Plate Tectonics Questions Page 4
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4Section 7 Volcanic Landforms
A composite cone, as
shown in Figure 4, forms by
many eruptions of material
with medium- or high-
silica content. They erupt
violently when pressure
builds up in the magma.
After the explosion, gooey
(viscous) lava oozes out
of the top. The volcano
becomes quiet. Over time,
pressure may build up and
Figure 3 Silica-rich magma does not flow readily
repeat the cycle. Composite
and often forms a volcanic dome, such as the
volcanoes are tall and have
one shown in this photograph.
steep slopes because the
lava does not flow easily.
When a very large volume
of magma is erupted,
the overlying rocks may
collapse. The collapse
produces a hole or
depression at the surface
called a caldera, shown in
Checking Up
Figure 5. A caldera is much
larger than the original
1. Explain in your own
words the meaning
Figure 4 Composite cones include the beautiful,
vent from which the
yet potentially deadly Cascades in the northwestern
of a contour
magma erupted.
United States (which includes Mt. Shasta, Mt.
line, contour
Rainier, and Mt. St. Helens).
interval, relief, and
topographic map.
2. Arrange corn
syrup, water, and
vegetable oil in
order of low to
high viscosity.
3. What is the silica
Figure 5 Calderas are
content of magma
deceptive volcanic
that has a low
structures. They are
viscosity?
large depressions
4. Why do silica-poor
rather than conical
magmas produce
peaks. Oregon’s
broad volcanoes
Crater Lake, formed
with gentle slopes?
nearly 7000 years ago,
is an example of this
5. Why does high-silica
type of volcano.
magma tend to
form volcanic domes
with steep sides?
6. How is a caldera
formed?
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