Geoscience Reference
In-Depth Information
acting on the magma to dissolve these bubbles can
decrease rapidly, causing a chain reaction of new
bubble formation. If the magma is gas-rich, as
andesitic lavas are, and the chain reaction is rapid
enough, an explosive eruption can occur. Once explo-
sive magma reaches the atmosphere, it is fragmented
into small particles by the gas and by the force of the
eruption. These pieces are called
pyroclastic ejecta
or
tephra and are classified according to size as follows:
bombs (liquid, boulder-sized blobs),
blocks
(angular,
solid boulders),
scoriae
(cinder material of any size),
lapilli
(fragments 2-60 mm in diameter), and sands
and ash (< 0.1 mm). Depending upon the size and
distance hurled, much of this material falls back to the
ground around the vent to build up a cone.
The study of the gas phase of magma is crucial in
determining whether the volcano will become explosive.
If the magma is rising slowly, a small quantity of gas will
continuously escape through vents to the surface
through
fumaroles
. This movement tends to be slow
because the viscosity of the magma inhibits bubble
release. Bubbles, once formed, tend to stick to
liquid-solid interfaces no matter how buoyant they
become. These gases also have sufficient time to dissolve
in groundwater. The monitoring of groundwater or
escaping gas can give an accurate indication of the
maturity of the magma, and its closeness to the surface.
Some of these
geochemical
techniques for prediction
were discussed in Chapter 10.
In some volcanic eruptions, only this
degassing
process initially affects the upper magma. As this
magma is forced out, the magma immediately beneath
degasses explosively. In this way, there may not be one
big explosion, but a series of continual blasts, each
lasting a few seconds to minutes, over several days.
Degassed magma may also pour out of the volcano as
lava, which (depending upon viscosity, output and the
slope of the ground) can flow from a few meters to
several hundreds of kilometers from the vent. All lava
soon solidifies to form hard rock, although extremely
thick lava beds may take years to cool down completely
to surrounding environmental temperatures. There are
three types of basaltic lava that can be ejected from a
volcano: pahoehoe, aa, and block lava. Pahoehoe is
basaltic lava that solidifies at the top, but which is still
fed from below by pipe vesicles. Such lava has a
smooth, wrinkled surface, is less than 15 m thick and
flows at rates of a few metres per minute. Aa forms as
a thin river of lava, less than 10 m thick, with a spiky,
cinder-like topping. Block lava consists solely of
andesite or rhyolite up to 300 m thick, and has a
surface coating consisting of smooth-sided fragments.
It has the slowest traveling speed of all lava flows,
moving only a few metres per day. Both aa and block
lava behave in all respects like a river and can be
bordered by levee banks.
The total energy of a volcanic event can be broken
down into four processes as follows:
• energy released by volcanic earthquakes,
• energy necessary to fracture the overburden,
• energy expended in ejecting material, and
• energy expended in producing atmospheric shock
waves and, occasionally, tsunami.
Volcanic tremor and earthquake energy can be
assessed in similar ways to any other earthquake event.
These methods of measuring energy have been dis-
cussed under earthquake magnitude in Chapter 10.
Earthquakes of magnitude 7 or larger on the Richter
scale have been produced by major explosive erup-
tions, and contain more than 4
10
15
joules of energy.
The energy required to break up overburden is
typically greater than 1
10
11
joules. Energy spent in
ejecting material consists of kinetic energy, potential
energy, and thermal energy. These terms can be
calculated for most volcanic eruptions, and indicate
that volcanic explosions release as much energy as
large nuclear devices - typically 1
10
15
joules. The
energy used in generating the atmospheric shock wave
can be expressed as follows:
10
13
sin
0.5
At
2
E
s
= 1.25
(11.1)
where
E
s
= energy to generate atmospheric
shock wave
sin
= distance from source of the
explosion in degrees on the
Earth
A
= amplitude of shock wave in
millibars
t
= wave period in seconds
A similar type of equation as 11.1 can be used to
measure the energy contained in any tsunami wave
generated by an eruption. These shock waves require
energy expenditure greater than 1
10
15
joules. In
total, volcanic eruptions contain energy in excess of
1
10
16
joules. Some of the larger eruptions, such as
Krakatau in 1883, or Tambora in 1815, contained
energy in excess of 1
10
18
joules (see Figure 11.1 for
