Chapter 11: Mountain Building 11.1 Rock Deformation Text pp 308-313

Factors Affecting Deformation Every body of rock, no matter how strong, has a point where it will bend or break.

Deformation is a term that refers to all changes in the original size and/or size of a rock body. When rocks deform they are said to strain. A strain is a change in size, shape, or volume of a material.

Deformation is caused by stress, the force per unit area acting on a solid. Rocks, under stresses greater than their own strength, begin to deform, usually by folding, flowing or fracturing.

The factors that influence the strength of a rock and how it will deform include: temperature, rock type, confining pressure, …and time.

Temperature and Pressure Rocks deform permanently in two ways: Ductile deformation Brittle deformation

Brittle Deformation Rocks near the surface, where temperatures and confining pressures are low, usually behave like brittle solids and fracture once their strength is exceeded.

Ductile Deformation Deep under the surface, where temperatures and confining pressures are high, rocks will bend, like modeling clay or bee’s wax.

Rock Type The mineral composition and texture of a rock also affects how it will deform. Rocks like granite with strong internal bonds usually fail by brittle fracture.

Sedimentary rocks that are weakly cemented are more likely to deform by ductile flow.

Time: it wins out over everything Forces that are unable to deform rock at first may cause rock to flow if the force is maintained over a long period of time.

Types of Stress The three types of stresses that rocks commonly undergo are: Tensional stress (stretched) Compressional stress (squeezed) Shear stress (distorting)

Folds The two most common types of folds are anticlines and synclines.

An anticline is formed by the upfolding – or arching – of rock layers.

Synclines are downfolds - or troughs - of rock layers Synclines are downfolds - or troughs - of rock layers. (They are often found in association with anticlines.)

Can you see the anticline and syncline in this mountain formation Can you see the anticline and syncline in this mountain formation? (Note: The green fringe at the bottom are full grown trees!)

Faults Faults are fractures in the crust where movement has taken place.

The major types of faults are … 1. Normal faults, 2. Reverse faults, 3. Strike-slip faults.

Normal Faults A normal fault occurs when the hanging wall block moves DOWN the footwall block. Because of the downward motion of the hanging wall block, normal faults result in the lengthening, or extending of the crust.

Reverse Faults A reverse fault is a fault in which the hanging wall block moves UP relative to the footwall block. Because the hanging wall block moves up and over the footwall block, reverse faults result in a shortening of the crust.

Thrust fault A fourth kind of fault is the thrust fault, which is a reverse fault with dips of less than 45°.

There aren’t that many large reverse faults There aren’t that many large reverse faults. Large thrust faults, however, can be found in the northern Rockies, Himalayas, and Appalachians. Mount Everest

Strike-Slip Faults Faults in which the movement is horizontal to the fault surface are called strike-slip faults. Most strike slip faults consist of roughly parallel fractures, instead of just a single fracture.