectonic or crustal plates. These plates, which vary in size and shape, "float" on the soft, semi-molten (melted) asthenosphere. As the convection currents move under the lithosphere, they exert dragging forces on the rigid tectonic plates above them. This causes the plates to move.

Fitting together like pieces in a jigsaw puzzle, the plates are in constant contact with each other. Because they are interconnected, no single plate can move without affecting others. The movement or activity of one plate can influence another plate located thousands of miles away. The scientific theory explaining plates and their movements and interactions is called plate tectonics (a hypothesis is an educated guess; a theory is a principle supported by extensive scientific evidence and testing). In general, tectonic plates inch their way across the surface of Earth at a rate no faster than human fingernails grow, roughly 2 inches (5 centimeters) Earth's surface is broken into seven large and many small tectonic plates. These plates, each about 50 miles thick, move relative to one another an average of a few inches a year . per year. As they move, the plates slide along each other (transform), move into each other (converge), or move away from each other (diverge). Plate margins are the boundaries or areas where the plates meet and interact. The stresses that produce faulting are usually related to movements along plate margins. Although associated mainly with plate margins, faults can occur anywhere on Earth where the crust is weak.

Normal faults are common in regions where the crust is being stretched and thinned as a result of plates diverging. Reverse faults are common in regions where the crust is being compressed and thickened as a result of plates converging. Thrust faults are even more common at convergent plate margins, where plates are moving toward one another, compressing, and often forming high mountains. Strike-slip faults are formed where unequal pressure causes rock sections to slip past each other. Major strike-slip faults occur between plates that are transforming. These special strike-slip plates are known as transform faults. Spotlight on famous forms Denali fault system, Alaska Denali (pronounced de-NAHL-ee) is the name given by the Athabascan-speaking native people of Alaska to the mountain known as Mount McKinley, the highest mountain in North America at 20,320 feet The Denali Fault runs for about 750 miles in Alaska and marks the boundary where the Pacific Plate and the North American Plate meet . PHOTOGRAPH REPRODUCED BY PERMISSION OF THE CORBIS CORPORATION . (6,194 meters).

Translated, it means "the Great One." Denali is also the name of a great fault system (a network of connected faults) that extends for about 750 miles (1,200 kilometers) from southeast to southcentral Alaska. Predominantly composed of right-lateral strike-slip faults, Denali is one of the longest strike-slip fault systems in the world. The Denali Fault marks the boundary where the Pacific Plate and the North American Plate meet and interact. Land south of the fault moves westward, and land north of the fault moves eastward. Geologists estimate that the two sides on the fault have slipped roughly 240 miles (386 kilometers) over the last 60 million years. On average, the plates move about 0.4 inch (1 centimeter) a year along the fault. At times during the Denali Fault's history, the plates along it have locked up. Subsequent pressure builds up until it is finally released in an earthquake. This is what occurred on November 2, 2002, when an earthquake struck the fault about 90 miles (145 kilometers) south of Fairbanks, Alaska.

One of the largest ever recorded in the United States, the earthquake measured 7.9 on the Richter scale (a scale developed by American seismologist Charles Richter to describe the amount of energy released by an earthquake). The earthquake ruptured 155 miles (260 kilometers) of the fault. Movement on the fault caused ground north of the fault to move eastward up to 26 feet (8 meters) relative to ground south of the fault. New Madrid fault system, Central United States The New Madrid fault system is a series of faults beneath the continental crust in the central United States. The system exists in the middle of the North American plate where tensional stress created a weak spot known as the Reelfoot rift. Called an intraplate fault system, it is a type of fault system that is still not clearly understood by geologists. The faults along the system—normal, reverse, and strike-slip—lie 10 to 12 miles (16 to 19 kilometers) beneath Earth's surface. They create no fault lines. The New Madrid fault system extends at least 150 miles (240 kilometers) from Cairo, Illinois, to Marked Tree, Arkansas.

It runs beneath the lines of five states: Illinois, Kentucky, Missouri, Tennessee, and Arkansas. It runs beneath the Mississippi River in three areas and the Ohio River in two areas. Movement along the system ranges from 1.5 to 2.8 inches (4 to 7 centimeters) a year. More earthquakes occur in the area of the New Madrid fault system than in any other area of the United States east of the Rocky Mountains. More than 200 earthquakes are recorded on the system each year, but the vast majority of those cannot be felt. Only about 8 to 10 earthquakes a year are large enough to be felt (measuring 3.0 or more on the Richter scale). In the winter of 1811–1812, three earthquakes centered on the New Madrid fault system were felt across the continent, from Canada to the Gulf of Mexico, and from the Rocky Mountains to the Atlantic coast. Present-day geologists estimate the largest of the earthquakes measured 8.8 on the Richter scale. The effects were widespread and severe. The most intense earthquake altered the course of the Mississippi River and created a depression in northwest Tennessee that filled with water to become Reelfoot Lake.

