Lecture #13: Intermediate and Felsic Volcanism
(Abbott, pp. 194-199, 201-214, 219-244, 308-311)
Formation of Felsic and Intermediate Magmas
Intermediate and felsic magmas often form in subduction zones.
Felsic, and perhaps
some intermediate, magmas are also associated with hot spots within continental areas.
In continent-ocean convergence boundaries (such as along the west coast of South
America), water from the subducting oceanic plate partially melts overlying mantle
materials to produce mafic magmas (Figure 8.6, p. 189).
As the rising mafic magmas
pass through the overlying continental crust, they partially melt the felsic and
intermediate materials to produce intermediate and felsic magmas (Figure 8.6, p. 189;
Figure 9.6, p. 221; Figure 8.11, p. 194).
Similarly, in continental
(such as at
Yellowstone National Park
), mafic magmas extensively melt continental
materials to form felsic and perhaps some intermediate magmas as they pass through
thick continental crusts.
Besides forming in continent-ocean subduction zones and some continental hot spots,
intermediate magmas may also occur in ocean-ocean convergence zones, such as the
Aleutian Islands of Alaska, the Philippines, and Indonesia.
Water released from the
heated subducting oceanic plate rises into the overlying mantle (Figure 3.15, p. 61).
water partially melts the mantle materials to form mafic magmas. As the mafic magmas
rise into the overlying oceanic crust, they partially melt the crust to form some andesitic
(intermediate) magmas (Figure 8.11, p. 194).
Formation of Felsic and Intermediate Magmas in Subduction Zones and Hot Spots:
Felsic and Intermediate Volcanoes
Some intermediate and mafic volcanoes are
(also called scoria cones;
Figure 8.25, p. 202).
Cinder cones are entirely composed of
and #11) and contain little or no lava flows.
Cinder cones may contain considerable
amounts of ash, blocks and bombs, and not just cinders.
As expected, the larger bombs
and blocks are located closer to the summit of the cinder cone, whereas at least some of
the ash may be blown farther away from the eruption site.
Cinder cones are usually no