Download Volcanoes, earthquakes,landslides, and floods can change Earth`s

eruptionsoccurqui
is a volcanic moun
Volcanoes,
earthquakes,landslides,
and floodscan change
Earth'ssurfacevery quickly.
Volcanoes
Temperatures
are so high at the bottom of Earth'scrust
that rocksmelt. Melted rock belowEarth'ssurfaceis
calledmagma.
Earth's
Moving
Plates
pages
176-177
As magmaheatsand expands,it pushesin all directions.
It movesto Earth'ssurfacewherethe crust is weakest.
weakplacesexistwhereEarth'srocky platesmeet.At t
places,magmamay shootor pour out of the surfacein
eruption. Magmathat reachesthe surfaceis calledlava.
An eruptioncan alsothrow hot boulders,ash,gases,
and
cindersinto the air A volcano is a mountain built up fro
hardenedlava,rocks,and ashthat eruptedout of Earth.
Someeruptionsoccurslowly.The volcanoesof Hawaii
produceslow eruptions.The magmarisesto the surface
forms lakesof lava.During an eruption,the lakes
and lavaflows down the sidesof the volcano.
Kilauea
in
Hawaii
n Srate.In 1980
and gaseshad buil
in. The pressureg
that in an instant
in. The explosionr
trees25 kilometersa
into the skv.
eruptionsoccurquickly asexplosions.Mount St.
is a volcanic mountain in the CascadeRange in
State.In 1980,the mountain exploded.
and gaseshad built up and weretrapped inside the
in. The presquregrew to be enormous. It became
that in an instant it blew away one side of the
in. The explosionwasso powerfrrlthat it knocked
trees25 kilometersaway.It shot steamand ash20
into the skv.
St Helens
Kevword:
HowDoVolcanoes
Form?
wwirv.scilinks.org
Code:CS95070
Earthquakes
An earthquakehappenswhen hugeslabsof rock move
againsteachother deepbelow Earth'ssurface.The slabs
touch at a fault. A fault is a crackin Earth'scrust.
###,
The rock slabsdo not move slowly and steadily along the
fault. Instead, they stick together until the forces pushing
on them becomevery great.Then one of the slabssu
,ft$#,ffi movesa short distance.This jolt produceswavesin the
Mountain
crust like ripples in a pond. Thesewavescanbe felt as
Building
pages
174-175 an earthquake.
In
In someplaces,landmay drop during an earthquake.
canbuild up
other places,land may rise.So earthquakes
or destrovland.
Slabsof rock
alonga fault.
Epicenter
The epicenteris the point
on Earth'ssurfacedirectlv
abovethe focus.
uakesare not ali
others. Some are mor
of an earthquake r
or by the destru
5, an American scier
a way of compa
uakes.His invention
ter scalehas the n
weakestearthquak
largernumbermean
the numberbeforeit.
3.0 on the Richte
uakethat measu
4.0is 10timesst
100timesstronge
Scale
Waves
Wavesmoveout in all
directions
fromthefocus.
Notfelt by peop
Feltby somepeq
Feltby mostpeo
Damage
caused
I
withinaboutlO I
Thefocusis the point
where
belowthe surface
theearthquake
starts.
Creatdamage
to
epicenter
Verydestructive
I
few hundredkikr
Veryrare;greatdr
epicenter
,qD
l-
uakesare not alike. Some releasemore energy
rs. Some are more destructivethan others.The
of an earthquakecanbe measuredby the energy
or by the destructionit produces.
an American scientistnamed CharlesRichter
awayof comparingthe strengthsof
His invention is called the Richter Scale.
ter scalehasthe numbersI through 9. Number
weakestearthquake,and number 9 is the strongest.
numbermeansan earthquake10timesstronger
numberbeforeit. For example,an earthquakethat
3.0 on the RichterScaleis 10timesstrongerthan
2.0.An earthquakethat
uakethat measures
4.0 is 10timesstrongerthan one that measures
2.0.
100timesstrongerthan one that measures
Scale
Notfelt by people
Feltby somepeople;littledamage
Feltby mostpeople;
causes
slightdamage
nearepicenter
Damage
caused
to weakbuildings
andotherstructures
withinaboutl0 km of theepicenter
to structures
up to 100kmfromthe
Creatdamage
epicenter
mayinjureandkillpeoplemorethana
Verydestructive;
fromthe epicenter
few hundredkilometers
Veryrare;greatdamage
to areasup to 1,000kmfromthe
eoicenter
Tsunamis A tsunami(tsooNAH mee)is a giant ocean
wavecausedby an underseaearthquake.When an
earthquakeoccurson the oceanfloot it releases
a lot
of energy.The energytravelsthrough the water and
producesa smallwaveon the surface.
The wave moves outward in all
directions. Far out at sea,the wave may
be lessthan a meter tall. But as it nears
land, it piles up into a huge, tall wave.
When it hits the shore,it may be more
than 20 meterstall.
Landslidesand Floods
Theforceof gravitycancauserapid
changesin the land.For example,
gravity pulls downwardon the rocks
and soil on the slopeof a hill. During
a rainstorm,earthquake,
or volcanic
eruption,the rocksand soil canbe
loosened.Then the forceof gravitypulls
the rocks,soil,and mud down the hill
in a landslide.
People
sometimes
"tidal
calltsunamis
waves."
Butbunamb
havenothing
to do
withtides.
destroy structut
land is built up at the
is quickly worn away
ek
ited somewhere
parts of the world
of steephills.To help
into the hills. Plan
es.The trees'root
also changethe
suddenly. They
land away from
place and deposit it
her place,suchas
mouth of a river.
floods are usually
by long, steady,
rainsor by rapid
g of large
ts of snow.The
cannot absorb the
fastenough.The wa
rivers,which then ou
vy rainstorm can cal
In a flash flood, wal
stream cannot catry a
-revrr
uv
v
:+)
flows over the bank
canyon.
all river floods can be
flood-control meas
dslidesdestroystructuressuchashills and cliffs.But
land is built up at the bottom of the landslide.The
is quickly worn away,or eroded.But just asquickly,it
depositedsomewhereelse.
many parts of the world, farmers plant crops on the
they dig
of steephills.To help preventlandslides,
cesinto the hills. Plantingtreesalsohelpsprevent
The trees'rootshold on to the soil.
dslides.
Itii+,
li 'li l":
Erosion
and
Deposition
pages
172-173
s also changethe
suddenlv.Thev
land away from
placeand depositit
anotherplace,suchas
the mouth of a river.
r floodsareusually
by long, steady,
vy rainsor by rapid
ting of large
untsof snow.The
cannot absorb the
fastenough.The waterruns
rivers,which then overflow.
heavyrainstorm can causea sudden flood called a flash
l. In a flashflood,waterentersa streamvery rapidly.
streamcannot caffy awaythe water fast enough.The
flows over the banksof the streamor fills a steepcanyon.
all river floods can be prevented.But dams,levees,and
r flood-control measurescan help prevent damage.
Ohio River
flood in 1997