Environmental Engineering Reference
In-Depth Information
Figure 12.6 General processes, rocks and landforms of
stratovolcano eruptions.
Source: After Howells et al. (1981).
Colour Plate 7 between pp. 272 and 273). Far larger calderas have been identified
recently from satellite imagery (see box, p. 251).
For every modern stratovolcano there are hundreds disguised by subsequent events in
older continental crust. They reveal important clues to earlier volcano-tectonic cycles and
volcanic arc landsystems can still be identified in the modern landscape which they help
to shape. The history of the closing Iapetus Ocean is seen in Wales as clearly as
anywhere. The Welsh basin occupies most of Wales, consisting of a 10 km thick pile of
Lower Palaeozoic marine sediments and ocean basalts. As the ocean subducted south-
eastwards, they were intruded and coated by arc-collision magmas and ashes in two
eruptive cycles, c . 485 Myr and 455 Myr ago. In the younger, Caradoc, period over 200
km 3 of subaerial and subaqueous igneous rocks, more than 2 km thick in places,
accumulated in just 3-5 Ma. Six hundred metres of tuffs, breccias and basalts formed the
thickest single formation of the Snowdon Volcanic Group, associated with a 40 km wide
caldera centred on Snowdon (Figure 12.7; see Table 1 in Key Processes box on p. 230).
Caldera subsidence was probably caused by back-arc extensional rifting, adjusting to
crustal compression, which triggered fissure rather than central vent eruptions. There are
600 active/dormant stratovolcanoes of Late Cenozoic age world-wide today. Eighty per
cent are located within the Pacific Ring of Fire, outside the andesite line which divides
the Pacific into stratovolcano and shield volcano provinces (see box, Figure 3, p. 199).
Subduction magmatism drives plate-boundary volcanoes but isolated hot spots or sub-
rift diapirism are the key to intra-plate volcanoes. Basaltic magmas generate shield
volcanoes in ocean intra-plate settings. Thousands of them, 1-2 km high and extinct or
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