Chapter 18
Volcanoes
Name ________________________________________ Hour _______
Cinder Cone Volcanoes
A cinder cone is a steep conical hill formed above a vent. Cinder cones are
among the most common volcanic landform found in the world. They are not as famous
and their eruptions usually don’t cause any loss of life. Cinder cones are chiefly formed
by short, explosive 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. Their eruptions are
generally explosive or pyroclastic. The lava fragments are ejected from a single vent and
accumulate around the vent when they fall back to earth. Their lava is high in silica,
thick, chunky and slow moving. Lava high in silica is called felsic lava and is generally
light in color. Cinder cones are located around subduction boundaries.
Cinder cones grow rapidly and soon approach their maximum size. They rarely
exceed 250 meters in height and 500 meters in diameter.
A great example of a cinder cone is Paricutin in Mexico.
Februrary 20, 1943 in a corn field and grew to 300 feet in 5 days.
It was born on
Composite Cone 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. Their
eruptions can be pyroclastic one time and non-pyroclastic another time. This results in
alternating layers of cinders and lava.
Strato-volcanoes often form impressive, snow-capped peaks, which often exceed
2500 meters in height, 1000 km2 in surface and 400 km3 in volume.
Between eruptions they are often so quiet they seem extinct. To witness the start
of a great eruption requires luck or very careful surveillance. Mt. Rainier in Washington
is a composite volcano that has seems to be extinct.
Composite volcanoes can erupt in an explosive way. This is usually caused by
subduction boundaries creating great amounts of heat and pressure that is applied to the
rocks. This eruption results in viscous, or thick, magma. When very viscous magma rises
to the surface, it usually clogs the crater pipe, and gas in the crater pipe gets locked up.
Therefore, the pressure will increase resulting in an explosive or pyroclastic eruption.
This lava, when it erupts, is high in silica, chunky and thick. Strato-volcanoes are formed
along subduction zones. The high silica lava erupted by composite volcanoes is called
felsic lava.
Shield Cone Volcanoes
Shield volcanoes are huge in size. They are built by many layers of runny lava
flows. This lava is thin, flows easily and is high in iron and magnesium. This lava is
called mafic lava and tends to be darker in color. Lava spills out of a central vent or
group of vents. A broad shaped, gentle sloping cone is formed. This is caused by the
very fluid (thin), 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 eruption of shield volcanoes are characterized by low-explosivity (also
known as non-pyroclastic) lava fountains that form cinder cones at the vent (top).
Famous shield volcanoes (Kilauea, for example) can be found in Hawaii.
Volcano Summary
**Use the information that you have just learned to complete the chart below.
Summary
What does
it look like,
and what is
it made of?
What kind
of
eruptions
does it
have?
Where does
it form?
How and
why does it
erupt?
What is one
real
example?
Cinder
Composite
Shield