Greetings All! These are pages 87 thru 91 in your Reading Workbook.
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Before You Read
Think of a time when you had soil on your hands. That soil
might once have been at the top of a mountain. Describe
how soil might make its way down a mountain.
Weathering and Its Effects
Tiny moss plants, earthworms, and even oxygen weaken
and break apart rocks at Earth’s surface. The surface
processes that break down rock are called weathering.
Weathering breaks rock into smaller and smaller pieces
called sediment. Sand, silt, and clay are three different sizes
of sediment. Sand grains are larger than silt. Silt is larger
Over millions of years, weathering has changed Earth’s
surface. The process continues today. There are two
different types of weathering—mechanical weathering and
chemical weathering. Both types work together to change
Mechanical weathering occurs when rocks are broken
apart by physical processes. Mechanical weathering breaks
rocks into small pieces, changing only the size and shape of
the rock. The chemical makeup of the small pieces is the
same as the chemical makeup of the original rock.
How do plants and animals cause weathering?
Water and nutrients collect in the cracks of rocks. Seeds
that land in the cracks are able to grow. As a plant grows,
its roots grow larger and move deeper into the crack in the
rock. As the roots get bigger, they make the crack larger.
You may have seen how the roots of a tree can lift and
crack a piece of sidewalk. This is one way plants cause
Animals also cause mechanical weathering. Look at the
figure above. Small burrowing animals, such as voles, dig
tunnels in the ground. Burrowing loosens small rocks and
sediment in soil. The animal pushes these small pieces of
rock to the surface. Once these small rocks and sediment are
out of the ground, other weathering processes act on them.
What is ice wedging?
Ice wedging is the mechanical
weathering process that occurs
when water freezes in the cracks
of rocks. Water may seep into a
crack in a rock. As the water turns
to ice, it expands and pushes
against the sides of the crack. The
crack gets wider and deeper, as
shown in the figure. The pressure
of the ice in the crack is so great it can break the rock apart.
When temperatures rise, the ice melts. Because the crack
is larger now, more water can enter the crack. When the
water freezes again, the ice will again put pressure on the
crack. After many years of this freezing and melting cycle,
the rock will break up completely.
88 Weathering and Soil
Where does ice wedging occur?
Ice wedging is often seen in mountains, where warm days
and cold night are common. Ice wedging is a process that
wears down mountain peaks. The cycle of freezing and
thawing also breaks up roads. When water seeps into cracks
in the pavement and freezes, it forces the pavement apart.
Ice wedging in roads is one cause of potholes.
How does mechanical weathering affect
Mechanical weathering by plants, animals, and ice
wedging breaks rocks into smaller pieces. These small pieces
have more surface area than the original rock had. As the
surface area increases, more rock is exposed to water and
oxygen. This speeds up a different kind of weathering, called
Chemical weathering occurs when chemical reactions
dissolve or alter the minerals in rocks or change them into
different minerals. Like mechanical weathering, chemical
weathering changes the size and shape of rocks. But it also
changes the chemical makeup of rock. These chemical
changes weaken the rock.
How do natural acids weather rock?
Naturally formed acids can weather rock. Carbonic acid is
a natural acid formed when water reacts with carbon
dioxide gas in the air or soil. Even though carbonic acid is a
weak acid, it causes chemical weathering in rocks.
Over thousands of years, carbonic acid can form caves in
limestone. Calcite is the main mineral in limestone. When
carbonic acid reacts with calcite, it causes the calcite to dissolve.
Over time, enough calcite in the limestone may dissolve to
form a cave.
Other naturally occurring acids weather other types of
rock. Granite, some types of sandstone, and other rocks
all contain the mineral feldspar. Over many years, feldspar
is broken down into a clay mineral called kaolinite
(KAY oh luh nite). Kaolinite clay is found in some soils.
Clay is an end product of weathering.
How do plant acids cause chemical
Some plant roots give off acids. Rotting or decaying
plants also give off acids. These natural acids can dissolve
minerals in rock. When the minerals dissolve, the rock is
weakened. Over time, the rock cracks and breaks into
smaller pieces. As the rock weathers, nutrients become
available to plants.
How does oxygen cause chemical weathering?
Oxygen also causes chemical weathering. Oxidation (ahk
sih DAY shun) occurs when some materials are exposed to
oxygen and water. When minerals containing iron are
exposed to water and oxygen, the iron in the mineral reacts
to form a new material. This new material looks like rust.
Oxidation occurs in the mineral magnetite. When the iron
in magnetite is exposed to water and oxygen, it forms
limonite, a rustlike material. Oxidation of minerals gives
some rock layers a red or rust color.
Effects of Climate
Climate affects how quickly weathering occurs. Climate is
the pattern of weather in a region over many years. In cold
climates, with frequent freezing and thawing, mechanical
weathering happens rapidly. Freezing and thawing cause ice
wedging that breaks down rock.
Chemical weathering is more rapid in warm, wet climates.
So, chemical weathering occurs quickly in tropical areas, such
as rain forests. Chemical weathering is slower in deserts
where there is little water. The constant low temperatures in
polar regions also slow down chemical weathering.
Do all rocks weather at the same rate?
Different types of rock weather at different rates. In
wet climates, marble weathers rapidly and discolors. Granite
weathers more slowly in humid climates.
The weathering of rocks and the process of soil formation
alter rock minerals so that soil minerals are mostly of the
parent rock type. Weathering begins the process of forming
soil from rock and sediment and also affects particle size and
soil texture. Recall that sand, silt, and clay simply describe
the different particle sizes of the soil’s mineral content.