Aftershocks (earthquake tremors that occur after the main shock) were felt around the region for a year afterward. San Andreas Fault, California The well-known San Andreas Fault in California is perhaps the most-studied fault in the world. It is the main fault in an intricate fault system; many smaller faults branch from and join the larger fault. The fault system is located along almost the entire Pacific coast of California, extending for more than 800 miles (1,287 kilometers). Its fault plane reaches down at least 10 miles (16 kilometers) into Earth's crust. Its fault line is a zone of crushed and broken rock that ranges from a few hundred feet to 1 mile (1.6 kilometers) wide. A right-lateral strike-slip fault, the San Andreas Fault represents the boundary where the North American Plate and the Pacific Plate meet. Along the fault, the two plates scrape against each other as the North American Plate slips to the southeast and the Pacific Plate slips to the northwest. The rate of movement along the fault is about 0.8 to 1.4 inches (2 to 3.5 centimeters) a year.

Because it is situated between plates that are transforming, the San Andreas Fault is also considered a transform fault. Geologists believe the San Andreas Fault came into existence about 15 to 20 million years ago. They estimate that the total movement along the fault since that time has been at least 350 miles (563 kilometers). Thousands of earthquakes, most of which are too small to be felt, occur along the fault regularly. Over the last 1,500 years, large earthquakes along the fault have occurred every 150 years or so. The last recorded large earthquake on the southern portion of the fault took place in 1857. In 1906, a devastating earthquake occurred in the San Francisco region. The plates, which had been locked along the fault, ruptured and tore the ground apart along a 290-mile (467-kilometer) stretch. The ground west of the fault shifted northward as much as 21 feet (6.4 meters) in places. For More Information Books Collier, Michael. A Land in Motion: California's San Andreas Fault . Berkeley, CA: University of California Press, 1999.

Harden, Deborah R. California Geology . Englewood Cliffs, NJ: Prentice Hall, 1997. Knox, Ray, and David Stewart. The New Madrid Fault Finders Guide . Marble Hill, MO: Gutenberg-Richter Publications, 1995. Web Sites High School Earth Science/Stress in the Earth's Crust - Wikibooks, open books for an open world High School Earth Science/Stress in the Earth's Crust From Wikibooks, open books for an open world This is the latest reviewed version , checked on 2 November 2016. (+) Quality: minimal 8 Points to Consider When you think about enormous plates of lithosphere traveling around on the planet's surface, you can probably imagine that the process is not smooth. Most geological activity takes place where two plates meet, at plate boundaries. In the Earthquakes chapter, you will learn that nearly all earthquakes, volcanic eruptions, and mountain building occur at plate boundaries. In this chapter, you will learn more about the geological activity that occurs because of plate tectonics, specifically mountain building and earthquakes. When plates are pushed or pulled, the rock is subjected to stress.

Stress can cause a rock to change shape or to break. When a rock bends without breaking, it folds. When the rock breaks, it fractures. Mountain building and earthquakes are some of the responses rocks have to stress. List the different types of stresses that cause different types of deformation. Compare the different types of folds and the conditions under which they form. Compare fractures and faults and define how they are related to earthquakes. Compare how mountains form and at what types of plate boundaries. Causes and Types of Stress[ edit ] Stress is the force applied to an object. In geology, stress is the force per unit area that is placed on a rock. There are four types of stresses that act on materials. A deeply buried rock is pushed down by the weight of all the material above it. Since the rock is trapped in a single spot, it is as if the rock is being pushed in from all sides. This pushing causes the rock to become compressed, but it cannot deform because there is no place for it to move. This is called confining stress. Compression is the stress that squeezes rocks together. Compression causes rocks to fold or fracture (break)(Figure 7.1).

