Environmental Engineering Reference
In-Depth Information
good estimates of the extent and rate of erosion since emplacement. Tens of kilometres of
crust stripped off since the late Palaeozoic have
partially exhumed the Cornubian granite batholith 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).
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 contaminates and gas-water volatiles. Consequently, there are broad
volcano-orogenic
associations. B-subduction shows an evolutionary progression, from
basalt → andesite → dacite →
Figure 12.5
Mode of formation and the modern landsystem of
the Ardnamurchan ring complex, Scottish Highlands. (a) An
active stratovolcano with associated dyke and sill complexes.
(b) Post-eruptive subsidence above the former diapir forms
concentric cones of the original sills whilst minor renewed
eruption intrudes ring dykes along fault lines. (c) The
Ardnamurchan landscape is dominated by three such phases
