11.1 Mountain Building : Forces in Earth’s Crust

Forces in Earth’s Crust Earth has 14 mountains higher than 8000 meters. Four of these are in Pakistan’s Karakoram range which is part of the Himalayas. 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 Every body of rock, no matter how strong, has a point where it will 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. Plate motions and plate interactions produce forces that can deform rock. Deformation Rock layers crumple when the Earth’s crust is subject to stresses. These stresses may result in folds (warping or bending of rock layers, such as in the diagram below) or faults (fractures in the crust).

Deformation Deformation occurs because of stress in a body of rock. Rock layers crumple when the Earth’s crust is subject to stresses. These stresses may result in folds (warping or bending of rock layers, such as in the diagram below) or faults (fractures in the crust). Deformation occurs because of stress in a body of rock. Stress is the force “per unit area” acting on a mountain. ( pressure per square inch, foot, or meter for example) 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

Deformation Deformation Rock layers crumple when the Earth’s crust is subject to stresses. These stresses may result in folds (warping or bending of rock layers, such as in the diagram below) or faults (fractures in the crust). The change in shape or volume of a body of rock is called strain. How can rock masses be bent into folds without breaking? 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.

Temperature Pressure Rock type Time Deformation Rock layers crumple when the Earth’s crust is subject to stresses. These stresses may result in folds (warping or bending of rock layers, such as in the diagram below) or faults (fractures in the crust). The factors that affect the deformation of rock include Temperature Pressure Rock type Time

Temperature and Pressure Deformation Brittle Material breaks Temperature and Pressure 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. Brittle materials include materials like glass or ceramics. They fracture into pieces when force is applied to them. Ductile Material Bends or buckles

Deformation Brittle Material breaks 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 include materials like modeling clay and bee’s wax. They mold but do not break. Most metals are ductile materials. If you put a penny on a railroad track, it will be flattened and deformed but it will not break. Ductile Material Bends or buckles

Deformation Brittle Material breaks Rock Type 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. They break or shatter like glass or ceramics. Sedimentary rocks that are weakly cemented or metamorphic rocks that contain zones of weakness are more likely to deform by ductile deformation; bending or buckling. Ductile Material Bends or buckles

Deformation Brittle Material breaks Rock Type Rocks that are weak and most likely to become ductile under pressure include rock salt, gypsum, and shale. Limestone, schist and marble are of intermediate strength and may become ductile under enough pressure. Ductile Material Bends or buckles

Deformation Time Small stresses applied over long time spans eventually cause the deformation of rock You can see the affects of time on deformation in everyday circumstances. Marble benches have been known to sag under their own weight over a span of a hundred years or so. 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.

Types of Stress Deformation 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

Types of Stress Deformation When rocks are squeezed or shortened, the stress is compressional. Converging continental plates cause large land masses to collide over time, slowly applying compressional stress to buckle and bend the Earth’s surface. When rocks are pulled in opposite directions, the force is tensional. Shear stress causes a body of rock to be distorted.

Principal of Isostosy Deformation In addition to the horizontal side-to-side motion of the Earth’s crust, there is also up and down motion. Much of this vertical movement occurs along plate boundaries and is linked to the process of mountain building. However, the up-and-down motions often occur in the interiors of continental plates far from plate boundaries.

Principal of Isostosy Deformation 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.

Principal of Isostosy Deformation 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.

Principal of Isostosy Deformation 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. try this interactive model of isostosy to see how density affects how much a substance sinks

Principal of Isostosy Deformation 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. During the last ice age, the weight of a three kilometer thick mass of glacial ice depressed earth’s crust by hundreds of meters. In the 8000 years since those glaciers vanished, uplift of as much as 300 meters has occurred in Canada’s Hudson Bay area where the glaciers were thickest.

Principal of Isostosy Deformation Isostatic adjustment can account for considerable vertical movement. Most compression stress mountain building causes the crust to shorten and thicken. The crust rises higher and goes deeper below the surface as it is squeezed. Because of isostosy, deformed and thickened crust will undergo regional uplift both during mountain building and for a long time afterward. As the crust rises, the processes of erosion increase, and the deformed rock layers are carved into a mountain landscape.

Principal of Isostosy Deformation As erosion reduces the summits of mountains, the crust will rise in response to the reduced load. The process of erosion and uplift together will continue until the mountain block reaches it’s normal crust thickness; it’s balancing point of 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.

Principal of Isostosy Deformation This process allows igneous batholiths, which form within the cores of mountains far underground, to be uplifted and exposed at the surface over time.