IGNEOUS ROCKS AND PLATE BOUNDARIES
Convergent plate boundary
(ocean-ocean).
Andesite & diorite is
characteristic (basalt &
gabbro are also found;
rhyolite is rare)
(e.g. the Caribbean)
Divergent plate boundary.
Basalt & gabbro are
characteristic.
(e.g. the Mid-Atlantic Ridge)
Convergent plate boundary
(ocean-continent).
Andesite & diorite are
characteristic. Rhyolite &
granite are common. Basalt
is less common (e.g. the Andes)
= locations where
magma is generated
Oceanic crust
Sea Level
lithosphere
plate
F
A
Oceanic (arc) crust
C
B
H 2O
Mantle
asthenosphere
(peridotite)
Box 2
Dehydration of water from the subducting plate
occurs as the plate descends into the hot
asthenosphere (see Box 1).
The water released from the subducting plate
causes hydration of the asthenosphere above
the subducting plate. This hydration promotes
partial melting of the overlying peridotite
mantle. 'Wet' partial melting (in the presence of
water) of the overlying peridotite mantle in the
asthenosphere will produce basalt (and
sometimes andesite) magma at B.
Rising basalt magma from the asthenosphere
can also induce partial melting of the basaltic
oceanic arc crust of the overiding plate. This is
because the basalt will tend to stall at the bottom of
the crust and solidify. As it does it releases its heat
into the surrounding arc crust. Over time, enough
heat may be pumped into the arc crust by the
rising basalt that it will initiate partial melting of the
arc crust at C. This will produce andesite magma
(and very rarely also rhyolite magma).
E
asthenosphere mantle
rising as plates spread apart
Box 1
Continental crust
D
Mantle
lithosphere
Partial melting of the
peridotite mantle in the
asthenosphere forms
basalt magma by
decompression melting
at A as asthenosphere
flows into the space created
by the diverging plates. As
the hot basalt and gabbro
solidify they release heat,
which heats the overlying
ocean water, which in turn
circulates through the crust
at the ridge. This hot, circulating water (hydrothermal
activity) will add water to
the minerals found in the
rocks (it "hydrates" them).
For example, when olivine
is hydrated due to this
hydrothermal circulation at
the ridge it reacts to form
a new mineral called serpentine. A rock made up
mostly of serpentine is called
a serpentinite, and serpentinite is the state rock of
California!
lithosphere
plate
H 2O
Box 3
Magma generation processes that occur in
ocean-ocean subduction zones also occur in
ocean-continental margin subduction zones
(see Box 2 description). Basalt and rarely
andesite magma are produced by 'wet' partial
melting at D.
And, as in ocean-ocean subduction zones
heat from the hot rising basalts can
induce partial melting of the overlying
continental crust at E, which melts at
relatively low temperature to produce rhyolite
magma. Some of this rhyolite magma
may blend with later rising basalt to produce
andesite magma at E (magma mixing),
or even higher up in a magma chamber in
the shallow crust at F.