Geology Reference
In-Depth Information
Once again, geologists invoke the phenomenon of partial
melting to explain the origin and composition of magma at sub-
duction zones. As a subducted plate descends toward the asthe-
nosphere, it eventually reaches the depth where the temperature
is high enough to initiate partial melting. In addition, the oce-
anic crust descends to a depth at which dewatering of hydrous
minerals takes place, and as the water rises into the overlying
mantle, it enhances melting and magma forms (Figure 4.5).
Recall that partial melting of ultramafi c rock at spread-
ing ridges yields mafi c magma. Similarly, partial melting of
mafi c rocks of the oceanic crust yields intermediate (53-65%
silica) and felsic (>65% silica) magmas, both of which are
richer in silica than the source rock. Moreover, some of the
silica-rich sediments and sedimentary rocks of continental
margins are probably carried downward with the subducted
plate and contribute their silica to the magma. Also, mafi c
magma rising through the lower continental crust must be
contaminated with silica-rich materials, which changes its
composition.
Many geologists now think that hot-spot volcanism
results from a rising
mantle plume
, a cylindrical plume
of hot mantle rock that rises from perhaps near the core-
mantle boundary. As it rises toward the surface, the pressure
decreases on the hot rock and melting begins, thus yielding
magma. Hot-spot volcanism may also account for vast fl at-
lying areas of overlapping lava fl ows or what geologists call
fl ood basalts
(Figure 4.6b). Figure 2.15 shows the locations of
many hot spots, but of particular interest to us is the Yellow-
stone hot-spot in Wyoming. We will have more to say about
the Yellowstone region in Chapter 23.
Not all geologists agree with the mantle plume theory.
They cite evidence based on studies of earthquake waves that
do not seem to be consistent with the theory. Nevertheless,
mantle plumes and hot-spot volcanism are the most widely
accepted explanation for chains of volcanoes in the oceans,
as well as linear associations of volcanic rocks on land.
Changes in Magma
Once magma forms, its composition may change by
crystal settling
, which involves the physical separation
of minerals by crystallization and gravitational settling
(
Most volcanism occurs at divergent and convergent plate
boundaries; however, there are some chains of volcanic out-
pourings in the ocean basins and on continents that are not
near either of these boundaries. The Emperor Seamount-
Hawaiian Islands, for instance, form a chain of volcanic
islands 6000 km long, and the volcanic rocks become pro-
gressively older toward the northwest (see Figure 2.22). In
1963, Canadian geologist J. Tuzo Wilson proposed that the
Hawaiian Islands and other areas showing similar trends lay
above a
hot spot
over which a plate moves, thereby yielding
a succession of volcanoes (
Figure 4.7). Olivine, the fi rst ferromagnesian silicate to
form in the discontinuous branch of Bowen's reaction se-
ries, has a density greater than the remaining magma and
tends to sink. Accordingly, the remaining magma becomes
richer in silica, sodium, and potassium because much of the
iron and magnesium were removed as olivine and perhaps
pyroxene minerals crystallized.
Although crystal settling does take place, it does not do
so on a scale that would yield very much felsic magma from
◗
◗
Figure 4.6a).
◗
Figure 4.6
Mantle Plume and Hot Spot
Direction of Plate Motion
Rhyolite flows
Flood basalts
Continental crust
Oceanic crust
Granitic plutons
Basalt dikes
Basaltic
magma
Basaltic magma
Plate motion
Mantle
Mantle
Plume
Asthenosphere
Plume
Asthenosphere
A mantle plume with an overlying hot spot yields fl ood basalts
and some of the continental crust melts to form felsic magma.
b
A mantle plume beneath oceanic crust with a hot spot. Rising
magma forms a series of volcanoes that become younger in the
direction of plate movement.
a
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