Mountain Building

Deformation Every body of rock, no matter how strong, has a point at which it will bend or break. Deformation is a general term that refers to all changes in the original shape and/or size of a rock body. Rocks bend, twist, and fracture. Most crustal deformation occurs along plate margins because plate motions create forces that cause rock to deform. Most deformation occurs much too slowly for human observation.

Deformation Stress is the force per unit area acting on a solid. Uniform stress is pressure. When rocks are under stresses greater than their own strength, they begin to deform, usually by FOLDING, FLOWING, or FRACTURING. The change in shape or volume of a body of rock as a result of stress is called strain.

Deformation When stress is gradually applied, rocks can be bent into folds without breaking because they deform elastically. These changes are recoverable, and the rock will return to almost its original size and shape when the force has been removed. However, once the elastic limit or strength of a rock is surpassed, it either flows (ductile deformation) or fractures (brittle deformation). These kinds of deformation are permanent.

Factors That Influence Deformation: Temperature Confining pressure Near the surface, where temperatures and confining pressures are LOW, rocks usually behave like brittle solids and fracture once their strength has been exceeded resulting in BRITTLE FAILURE or BRITTLE DEFORMATION

Factors that Influence Deformation Deep inside the Earth, temperatures and confining pressures are HIGH so rocks undergo DUCTILE DEFORMATION that produces a change in the size and shape of an object without breaking it.

Factors That Influence Deformation: Rock Type Another factor that influences the type of deformation is the ROCK TYPE because of mineral composition and texture. Rocks with strong internal molecular bonds usually fail by brittle fracture. Examples: GRANITE and BASALT (igneous). Sedimentary rocks that are weakly cemented or metamorphic rocks with zones of weakness (such as foliation) are more likely to deform by ductile flow. Examples: GYPSUM, SHALE, ROCK SALT; sometimes LIMESTONE, MARBLE, and SCHIST.

Factors That Influence Deformation: Time TIME is another factor that influences deformation. Small, short-term stresses that do not cause deformation can cause rock to flow when maintained over long time periods. Marble benches have been known to sag under their own weight after time spans of nearly a century.

Types of Stress on Rocks Tensional Pulled in opposite directions Compressional Squeezed or shortened Shear Distorted due to forces acting in opposite directions

Types of Folds that Rocks Form During mountain building, flat-lying sedimentary and volcanic rocks are often bent into folds:

Types of Folds That Rocks Form Anticline Layers bend downward in opposite directions from the crest (letter “A” shape) Syncline Layers dip upward toward each other Monocline Large, step-like fold in otherwise horizontal sedimentary strata, usually over an underlying faulted block.

Types of Folds That Rocks Form

Fault Types Faults are fractures in the crust along which movement has taken place. For faults with vertical offsets, the footwall is the block beneath the fault plane. The hanging wall is the block above the fault plane.

Types of Faults: Dip-Slip NORMAL Hanging wall block moves DOWN relative to the footwall block (vertical offset). Crust is stretched and extended. (Tensional forces and gravity, @ divergent plate boundaries)

Types of Faults : Dip-Slip REVERSE Hanging wall block moves UP relative to the footwall block (vertical offset). Crust is shortened. (Compressional forces @ convergent plate boundaries)

Types of Faults : Dip-Slip THRUST Reverse faults with dips of less than 45° (shallow). Hanging wall block moves UP relative to the footwall block. Crust is shortened (compressional forces @ convergent plate boundaries).

Types of Faults : Dip-Slip Dip-slip faults can be OBLIQUE which means they have horizontal as well as vertical offsets.

Types of Faults: Strike-Slip Movement is predominantly horizontal, parallel to the trend or strike of the fault surface Crust is displaced without extending or compressing by shear forces (@transform plate boundaries)

Source: http://www.livescience.com/37052-types-of-faults.html

Can You Identify the Types of Faults? B C D Seen from above

Mountain Formation Geologists refer to the collection of processes involved in mountain building as orogenesis. The term is derived from the Greek words oros meaning “mountain” and -geny meaning “born”.

Mountain Ranges and Systems 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. The Brooks Range (AK), Canadian Rockies, Absarokas (MT/WY), Grand Tetons (WY), and Sangre de Cristo (CO) mountain ranges form part of the Rocky Mountain system.

Types of Mountains Dome or upwarped Volcanic Fault-block Folded There are four main types of mountains based upon their formation process: Dome or upwarped Volcanic Fault-block Folded (complex)

Dome or Upwarped Mountains Broad upwarping in the rock underlying an area may deform sedimentary layers. You can think of the upwarped layers that make up a dome as a large fold. Normal faults can develop as the rock layers stretch forming fault scarps.

Dome or Upwarped Mountains Examples are the Black Hills of SD and Adirondack Mountains of NY

Volcanic Mountains Form from the buildup of lava and pyroclastic material Continental volcanic arcs and island volcanic arcs along convergent plate boundaries are volcanic mountain ranges such as the Andes and Cascade Ranges and the Aleutian Islands Hot spots such as Mauna Loa in Hawai’i Continental rift volcanoes such as Mt. Kilimanjaro in Africa

Fault-Block Mountains Fault-block mountains form as large blocks of crust are uplifted and tilted along large-scale normal faults. Some form by tilting of fault blocks by broad crustal uplift while others occur due to tensional stresses. Often show one steep flank (fault scarp) while the other flank is more gradual.

Fault-Block Mountains 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. A graben produces an elongated valley bordered by relatively uplifted structures called horsts.

Fault-Block Mountains In the western US, examples of fault block mountains include the Grand Tetons (WY) and the Sierra Nevadas in California.

Folded (Complex) Mountains Mountains that are formed primarily by folding of rock strata are called folded mountains. Compressional stress is the major cause of folded mountains. Examples are the Himalayas in Asia, the northern Rockies and the Appalachians in North America, and the Alps in Europe.

Plateau A plateau is a large highland area of fairly level land separated from surrounding land by steep slopes. Some plateaus, like the plateau of Tibet, lie between mountain ranges. Others are higher than surrounding land. Plateaus are widespread, and together with enclosed basins they cover about 45 percent of the Earth's land surface. http://www.edu.pe.ca/southernkings/plateaurh.htm

Plateau