Mountains and Mountain Building: Chapter 11

Rock Deformation Deformation is a general term that refers to a change in size or shape of rocks in the earth's crust. Deformation occurs when stress, or a force over a given area occurs. The amount of deformation that occurs and can be measured, either by a change in shape or volume of rocks in the earth's crust is referred to as strain. The four factors that influence rock deformation are temperature, pressure, rock type, and time.

Temperature and Pressure When rocks are near the earth's surface and temperatures and pressures are normal, rocks undergo brittle deformation and fracture. When rocks are at great depths, the temperatures and pressures are very high. Under extreme temperatures and pressures rocks can behave elastically. When rocks are elastic the type of deformation that occurs is referred to as ductile deformation.

Rock Types and Time Igneous Rocks like Granite and Basalt usually deform by brittle deformation. Sometimes they deform by ductile deformation. Metamorphic Rocks are more likely to deform by ductile deformation. Often times Sedimentary Rocks show that ductile deformation has occurred. One possible explanation for the ductile deformation is the sedimentary rocks folded before they hardened (lithification). The longer stress is applied to rocks, the more deformation will occur.

Types of Stress The three types of stress can be applied to rocks. Tensional Stress – When rocks pull apart. Compressional Stress – When rocks push together. Shear Stress – When rocks twist or push in opposite directions.

Stresses

Folds Often times when Compressional Stresses and ductile deformation occurs during mountain building, rocks will fold or bend much like sheets of paper. When a fold bends upward in an arching shape, an anticline forms. When a fold bends downward in the shape of a trough, a syncline forms. When a fold occurs in a step or stair-like fashion a monocline can occur.

Folds

Faults Stresses can also cause a fault or fracture in the earth's crust to occur due to brittle deformation. There are four main types of faults. Normal faults – these are caused by tensional stresses. When this occurs the head (hanging) wall moves down relative to the foot wall. When discussing the relative motion of the earth's crust along faults, the hanging wall or head wall refers to the side of the fault that rests on the foot wall. The foot wall is the side of the fault that appears to support the head wall or hanging wall.

Faults Reverse Faults – faults that are caused by compressional stress. In the case of a reverse fault, the hanging wall moves up relative to the footwall. Thrust Faults – these faults are low angle (less than a 45 degree angle) reverse faults. All faults that move up and down (normal, reverse, and thrust faults) are referred to as dip-slip faults. Strike-Slip Faults – These faults move side to side.

Faults

Mountain Building The formation of mountains is called orogenesis. Mountains that are formed by compressional stresses are called folded mountains. In folded mountains, compressional stresses can cause anticlines and synclines to occur, or they can produce reverse faults and thrust faults. Examples of folded mountain ranges are the Appalachian Mountains, The northern Rocky Mountains, and the Alps in Europe.

Folded Mountains

Mountain Building Mountains that are formed by tensional stresses are called fault-block mountains. Most fault-block mountains are formed as a result of normal dip-slip faults. When a normal fault occurs, a block of the earth's crust (the hanging wall) collapses, leaving behind a low-lying valley called a graben. Graben is a German word for a ditch or trench. The uplifted or higher elevated foot wall(s) is/are referred to as a horst(s).

Fault-Block Mountains

Mountain Building Examples of fault-block mountains include the Teton Range of Wyoming, the Sierra Nevada Range of California, and the Basin and Range area of Nevada, Utah, and California. A circular bulge or upwarping of the earth's crust is called a dome. A circular dip or downwarping of the earth's crust is called a basin.

Fault-Block Mountains

Domes and Basins