Page 114 - Fundamentals of Physical Volcanology

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STEADY EXPLOSIVE ERUPTIONS 91

eruption generates a stable Plinian column greater • As the gas–magma mixture rises towards the
than 30 km in height (Fig. 6.10b). As the gas con- surface its pressure decreases. This causes an
tent decreases a point is eventually reached, in this expansion and cooling of the gas and a resulting
case when the gas content is ∼2.3 wt%, where the release of energy. This released energy causes
plume can no longer entrain enough air to become acceleration of the rising material and does work
thermally buoyant. From this time onward, the against gravity and friction.
new material emerging from the vent feeds a pyro- • The speed at which the gas–magma mixture
clastic fountain. The gas and pyroclasts already in emerges from the vent is called the exit velocity
the eruption cloud continue to climb convectively and is typically tens to hundreds of meters per
into the atmosphere, although their motion, and second. The exit velocity is dependent on a num-
the shape of the eruption cloud, change as the ber of factors but is most strongly controlled by
supply of new heat from below is cut off, and the gas content of the magma. The higher the
eventually disperse downwind. As the gas content initial gas content of the magma, the higher the
declines further, the height of the fountain will also exit velocity.
decline because its height is critically dependent • As the gas stream exits from the vent, air is mixed
on the exit velocity of the material, and, therefore, or entrained into the rising eruption plume. En-
on the gas content as described in section 6.4.1. trainment causes two important effects. It causes
Many magma chambers, especially those containing the rise speed of the material in the plume to
rhyolitic magma, evolve in a way that causes the decline progressively above the vent. This hap-
volatiles to be concentrated into the shallowest pens because the initial rise of the material is due
part of the system. It is this part of the system that to the momentum the material has on eruption.
erupts first, and so a decrease in volatile content As air is entrained the mass of material in the
during an eruption will be more common than an plume increases, and so conservation of mom-
increase. entum requires that the rise speed decreases
The pyroclastic fountains produced by convec- (eqn 6.6). The erupted material has an eruption
tive instability in eruption columns feed various temperature of ∼900–1150°C whereas the en-
kinds of pyroclastic density currents, the largest of trained air has a temperature of about 0°C. This
which generate deposits called ignimbrites, as means that as the air is entrained it is heated. The
described in Chapter 8. volume of air which is entrained is large com-
pared with the volume of magmatic material
erupted and this means that the temperature of
6.8 Summary the plume is eventually only slightly higher than
that of the surrounding air. This heating is, how-
• As gas bubbles form and grow within a rising ever, sufficient to cause the plume to be thermally
magma they become increasingly close-packed. buoyant compared with the surrounding air.
With continued growth the bubble walls become • In the lowest few kilometers of the eruption
so thin that they collapse causing the individual plume rise is due to the initial momentum of the
bubbles to coalesce into a continuous gas stream. erupted material. This section of the eruption
This process is called fragmentation as it causes plume is called the inertial or gas-thrust region.
the tearing apart of the magma into individual As entrainment proceeds, the rise speed due to
clots and clasts. Below the fragmentation level the initial momentum becomes minimal. By the
the rising fluid consists of a continuous stream of time this point is reached, however, the entrain-
magmatic liquid which contains individual gas ment and heating of air will mean that the plume
bubbles within it. After fragmentation the rising material is of slightly lower density than the
fluid is a continuous stream of gas with individual surrounding air and so rise continues through
clots and clasts of magma within it. The clots and thermal buoyancy. The section of the plume in
clasts may still contain trapped – and even inter- which rise is due to thermal buoyancy is known
connected – gas bubbles. as the convective region. With continued rise