a path of existing crustal weakness and will propagate through irregular fissures
and faults (Figs. 5.3 and 5.4), which become conduits of magma that could be
released to form volcanic edifices, lava ponds, pillow mounds and other volcanic
features.
Lava Morphology
The types of lava morphology encountered on the sea floor vary according to the
geological environment, the depth from which the lava has come, its temperature,
rate of eruption and its viscosity, which also depends on the lava’s composition.
The major parameters involved in modifying the morphology of lava flows
are listed below:
(a) Magma’s viscosity depends on its flow rate, the amount of crystals and its
volatile content other than H 2 O (mainly for silica-rich lava). A magma’s
viscosity will affect its density. Viscosity is defined as a substance’s
internal resistance to flow when shear stress is applied. For example,
water flows in response to a small amount of stress therefore we say it has
a low viscosity. Viscosity is measured in Pascal-seconds (1 kg/1 m/
s = 1 Pa s = 10 poise). Most lavas including basalt, andesite, rhyolite
and others such as trachy-andesite, or trachyte all flow at a rate between
10
2 and 10
5 Pa s. For comparison, pure water has a viscosity of 10
-3 Pa s.
Viscosity decreases with an increase in temperature and an increase in the
water content of the magma. Murase and McBirney (1963, 1973) have
discussed the relationship between temperature, water content and
viscosity.
(b) Cooling rate depends on the conditions of thermal diffusivity. Thick crust
will insulate the magma flow, and thereby decrease the cooling rate. Large
magmatic conduits and magma chambers will cool more slowly than small
veins and thin horizontal sheets forming sills. A small conduit such as a
lava tube (\2 m in diameter) will channel a limited amount of lava for a
short distance.
(c) Extrusion rate depends on the size of a conduit, the volume of magma, and
the rate of magma delivery. Magma conduits supplied by large magma
chambers will take more time to cool (several days or even several tens of
years).
Volcanic Landscape
113
and faults (Figs. 5.3 and 5.4), which become conduits of magma that could be
released to form volcanic edifices, lava ponds, pillow mounds and other volcanic
features.
Lava Morphology
The types of lava morphology encountered on the sea floor vary according to the
geological environment, the depth from which the lava has come, its temperature,
rate of eruption and its viscosity, which also depends on the lava’s composition.
The major parameters involved in modifying the morphology of lava flows
are listed below:
(a) Magma’s viscosity depends on its flow rate, the amount of crystals and its
volatile content other than H 2 O (mainly for silica-rich lava). A magma’s
viscosity will affect its density. Viscosity is defined as a substance’s
internal resistance to flow when shear stress is applied. For example,
water flows in response to a small amount of stress therefore we say it has
a low viscosity. Viscosity is measured in Pascal-seconds (1 kg/1 m/
s = 1 Pa s = 10 poise). Most lavas including basalt, andesite, rhyolite
and others such as trachy-andesite, or trachyte all flow at a rate between
10
2 and 10
5 Pa s. For comparison, pure water has a viscosity of 10
-3 Pa s.
Viscosity decreases with an increase in temperature and an increase in the
water content of the magma. Murase and McBirney (1963, 1973) have
discussed the relationship between temperature, water content and
viscosity.
(b) Cooling rate depends on the conditions of thermal diffusivity. Thick crust
will insulate the magma flow, and thereby decrease the cooling rate. Large
magmatic conduits and magma chambers will cool more slowly than small
veins and thin horizontal sheets forming sills. A small conduit such as a
lava tube (\2 m in diameter) will channel a limited amount of lava for a
short distance.
(c) Extrusion rate depends on the size of a conduit, the volume of magma, and
the rate of magma delivery. Magma conduits supplied by large magma
chambers will take more time to cool (several days or even several tens of
years).
Volcanic Landscape
113