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Layers of basalt New lava

Sea level Sea level

Feeders
A

FIGURE 19.20 This is the top of Mount St. Helens several
years after the 1980 explosive eruption.

that forms this type of volcano has a low viscosity, spreading B
widely from a vent. The islands of Hawaii are essentially a series
FIGURE 19.21 (A) A schematic cross section of an idealized
of shield volcanoes built upward from the ocean floor. These
shield volcano. (B) A photo of a shield volcano, Mauna Loa in Hawaii.
enormous Hawaiian volcanoes range up to elevations of 8.9 km
(5.5 mi) above the ocean floor, typically with slopes that range
from 2° to 20° (Figure 19.21).
The Hawaiian volcanoes form from a magma about 60 km
(about 40 mi) deep, probably formed by a mantle plume. The
magma rises buoyantly to the volcanic cones, where it erupts
by pouring from vents. Hawaiian eruptions are rarely explosive.
Shield volcanoes also occur on the oceanic ridge (Iceland) and
occasionally on the continents (Columbia Plateau).
A cinder cone volcano is constructed of, as the name states,
cinders. The cinders are rock fragments, usually with sharp edges
since they formed from frothy blobs of lava that cooled as they
were thrown into the air. The cinder cone volcano is a pile or piles
of red or black cinders that have been ejected from a vent. Such
cones can have steep sides, with slope angles of 35° to 40°. This
FIGURE 19.22 Sunset Crater, a cinder cone volcano near
is the maximum steepness at which the sharp-edged, unconsoli-
Flagstaff, Arizona.
dated cinders will stay without falling. The cinder cone volcanoes
are much steeper than the gentle slopes of the shield volcanoes.
They also tend to be much smaller, with cones that are usually no addition to the alternating layers of solid basaltic rock and depos-
more than about 500 m (1,600 ft) high (Figure 19.22). its of cinders and ash, a third type of layer is formed by volcanic
A composite volcano (also called stratovolcano) is built up of mudflow. The volcanic mud can be hot or cold, and it rolls down
alternating layers of cinders, ash, and lava flows (Figure 19.23), the volcano cone like wet concrete, depositing layers of volcanic
forming what many people believe is the most imposing and conglomerate. Volcanic mudflows often do as much or more
majestic of Earth’s mountains. The steepness of the sides, as you damage to the countryside than the other volcanic hazards.
might expect, is somewhere between the steepness of the low Shield, cinder cone, and composite volcanoes form when
shield volcanoes and the steep cinder cone volcanoes. The Cas- magma breaks out at Earth’s surface. Only a small fraction of all
cade volcanoes are composite volcanoes, but the mixture of lava the magma generated actually reaches Earth’s surface. Most of
flows and cinders seems to vary from one volcano to the next. In it remains below the ground, cooling and solidifying to form

19-15 CHAPTER 19 Building Earth’s Surface 491