Science High School Reviewer | Grade 4 Science | Earth Science | Shaping Earth | Lesson 18: Weathering and Erosion

What is weathering?
Every day, rocks move and change shape as
the wind blows and rain falls. Look at the rock
below. What happened to it?
Even the largest boulder can break apart over
time. Weathering is the slow process that breaks
rocks down into smaller pieces. Flowing water,
rain, and wind are some causes of weathering.
In the distant future, the rock below will
probably look very different. It may break into
pieces the size of pebbles, or even smaller!

Physical Weathering
Rocks can change size and shape
without changing their chemical
makeup. This process is called
physical weathering.
Flowing water from streams
and rivers can make sharp rocks
smooth. Waves crashing onto a cliff
can break off small pieces of land.
Rainfall may seep through small
cracks in a rock. Cycles of freezing
and melting can widen the cracks.
Living things also cause physical
weathering. Plant roots can force their
way through cracks in a rock. As the
plant grows, its roots get stronger. The
strong roots can break the rock apart.
Chemical Weathering
Chemical weathering changes
the minerals that make up rocks.
Oxygen, acids, and carbon dioxide
all cause chemical weathering. They
change minerals into new substances.
Have you ever seen an iron chain
get rusty? Water and air change iron
into rust. Rocks that have iron in
them can rust too. Water and carbon
dioxide can form limestone caves.
Even living things, such as lichens,
can soften the rocks they grow on.

What is erosion?
Have you ever built a sand castle at the
beach? Did waves wash it away? Waves crash
against the shores of oceans and lakes all the
time. They pick up beach sand and small rocks.
Then they carry the pieces away.
The transport of weathered rock is called
erosion. Weathering and erosion work together
to shape the land.
Causes of Erosion
Flowing water is one cause of erosion.
Rainwater carries particles into streams and
rivers. The moving water then flows downhill
and carries them along. Eventually, the
particles are dropped off in a new place.
Waves and wind also pick up small pieces
of rock and put them somewhere else. Gravity
moves rocks downhill. Glaciers carve through
land as they slide over it.
Evidence of erosion is all around you. You
can find grooves carved into ancient rocks by
erosion. Or look closely at a handful of sand.
Over time, weathering and erosion can turn a
sharp, rocky peak into a smooth, round one.

Rivers Erode the Land
The Grand Canyon in Arizona
shows how powerful a river can be.
The canyon is 446 km (277 mi)
long. It has an average depth of 1.6
km (1 mi). This huge space was
carved over thousands of years by
the Colorado River.
Rivers and streams pick up bits of
rock and soil as they flow over land.
Some of the pieces get deposited, or
dropped off, on the banks. Others
get carried to the mouth of the river.

Deposition
Deposition is the dropping off
of weathered rock. Deposition by
water builds up deltas, riverbanks,
and beaches. Deposition by wind
forms sand dunes.

How do glaciers
shape the land?
In very cold places, thick sheets
of ice called glaciers (GLAY•shuhrz)
creep over the land. About a million
years ago, glaciers began to cover
much of Earth. Few places are cold
enough for glaciers today.
Glaciers form where snow collects
quickly and melts slowly. Year after
year, the snow builds higher. The
weight on top of the mound puts
pressure on the snow below. The
bottom of the glacier slowly turns to
ice. Near the ground, some ice melts.
Carving the Land
Melting makes the bottom of the
glacier slippery. It begins to flow
downhill. The bottom and sides
freeze onto rocks. As the glacier
continues to move, it tears rocks
from the ground. It scratches,
flattens, breaks, or carries away
the things in its path. A glacier can
make a valley wider and steeper.

What Glaciers Leave Behind
You have read how glaciers erode
the land. Glaciers also deposit land.
As glaciers melt, they leave behind
the rocks they carried. The leftover
rocks are called glacial debris
(GLAY•shuhl duh•BREE).
Glacial debris can be made of
large boulders or small rocks. They
can have bits of gravel, sand, soil,
and clay. The glacier drops most of
this debris at its downhill end, or
terminus.
Have you ever seen a giant
boulder all by itself in a valley? It was
probably glacial debris. More often,
glacial debris is a mix of gravel, small
rocks, sand, and clay. The mixture is
called glacial till.
Materials that a glacier picks
up or pushes can forms mounds.
These mounds are called moraines.
Today, you can find glacial till
and moraines across Canada and
northern parts of the United States.

How do people
shape the land?
Most processes in nature change
Earth’s land very slowly. People can
make faster changes.
Mining
One way people change the land
is by mining it. Mining is digging
into the land for useful resources
like minerals, metals, or fuel.
Landfills
Landfills are places where people
pile trash. Some form large mounds
or hills. Some are covered with soil
and plants to blend in with the land.
Forests
People need land to build farms,
homes, resorts, and factories. Often,
people cut down or burn forests to
clear the land. The trees are used
to make products. Scientists are
concerned about losing our forests.