When cars driving around a parking lot collide, compression causes the cars to crumple. Compression is the most common stress at convergent plate boundaries. Figure 7.1: Stress caused these rocks to fracture. Rocks that are being pulled apart are under tension (also called extension). Tension causes rocks to lengthen or break apart. Tension is the major type of stress found at divergent plate boundaries. When forces act parallel to each other but in opposite directions, the stress is called shear (Figure 7.2). Shear stress causes two planes of material to slide past each other. This is the most common stress found at transform plate boundaries. Figure 7.2: Rocks showing dextral shear. Note how the white quartz vein has been elongated by shear. If the amount of stress on a rock is greater than the rock's internal strength, the rock bends elastically. This type of change is called elastic because when the stress is eliminated the rock goes back to its original shape, like a squeezed rubber ball. If more stress is applied to the rock, it will eventually bend plastically. In this instance, the rock bends, but does not return to its original shape when the stress is removed.

If the stress continues, the rock will fracture; that is, it breaks. When a material changes shape, it has undergone deformation. Deformed rocks are common in geologically active areas (Figure 7.3). Figure 7.3: When stress is applied to a material, it initially deforms elastically. With more stress, the material deforms plastically and when the material's strength is exceeded, it fractures. The amount of stress that can be applied before the material transitions to the next type of deformation depends on the material and the conditions where it is located. What a rock does in response to stress depends on many factors: the rock type; the conditions the rock is under, primarily the surrounding temperature and pressure; the length of time the rock is under stress; and the type of stress. It seems difficult to imagine that rocks would not just simply break when exposed to stress. At the Earth's surface, rocks usually break quite quickly once stress is applied. But deeper in the crust, where temperatures and pressures are higher, rocks are more likely to deform plastically. Sudden stress, like a hit with a hammer, is more likely to make a rock break. Stress applied over time often leads to plastic deformation.

Geologic Structures[ edit ] Sedimentary rocks are often found in layers. This is most magnificently displayed at the Grand Canyon, where the rock layers are exposed like a layer cake (Figure 7.4). Each layer is made of sediments that were deposited (laid down) in a particular environment, perhaps a lake bed, shallow offshore region, or a sand dune. Sediments are deposited horizontally, the lowest layers are the oldest and the highest layers are the youngest. Some volcanic rocks, like ash falls, resemble sedimentary rocks because they are laid down horizontally as well. Figure 7.4: The layered rocks of the Grand Canyon from the rim. Figure 7.5: Geologic column of the Grand Canyon. The sedimentary rocks of groups 3 through 6 were deposited horizontally and remain horizontal. Group 2 rocks were deposited horizontally but have been tilted. Group 1 rocks are not sedimentary. The rock layers at the bottom of the stack are the oldest while the ones at the top are the youngest. It's important to remember that sediments are deposited horizontally when thinking about geologic structures.

This is because you can trace the deformation the rock has experienced by seeing how it differs from its original horizontal, oldest-on-bottom, position (Figure 7.5). Geologic structures are the folds, joints, and faults that are caused by stresses. Folds[ edit ] When rocks experiencing compressive stress deform plastically, the rocks crumple into folds. Folds are just bends in the rock. You can easily make folds by placing your hands on opposite edges of a piece of cloth and pushing the cloth together. In layered sedimentary rocks, you can trace the folding of the layers with your eyes (Figure 7.6). Figure 7.6: Snow accentuates the fold exposed in these rocks in Provo Canyon, Utah. Once rocks are folded, they do not return to their original shape. If the rocks experience more stress, they may undergo more folding, or even fracture. Folds often occur in groups. There are three types of folds: monoclines, anticlines, and synclines. A monocline is a simple bend in the rock layers so that they are no longer horizontal but are inclined (Figure 7.7). In a monocline, the oldest rocks are at the bottom and the youngest are at the top.

In the Grand Canyon geologic column, the rocks in group 2 have been folded into a monocline. Figure 7.7: A monocline can be spotted in the photo taken at Colorado National Monument where the rocks plunge toward the ground. Figure 7.8: Diagram of an anticline. An anticline is a convex upward fold. Figure 7.9: An anticline exposed in a road cut in New Jersey. An anticline is a fold that arches upward. The rocks dip away from the center of the fold (Figure 7.8). The oldest rocks are found at the center of an anticline and the youngest ones are draped over them at the top of the structure (Figure 7.9). When rocks arch upward to form a circular structure, that structure is called a dome. If the top of the dome is eroded off, the oldest rocks will be exposed at the center. A syncline is a fold that bends downward. The rocks curve down to a center (Figure 7.10). Figure 7.10: A syncline in Rainbow Basin in California. In a syncline, the youngest rocks are at the center and the oldest at the outsides (Figure 7.11).

