Rock formation and deformation - Fundamentals of the Physical Environment - page 258

(a)

Figure 12.4
(a) Principal forms of igneous
intrusion and extrusion and (b)
modern annual rates of global
magma production.
Source: Partly after Smith (1982)

Eruptive products

MAIN VENT

Satellite cone

Plug

Lava flow

EXTRUSIVE

VOLCANIC

Ash layers

Dyke

Sill

INTRUSIVE

HYPABYSSAL

Feeder dyke

Sill complex

METAMORPHIC
AUREOLE

BATHOLITH

PLUTONIC

(b)

Extrusive magma
0.1 km 3

1.0 km 3

0.9 km 3

4 km 3

Intrusive magma

1.2 km 3

9.5 km 3

?

Island arc

Continental Collision Intra-plate Mid-ocean ridge

14 km 3
CONSUMED
CRUST

Lithosphere

Asthenosphere

a solid, rhyolitic plug in the vent enhances explosivity and
may cause lateral blasts of the sort displayed by Mount St
Helens in 1980 ( Plate 12.2 and see Plate 10.1 ).
The violent exhalation of gases, magma and frag-
mented 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 ( Plate 12.3 ). 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
magmas 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 com-
monly vent 10 1-2 km 3 of ash and magma per eruption and