Inner Earth processes and systems - Physical Processes in Earth and Environmental Sciences - page 209

dissipation of heat upward along a fault zone may decrease
the efficiency and occurrence of the mechanism.
A final mechanism is thought to be responsible for
widespread deep melting and continental crustal fusion in
mountain belts caused by massive overthrusting of one
crustal terrane upon another on deep thrust faults. This
process acts quickly, at horizontal velocities appropriate to
colliding plates (order of 10 1 ma 1 ), and places crustal
rocks rich in radioactive elements under other crustal rocks
whose ambient temperature is that of their truncated sub-
surface geotherms. As always, any crustal melting that
might result will be aided by the presence of water in the
system and also by the rapidity of the faulting movements
in relation to the thermal diffusivities of the rocks
involved. The process is thought to have caused the
fusion of continental crust under mountain belts like the
Himalayas and the production of viscous acidic magmas
that slowly crystallize to granitic rocks rich in potassium-
bearing radioactive minerals like the mica and muscovite.
The approximate annual amounts of melt produced and
attributed to the first two mechanisms above are indicated
in Box 5.2. Fluxes from third and fourth are unknown
since the melt remains subsurface.

TRENCH VOLCANIC ARC

Sea level

OVERIDING PLATE

Magma
ascent

MANTLE

WEDGE

Partial melting
from serpentinite
dehydration

DIPPING

SLAB

Fig. 5.11 Volcanic arc magmatism results from the fluxing effects of
water released into the overiding plate as serpentinite dehydrates in a
descending lithospheric slab.

5.1.5 Melt material properties

earthquakes (called “seismic pumping”) and mixes with
the plastic mantle olivine of the continental lithosphere of
the overriding plate. This causes the melting point of the
mantle to fall and its mechanical strength to drop drasti-
cally. The resulting partial melting and melt migration
eventually leads to generation of water-rich intermediate
magmas characteristic of volcanic arcs.
The third melting mechanism notes the local coinci-
dence of certain magmatic bodies, chiefly ancient mag-
matic plutons exposed by deep erosion, with strike-slip
faults and appeals to the transformation of mechanical
work to heat energy during deep faulting to cause melting.
The magnitude of thermal energy produced is given by the
mechanically equivalent acceleration times the velocity of
the fault surface motion. This shear heating during earth-
quakes is of order

Adjacent quadrivalent silicon cations, Si 4 , in silicate melts
enter into shared coordination with four surrounding
oxygen ions to form silica-oxygen tetrahedra. Adjacent
tetrahedra share O ions and also join to aluminum ions in
linked rings. The linked groups are said to be in a state of
polymerization and are a feature of silicate melts. It is the
continuous, polymer-like, linkage of oxygen ions (up to 15
or so tetrahedral lengths may be involved) that seems to
control important physical properties; the greater the silica
content and degree of group polymerization, the greater
the viscosity and higher the solidus temperature. Alkali
and alkali earth cations like Ca, Na, and K, together with
nonbridging O anions and OH reduce the degree of

is the frictional
shear stress on the fault surface and u is the mean velocity
of its motion. As long as the heat energy is retained locally
due to low thermal diffusivities of the rocks involved, then
the temperature can build up with the possible occurrence
of local melting. Temperature build up is aided by a ther-
mal feedback process such that any increase in local strain
rate caused by lowering of viscosity at the heightened tem-
perature releases even more heat and this continues
until melting occurs after a few million years. However,
the presence of circulating fluids and their role in the

u, in Watts, where

Box 5.2 Global melt fluxes.

Total volume of oceanic plate added as melt at MORs:
c .25 km 3 a 1
Total volume of oceanic plume-related intraplate volcanic melt:
c .1-2 km 3 a 1
Total of volcanic arc melt: c .2.9-8.6 km 3 a 1
Total of continental intra-plate melt: c .1.0-1.6 km 3 a 1