Page 141 - Fundamentals of Geomorphology

124 STRUCTURE

a
( ) Lava shield derive their name from the Greek tholos, a ‘domed build-
ing’ (Plate 5.4). Their growth is often associated with
nuée ardente eruptions, which wipe out towns unfortu-
nate enough to lie in their path. A tholoid sits in the crater
of Mount Egmont, New Zealand. Coulées are dome–
b
( ) Lava dome lava-flow hybrids.They form where thick extrusions ooze
on to steep slopes and flow downhill (Figure 5.7b). The
Chao lava in northern Chile is a huge example with a lava
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volume of 24 km . Peléean domes (Figure 5.7c) are typ-
ified by Mont Pelée, Martinique, a lava dome that grew
c
( ) Lava cone
in the vent of the volcano after the catastrophic erup-
tion that occurred on 8 May 1902, when a nuée ardente
destroyed Saint Pierre. The dome is craggy, with lava
spines on the top and a collar of debris around the sides.
( ) Lava mound Upheaved plugs, also called plug domes or pitons,are
d
produced by the most viscous of lavas. They look like a
monolith poking out of the ground, which is what they
are (Figure 5.7d). Some upheaved plugs bear a topping
of country rock. Two upheaved plugs with country-rock
e
( ) Lava disc cappings appeared on the Usu volcano, Japan, the first in
1910, which was named Meiji Sin-Zan or ‘Roof Moun-
tain’, and the second in 1943, which was named Showa
Sin-Zan or ‘New Roof Mountain’.
Figure 5.6 Types of basaltic volcanoes, not drawn to scale.
Source: Adapted from Ollier (1969, 21)
Calderas
Calderas are depressions in volcanic areas or over vol-
thick, steep-sided, dome-shaped extrusions. Volcanoes canic centres (Figure 5.8). They are productions of
erupting acidic lava often explode, and even where extru- vast explosions or tectonic sinking, sometimes after an
sion takes place it is often accompanied by some explosive eruption (Figure 5.9). An enormous caldera formed in
activity so that a low cone of ejecta surrounds the extru- Yellowstone National Park, USA, some 600,000 years
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sions. Indeed, the extrusion commonly represents the last ago when some 1,000 km of pyroclastic material was
phase in an explosive eruptive cycle. erupted leaving a depression some 70 km across. Another
Extrusions of acid lava take the form of various large caldera formed some 74,000 years ago in northern
kinds of lava dome: cumulo-domes and tholoids, Sumatra following a massive volcanic eruption, the ash
coulées, Peléean domes, and upheaved plugs (Figure 5.7). from which was deposited 2,000 km away in India. The
Cumulo-domes are isolated low lava domes that resem- Toda caldera is about 100 km long and 30 km wide
bleupturnedbowls(Figure5.7a).ThePuyGrandSarcoui and now filled by Lake Toba. It is a resurgent caldera,
in the Auvergne, France, the mamelons of Réunion, in which means that, after the initial subsidence amounting
the Indian Ocean, and the tortas (‘cakes’) of the central to about 2 km, the central floor has slowly risen again to
Andes are examples. A larger example is Lassen Peak, produce Samosir Island. Large silicic calderas commonly
California, which has a diameter of 2.5 km. Tholoids, occur in clusters or complexes. A case is the caldera com-
although they sound like an alien race in a Star Trek plex found in the San Juan volcanic field, south-western
episode, are cumulo-domes within large craters and Colorado, USA, which contains at least eighteen separate