Geoscience Reference
In-Depth Information
volcanoes are inactive for some time and erupt only when the
pressure of material rising from the mantle becomes excessive.
The length of inactivity varies dramatically between volcanoes.
Some volcanoes lie dormant for hundreds or thousands of years
before they erupt, whereas others are in a near-constant state of
eruption.
Although no hard and fast classification scheme exists for
volcanoes, most geologists distinguish three basic kinds: cin-
der cones, composite volcanoes, and shield volcanoes. This
classification depends in large part on whether the eruption is
explosive or fluid. The first part of the discussion will focus on
the volcanoes associated with explosive eruptions.
Explosive Volcanoes
As the name implies, an explosive volcano is one that erupts very
quickly and with great force. Of the two kinds of explosive vol-
canoes that occur on Earth, the easiest kind to understand is a
cinder-cone volcano
, which usually forms very quickly after a
single eruption. These volcanoes are small relative to other types
of volcanoes, have steep sides (~30°), and consist of solidified
magma fragments, rock debris, and ash that are ejected from a
central vent. A great example of a cinder-cone volcano can be
seen in northeastern New Mexico at Mount Capulin (Figure
13.34). This volcano has erupted only once, about 62,000 years
ago, and is approximately 300 m (1000 ft) high.
In contrast to cinder-cone volcanoes,
composite volca-
noes
are volcanoes that build up and grow over the course of
several eruptions. Composite volcanoes are typically inactive
for long periods of time between eruptions, but when they do
erupt, they tend to do so quite violently. Such an eruption occurs
because the magma within composite volcanoes is rich in sili-
cas, which are minerals containing silicon (Si), and therefore
highly viscous (meaning sticky and slow flowing). As a result,
gases are gradually trapped in the magma during the inactive
phase and build up pressure within the volcano until it explodes.
Such an eruption sends volcanic ash high into the atmosphere,
and thick layers of volcanic debris will accumulate on the
slopes of the volcano, causing it to enlarge. This debris may
consist of alternating layers of lava, which is magma flowing
on the surface, and fragmented rock debris called
pyroclastic
material
(or
tephra
), such as volcanic ash, cinders, and boul-
ders (Figure 13.35a). Also called
stratovolcanoes
because they
contain strata (or layers) of volcanic debris, these volcanoes
typically have moderately steep cones with a semi-horizontal
Figure 13.34 Cinder-cone volcanoes.
Cinder-cone volcanoes
are relatively small volcanoes that build up by the accumulation of
solidified magma fragments, rock, and ash over a short period of
time. Mount Capulin is a typical cinder-cone volcano and is about
305 m (1000 ft) high.
top containing the crater. Composite volcanoes are much larger
than cinder cones, perhaps over 3000 m (10,000 ft) high. A
beautiful example of a composite volcano is the famous Mount
Fuji in Japan (Figure 13.35b).
Sometimes the eruption of a composite volcano is so explo-
sive that it literally blows the top off of a mountain, creating a
large crater. After this massive kind of eruption occurs, the crater
may partially fill with a
lava dome
, which forms slowly in the
ensuing decades. Such a volcanic feature consists of a steep-sided
mound built by highly viscous magma that gradually oozes from
the central vent. Although these features sometimes occur alone
and are thus classified as a particular type of volcano by many
geologists, the vast majority of the recently active lava domes
occur in association with composite volcanoes.
Volcanic Arcs at Plate Boundaries
Composite volcanoes
are most commonly associated with plate boundaries in places
where subduction is occurring. This geographical relationship
occurs because the subducting plate melts when it encounters the
upper mantle and the molten material gradually moves upward
through the crust. Given that subduction tends to occur along the
length of certain plate boundaries, these zones are places where a
chain of volcanoes is typically located at the surface. Such a chain
is called a
volcanic arc
and is genetically similar to the island arc
Cinder-cone volcano
A small, steep-sided volcano that con-
sists of solidified magma fragments and rock debris that may
form in only one eruption.
Pyroclastic material
Fragmented rock materials resulting
from a volcanic explosion or ejection from a volcanic vent.
Lava dome
A steep-sided volcanic landform consisting of
highly viscous lava that does not flow far from its point of ori-
gin before it solidifies.
Composite volcano
A large, steep-sided volcano that grows
through progressive volcanic eruptions, which are usually
explosive, and consists of layers of volcanic debris.
Volcanic arc
A chain of volcanoes created by rising magma
derived from a subducting tectonic plate.
