Geology Reference
In-Depth Information
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-
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
volume of 24 km
3
.
Peléean domes
(Figure 5.7c) are typ-
ified by Mont Pelée, Martinique, a lava dome that grew
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.
Upheaved plugs
, also called
plug domes
or
pitons
,are
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
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'.
( ) Lava shield
a
( ) Lava dome
b
( ) Lava cone
c
( ) Lava mound
d
( ) Lava disc
e
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-
canic centres (Figure 5.8). They are productions of
vast explosions or tectonic sinking, sometimes after an
eruption (Figure 5.9). An enormous caldera formed in
Yellowstone National Park, USA, some 600,000 years
ago when some 1,000 km
3
of pyroclastic material was
erupted leaving a depression some 70 km across. Another
large caldera formed some 74,000 years ago in northern
Sumatra following a massive volcanic eruption, the ash
from which was deposited 2,000 km away in India. The
Toda caldera is about 100 km long and 30 km wide
and now filled by Lake Toba. It is a
resurgent caldera
,
which means that, after the initial subsidence amounting
to about 2 km, the central floor has slowly risen again to
produce Samosir Island. Large silicic calderas commonly
occur in clusters or complexes. A case is the caldera com-
plex found in the San Juan volcanic field, south-western
Colorado, USA, which contains at least eighteen separate
thick, steep-sided, dome-shaped extrusions. Volcanoes
erupting acidic lava often explode, and even where extru-
sion takes place it is often accompanied by some explosive
activity so that a low cone of ejecta surrounds the extru-
sions. Indeed, the extrusion commonly represents the last
phase in an explosive eruptive cycle.
Extrusions of acid lava take the form of various
kinds of lava dome: cumulo-domes and tholoids,
coulées, Peléean domes, and upheaved plugs (Figure 5.7).
Cumulo-domes
are isolated low lava domes that resem-
ble upturned bowls (Figure 5.7a). The Puy Grand Sarcoui
in the Auvergne, France, the mamelons of Réunion, in
the Indian Ocean, and the tortas ('cakes') of the central
Andes are examples. A larger example is Lassen Peak,
California, which has a diameter of 2.5 km.
Tholoids
,
although they sound like an alien race in a
Star Trek
episode, are cumulo-domes within large craters and