Figure 7.11: Diagram of a syncline. A syncline is a concave downward fold. Figure 7.12: Geologic map of the Michigan Basin. When rocks bend downward in a circular structure, that structure is called a basin. If the rocks are exposed, the youngest rocks will be at the center. Basins can be enormous. For example, the Michigan Basin is centered on the state of Michigan, but extends into four other states and a Canadian province (Figure 7.12). Folds are sometimes, but not always the cause of geographic features such as hills or valleys. Geologic processes that work at the surface, like erosion, are important in creating geographic features as well. Faults[ edit ] A rock under enough stress will fracture, or break. When there is a block of rock still standing on either side of a fracture line, as shown in Figure 7.13, the fracture is called a joint. One example of how joints form is when confining stress is removed from an underlying granite. Figure 7.13: Granite rocks in Joshua Tree National Park showing horizontal and vertical jointing.

Over millions of years, wind and water have broken down the granite, enlarging the joints and making the pattern of jointing more obvious. If the blocks of rock on one or both sides of a fracture move, the fracture is called a fault (Figure 7.14). Earthquakes happen when there are sudden motions along faults. When rocks break and move suddenly, the energy released causes an earthquake. Faults may occur at the Earth's surface or deeper in the crust. Faults are found alone or in clusters, creating a fault zone. Figure 7.14: Faults are easy to recognize as they cut across bedded rocks. Slip is the distance rocks move along a fault. Slip is said to be relative, because there is usually no way to know whether both sides moved or only one. The only thing we can say for sure, is that one block of rock moved passed the other. Faults lie at an angle to the horizontal surface of the Earth. That angle is called the fault's dip. The dip defines which of two basic types a fault is. If the fault's dip is inclined relative to the horizontal, the fault is a dip-slip fault. Slip can be up or down the fault plane.

In the following images, you are looking at the fault straight on, as if you are standing on a road and the fault is exposed in the road cut. The hanging wall is the rock that overlies the fault, while the footwall is beneath the fault. You can remember which part is the hanging wall and which is the footwall by imagining you are walking along a fault. The hanging wall is above you and the footwall is where your feet would be. Miners often extract mineral resources along faults. They used to hang their lanterns above their heads. That is why these layers were called the hanging wall. In normal faults, the hanging wall drops down relative to the footwall. Normal faults are caused by tensional stress that pulls the crust apart, causing the hanging wall to slide down relative to the footwall. When compression squeezes the crust into a smaller space, the hanging wall pushes up relative to the footwall. This creates a reverse fault (Figure 7.15). Figure 7.15: The two types of dip-slip faults. In normal faults the hanging wall drops down relative to the footwall. In reverse faults, the footwall drops down relative to the hanging wall.

A type of reverse fault is called a thrust fault. At a thrust fault, the fault plane angle is nearly horizontal and rocks can slip many miles along thrust faults (Figure 7.16). Figure 7.16: At Chief Mountain in Montana, stresses that raised up the Rocky Mountains caused a block of ancient Precambrian crust to be thrust more than 80 kilometers (50 miles) over much younger Cretaceous rocks. The result is that the upper rocks at the Lewis Overthrust are more than 1 billion years older than the lower rocks. Normal faults can be huge. They can be responsible for uplifting mountain ranges in regions experiencing tensional stress (Figure 7.17). Figure 7.17: The Teton Range in Wyoming rose up along a normal fault. Strike-Slip[ edit ] A strike-slip fault is a dip-slip fault where the dip of the fault plane is vertical. Strike-slip faults result from shear stresses. If you stand with one foot on one side and one foot on the other side of a strike-slip fault, the block on one side will be moving toward you and the block on the other side will be moving away from you.

If the block moving toward you is the block that your right foot is on, the fault is known as a right-lateral strike-slip fault. If the block moving toward you is the one your left foot is on, the fault is a left-lateral strike-slip fault (Figure 7.18). Figure 7.18: The two types of strike-slip faults. The world's most famous strike-slip fault is the San Andreas Fault in California, which is a right-lateral strike slip fault (Figure 7.19). Because the San Andreas is a plate boundary, it is also called a transform fault. People sometimes say that California will fall into the ocean someday, but this is a joke. The portion west of the San Andreas Fault is moving northeastward and someday Los Angeles will be a suburb of San Francisco, but the land west of the San Andreas Fault is a solid piece of continental crust that will not disappear entirely. Figure 7.19: The San Andreas is a transform fault separating the Pacific from North American Plates. The fault creates a scar on the land as it moves across the Carizzo Plains in eastern San Luis Obispo County, California.

