Geoscience Reference
In-Depth Information
eventually construct volcanoes 1-3 km high with 10
2-3
km
3
of magma products on base areas of 10
3-4
km
2
in a
few hundred thousand years. As eruption empties the
near-surface magma chamber, the main vent collapses to
form a
caldera
, varying in size from Crater Lake in
Oregon, 8 km in diameter, to Lake Taupo in North Island,
New Zealand, over 70 km in diameter (see
Plate 12.4
).
Far
larger calderas have been identified recently from satellite
imagery (see box, p. 272).
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, accu-
mulated in just 3-5 Ma. Six hundred metres of tuffs,
on 22 July 1980, more than two months after the initial May
eruption. The lower, lighter plume is the ash cloud rising from
pyroclastic flows, compared with the darker, rising eruptive
plume.
Photo: R. Hoblitt, US Forest Service
Ash column
Downwind plume
Tephra
Pyroclastic flow
(nuée ardente)
Ash-fall
tuff
Ash-flow tuff
(ignimbrite)
Rhyolite
lava
flow
Volcanic
breccia
Welded tuff
Debris flow
Submarine
pillow basalt
Sea level
Source: After Howells et al. (1981)
