Rock formation and deformation - Fundamentals of the Physical Environment - page 261

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 Ta b l e
12.2 ). Caldera subsidence was probably caused by back-
arc extensional rifting, adjusting to crustal compression,
which triggered fissure rather than central vent eruptions.
The structural imprint of the caldera can be seen in Plate
10.7 . There are 600 active/dormant stratovolcanoes of
Late Cenozoic age worldwide 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 Figure 10.12 ).
Subduction magmatism drives plate-boundary volca-
noes 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 dormant, do not
break surface. Those which do often form long volcanic
island chains, best seen in the Hawaiian and other Pacific
islands. Slow, intermittent sea-floor effusion of magma
above a 'geostationary' hot spot has studded the main
Pacific plate with a line of volcanoes 6,000 km long in its
journey north-west during the past 70 Ma. A shift in plate
direction away from the East Pacific Rise, adjusting to
neighbouring plates c . 40 Ma ago, realigned islands in the
Hawaiian and adjacent chains. Hawaii itself is at the active
point in the chain. Its shield volcanoes of Mauna Loa,
Kilauea and three others rise 10 km from the sea floor at
shallow angles, due to low-viscosity magmas, with the

Plate 12.4 The caldera of Aniakchak, a rhyolite-andesite
volcanic caldera 10 km in diameter and 500-1,000 m deep, in
the Alaskan peninsula. Over 50 km 3 of magma and rock were
erupted 3,400 years ago and subsequent eruptions have
formed cinder cones and resurgent domes on the caldera
floor.
Photo: US Geological Survey

Non-welded
ash-flow tuff

Ordovician marine
sediments

Volcanic
breccia

Welded ash-flow
tuff

Figure 12.7 A reconstruction of Ordovician fissure eruptions and caldera collapse, Snowdonia, North Wales.
Source: After Howells et al. (1991)