Volcanoes
Volcanoes form in
two main zones: rift valleys and subduction zones.
Rift Valleys
Rift valleys are created where two
plates are pulling apart. The result of this movement is an area of less dense
crust, and the land eventually sinks under its own weight and lowered density.
This creates cracks in the crust where magma pushes its way through and up to
the surface. It eventually leaks out and over time may form a volcano.
Subduction Zones
The other place volcanoes form is subduction zones. This is the exact opposite of a rift
valley. A subduction zone forms when two plates are
forced into each other. One of the plates dives under the other, since it is
the only place that it can go. The lower of the two melts from the pressure,
and molten magma is squirted out of the boundary between the plates. When
enough pressure builds up, the magma erupts and it becomes a volcano. The Ring
of Fire in
Cinder cone Volcano
Cinder cone volcanoes have very explosive
eruptions; they throw lava and rocks high into the air. Bits
of rock and solidified lava dropped from the air is called tephra. Tephra varies in size
from volcanic ash, to cinders, to larger rocks called bombs and blocks. Tephra falls to the ground and forms a steep-sided, loosely
packed cinder cone volcano. One example of this volcano is Paricutin in
Shield Volcano
Shield volcanoes
have quiet eruptions of basaltic lava that spreads out in flat layers. The
build up of these layers form a broad volcano with gentle slopes. Basaltic lava
also flows onto Earth’s surface through large cracks called fissures. This type
of eruption forms flood basalts, not volcanoes. This accounts for the greatest
volume of erupted volcanic material. Basaltic lava flows over the Earth’s
surface, covering large areas with thick deposits of basaltic igneous rock when
it cools. Examples of shield volcanoes can be found on the
Composite Volcano
Composite
volcanoes vary between violent and quiet, depending on the amount of trapped
gas and how rich in silica content is in the magma. Explosive
periods of a composite volcano result in gas and ash being released, and
forming a tephra layer. It can switch to a
quieter period when lava erupts overtop of the tephra
layer. An example of a composite volcano is Soufriere Hills in Montserrat and Mount Rainier in
