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12.1 Weathering
Key Idea: Over time, the rocks are
broken down by mechanical and
chemical weathering.
Lesson objectives:
1. Describe how mechanical weathering
breaks down rocks.
2. Describe how chemical weathering
breaks down rocks.
3. Name three factors that affect weathering
rates.
Weathering? What’s That?
• Weathering is the breakup of rock due to
exposure to processes that occur at
Earth’s surface. The weathering processes
belong to two major groups:
A) Mechanical weathering, also called
Physical weathering
B) Chemical weathering
Mechanical Weathering
• Mechanical weathering or Disintegration
takes place when rock is split into smaller
pieces of the same material without
changing its composition. This type of
change (in size and shape) is also called
physical weathering.
Mechanical Weathering
Mechanical Weathering
What Causes Mechanical
Weathering?
Mechanical of physical weathering
processes include:
1. Frost wedging
2. Wetting and drying abrasion
3. Plant and animal activity
4. Exfoliation as a result of upward
expansion
1. Frost Wedging
• When water freezes its volume increases
by about 10%. Most natural materials
shrink as the temperature drops, but water
does the opposite.
• When rain water enters existing cracks in
rocks and freezes, the ice expands,
wedging the rock apart.
Frost Wedging
Frost Wedging
• Frost wedging occurs in places where there are
frequent freezes and thaws. Bare mountaintops
are especially susceptible to frost wedging.
• The vast fields of sharp cornered boulders on
mountaintops are pieces of the mountain that
has been broken off by frost wedging.
• Frost wedging also creates potholes in the
pavement; when the water in the ground
freezes, it lifts up the pavement; when the ice
thaws, the pavement collapses, leaving a
pothole.
2. Abrasion
• Water, wind, and ice are capable of moving rocks.
• Water can tumble boulders, pebbles, and sand along
streambeds.
• Wind can blow sand and small pebbles across a rocky
plain. Glaciers, which are moving masses of ice carry
rocks over great distances.
• As moving sand, pebbles, and larger rocks grind and
scrape against one another, thee rock materials are worn
away. This type of mechanical weathering is called
abrasion.
• The sand on beaches is a product of abrasion.
3. Plants and Animals
(Biological Weathering)
• The growth of plants and the activity of animals
contribute to mechanical weathering of rocks.
• When mosses and other small plants grow on
rocks they wedge their tiny roots into pores and
crevices. As the roots grow, the rock splits;
larger plants may grow in the cracks, continuing
to open the cracks.
• Insects, worms, and larger animals dig holes in
the soil. These holes allow air and water to
reach the bedrock and weather it.
3.Biological Weathering
3.Biological Weathering
4. Upward Expansion
• The upward expansion of rocks that are formed
deep underground may result in a mechanical
weathering process called exfoliation. For
example magmatic rocks formed deep in the
ground eventually become exposed when the
magmatic rocks are lifted up and the overlaying
rocks are eroded away.
• The removal of the overlaying rocks reduces the
pressure along the surface of the granite. ,
causing the granite to break along curved joints
that are parallel to the surface. Such joints can
be seen in exposed peaks.
Exfoliation
B). Chemical weathering
•
Chemical weathering involve a change in the
chemical makeup of the rock. Such changes occur
under the influence of water, either in liquid or
gaseous state (water vapors), acids, and oxygen.
•
Water interacts with the minerals in the rocks in two
ways:
a) it dissolves some minerals and removes them from the
rock and
b) Water is added to the molecules of some minerals
through a process hydrolysis. Through hydrolysis the
newly formed minerals become softer, and are
removed easier.
Chemical Weathering
• The water often combines chemically with
gasses from the atmosphere and forms
acids, such as:
-carbonic acid, (water + CO2) which
dissolves limestone.
-sulfuric acid (water + SO2 ), which is
pretty aggressive, and dissolves a larger
array of minerals, as well as concrete,
metal structures, etc.
Caves form due to chemical
weathering
Oxygen and Chemical weathering
• Iron is present in the chemical make up of
many minerals. When iron combines with
oxygen, it forms a iron oxide, or rust.
• Many rock formations display beautiful
yellowish, or reddish colors when iron
oxides are present. There are two
common iron oxides: limonite (yellow) and
hematite (red).
The red coloration of the rocks is
given by iron oxides
Rates of Weathering
• The weathering is a very slow process.
The following conditions conditions can
accelerate the decay of the rocks though:
1. surface exposure
2. composition of the rock
3. Climate
1. Surface Exposure
• The rate at which a rock is affected by
weathering increases as the surface
exposed to weathering becomes larger.
Surface Exposure
Rates of Weathering
• The composition of the rock
Some rocks are more resistant to weathering
than others, depending on the minerals that are
found in these rocks. Quartz is a very resistant
mineral, therefore rocks that contain a lot of
quartz are very resistant to weathering.
Calcite reacts with weak acids and the rocks
containing calcite (limestone) are easily
weathered.
The rock formation was exposed to the same conditions,
yet different layers of rocks responded differently to
weathering (Palo Duro Canyon, Texas)
3. Climate Influences Weathering
-Humid and climates, with lots of rain favor
chemical weathering; water and heat accelerate
chemical reactions, and some minerals are
rapidly removed from rocks.
-Dry climates, on the other hand favor mechanical
weathering. In deserts, for example, the
temperature drop below freezing, and during day
time is very, very hot. The rocks dilate and
contract repeatedly, and crumble into small
pieces. Ice wedging is also an important factor.