Earth Science Ch 11 Review : Mountains

1. Earth Science Ch 11 Review : Mountains Mountain Building: Review

2. Mountain Building : Deformation • Over millions of years, these mountains formed when plate motion and other forces uplifted the crust of the Earth. At the same time, weathering and erosion shape the crust into peaks and other formations. The process begins when plate motions produce forces in rock that cause it to bend or break. • Deformation is any change in the original shape and/or size of a rock body. In Earth’s crust, most deformation takes place along plate boundaries. • Deformation occurs because of stress in a body of rock. Stressis the force “per unit area” acting on a mountain. ( pressure per square inch, foot, or meter for example)

3. Elastic deformation • When rocks are under stress that is greater than their own strength, they begin to deform. • Usually they deform by one of the following: Folding, Flowing, Fracturing • The change in shape or volume of a body of rock is called strain.When stress is gradually applied, rocks first respond by responding elastically. • A change that results from elastic deformation can be reversed. Like a rubber band; the rock will return to it’s original size and shape once the force upon it is removed. • Once the elastic limit or strength for a rock is surpassed, the rock either flows or fractures.

4. Ductile and brittle deformation • The factors that affect the deformation of rock include: Temperature, Pressure, Rock type, Time. • Rocks deform permanently in two ways: Brittle deformation, Ductile deformation • Rocks near the surface, where temperatures and pressures are low, usually behave like brittle solids and fracture once their strength is exceeded • Ductile deformation is a type of solid-state flow that produces a change in the size and shape of an object without fracturing the object. Ductile materials buckle or bend. Brittle materials break Ductile materials bend Or buckle

5. Ductile deformation and brittle fracture The mineral composition and texture of a rock also greatly affects how it will deform. • Rocks like granite and basalt that have a strong internal molecular bonds usually fail by brittle fracture. • Sedimentary rocks or metamorphic rocks are more likely to deform by ductile deformation; bending or buckling. • Small stresses applied over long time spans eventually cause the deformation of rock. Forces that are unable to deform rock when first applied, may cause rock to flow if the force is maintained over a long period of time. Brittle materials break Ductile materials bend Or buckle

6. Types of Stress Plate motions cause different types of stress in the rocks of the lithosphere. The three types of stress that cause deformation of rocks are • Tensional stress • Compressional stress • Shear stress • When rocks are squeezed the stress is compressional stress. • When rocks are pulled in opposite directions, the force is tensional stress. • Shear stress causes a body of rock to be distorted.

7. Isostosy • Earth’s crust floats on top of the denser more flexible rocks of the mantle. The concept of a floating crust in gravitational balance is called isostosy. • One way to understand the concept of isostosy is to think bout a series of wooden blocks of different heights floating in water. The thicker blocks float higher than the thinner blocks. • In a similar way, many mountain belts stand high above the surface because they have less dense “roots” that extend deep into the denser mantle. The denser mantle supports the mountains from below. • What would happen if another small block of wood was placed upon one of the floating blocks? The combined blocks would sink until a new balance of gravity was reached. • However, the top of the pair of blocks would be higher than it was before and the bottom would be lower. This process of finding a new level of gravitational balance is called isostatic adjustment. Over time, the roots of the mountains rise up in Isostatic adjustment

8. Isostatic adjustment • Applying this concept of isostosy, we should expect that when weight is added to the crust, the crust responds by sinking lower. Also , when weight is removed the crust will rebound and rise again. • As erosion reduces the summits of mountains, the crust will rise in response to the reduced load of weight. • The process of erosion and uplift together will continue until the mountain block reaches it’s normal crust thickness; it’s balancing point or equilibrium. • When this occurs, the mountain will be eroded to near sea-level and the once deeply buried interior of the mountain will be exposed at the surface. Over time, the roots of the mountains rise up in Isostatic adjustment

9. Folds and Faults • Over millions of years, stress forces can bend rock like a ribbon or soft dough. Steady pressures of stress over long periods of time affect sedimentary layers and can fold them into dramatic forms. • During mountain building, compressional stresses often bend flat-lying sedimentary rocks into wavelike ripples called folds. Folds of sedimentary strata come in three main types: Anticlines / Synclines / Monoclines • An anticlineis usually formed by the upfolding, or arching of rock layers. Often found in association with anticlines are downfolds, or troughs, called synclines. The angle that a fold or fault makes with the horizontal is called the dip of the fold or fault. A vertical cliff straight up would be a 90 degree dip.

