Earth’s Materials and Processes-Part 10 Mountain Building and Faults

Stress and Deformation The movement of tectonic plates causes stress on rock structures. Stress is the amount of force per unit area placed on an object. When a rock is placed under stress, it deforms, or changes shape. Deformation is the process by which rocks change shape when under stress. When rocks bend, folds form; when rocks break, faults form.

Types of Folds Folding occurs when rock layers bend under stress. The bends are called folds. Scientists assume that all rock starts in horizontal layers deposited on top of earth over time. A fold, or bend, in the rock layers means that deformation has happened.

Types of Folds Two common types of folds are syncline and anticlines. In a syncline, the youngest layers of the rock are the core of the fold. The rock layers are arched upward. In an anticline, the oldest layers of rock are the core of the fold. The rock layers are usually arched downward.

Types of Faults Sometimes, rock is under so much stress, it breaks. A crack called a fault forms when large blocks of rock break and move past each other. The blocks of rock on either side of a fault are called fault blocks. The movement of fault blocks create mountains and other landforms.

Types of Faults The three main kinds of faults are strike-slip faults, reverse faults, and normal faults. A fault plane is the location where two faults meet. For any fault except a perfectly vertical fault, the block above the fault plane is called the hanging wall. The block below the plane is called the footwall.

Strike-Slip Faults Fault blocks move past each other horizontally. Form when rock is under shear stress, or the stress that pushes rocks in parallel opposite directions. Common along transform boundaries, where tectonic plates move past each other.

Normal Fault The hanging wall moves relative to the footwall, in a way you would normally expect as a result of gravity. Form when rock is under stress that stretches or pulls rock apart. Common along divergent boundaries.

Reverse Faults The hanging wall moves up relative to the footwall. Reverse faults form when rocks undergo compression, which is stress that pushes rock together. Common along convergent boundaries, where two plates collide. Sometimes called thrust faults.

Mountain Building Mountains can form through folding and faulting. Uplift is a process that can cause land to rise. It can also contribute to mountain building. Because tectonic plates are always in motion, some mountains are constantly being uplifted.

Types of Mountains Folded mountains form when rock layers are squeezed together and pushed upward. Most folded mountains form at convergent boundaries where plates collide. Volcanic mountains form when melted rock erupts onto Earth’s surface. Many volcanic mountains are located at convergent boundaries. Volcanic mountains can form on land or on the ocean floor.

Fault-block mountains form when tension makes the lithosphere break into many normal faults. Along the faults, some pieces of the lithosphere drop down compared with other pieces. The pieces left standing form fault-block mountains.

Can also have dome mountains and folded mountains. These are formed by Earth’s Processes.