Environmental Engineering Reference
In-Depth Information
of Tertiary eruption and collapse eroded down to the line A-
B in (b).
rhyolite volcanoes with increasing age and distance from the trench. Dacite is
intermediate between andesite- rhyolite, with 63-69 per cent silica. Rare A-subduction
volcanoes are usually of dacite-rhyolite composition.
The presence of dissolved gases and cooler, more viscous magma is the recipe for
spectacular eruptions. Water (forming over 90 per cent by mass) together with smaller
quantities of sulphur, hydrogen, chlorine and carbon gases (SO
2
, H
2
S, H
2
, HCl, CO
2
)
vaporizes in the near-surface lower-pressure environment. This forms an explosive
mixture - rather like the decompressive effects of uncorking a champagne bottle. Sea
water may enter the vent in marine environments and create a
hydrovolcanic
effect, with
an explosive expansion of steam. Surtsey (1963) developed in this way in Iceland. The
existence of a solid, rhyolitic plug in the vent enhances explosivity and may cause lateral
blasts of the sort displayed by Mount St Helens in 1980.
The violent exhalation of gases, magma and fragmented rock dramatically transforms
volcanic products from lava flows over surrounding landscapes into airborne tephra,
rained out over a large area. Fine ash is forced several kilometres into the atmosphere as
an
ash
column, extending a
plume
and
ash fall
often thousands of kilometres downwind.
Column collapse of heavier ash,
lapilli
and tephra
bombs
on to the volcano develops into
fast-moving, incandescent (fiery) ash flows or
nuées ardentes
at ground level (see
Colour Plate 8 between pp. 272 and 273). Sedimentation from the atmosphere and
through water, from flows or plumes extending seawards, forms
pyroclastic rocks
. Heat
often welds fine-grained
ash
-
fall tuffs
, even under water, and coarse debris forms
volcanic breccia
. Lightweight, highly vesicular and vitreous (glassy) magma cools as
cinders
and
pumice
. Pyroclastic flows at temperatures between 250° C and 700° C form
ash-flow tuffs or
ignimbrites
on cooling (Figure 12.6). Widespread distribution of
mineralogically distinctive tephra from single events gives
tephro-chronology
a
powerful role in geological dating.
Explosive activity from andesite-dacite-rhyolite mag-mas is concentrated in 'mature'
arc-margin and island-arc orogens at stratovolcanoes. They are the largest terrestrial
forms and epitomize the classic view of volcanoes as steep-sided, composite cones of
stratified (layered) lava flows, tuffs and breccias (Figure 12.6). Several generations of
satellite cones surround a main vent, each with its feeder dykes. Stratovolcanoes
commonly vent 10
1-2
km
3
of ash and magma per eruption and eventually construct
volcanoes 1-3 km high with 10
2-3
km
3
of magma products on base areas of 10
3-4
km
2
in a
few hundred thousand years. As eruption empties the near-surface magma chamber, the
main vent collapses to form a
caldera
, varying in size from Crater Lake in Oregon, 8 km
in diameter, to Lake Taupo in North Island, New Zealand, over 70 km in diameter (see
