Transcript Physical weathering breaks up rock

Weathering, in geology, processes of physical
disintegration and chemical decomposition
of solid rock materials at or near the earth's
surface. Physical weathering breaks up
rock without altering its composition, and
chemical weathering decomposes rock by
slowly altering its constituent minerals.
Both processes work together continuously to
produce debris that is then transported away
mechanically or in solution (see Erosion).
Weathering processes also aid in the
formation of soil.
Physical weathering results primarily from
temperature changes, such as intense
heat; the action of water freezing in
rock crevices; and living organisms, such
as tree roots and burrowing animals.
Temperature changes alternately expand
and contract rocks, causing
granulation, flaking, and massive
sheeting of the outer layers. Frost
action and organisms widen cracks, exposing
deeper layers to chemical weathering.
Chemical weathering alters the original
mineral composition of rock in a number of
ways, such as by dissolving minerals by
water and weak soil acids; by oxidation
(see Chemical Reaction); by producing a
reaction with carbon dioxide; and by
hydration, which is a process in which water
chemically combines and reacts with minerals.
Plants, such as lichens, also decompose
certain rocks by extracting soluble
nutrients and iron from the original
minerals.
Weathering is often a combination of
three processes: the mechanical
process, as in the growth of ice or salt
crystals or in thermal heating and
cooling; the chemical process, as in
acid-water solutions that tend to
dissolve minerals such as calcite and
feldspar; and the biological process, as
in the effect of plant roots, which
generate both mechanical and chemical
energy.
Erosion is the dislodging, removal,
and transport of material, either in
solution or in particle form. The
energy to accomplish this may be
provided in the form of raindrops,
running water, wind, waves, or simply
gravity (as in a landslide).
Human-induced soil erosion is a feature of the
present day and of the last few millennia, because
clearing land of native vegetation or excessive
grazing by domesticated animals exposes the
soil to massive erosion. In this way some 3 billion
metric tons of particulate material are washed from
the surface of the U.S. alone each year. In
undisturbed natural settings, on the other hand—
notably in low-relief continental interiors—erosion
rates are very slow (except in semiarid areas where
thunderstorms produce flash floods). In structurally
active belts such as in youthful mountains, which as a rule
coincide with plate boundaries that recently collided or
rifted, erosion rates may be enormous.
Of all the different processes acting on the
earth's surface, rain and rivers are the most
vigorous erosive agents. By contrast, although
wave action on a rocky coast is often
impressive, the rate of retreat of the
shoreline is generally very slow. Sand dunes
in the Sahara are also impressive, but the sand is
only a relatively thin veneer; and the moraines
left by giant continental glaciers are
likewise only superficial scrapings of ancient
soils. In general, without human
interference, the landscape is stable.