Volcanoes, earthquakes,landslides, and floods can changeEarth's surface very quickly.

VolcanoesTemperatures are so high at the bottom of Earth's crustthat rocks melt. Melted rock below Earth's surface iscalled magma.

As magma heats and expands, it pushes in all directions.It moves to Earth's surface where the crust is weakest.weak places exist where Earth's rocky plates meet. At tplaces, magma may shoot or pour out of the surface ineruption. Magma that reaches the surface is called lava.

An eruption can also throw hot boulders, ash, gases, andcinders into the air A volcano is a mountain built up frohardened lava, rocks, and ash that erupted out of Earth.

Some eruptions occur slowly. The volcanoes of Hawaiiproduce slow eruptions. The magma rises to the surfaceforms lakes of lava. During an eruption, the lakesand lava flows down the sides of the volcano.

eruptions occur quiis a volcanic moun

n Srate. In 1980

and gases had builin. The pressure gthat in an instantin. The explosion r

trees 25 kilometers ainto the skv.

Earth's MovingPlatespages 176-177

Kilauea inHawaii

eruptions occur quickly as explosions. Mount St.is a volcanic mountain in the Cascade Range in

State.In 1980, the mountain exploded.

and gases had built up and were trapped inside thein. The presqure grew to be enormous. It becamethat in an instant it blew away one side of thein. The explosion was so powerfrrl that it knocked

trees 25 kilometers away. It shot steam and ash 20into the skv.

St Helens

Kevword: How Do Volcanoes Form?wwirv.scilinks.orgCode: CS95070

EarthquakesAn earthquake happens when huge slabs of rock moveagainst each other deep below Earth's surface. The slabstouch at a fault. A fault is a crack in Earth's crust.

The rock slabs do not move slowly and steadily along thefault. Instead, they stick together until the forces pushing

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on them become very great. Then one of the slabs sumoves a short distance. This jolt produces waves in the

The focus is the pointbelow the surface wherethe earthquake starts.

uakes are not alilothers. Some are mor

of an earthquake ror by the destrur

5, an American sciera way of compa

uakes. His invention

ter scale has the nweakest earthquakr

larger number meansthe number before it.

3.0 on the Richteuake that measur4.0 is 10 times st

100 times stronger

Scale

Not felt by peopl

Felt by some peq

Felt by most peof

Damage caused Iwithin about lO I

Creat damage toepicenter

Very destructive; Ifew hundred kikr

Very rare; great drepicenter

MountainBuildingpages 174-175

crust like ripples in a pond. These waves can be felt asan earthquake.

In some places,land may drop during an earthquake. Inother places, land may rise. So earthquakes can build upor destrov land.

Slabs of rockalong a fault.Epicenter

The epicenter is the pointon Earth's surface directlvabove the focus.

WavesWaves move out in alldirections from the focus.

,qD l-

uakes are not alike. Some release more energyrs. Some are more destructive than others. The

of an earthquake can be measured by the energyor by the destruction it produces.

an American scientist named Charles Richteraway of comparing the strengths of

His invention is called the Richter Scale.

ter scale has the numbers I through 9. Numberweakest earthquake, and number 9 is the strongest.

number means an earthquake 10 times strongernumber before it. For example, an earthquake that3.0 on the Richter Scale is 10 times stronger thanuake that measures 2.0. An earthquake that4.0 is 10 times stronger than one that measures

100 times stronger than one that measures 2.0.

Scale

Not felt by people

Felt by some people; little damage

Felt by most people; causes slight damage near epicenter

Damage caused to weak buildings and other structureswithin about l0 km of the epicenter

Creat damage to structures up to 100 km from theepicenter

Very destructive; may injure and kill people more than afew hundred kilometers from the epicenter

Very rare; great damage to areas up to 1,000 km from theeoicenter

Tsunamis A tsunami (tsoo NAH mee) is a giant oceanwave caused by an undersea earthquake. When anearthquake occurs on the ocean floot it releases a lotof energy. The energy travels through the water andproduces a small wave on the surface.

People sometimescall tsunamis "tidalwaves." But bunambhave nothing to dowith tides.

destroy structutland is built up at theis quickly worn away,

ited somewhere ek

parts of the worldof steep hills. To help

into the hills. Planes. The trees'roots

also change thesuddenly. They

land away from

The wave moves outward in alldirections. Far out at sea, the wave maybe less than a meter tall. But as it nearsland, it piles up into a huge, tall wave.When it hits the shore, it may be morethan 20 meters tall.

Landslides and Floods

The force of gravity can cause rapidchanges in the land. For example,gravity pulls downward on the rocksand soil on the slope of a hill. Duringa rainstorm, earthquake, or volcaniceruption, the rocks and soil can beloosened. Then the force of gravity pullsthe rocks, soil, and mud down the hillin a landslide.

place and deposit ither place, such as

mouth of a river.

floods are usuallyby long, steady,

rains or by rapidg of largets of snow. The

cannot absorb thefast enough. The wa

rivers, which then ou

vy rainstorm can calIn a flash flood, wal- r e v r r u v v : + )

stream cannot catry aflows over the bankcanyon.

all river floods can beflood-control meast

dslides destroy structures such as hills and cliffs. Butland is built up at the bottom of the landslide. Theis quickly worn away, or eroded. But just as quickly, it

deposited somewhere else.

many parts of the world, farmers plant crops on theof steep hills. To help prevent landslides, they digces into the hills. Planting trees also helps prevent

dslides. The trees'roots hold on to the soil.

s also change thesuddenlv. Thev

land away fromplace and deposit it

another place, such asthe mouth of a river.

r floods are usuallyby long, steady,

vy rains or by rapidting of largeunts of snow. Thecannot absorb the

fast enough. The water runsrivers, which then overflow.

heavy rainstorm can cause a sudden flood called a flash

l. In a flash flood, water enters a stream very rapidly.stream cannot caffy away the water fast enough. The

flows over the banks of the stream or fills a steep-canyon.

all river floods can be prevented. But dams,levees, and

r flood-control measures can help prevent damage.

Itii+,l i ' l i l " :

Erosion andDepositionpages 172-173

Ohio Riverflood in 1997