830 Encyclopedia of the Solar System
FIGURE 1 A Hawaiian-style lava
fountain feeding a lava flow and
building a cinder cone (Pu’u ‘O’o
on the flank of Kilauea volcano in
Hawai’i). Steaming ground marks
the location of the axis of the rift
zone along which a dike
propagated laterally to feed the
vent. (Photograph by P. J.
Mouginis-Mark.)locations of “hot spots” in the underlying mantle, vigorously
rising plumes of mantle material from which magmas mi-
grate through the overlying plate. Because the plate moves
over the hot spot, a chain of shield volcanoes can be built
up in this way, marking the trace of the relative motion. The
largest shield volcanoes on Earth form such a line of vol-
canoes, the Hawaiian Islands, and the two largest of these
edifices, Mauna Loa and Mauna Kea, rise∼10 km above
the ocean floor and have basal diameters of about 200 km.
Eruptive activity on shield volcanoes tends to be concen-
trated either at the summit or along linear or arcuate zones
radiating away from the summit, called rift zones. The low
viscosity of the basaltic magmas released in Hawaiian-style
eruptions on these volcanoes (Fig. 1) allows the lava flows
produced to travel relatively great distances (a few tens of
kilometers), and is what gives shield volcanoes their charac-
teristic wide, low profiles. It is very common for a long-lived
reservoir of magma, a magma chamber, to exist at a depth
of a few to several kilometers below the summit. This reser-
voir, which is roughly equant in shape and may be up to
1 to 3 km in diameter, intermittently feeds surface erup-
tions, either when magma ascends vertically from it in the
volcano summit region or when magma flows laterally in
a subsurface fracture called a dike, which most commonly
follows an established rift zone, to erupt at some distance
from the summit. In many cases, magma fails to reach the
surface and instead freezes within the fracture it was fol-
lowing, thus forming anintrusion. The summit reservoir
is fed, probably episodically, from partial melt zones in the
mantle beneath. Rare but important events in which a large
volume of magma leaves such a reservoir lead to the collapse
of the rocks overlying it, and a characteristically steep-sided
crater called a caldera is formed, with a width similar to that
of the underlying reservoir.
Volcanoes erupting silica- and volatile-rich magma (an-
desite or, less commonly, rhyolite) mark the destructive
margins of plates, where the plates bend downward to be
subducted into the interior and at least partly remelted.
These volcanoes tend to form an arcuate pattern (called an
island arc when the volcanoes rise from the sea floor), mark-
ing the trace on the surface of the zone where the melting
is taking place, at depths on the order of 100–150 km. The
andesitic magmas thus produced represent the products of
the melting of a mixture of subducted ocean floor basalt,
sedimentary material that had been washed onto the ocean
floor from the continents (which are themselves an older,
silica-rich product of the chemical differentiation of Earth),
seawater trapped in the sediments, and the primary man-
tle materials into which the plates are subducted. Thus,
andesites are much less representative of the current com-
position of the mantle. Andesite magmas are rich in volatiles
(mainly water, carbon dioxide, and sulfur compounds), and
their high silica contents give them high viscosities, mak-
ing it hard for gas bubbles to escape. As a result, andesitic
volcanoes often erupt explosively in Vulcanian-style erup-
tions, producing localizedpyroclasticdeposits with a range
of grain sizes; alternatively, they produce relatively viscous
lava flows that travel only short distances (a few kilometers)
from the vent. The combination of short flows and localized
ash deposits tends to produce steep-sided, roughly conical
volcanic edifices.
When large bodies of very silica- and volatile-rich magma
(rhyolite) accumulate—in subduction zones or, in some
cases, where hot spots exist under continental areas, leading