Volcanos occur all over the world.The best know volcano zones occur on the destructive plate
margin around the Pacific Ring of Fire.This chain of volcanos that lie along the west coast of central and
south America,Japan and the Philippines,across New Zealand and into the Atlantic.Volcanos also occur in
mid-ocean along the line of ocean ridges.The Mid-Atlantic Ridge is an example of ocean floor
volcanos.When these volcanos erupt lava flows from the cracks building up the ocean floor.Volcanos can
also occur away from plate boundaries.These are the Hawaiian Volcanos.As you can see there are lots of
places were volcanos occur.Sometimes these volcanos are where humans have settled.To help protect our
belongings and save human lives we have developed ways to predict when volcanos are about to erupt.Here
are some.

Two main methods are used to predict when volcanos are about to erupt.These methods are called
geophysical techniques,and chemical techniques.

The geophysical method can detect any changes in the rock below.As the magma rises it forces the rocks
apart,Which can create minor earthquakes on and or around the mountain.These can be detected on a
seismographs.A seismograph picks up the seismic waves the earthquake gives off and records them.A
seismograph works by a pen and a weight suspended on a spring.As the earth shakes the pen writes the
waves,called a seismogram,on a revolving drum.A comparison can be used.They take two or more
seismographs readings to pin point exactly where the magma is trying to surface.This technique is
successful in Hawaii where they are a lot of volcanos.The place and the time of eruptions have been
forecast accurately.Before a volcano is about to explode the ground around it swells with magma.This
causes the ground to start to tilt.The tilt can be measured by an instrument called a tiltmeter.Two containers
are half filled with water.And a tube connects the two.As magma rises and changes the a!
ngle of the earth.Water goes into the lower container.Scientist can tell if the volcano is getting more
active.When magma moves its gravity changes.This change in gravity can be detected by using
gravimeters.When rock is heated to a certain temperature.It looses its magnetism.And this can be detected
with a magnetometer.When magma rises to the surface it gives off heat.This heat can be detected with a
intfa-red temperature surveys.Sometimes chemical methods are used.They measure the volcanic gases at
hit springs and at fumaroles which are openings in the ground were gases escape from.Just before a volcano
is about to erupt there is an increase in certain gases.These gases are chlorine compounds,sulphor
oxides,carbon dioxide,hydrogen, and radon.
When a volcano erupts huge amounts of lava are spewed out of the volcano.These huge flows of
lava can wipe out towns.This type of volcanic eruption is called Hawaiian.These volcanos put on fire
shows .They are rarely violent or explosive ,because the lava is fluid enough to let the gases escape
easily.The lava flows out of boccas.These volcanos are not really dangerous.But they can cause alot of
damage if people or buildings get in its way.A strombolian eruptions are more violent producing a lot
thicker lava.Because the lava is thicker the gas can not escape easily from it making the lava hostile.The
plinain eruptions ar the most dangerous .They hurl huge amounts of pumice as high as fifty kilometers into
the atmosphere.Then is ash falls over wide areas.Then this is followed by pyoclastic flows,where hot gases
and ash pour out of the vent.These flows rush across the landscape ,burning and burying anything in its
path.This is the eruption that happened to Mount St. Helen in th!
e U.S.A. Wisps of steam were coming out of the vents on the side of the mountain.During a period of
several weeks a three hundred foot high wedge shaped bulge developed on the south side of the mountain
as a result of rock and ice being pushed up.There were gaping cracks in the glaciers.They were gray with a
dusting in volcanic ash.Weeks later the bulge blew sending ash and mud slides down the mountain wiping
any thing that got in its way.The eruption cause a lot of damage.It created the worlds biggest clear cut.Two
hundred and thirty square miles of trees lay in a tangled mess after the eruption.But people were trying to
save as much of the lumber as they could.They were hauling out