10. Folds and Faults • Foldsare generally closely related to faults in the Earth’s crust. Examples of this close association can be found in monoclines. • Monoclinesare large step-like folds in otherwise horizontal sedimentary layers. Monoclines occur as sedimentary layers get folded over a large faulting-block of underlying rock. Faults: Recall thatfaultsare fractures in the Earth’s crust along which movement has taken place. The rock surface immediately above the fault is called the hanging wall. The rock surface below the fault is called the footwall. • The major types of faults are • Normal faults • Reverse faults • Thrust faults • Strike-slip faults

11. Folds and Faults Faults: Normal faults occur due to tensional stress and reverse and thrust faults occur due to compressional stress. • Compressional forces generally produce folds as well as faults, resulting in a thickening and shortening of rocks. Shearing stresses produce strike-slip faults. Faults are classified according to the type of movement that occurs along the fault. Normal Faults: A normal fault occurs when the hanging wall block moves down relative to the footwall block. • A reverse fault is a fault in which the hanging block moves up (instead of down) relative to the footwall block. Reverse faults are high angle compressional faults with dips greater than 45 degrees.

12. Thrust Faults and Strike-Slip Faults • Thrust faults are reverse faults with dips of less than 45 degrees. Because the hanging wall block moves up and over the footwall block, reverse and thrust faults result in a compression of the crust. • Most high-angle reverse faults are small in scale. Thrust faults, however, exist at all scales. Many can be quite large and account for some of the largest mountain ranges in the world such as the Alps in Europe. The result of this type of movement is that older rocks end up on top of younger rocks. • Faults in which the movement is horizontal and parallel to the line of the fault is called a strike-slip fault. Because of their large scale, and linear nature ( in a line) many strike-slip faults produce a trace that can be seen over a great distance. Strike-slip fault

13. 4 types Mountain building • Folding and faulting produce many but not all of Earth’s mountains. In general, mountains are classified by the processes that formed them. • The major types of mountain types include • Volcanic mountains • Folded mountains • Fault-block mountains • Dome mountains • Earth’s mountains do not occur at random. Several mountains of similar shape, age, size and structure form a group called a mountain range. • A group of different mountain ranges in the same region form a mountain system. Rocky Mountain System

14. Volcanic and Folded Mountains • Volcanic Mountains : Recall from the previous chapters that volcanic mountains form along plate boundaries and at hot spots. In addition, igneous activity forms rock deep in the crust that can be uplifted as a result of plate motions and isostatic adjustment. • Mountains that are formed primarily by folding are called folded mountains. Compressional stress is the major cause of folded mountains. Compressional stress helped to form the Alps in Europe. • Thrust faulting is also important in the formation of folded mountains, which are often called fold-and-thrust belts. Folded mountains often contain numerous stacked thrust faults that have displaced the folded rocks layers many kilometers horizontally. Stacked thrust faults

15. Fault Block Mountains • Fault block mountains; another type of mountain formation, is the result of movement along normal faults. • Large scale normal faults are associated with fault-block mountains. Fault-block mountainsform as large blocks of crust are uplifted and tilted along normal faults. • Grabens and Horsts: Normal faulting occurs where tensional stresses cause the crust to be stretched or extended. As the crust is stretched, a block called a graben, which is bounded by normal faults, drops down. Grabens produce an elongated valley bordered by relatively uplifted structures called horsts.

