Page 43 - 4571

Shield volcanoes are produced by the accumulation of fluid basaltic lavas and
exhibit the shape of a broad, slightly domed structure that resembles a warrior’s shield.
Most shield volcanoes begin on the ocean floor as seamounts, a few of which grow
large enough to form volcanic islands. In fact, with the exception of the volcanic islands
that form above subduction zones, most other oceanic islands are either single shield
volcanoes, or more often the coalescence of two or more shields built upon massive
amounts of pillow lavas. Examples include the Canary Islands, the Hawaiian Islands,
the Galapagos, and Easter Island.

2. Cinder Cones
As the name suggests, cinder cones (also called scoria cones) are built from
ejected lava fragments that take on the appearance of cinders or clinkers as they begin to
harden in flight. These pyroclastic fragments range in size from fine ash to bombs that
may exceed a meter in diameter. However, most of the volume of a cinder cone consists
of pea- to walnut-sized lapilli that are markedly vesicular and have a black to reddish-
brown color (these vesicular rock fragments are called scoria.) Although cinder cones
are composed mostly of loose pyroclastic material, they sometimes extrude lava. On
such occasions the discharges most often come from vents located at or near the base
rather than from the summit crater. Cinder cones have very simple, distinctive shapes
determined by the slope that loose pyroclastic material maintains as it comes to rest.
Because cinders have a high angle of repose (the steepest angle at which material
remains stable), cinder cones are steep-sided, having slopes between 30 and 40 degrees.
In addition, cinder cones have large, deep craters in relation to the overall size of the
structure. Although relatively symmetrical, many cinder cones are elongated and higher
on the side that was downwind during the eruptions.
3. Composite Cones
Earth’s most picturesque yet potentially dangerous volcanoes are composite
cones or stratovolcanoes. Most are located in a narrow zone that rims the Pacific Ocean,
called the Ring of Fire. This active zone consists of a chain of continental volcanoes that
are distributed along the west coast of the Americas, including the large cones of the
Andes in South America and the Cascade Range of the western United States and
Canada.
The classic composite cone is a large, nearly symmetrical structure consisting of
alternating layers of explosively erupted cinders and ash interbedded with lava flows. A
few composite cones, e.g. Italy’s Etna, display very persistent eruption activity, and
molten lava has been observed in their summit craters for decades. Mount Etna has
erupted, on average, once every 2 years since 1979. Just as shield volcanoes owe their
shape to fluid basaltic lavas, composite cones reflect the viscous nature of the material
from which they are made. In general, composite cones are the product of gas-rich
magma having an andesitic composition. However, many composite cones also emit
various amounts of fluid basaltic lava and occasionally pyroclastic material having
rhyolitic composition. Relative to shields, the silica-rich magmas typical of composite
cones generate thick viscous lavas that travel less than a few kilometers. In addition,
composite cones are noted for generating explosive eruptions that eject huge quantities
of pyroclastic material. A conical shape, with a steep summit area and more gradually
sloping flanks, is typical of many large composite cones.
During the early stages of growth, lavas tend to be more abundant and flow
greater distances from the vent than lavas do later in the volcano’s history. This
contributes to the cone’s broad base. As the volcano matures, the shorter flows that
come from the central vent serve to armor and strengthen the summit area.
Consequently, steep slopes exceeding 40 degrees are sometimes possible. Two of the

42