Rock formation and deformation - Fundamentals of the Physical Environment - page 257

VOLCANIC
(fine-grained
or porphyritic)
HYPABYSSAL
(medium-grained)
PLUTONIC
(coarse-grained)

BASALT ANDESITE DACITE RHYOLITE

DOLERITE GRANOPHYRE

MICROGRANITE

PERIDOTITE GABBRO DIORITE GRANODIORITE GRANITE

Minerals

Increasingly dark Increasingly viscous

100

QUARTZ

Anorthite
(calcium-
rich)

80

ORTHOCLASE
FELDSPAR
(potassium-rich)

60

PLAGIOCLASE
FELDSPAR

PYROXENE

40

OLIVINE

e

Albite (sodium-rich)

20

HORNBLENDE

0

ULTRA-
MAFIC

MAFIC INTERMEDIATE ACID

40 45 50 55 60 65 70 75

% SiO 2 by weight

Figure 12.3 Mineral composition, texture, viscosity and emplacement environment of the more common igneous rocks.
Volcanic rocks are eruptive or extruded at the surface; hypabyssal and plutonic rocks intrude existing rocks at intermediate and
greater depth.

of south-west England. This diapiric batholith, 250 km
long and over 55,000 km 3 in volume, is a residual part of
the European Hercynian orogen and now forms high
ground on Dartmoor, other Cornish upland and the Isles
of Scilly. For comparison, erosion has exposed a shallow
Tertiary sub-volcanic landscape with cone sheets, ring
dykes and volcanic plugs in the Ardnamurchan ring
complex of the Scottish Highlands ( Figure 12.5c ) .

contaminates and gas-water volatiles. Consequently, there
are broad volcano-orogenic associations. B-subduction
shows an evolutionary progression, from basalt q
andesite qdacite qrhyolite 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

Extrusive and eruptive activity

Volcanoes are the rock stars of the supercontinental cycle!
However, less than 10 per cent of new magma erupts
annually from terrestrial volcanoes and we know that
basaltic lava effusion at mid-ocean ridges accounts for
most extrusive activity ( Figure 12.4b ) . Progression from
effusive lava flow to explosive volcanic eruption closely
follows the fall in viscosity with increased silicate content,
falling melt temperature and parallel increases in crustal