Creating Landforms: Volcanoes
Volcanism and volcanic activity are an important part of
recycling the earth’s crust as well as building many of the
landform regions on earth.
Magma:
Magma is located below the earth’s surface – once it
breeches the crust it is then considered lava.
Lava Characteristics:
Low viscosity lava is very hot and contains little amounts of
silica allowing it to flow a long way before it solidifies. This
creates flat plateaus of basalt (Igneous Rock)
Viscose lava (resistant to flow), eg. Andesitic, and Rhyolitic
has a high amount of silica (the principal ingredient in all
lava) thus does not travel far and solidifies more rapidly, thus
creating cone shaped mountains.
Magma also contains dissolved gasses such as: water vapour
and carbon dioxide, when basaltic magma reaches the
surface the gases release easily. More viscous magma tends
to trap large volumes of gas and when it reaches the surface
the gases expand in a huge explosion of gas and debris.
Creating Pressure:
When two plates collide in a subduction zone the great
pressure and force actually melt the rock. This magma is
generally stored in underground magma chambers. If the
pressure is great enough, it flows to the surface through a
magma conduit.
Types of Volcanoes (Images on Page 75)
1) Shield Volcano - 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.
a. Kilauea and Mauna Loa on the island of Hawaii
2) Cone Volcano - Cinder cone volcanoes are the most
common kind of volcanoes. They are steep sided
cones of basaltic fragments and are smaller and simpler
than composite volcanoes. Streaming gases explode
liquid lava blobs into the atmosphere that fall back to
earth around a single vent to form the cone. The
volcano forms when ash, cinders and bombs pile up
around the vent to form a circular or oval cone.
3) Composite Cone - Some of the Earth's grandest
mountains are composite volcanoes -- sometimes called
stratovolcanoes. They are typically steep-sided,
symmetrical cones of large dimension built of
alternating layers of lava flows, and volcanic ash. Most
composite volcanoes have a crater at the summit which
contains a central vent or a clustered group of vents.
Lava flows through breaks in the crater wall or from
fissures on the flanks of the cone. Lava, solidified
within the fissures, act as ribs which greatly strengthen
the cone. When these volcanoes erupt they can often
be cataclysmic events, because these mountains often
have snow covered peaks that melt to create Lahars
(mudflows)
a. Mount St Helens and Mount Fuji
4) Calderas – When a volcano completely empties it’s
magma chamber (beneath the surface) after an
explosion the volcano will collapse resulting in a crater
shaped basin called a caldera.