Geology Reference
In-Depth Information
Crater
Lava flow
Dyke
Lateral cone
Sill
Lava flow
Buried cinder cone
Tephra
Ash deposits
Pipe conduit
Dyke
Magma reservoir
Figure 5.5
The structure of a typical strato-volcano.
Source:
Adapted from MacDonald (1972, 23)
Basic-lava volcanoes - shields
cone (now Monte Somma) was demolished by the erup-
tion of
AD
79 and a younger cone grew in its place. Mount
Etna is a huge composite volcano, standing 3,308 m
high with several summit vents and innumerable parasitic
monogenetic vents on its flanks.
Another level of complexity is found in
compound
or
multiple volcanoes
. Compound volcanoes consist, not
of a single cone, but of a collection of cones intermixed
with domes and craters covering large areas. Nevado Ojos
del Salado, at 6,885 m the world's highest volcano, covers
an area of around 70 km
2
on the frontier between Chile
and Argentina, and consists of at least a dozen cones.
Volcano complexes
are even more complex than com-
pound volcanoes. They are so muddled that it is difficult
to identify the source of the magma. In essence, they
are associations of major and minor volcanic centres and
their related lavas flows and pyroclastic rocks. An example
is Cordon Punta Negra, Chile, where at least twenty-
five small cones with well-developed summit craters are
present in an area of some 500 km
2
. None of the cones
is more than a few hundred metres tall and some of the
older ones are almost buried beneath a jumbled mass of
lavas, the origin of whose vents is difficult to trace.
Basic lava, such as basalt, is very fluid. It spreads read-
ily, so raising volcanoes of low gradient (often less
than 10
◦
) and usually convex profile. Basic-lava volca-
noes are composed almost wholly of lava, with little
or no addition of pyroclastic material or talus. Several
types of basic-lava volcano are recognized: lava shields,
lava domes, lava cones, lava mounds, and lava discs
(Figure 5.6). Classic examples of
lava shields
are found
on the Hawaiian Islands. Mauna Loa and Mauna Kea
rise nearly 9 km from the Pacific floor.
Lava domes
are
smaller than, and often occur on, lava shields. Individual
peaks on Hawaii, such as Mauna Kea, are lava domes.
Lava cones
are even smaller. Mount Hamilton, Victoria,
Australia, is an example.
Lava mounds
bear no signs of
craters.
Lava discs
are aberrant forms, examples of which
are found in Victoria, Australia.
Acid-lava volcanoes - lava domes
Acid lava, formed for instance of dacite or rhyolite or
trachyte, is very viscous. It moves sluggishly and forms
