Geoscience Reference
In-Depth Information
5.1.1
Difficult initial questions and early clues
We need to ask a number of exploratory questions about
magma genesis. Why, where, and how does melting of
Earth's crust and mantle occur? Does magma exist as con-
tinuous or discontinuous pockets? Why and how does
magma rise to the surface?
We know heat escapes from the Earth at a mean flux of
some 65 mW m 2 (Chapter 8). But this global mean value
allows for local areas of much higher flux. The geographi-
cal distribution of active volcanoes and geothermal areas
shows that the local production of enhanced heat energy
and subsurface melting is far from accidental or random: it
usually occurs associated with areas of plate creation along
the midocean ridges (Iceland) or destruction along the
subduction zone trenches (Section 5.2; Fig. 5.1).
Therefore we conclude that melting is also associated with
these large-scale processes. Exceptions, as always, disprove
this rule and so we also need to look with particular inter-
est at those prominent volcanic edifices that occur far from
plate boundaries, like the Canary Islands and Hawaii. Why
does melting occur there?
We can gather clues as to the nature of magma from
observing different styles of volcanic activity. Quiescent
volcanoes often gently discharge gases like steam, CO 2 ,
and SO 2 from craters or subsidiary vents called fumaroles .
So, we infer that magma must also contain such gas phases,
presumably in dissolved form under pressure, and that the
gases can discharge passively. Volcanic eruptions of lava
(Fig. 5.2) are themselves often passive; thus a Hawaiian
volcano emits molten lava easily as rapidly moving flows.
On the other hand, eruption may be far from passive;
Vesuvian or Surtseyan explosions (Fig. 5.3) blast material
vertically into the stratosphere as massive plumes or later-
ally as horizontal jets hugging the ground. Strombolian
eruptions (Fig. 5.4) shower molten material periodically
skywards for a few hundred meters in a fire fountain. Why
this diversity of volcanic behavior into flow, blast, and
fountain? A first clue came from observations made by
geologists of the types of rock produced by these various
styles of eruption. There is a wide range of possible chem-
ical composition of magma, with more than a dozen main
chemical elements and a score or more of minor (trace)
elements involved, for our purposes we need simply to
divide magmas and igneous rocks into three types
(Fig. 5.5), according to their silica content - acid , interme-
diate , and basic . Acidic volcanic rocks rich in silica (
1 km
Fig. 5.2 Thermal imaging view of three cinder cones and associated
breaching lava flow A. Note the lava levees bordering the upper
channel conduit and flow wrinkles on the lobate lava fan margin.
A younger flow (black) has breached the end of the levee system at
B. C-E are older flows. Kamchatka, Russia.
Fig. 5.3 Explosive eruption column (2 km high) and accompanying
base surge blast, Capelinhos volcano, Azores, October 1957. The
central part of the Surtseyan eruption column is an internal core-jet
rich in dark-colored volcanic debris. The base surge is steam-
dominated.
63
percent SiO 2 ), called rhyolites , are comparatively rare as vol-
canic flows. Rocks with intermediate amounts of silica
(52-63 percent SiO 2 ), called dacites or andesites , often with
minerals containing tiny amounts of water in their atomic
lattices, tend to occur as the products of violent blasts.
Rocks solidified from melts that passively flow as lavas tend
to have the lowest amount of silica (
52 percent SiO 2 );
these are the ubiquitous basalts . Basalt flows are also the
products of submarine volcanoes at midocean ridges.
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