Download Weathering - Spring Branch ISD

Name ____________________________________ Date __________ Class ___________________
SECTION 7-1
SECTION SUMMARY
Rocks and Weathering
Guide for
Reading
◆ What causes
mechanical
weathering?
◆ What causes
chemical
weathering?
◆ What determines
how fast
weathering
occurs?
Unit 2 Resources
W
Science Explorer Grade 7
© Prentice-Hall, Inc.
10
eathering is the process that breaks down rock and other
substances at Earth’s surface. The forces of weathering break rocks
into smaller and smaller pieces. The forces of erosion carry the pieces
away. Erosion is the movement of rock particles by wind, water, ice, or
gravity. Weathering and erosion work together continuously to wear
down and carry away the rocks at Earth’s surface.
The type of weathering in which rock is physically broken into smaller
pieces is called mechanical weathering. These smaller pieces have the
same composition as the rock they came from. Mechanical weathering
breaks rock into pieces by freezing and thawing, release of pressure,
growth of plants, actions of animals, and abrasion. Rock particles
carried by wind, water, and ice can wear away rocks in the process called
abrasion. Water expands when it freezes, and acts as a wedge. This
process is called ice wedging. As a rock surface erodes, pressure on the
rock below is reduced, and this pressure release can cause rocks to crack.
Plant roots can pry apart cracked rocks.
Chemical weathering is the process that breaks down rock through
chemical changes. The agents of chemical weathering include water,
oxygen, carbon dioxide, living organisms, and acid rain. Chemical
weathering produces rock particles that have a different mineral makeup
from the original rock. Chemical and mechanical weathering often work
together. As mechanical weathering breaks rock into pieces, more surface
area becomes exposed to chemical weathering.
Water is the most important agent of chemical weathering. Water
weathers rock by dissolving it. The oxygen in air is an important cause of
chemical weathering. Iron combines with oxygen in the presence of
water in a process called oxidation. Rock that contains iron oxidizes, or
rusts. Another gas found in air, carbon dioxide, combines with rainwater
to form carbonic acid. Carbonic acid easily weathers marble and
limestone. Plant roots produce weak acids that slowly dissolve any rock
around the roots. Lichens also produce a weak acid that weathers rock.
Acid rain causes very rapid chemical weathering.
The most important factors that determine the rate at which
weathering occurs are type of rock and climate. Some types of rock
weather more rapidly than others. For example, some rock weathers
easily because it is permeable, which means that it is full of air spaces
that allow water to seep through it. Both chemical and mechanical
weathering occur faster in wet climates. Chemical weathering occurs
more quickly where the climate is both hot and wet.