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Weathering and Erosion
Section 1: Weathering
Section 2: The Nature of Soil
Section 3: Erosion
Weathering and Its Effects
• Together, surface processes that work to
break down rock are called weathering.
• Weathering breaks rock into smaller and
smaller pieces, such as sand, silt, and clay.
These smaller, loose pieces are called
sediment.
• The terms sand, silt, and clay are used to
describe specific sizes of sediment.
Mechanical Weathering
• Mechanical weathering occurs when rocks
are broken apart by physical processes. This
means that the overall chemical makeup of
the rock stays the same.
Click image to view movie.
Mechanical Weathering
• Growing plants, burrowing animals, and
expanding ice are some of the things that can
mechanically weather rock.
• As rock is broken apart by mechanical
weathering, the amount of rock surface
exposed to air and water increases.
Plants and Animals
• Burrowing animals also cause mechanical
weathering.
• As these animals
burrow, they loosen
sediment and push it
to the surface. Once
the sediment is
brought to the surface,
other weathering
processes act on it.
Ice Wedging
• Ice wedging occurs in temperate and cold
climates where water enters cracks in rocks
and freezes.
• When water enters
cracks in rock and
freezes, it expands,
causing the cracks
to enlarge and the
rock to break apart.
Chemical Weathering
• Chemical weathering occurs when chemical
reactions dissolve the minerals in rocks or
change them into different minerals.
• This type of weathering changes the chemical
composition of the rock, which can weaken
the rock.
Natural Acids
• Carbonic acid reacts
with minerals such as
calcite, which is the
main mineral that makes
up limestone.
• Over many thousands of
years, carbonic acid has
weathered so much
limestone that caves have
formed.
Oxygen
• Oxidation occurs when
some materials are
exposed to oxygen and
water.
• Oxidation of minerals
gives some rock layers
a red color.
Effects of Climate
• Climate is the pattern of weather that occurs
in a particular area over many years.
• In cold climates, where freezing and thawing
are frequent, mechanical weathering rapidly
breaks down rock through the process of ice
wedging.
Formation of Soil
• What is soil and where does it come from?
1. A layer of rock and mineral fragments produced by
weathering covers the surface of Earth.
2. Weathering gradually breaks rocks into smaller and
smaller fragments.
3. Plants and animals add organic matter, the remains
of once-living organisms, to the rock fragments.
• Soil is a mixture of weathered rock, decayed
organic matters, mineral fragments, water,
and air.
Composition of Soil
• Most organic matter in
soil comes from plants.
• Animals and
microorganisms provide
additional organic
matter when they die.
• The decayed organic matter turns into a darkcolored material called humus. Humus serves
as a source of nutrients for plants.
Soil Profile
• The top layer typically is darker than the soil
layers below it. These different layers of soil
are called horizons.
• Most soils have three
horizons—A, B, and C,
with bedrock below, and
humus or litter layer on
top.
• All the horizons of a
particular soil form a
soil profile.
A Horizon
• The A horizon is the top
layer of soil.
• In a forest, the A horizon
might be covered with litter.
Litter consists of leaves,
twigs, and other organic
material that eventually can
be changed to humus by
decomposing organisms.
• The A horizon also is known as topsoil.
B Horizon
• Leaching is the removal
of minerals that have been
dissolved in water. In soil,
water seeps through the A
horizon where it reacts
with humus and carbon
dioxide to form acid.
• The acid dissolves some of the minerals in the
A horizon and carries the material into the B
horizon, or subsoil.
C Horizon
• The C horizon consists of partially
weathered rock and is the bottom horizon
in a soil profile.
• This horizon does not contain much organic
matter and is not strongly affected by
leaching.
Erosion and Deposition
• Erosion is a process that wears away surface
materials and moves them from one place to
another.
• An important
erosional force is
gravity. It’s power is
seen in this
landslide photo.
• Other causes of erosion, also called agents of
erosion, are water, wind, and glaciers.
Dropping Sediments
• Agents of erosion drop the sediments they
are carrying as they lose energy. This is
called deposition.
• When sediments are eroded, they are not lost
from Earth—they are just relocated.
Mass Movement
• Rocks and other materials, especially on
steep slopes, are pulled toward the center of
Earth by gravity.
• A mass movement is any type of erosion that
happens as gravity moves materials
downslope.
• Mudflows, rock slides,
rockfalls, creep, and
slump are most likely to
occur on steep slopes, and
they all depend on gravity
to make them happen.
Slump
• When a mass of material slips down along a
curved surface, the mass movement is called
a slump.
• A curved scar
is left where
the slumped
materials
originally
rested.
Creep
• Leaning trees and human-built structures
show another mass movement called creep.
• Creep occurs when sediments slowly shift
their positions downhill.
• Creep is common in areas of frequent
freezing and thawing.
How Glaciers Form and Move
• A large mass of ice and snow moving on land
under its own weight is a glacier.
• Glaciers form in regions where snow
accumulates. When snow doesn’t melt, it
piles up.
• As it accumulates slowly, the increasing
weight of the snow becomes great enough
to compress the lower layers into ice.
Glaciers
• Continental glaciers are huge masses of ice and
snow. Today, continental glaciers cover ten
percent of Earth, mostly near the poles in
Antarctica and Greenland.
• Valley glaciers erode
special landforms like
bowl-shaped basins,
called cirques, long
ridges called an
arêtes, and sharpened
peaks called horns.
Plucking
• The process called plucking, results in
boulders, gravel, and sand being added to
the bottom
and sides of
a glacier.
Ice Depositing Sediment
• When glaciers begin to melt, sediment drops
or is deposited, on the land.
• When a glacier melts and begins to shrink
back, it is said to retreat.
• As it retreats, a jumble of boulders, sand,
clay, and silt is left behind.
• This mixture of different-sized sediments is
called till.
Moraine Deposits
• Till is also deposited at the end of glacier
when it is not moving forward.
• Rocks and soil are moved to the end of the
glacier, much like items on a grocery store
conveyor belt.
• Because of this, a big ridge of material piles
up called a moraine.
Outwash Deposits
• Material deposited by the meltwater from a
glacier, most often beyond the end of the
glacier, is called outwash.
• Meltwater carries sediments and deposits
them in layers.
Deflation
• Wind erodes Earth’s surface by deflation and
abrasion.
• Deflation and abrasion happen to all land
surfaces but occur mostly in deserts,
beaches, and plowed fields.
• These areas have fewer
plants to hold the sediments
in place.
• When wind erodes by
deflation, it blows across
loose sediment, removing
small particles such as silt and
sand.
Abrasion
• When windblown sediment strikes rock, the
surface of the rock gets scraped and worn away by
a process called abrasion.
• These sand grains
strike against rock
and break off
small fragments.
• The rocks
become pitted
and are worn
down gradually.
Deposition by Wind—Loess
• Wind deposits of fine-grained sediments are
known as loess.
• Loess is as fine as talcum powder.
• Strong winds that blew across glacial outwash
areas carried the sediments and deposited
them.
• The sediments settled on hilltops and in
valleys.
Dunes
• A dune is a mound of
sediments drifted by
the wind.
• A sand dune has two
sides. The side facing
the wind has a gentler
slope. The side away
from the wind is
steeper.