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 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 fracture
strain. 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 plate tectonics.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
Earth.
The physical breaking up of rocks is called mechanical weathering.
Mechanical weathering occurs by wedging effects and the effects of reduced
pressure. 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 chemical
weathering. 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.