A fault may have broken and moved only once, but most faults are active repeatedly. There are two reasons for this. One is that plate tectonic processes continue in the same locations. The other is that a fault is a zone of weakness in the crust, and it is easier for movement to take place along an existing fault than for a new fault to be created in solid crust. Stress and Mountain Building[ edit ] Figure 7.20: The Indian plate collides with the Eurasian plate to form the world's largest mountain range, the Himalayas. Mountains can stand alone or in ranges that formed at a similar time and in a similar way. Many processes can create mountains. Although most mountains form along plate boundaries, some result from intraplate activity. For example, volcanoes build upwards at hotspots within the Pacific Plate. Most of the world's largest mountains result from compression at convergent plate boundaries. The largest mountains arise when two continental plates smash together. Continental lithosphere is too buoyant to get pushed down into the mantle or subduct, so when the plates smash together, the crust crumples upwards, causing uplift.

The stresses cause folds, reverse faults, and thrust faults, all of which allow the crust to grow thicker and rise upwards. The world's highest mountain range, the Himalayas, is growing from the collision between the Indian and the Eurasian plates. About 80 million years ago, the Indian plate was separated from the Eurasian plate by an ocean (Figure 7.20). As the Indian plate moved northward, a subduction zone formed beneath Eurasia. The seafloor was subducted and caused the formation of a set of continental arc volcanoes. When the oceanic lithosphere was completely subducted, about 40 million years ago, the Indian plate began to collide with the Eurasian plate. Some of the Indian plate was thrust beneath Asia and some of Asia was thrust onto India. Rock also folded, which thickened the crust and formed the mountains. In places, the old seafloor that was between the two slabs of continental crust have been thrust over the Asian continent and are found high in the Himalayas (Figure 7.21). Figure 7.21: The Himalayas are the world's highest mountain range. Subduction of oceanic lithosphere at convergent plate boundaries also builds mountain ranges.

The Andes Mountains are a chain of continental arc volcanoes that build up as the Nazca plate subducts beneath the South American plate (Figure 7.22). Figure 7.22: The Andes Mountains have arisen due to the convergence of two lithospheric plates. Figure 7.23: Where the crust is being pulled apart by tension, it breaks apart along normal faults. Some blocks are uplifted to form ranges, known as horsts, and some are down-dropped to form basins, known as grabens. Rifting at a divergent plate boundary can also build mountain ranges. When crust is pulled apart, it occupies more area. The crust breaks into blocks that slide up and down along the normal faults that separate them. The result is alternating mountain ranges and valleys, known as a basin-and-range (Figure 7.23). The state of Nevada is the center of a classic basin and range province (Figure 7.24). Figure 7.24: In this photo taken in the Basin and Range province, the photographer is standing at Emigrant Pass in the Nopah Range, and looking across a basin to the Kingston Range beyond.

Lesson Summary[ edit ] Stress is the force applied to a rock, which may cause deformation. The three main types of stress go along with the three types of plate boundaries: compression is common at convergent boundaries, tension at divergent boundaries, and shear at transform boundaries. Where rocks deform plastically, they tend to fold. Brittle deformation brings about fractures and faults. The two main types of faults are dip-slip and strike-slip. In dip-slip faults, the angle of the fault plane is inclined to the horizontal, in strike-slip faults the fault plane is perpendicular to the horizontal. The world's largest mountains grow at convergent plate boundaries, primarily by thrust faulting and folding. Review Questions[ edit ] Why don't rocks deform under confining stress? What type of stress is compression and at what type of plate boundary is this found? What type of stress is tension and at what type of plate boundary is it found? What type of stress is shear and at what type of plate boundary is it found? What is the difference between plastic and elastic strain? Under what conditions is a rock more likely to deform plastically than to break? You are a geologist walking around in the field, when you spot a monocline.

You inspect the fossils in each layer of the rock and you discover that the oldest rocks are at the top and the youngest at the bottom. How do you explain how the rocks came to be this way? Describe an anticline and name the age order of rocks. Describe a syncline and name the age order of rocks. What is the difference between a dome and a basin and what is the age order of rocks in each? Name one similarity and one difference between a fracture and a fault. What are the two types of dip-slip faults and how are they different from each other? California is plagued by earthquakes along the San Andreas Fault zone. Why are there so many earthquakes and why are they so severe? Volcanoes are mountains that form in two ways. Describe these two ways and how they are associated with a plate boundary. Describe the plate tectonics processes and associated stresses that have led to the formation of the Himalayas, the world’s largest mountain range. Vocabulary[ edit ] anticline A fold that arches upward, in which the older rocks are in the center and the younger rocks are at the outside. basin

Question: What is the name for a raised block of the earth's crust thrown up between two faults?
Answer: Horst