The principle of uniformity is the frame of reference that the same geologic processes you see changing rocks today are the same processes that changed them in the past.

Diastrophism is the process of deformation that changes the earth's surface, and the movement of magma is called vulcanism. Diastrophism,vulcanism, and earthquakes are closely related, and their occurrence can be explained most of the time by events involving plate tectonics.

Stress is a force that tends to compress, pull apart, or deform a rock, and the adjustment to stress is called strain. Rocks respond to stress by (1) withstanding the stress without change, (2) undergoing elastic strain, (3) undergoing plastic strain, or (4) by breaking in fracturestrain. Exactly how a particular rock responds to stress depends on (1) the nature of the rock, (2) the temperature, and (3) how quickly the stress is applied over time.

Deeply buried rocks are at a higher temperature and tend to undergo plastic deformation, resulting in a wrinkling of the layers into folds. The most common are an arch-shaped fold called an anticline and a troughshaped fold called a syncline. Anticlines and synclines are most easily observed in sedimentary rocks because they have bedding planes, or layers.

Rocks near the surface tend to break from a sudden stress. A break with movement on one side of the break relative to the other side is called a fault. The vibrations that move out as waves from the movement of rocks is called an earthquake. The actual place where an earthquake originates is called its focus. The place on the surface directly above a focus is called an epicenter. There are three kinds of waves that travel from the focus, S-, P-, and surface waves. The magnitude of earthquake waves is measured on the Richter scale.

Folding and faulting produce prominent features on the surface called mountains. Mountains can be classified as having an origin of folding, faulting, or volcanic. In general, mountains that occur in long narrow belts called ranges have an origin that can be explained by platetectonics.

Weathering is the breaking up, crumbling, and destruction of any kind of solid rock. The process of physically picking up weathered rock materials is called erosion. After the eroded materials are picked up, they are removed by transportation agents. The combined action of weathering, erosion, and transportation wears away and lowers the surface of the earth.

The physical breaking up of rocks is called mechanical weathering. Mechanical weathering occurs by wedging effects and the effects of reducedpressure. Frost wedging is a wedging effect that occurs from repeated cycles of water freezing and thawing. The process of spalling off of curved layers of rock from reduced pressure is called exfoliation.

The breakdown of minerals by chemical reactions is called chemicalweathering. Examples include oxidation, a reaction between oxygen and the minerals making up rocks; carbonation, a reaction between carbonic acid (carbon dioxide dissolved in water) and minerals making up rocks; and hydration, the dissolving or combining of a mineral with water. When the end products of complete weathering of rocks are removed directly by gravity, the erosion is called mass movement. Erosion and transportation also occur through the agents of running water, glaciers, or wind. Each create their own characteristic features of erosion and deposition.