Inner Earth processes and systems - Physical Processes in Earth and Environmental Sciences - page 203

5 Inner Earth processes and
systems

5.1 Melting, magmas, and volcanoes

The ancient Greeks supposed that a river of melt, shifting
according to Poseidon's whims, ran under the Earth's
surface, periodically rising to cause volcanic eruptions and
violent earthquakes. We have seen evidence (Section 4.17)
that most of the mantle and crust of the outer Earth is
solid, exhibiting elastic or plastic behavior and transmit-
ting P and S waves. Yet the Low Velocity Zone marking
the top of the asthenosphere has a tiny amount of melt,
sufficient to slow seismic waves somewhat and to enable
plate motion over it (see Section 5.2). On the other hand,
more than 1,500 Holocene-active volcanoes (Fig. 5.1) give

first hand evidence for localized accumulations of abun-
dant magma not far below the surface. Magma is a high
temperature, multiphase mixture of crystals, liquid, and
vapor (gas or supercritical fluid). It is impossible to meas-
ure its temperature or other physical properties directly,
for once it has flowed out of a volcanic vent as lava it will
have cooled somewhat, begun to crystallize, and would
have lost dissolved gas phases. We have to make recourse
to experiments that show at atmospheric pressure, typical
basalt magma is at about 1,280

C with a viscosity of

around 15 Pa s.

Iceland

Kamchatka

Aleutians

Vesuvius

Mt St
Helens

Yell´stone

Santorini

Azores

Jemez

Fuji

M´serrat
St Pierre

Canaries

S´boli

Unzen

Etna

Hawaii

Phillipines

Andes

New Hebrides

Kili´jaro

Tonga

Ta u p o

seismic zone
Holocene-active volcano or
volcanic arc
midocean ridge

Fig. 5.1 Map showing summary world seismic belts (14 year record of M 4.5) and the location of selected Holocene-active volcanoes and
the major volcanic arcs.