Science High School Reviewer | Grade 4 Science | Earth Science | Shaping Earth | Lesson 19: Changes Caused by the Weather

How do floods and fires
change the land?
You have learned how erosion and deposition
change the land slowly. What causes Earth’s
surface to change quickly?
Floods
Heavy rain or quickly melting snow can cause
water to flow over the land. The water may not
soak into the ground completely. It runs on top
of the land. It flows into streams and rivers. The
extra water may spill over the sides, or banks, of
streams and rivers. Then it floods (FLUHDZ) the
nearby land. A flood is an overflow of water onto
land that is normally dry.
Cities flood when water drains cannot carry
water away fast enough. The drains overflow.
The streets become flooded.
Floods can carry mud into homes and streets.
The mud and water cause damage. Floodwaters
erode the soil quickly. They can wash away trees
and anything else in their path.
Floods also serve a purpose in nature. After
a flood, new soil is deposited on the land. The
nutrients in this soil help plants grow.
Fires
When there is too little rain, fires are likely.
Many are caused by lightning. A fire can quickly
change a forest into a field of charred tree trunks.
Forest animals lose their habitats. Grassland fires
are fueled by dry plants and spread by winds.
Most places recover from natural fires.

Fire Safety
Carelessness also causes fires.
People can prevent wildfires by
being safe around campfires and
cookouts. Do not light fires in dry
areas. Never play with matches.

How do storms change the land? Hurricane Damage
Have you ever heard the saying “When it rains,
it pours”? A light shower might form a few puddles
here and there. A severe storm can change the land.
Tornadoes
A thunderstorm can spin off a violent storm
called a tornado (tor-NAY-doh). Tornadoes are
columns of spinning wind. They move across the
ground in a narrow path. As they move, tornadoes
whip up or destroy everything in their path.
Tornadoes are common in the Great Plains region
of the United States. In fact, a certain path through
that region is known as “Tornado Alley.”
Hurricanes
If you live near an ocean or the Gulf Coast, you
may have experienced a hurricane. A hurricane is
a very large, swirling storm. At its center, or eye, is
an area of very low pressure. Strong winds, walls of
clouds, and pounding rains surround the eye.
A hurricane is much bigger than a tornado. It can
span hundreds of kilometers. It also lasts longer.
Hurricanes form over warm oceans near the
equator. They whip up large waves as they travel.
When a hurricane moves toward a coast, winds and
waves can force water onshore. Massive floods can
occur. Heavy rains add to the flooding. The damage
does not stop there. Once it is over land, a hurricane
can uproot trees and flatten buildings. It can change
an entire ecosystem in one day.
Hurricanes are becoming more and more
common in some places. Scientists are finding that
higher temperatures are a factor.

How do landslides
change the land?
Have you ever seen a pile of
rocks at the bottom of a slope?
How did they get there? Part of
the answer is gravity. Gravity pulls
rocks and other objects from high
places to low places.
Heavy rains can cause loose
rock and soil to move quickly
down a slope. A landslide is the
sudden downhill movement of these
materials in large amounts.

An avalanche (AV•uh•lanch)
is similar to a landslide. In an
avalanche, tons of ice and snow rush
down a mountain.
Scientists work to predict when
and where landslides and avalanches
happen. They never know when one
will strike. It pays to be extra careful
when you are near mountains.