3. Composite Volcanoes
Composite volcanoes, also called
strato volcanoes, are formed by
alternating layers of lava and rock
fragments. This is the reason they
are called composite.
Strato-volcanoes often form
impressive, snow-capped peaks
which are often exceeding 2500m in
height, 1000sq.km in surface, and
400km3 in volume.
Although strato-volcanoes are
usually large and conical, we can
distinguish different shapes of them:
concave (like Agua), pyramidal (like
Stromboli), convex-concave (like
Vesuvius), helmet-shaped (like
Mount Rainier), collapse caldera
(like Graciosa), nested (like El Piton
in Teide), multiple summits (like
Shasta), elongated along a fissure
(like Hekla).
4. Shield Volcanoes
Shield volcanoes are huge in size. They
are built by many layers of runny lava
flows. Lava spills out of a central vent or
group of vents. A broad shaped, gently
sloping cone is formed. This is caused
by the very fluid, basaltic lava which
can't be piled up into steep mounds
Shield volcanoes may be produced by
hot spots which lay far away from the
edges of tectonic plates. Shields also
occur along the mid-oceanic ridge,
where sea-floor spreading is in progress
and along subduction related volcanic
arcs.
The eruptions of shield volcanoes are
characterized by low-explosivity lava-
fountaining that forms cinder cones and
spatter cones at the vent. Famous
shield volcanoes can be found for
example in Hawaii (e.g. Mauna Loa and
Kilauea).
5. Cinder Cones
A cinder cone is a steep conical hill formed above
a vent. Cinder cones are among the most
common volcanic landforms found in the
world. They aren't famous as their eruptions
usually don't cause any loss of life. Cinder cones
are chiefly formed by Strombolian eruptions. The
cones usually grow up in groups and they often
occur on the flanks of strato volcanoes and shield
volcanoes.
Cinder cones are built from lava fragments called
cinders. The lava fragments are ejected from a
single vent and accumulate around the vent when
they fall back to earth.
Cinder cones grow rapidly and soon approach
their maximum size. They rarely exceed 250m in
height and 500m in diameter.
The shape of a cinder cone can be modified
during its (short) life. When the position of the
vent alters, aligned, twin or secant cones
develop. Nested,buried or breached cones are
formed when the power of the eruption varies.
A great example of a cinder cone is Paricutín in
Mexico. It was born in February 20, 1943 in a
corn field and grew to 300 feet in 5 days.
6. Spatter Cones
When hot erupting lava contains just
enough explosive gas to prevent the
formation of a lava flow, but not
enough to shatter it into small
fragments the lava is torn by
expanding gases into fluid hot clots,
ranging in size from 1cm to 50cm
across, called spatter.
When the spatter falls back to Earth
the clots weld themselves together
and solidify forming steep-sided
accumulations. These
accumulations focused on an
individual vent are called spatter
cones.
7. Different Types of Volcanic
Eruptions
1. low water, low silica - runny
lava flows (not viscous)
2. low water, high silica (very
viscous) - pasty lava - often building
domes
3. high water, low silica(not
viscous) - fountain of runny lava
4. high water, high silica
(very viscous) - explosion
8. Impacts and Effects of
Volcanoes
The sudden violence of volcanic eruptions causes catastrophe
and devastation. Various notorious eruptions of volcanoes in
the past, such as Mount Pelée, Tambora, Krakatau and
Pinatubo have demonstrated the devastating impact of volcanic
activity on nearby landscapes and communities. Many people
got killed. A big number of people had to abandon their homes
and land forever. Even the whole world's climate was changed
for a while as a result of an eruption!
These effects of volcanic eruptions are mostly the result of
certain hazards. Volcanoes provide different hazards during an
eruption. Each hazard poses different risks affecting different
areas.
9. How Volcanoes Erupt
An eruption begins when pressure on a magma chamber forces magma up through the conduit and out the
volcano's vents. When the magma chamber has been completely filled, the type of eruption partly depends
on the amount of gases and silica in the magma. The amount of silica determines how sticky (level of
viscosity) the magma is and water provides the explosive potential of steam.
Obstacles also influence the type of eruption. When the pipe is blocked by a stopple or an accumulation of
pumice, the pressure in the pipe will build up very high resulting in an explosion.
When magma reaches earth's surface it is called lava. It may pour out in gentle streams called lava flows or
erupt violently into the air. Rocks ripped loose from the inside of the volcano or torn apart by the gas may be
shot into the air with the lava. These rocks blown out of a volcano are called pyroclastic rocks. The rock
fragments fall back to earth in many different shapes and sizes:
– Dust - particles less than 1/100 inch in diameter
Dust particles may be carried great distances. In a powerful eruption they may be carried around the
earth several times.
– Ash - fragments less than 1/5 inch in diameter
Most volcanic ash falls to the surface and cemented together by water to form a rock called volcanic tuff.
– Bomb - A rounded piece of newly hardened lava which takes shape while flying through the air.
– Block - A piece of lava that has sharp corners.
– Cinder - Bubbly rock formed by liquid lava cooling in the air.
– Pumice - Cinder so bubbly that it floats in water.
Volcanic activity is classified by how often a volcano erupts. A volcano may be active, intermittent, dormant,
or extinct. Active volcanoes erupt constantly. Intermittent volcanoes erupt fairly regularly. Dormant
volcanoes are inactive, but not long enough to determine whether they will erupt again or not. Extinct
volcanoes have been inactive since the beginning of recorded history.