16. Fault Block Mountains • The Basin and Range regions of Nevada, Utah, and California is made of elongated grabens. Above the grabens, tilted fault-blocks or horsts produce parallel rows of fault-block mountains. • In the western US, other examples of fault block mountains include the Grand Tetons and the Sierra Nevada Range in California. These steep mountain fronts were produced over 5 to 10 million years by many episodes of faulting. Sierra Nevada Range

17. Plateaus, Domes and Basins • Mountains are not the only landforms that result from forces in Earth’s crust. Up and down movements of the crust can produce a variety of landforms, including plateaus, domes, basins. _____________________________________ • A plateau is a landform with a relatively high elevation and more or less level surface. To form a plateau, a broad area of the crust is uplifted vertically; raised above the adjoining landscape. Plateaus can cover very large areas of land such as the Colorado Plateau which stretches over four states. • Broad upwarping in the rock underlying an area may deform sedimentary layers. When upwarping produces a roughly circular structure, the feature is called a dome.Domes often have the shape of an elongated oval. • Down-warped structures that have a roughly circular shape are called basins.The central United States contains a number of basins, including the large Michigan Basin. Colorado Plateau

18. Mountains and Plates: Ocean-Ocean Convergence • Mountain building still occurs in many places worldwide. The jagged mountain peaks of the Grand Teton Range in Wyoming began to form about a million years ago and is still rising to this day. In contrast, older mountain ranges, such as the eastern Appalachians, are deeply eroded. • With the development of the theory of plate tectonics, a widely accepted model for mountain building became available. Most mountain building occurs at convergent plate boundaries. Colliding plates provide the compressional forces that fold, fault, and metamorphose the thick layers of sediments deposited at the edges of landmasses. Ocean-Ocean convergence: The convergence of two oceanic plates mainly produces volcanic mountains. Recall that this process occurs where oceanic plates converge in a subduction zone. • The result of this is the formation of a volcanic island arc on the ocean floor.

19. Convergent Boundary Mountains • Ocean-Continental Convergence: The convergence of an oceanic plate and a continental plate produces volcanic mountains and folded and faulted mountains. Mountains develop in two belts that run parallel to the edge of a continent. Continental volcanic arcs form when an oceanic plate is subducted beneath a continental plate. • Convergent Boundary Mountains: Another process forms a belt of coastal mountains made up of folded and faulted rocks. During subduction, sediment is eroded from the land and scraped from the subducting plate. This sediment becomes stuck against the landward side of the trench. • Along with scraps of oceanic crust, the sediment forms an accretionary wedge. A long period of subduction can build an accretionary wedge that stands above sea level. California’s coastal ranges formed by this process.

20. Convergent Boundary Mountains Continent-Continent Convergence: • At a convergent boundary, a collision between two plates carrying continental crust will form folded mountains. The reason for this is the continental crust is not dense enough, compared with the denser crust of the mantle, to be subducted. An example of such a collision began about 45 million years ago when India collided with the Eurasian Plate to form the Himalayas. • Before this event, India was part of Antarctica. It slowly moved thousands of kilometers north of millions of years. The result of this collision was the formation of the Himalayan Mountains. • Today, these sedimentary rocks and slivers of oceanic crust are elevated high above sea-level. The closing up of the ocean between India and the Eurasian plate is an example of how plate motions can destroy a sedimentary basin.

21. Divergent Boundary Mountains: • Most mountains are formed at convergent boundaries, but some are formed at divergent boundaries, usually on the ocean floor. These mountains form a chain that curves along the ocean floor at the ocean ridges. This mountain chain is over 70,000 kilometers long and rises 2000 to 3000 meters above the ocean floor. • The mountains that form along ocean ridges at convergent plate boundaries are fault-block mountainsmade of volcanic rock. The mountains are elevated because of isostosy. Rock at the ridge is hotter and less dense, so it rises higher than older, colder oceanic crust. Non-Boundary Mountains: Some mountains occur well within plate boundaries. Volcanic mountains at hot spots, as well as some upward mountains and fault- block mountains, can form far from boundary plates. The Hawaiian islands are a well known example of volcanic mountains at a hot spot.

22. Non-Boundary Mountains: • Non-boundary mountains formed by upwarping and faulting include the southern Rocky Mountains. The southern Rocky Mountains began to form about 60 million years ago with the subduction of an oceanic plate more than 1600 kilometers away. • At first, compressional forces deformed the crust. Than the subducting plate separated from the lithosphere above. This allowed hot rock to upwell from the mantle, pushing up the crust and forming the southern Rockies. • As the crust bent upwards, tensional forces stretched and fractured it, forming the fault-block mountains of the Basin and Range region.