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, volcanic ash, cinders, blocks, and bombs and may
rise as much as 8,000 feet above their bases. Some of the most
conspicuous and beautiful mountains in the world are composite
volcanoes, including Mount Fuji in Japan, Mount Cotopaxi in Ecuador,
Mount Shasta in California, Mount Hood in Oregon, and Mount St. Helens
and Mount Rainier in Washington.
Most composite volcanoes have a crater at the summit which contains a
central vent or a clustered group of vents. Lavas either flow through
breaks in the crater wall or issue from fissures on the flanks of the
cone. Lava, solidified within the fissures, forms dikes that act as ribs
which greatly strengthen the cone.
[Illustration: Schematic representation of the internal structure of a
typical composite volcano.]
The essential feature of a composite volcano is a conduit system through
which magma from a reservoir deep in the Earth’s crust rises to the
surface. The volcano is built up by the accumulation of material erupted
through the conduit and increases in size as lava, cinders, ash, etc.,
are added to its slopes.
When a composite volcano becomes dormant, erosion begins to destroy the
cone. As the cone is stripped away, the hardened magma filling the
conduit (the volcanic plug) and fissures (the dikes) becomes exposed,
and it too is slowly reduced by erosion. Finally, all that remains is
the plug and dike complex projecting above the land surface—a telltale
remnant of the vanished volcano.
[Illustration: Shishaldin Volcano, an imposing composite cone, towers
9,372 feet above sea level in the Aleutian Islands, Alaska.]
An interesting variation of a composite volcano can be seen at Crater
Lake in Oregon. From what geologists can interpret of its past, a high
volcano—called Mount Mazama—probably similar in appearance to
present-day Mount Rainier was once located at this spot. Following a
series of tremendous explosions about 6,800 years ago, the volcano lost
its top. Enormous volumes of volcanic ash and dust were expelled and
swept down the slopes as ash flows and avalanches. These large-volume
explosions rapidly drained the lava beneath the mountain and weakened
the upper part. The top then collapsed to form a large depression, which
later filled with water and is now completely occupied by beautiful
Crater Lake. A last gasp of eruptions produced a small cinder cone,
which rises above the water surface as Wizard Island near the rim of the
lake. Depressions such as Crater Lake, formed by collapse of volcanoes,
are known as _calderas_. They are usually large, steep-walled,
basin-shaped depressions formed by the collapse of a large area over,
and around, a volcanic vent or vents. Calderas range in form and size
from roughly circular depressions 1 to 15 miles in diameter to huge
elongated depressions as much as 60 miles long.
[Illustration: Crater Lake, Oregon; Wizard Island, a cinder cone, rises
above the lake surface.]
The Evolution of a Composite Volcano:
[Illustration: a. Magma, rising upward through a conduit, erupts at the
Earth’s surface to form a volcanic cone. Lava flows spread over the
surrounding area.]
[Illustration: b. As volcanic activity continues, perhaps over spans of
hundreds of years, the cone is built to a great height and lava flows
form an extensive plateau around its base. During this period, streams
enlarge and deepen their valleys.]
[Illustration: c. When volcanic activity ceases, erosion starts to
destroy the cone. After thousands of years, the great cone is stripped
away to expose the hardened “volcanic plug” in the conduit. During this
period of inactivity, streams broaden their valleys and dissect the lava
plateau to form isolated lava-capped mesas.]
[Illustration: d. Continued erosion removes all traces of the cone and
the land is worn down to a surface of low relief. All that remains is a
projecting plug or “volcanic neck,” a small lava-capped mesa, and
vestiges of the once lofty volcano and its surrounding lava plateau.]