Download Chapter 5, Lesson 4

Chapter 5,
Changes Over Time
LESSON 4,
FORCES THAT SHAPE EARTH
Objectives
 Describe the two main types of
weathering.
 Summarize how soil is formed and
why it is important.
Main Idea
 Several forces cause changes to
Earth’s surface over time.
Vocabulary
• weathering
• till
• erosion
• moraine
• deposition
• soil
• sediment
• humus
• mass wasting
What is weathering?
Weathering is the breaking down of rock
into smaller pieces by natural processes.
Physical Weathering
 Physical weathering (mechanical
weathering) is the breaking down of rock
by physical changes, caused by freezing
water, moving water, plants, or animals.
 Frost wedging occurs
when water seeps into
cracks, freezes and
expands, forcing the rock
apart.
 Moving water carries
pieces of rock which
collide and break apart.
 Plant roots can enter a
crack in the rock, grow,
and force the cracks to
widen.
 Burrowing animals
bring rocks to the
surface and expose
them to weathering.
Chemical Weathering
Chemical weathering is the breaking
down of rocks by changes in its
chemical composition by oxygen and
acids.
Oxygen reacts with the iron that some
rocks contain creating rust which weakens
the rock, allowing it to be easily broken.
Carbon dioxide can dissolve in rain water
forming carbonic acid which reacts with
limestone, dissolving the limestone, which
in turn can form caverns.
Quick Check
Sequence
Describe the steps that occur when
freezing water breaks down rock.
Water seeps into cracks in rock, freezes,
and widens the cracks; then the water
melts, seeps in deeper, freezes and
expands, widening cracks further; the rock
eventually breaks down.
Critical Thinking
How do chemical changes produce
weathering?
Chemical changes produce new substances
with properties that can cause rock to
break apart.
Oxygen, for example, combines with iron
in rock to form rust; the rusty rock breaks
more easily.
Which forces carry and drop?
 Erosion is the picking up and removing
of rock pieces and other particles.
 Deposition
is the
dropping off
of particles
in another
location.
 Wind contributes by picking up and
depositing. An example of this is sand
dunes.
 Water can also erode the land by
picking up and depositing or freezing
and thawing.
Flowing Water
 Sediment is the loose pieces of
minerals, rock, and organic materials
deposited by water.
 Deposition can change the course of a
river, causing it to turn or meander.
 Waves can carry sand away from the
beach, causing erosion of that beach.
 Mass wasting is the downhill movement
caused by gravity.
 Glaciers
move over
the land
pushing and
scraping the
rocks and
soil in front
and to the
sides of
them.
Quick Check
Sequence
Explain the steps in the formation of a
sand dune.
Weathering rock produces sand particles;
the wind picks up these particles and
moves them.
The particles are then deposited in a new
location and the deposited sand builds up
into a dune.
Critical Thinking
What will eventually happen to every arch
in Arches National Park? Explain.
In time, all the arches will be destroyed by
weathering and erosion.
Even if new arches form, they, too, will
continue to weather and erode until
destroyed.
How can moving water change the
land?
 As a river travels, the flowing water slowly
erodes the riverbed cutting through the
softer layers of rock forming channels, such
as deep canyons, streams, and valleys.
 Sediment causes additional erosion and is
eventually deposited offshore.
How Glaciers Form
Glaciers are formed by accumulated
snowfall over the years.
Eventually the bottom layers, due to the
weight of the upper layers, turns to ice.
Glaciers can join other glaciers and
become a continental glacier.
Some eventually reach the edge of a
continent and break off into the water
becoming an iceberg.
Moving and Melting Glaciers
 Glaciers change the land as they move by
transporting rocks or carving the land.
 The mixture of
substances moved by a
glacier are called a till.
When these deposits
form a ridge or mound
it is called moraine.
The Great Lakes were
created this way.
Quick Check
Sequence
Describe the process by which glaciers form.
More snow falls each winter than melts in
summer.
The snow builds up and exerts pressure on
the snow underneath, causing it to change
to ice.
As the ice layer increases in thickness, mass,
and weight, the glacier begins to form.
Critical Thinking
How does gravity affect stream erosion?
On a steep slope, gravity causes water to
flow downhill quickly.
The fast moving water causes
considerable erosion by carving out a
channel and carrying sediment away.
How is soil formed?
 Soil is a mixture of weathered rock, air,
water, and humus.
 Humus is a material made up of decayed
plant and animal remains.
 Soil horizons are the distinct layers of
soil, such as topsoil, subsoil, parent
rock, and bedrock.
 Clay soils are made of very fine
particles and is nearly impermeable;
water cannot easily pass through it.
 Sandy soils are made of coarse
particles and are permeable; water can
easily pass through it.
Uses of Soil
 Most living organisms depend directly or
indirectly on soil.
 Plants need soil to grow in.
 Many animals live in the soil.
Quick Check
Sequence
Explain the steps by which soil forms.
Rock weathers, breaking into small pieces
and forming a thin soil layer;
organisms in the soil break down and
enrich the soil with humus when they die.
Critical Thinking
Why do gardeners appreciate having
earthworms in their gardens’ soil?
Earthworms promote the movement of air,
water, and nutrients through the soil as
they burrow, create tunnels, digest,
reproduce, and complete the life cycle.
This enriches the soil and helps plants
grow more successfully.
Why is soil important?
Soil contains the minerals that organisms need
to survive.
Wasteful Practices
 Growing too many plants in an area or
growing the same crops year after year can
deplete the soil of important nutrients.
 Other ways that we make soil unusable
are;
Paving over land,
cutting down forests,
dumping wastes,
and building dams.
Quick Check
Sequence
How might building dams damage soil?
The water and nutrient rich sediments
held in a dam might not be available to
plants downriver, so these plants might
die.
With less water and less plant life, soil can
become dry, and wind can blow it away.
Critical Thinking
How might a farmer help protect soil by
planting different crops from year to year?
Different plants have different nutritional
requirements and replace different
minerals in the soil as the plants grow and
die.
By planting varying crops, a farmer can
help ensure that the minerals and
nutrients in the soil are being replenished.
Gravity pulls water downhill. In flowing water,
hard grains of sand gradually carve through the
underlying rock layers.
As snow falls onto a glacier, its weight packs the
snow and turns it to ice. Then gravity pulls on
the glacier so it slowly grinds its way down the
slope. This contributes to mass wasting, which
is a form of erosion.
Gravity’s role in deposition is to make the
sediment fall to the ground or the bottom of a
body of water where it can be picked up and
carried away again.