Geoscience Reference
In-Depth Information
(b)
(a)
Epicentre
Depth in
'ooo km
Velocity km sec
-1
Crust
0
5
10
15
Focus
hos
0
M
eno
1
Mesosphere
2
S waves
G
3
P waves
Outer
Core
4
5
Inner
Core
6
6.37
0
5
10
15
Figure 10.10
Typical (a) velocities and (b) pathways of earthquake waves, M and G mark the Mohorovicic and Gutenberg
discontinuities.
Source: After Duff (1993) and Smith (1982)
above sea level. Accretion can be augmented by slivers of
layered oceanic crust sheared off during subduction and
known as
ophiolite
in the
mélange
, or chaotic mix of rock
material, forming the prism (
Figure 10.13
).
Ophiolite
found high in the Alps and Himalayas is important
evidence of the power of plate tectonics. So too are the
north-east-south-west parallel bands of rocks, ageing
northwards and now forming the Scottish Southern
Uplands, which represent the accretionary prism of the
closing Lower Palaeozoic Iapetus Ocean.
Trench-arc distances are determined by the angle and
rate of subduction, since the volcanic arc overlies the zone
of maximum melt. At angles below 25
eruptions of volcanic arcs, exhaling gases and ejecting ash
and larger debris or
tephra
, contrast with continuous,
effusive
outflows of basaltic lava at mid-ocean ridges. As
the arc matures the depth of its magma source increases,
causing geochemical compositional changes as differ-
ent mineral cocktails fractionate from the wet, sub-
ducted oceanic crust and adjacent melted asthenosphere.
Early melts produce silicate-poor, less viscous basalt-
andesite volcanoes, switching to silicate-rich, more viscous
andesite-rhyolite later. Intrusive, non-eruptive granitic
magmas solidify below the surface and become exposed
only by subsequent erosion. These processes are detailed
in
Chapter 12.
The back arc, which may be 200-600 km wide, develops
by crustal extension of the non-descending plate, in
contrast to general
crustal shortening
or compression
slab may be
resorbed without vulcanism; 30-60
provides a fore-arc
width of approximately 200 km, and steeper angles can
halve this distance. The episodic,
explosive
volcanic
