Volcanoes form when magma from the Earth's interior is able to make its way
to the surface via a vent like this one behind me on Mount Ngaruahoe. This is
usually associated with plate boundaries. Volcanoes vary greatly in their shape as
does the type of material which is emitted during an eruption.
Magma from the mantle enters weaknesses in the crust caused by plate movement. A
magma chamber may form. From the magma chamber the magma may do one of the
following things: The magma may stay underground in horizontal layers called
sills. It may cut through adjacent rock layers to form dykes. The magma may stay
underground and reservoirs called laccoliths. The magma might enter an existing
vent and blast out the plug. Alternatively it might enter an existing vent and be
unable to dislodge the plug and burst out the side creating a parasite cone.
Before an eruption takes place earthquakes often appear. An existing
volcano can begin to bulge. If a crater lake exists the temperature of its water
increases. When the eruption occurs material can move in many ways. There can
be lava flows. Lava is the hot molten rock or magma that flows down the sides
of volcanoes. Lava flows range from being extremely thick and viscous which we
call an Aa flow, to highly fluid which we call a Pahoehoe flow. I'm standing in
the middle of pyroclastic flow. Pyroclastic flows are clouds of hot ash, gas
and rock that flow down the side of the mountain. This black rock that I'm
standing on is the remains of a pyroclastic flow from the eruption in February 1975.
Now these are very dangerous. They can travel at about a hundred kilometers an
hour. Now if you are caught in this area when there is an eruption, the best thing you
could do is get out of the valleys and onto the ridges.
That is probably your best chance survival. Volcanoes also emit a lot of
airborne material. Ash, is debris less than two millimeters in diameter. Ash clouds
can reach a height of 50 kilometers and can be carried a long way by wind. Ash
can stay in the atmosphere causing global variations and weather patterns.
Ash Falls do not cause any deaths but can lead to breathing difficulties.
Lapilli is debris between 2 and 64 millimeters in diameter. Lava bombs is
debris more than 64 millimeters in diameter. Bombs really land more than 4
to 5 kilometers away.
Scoria is a pyroclastic rock which has been rapidly caught in flight and so is
pockmarked by ear bubbles. The darker the scoria, the richer it is in iron content.
Pumice is a light rhyolitic rock which has a porous nature due to the release
of a lot of gases during an eruption. Gas is emitted from volcanoes include water
vapor. carbon dioxide sulfur dioxide, hydrogen sulfide, helium and carbon
monoxide.
There are also secondary volcanic hazards. A lahar is a mixture of mud and water that
flows down from a volcano like wet concrete. Lahars can be caused by a
crater lake overflowing or collapsing. They can also be caused by heat from an
eruption melting the snow and ice on the mountain. On steep slopes
lahars can reach speeds of up to 22 meters per second. One of New Zealand's
worst natural disasters the Tangiwai disaster was caused by a lahar from Mount
Ruapehu in 1953. Debris building up in Ruapehu's crater lake collapsed causing 340
thousand cubic meters of water to flow down the Whangaehu River River, picking up
volcanic ash and boulders as it went. The lahar wiped out the railway bridge at
Tangiwai. When the train arrived it plunged into the river
killing 151 passengers. Where I am right now is the site of the toilet block that
used to exist here at Tangiwai. Then in about 2007 when a lahar swept down
the Whangaehu River the toilet block was actually dislodged from this place and
taken downstream.
Another secondary effect is a volcanic landslide. Volcanic landslides are not
always associated with eruptions. Heavy rainfall or a big earthquake can also
trigger a landslide on steep slopes.
Volcanoes are susceptible to landslides because they are composed of layers of
loose volcanic rocks. Another secondary effect of volcanoes is called a
tsunami.
An eruption of an offshore island such Mayor Island or White Island could result
in a tsunami affecting the Bay of Plenty. Ash clouds from volcanic eruptions can
generate powerful electrical fields producing intense and frequent lightning
discharges.