Page 117 - Fundamentals of Physical Volcanology

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7 Transient volcanic eruptions

7.1 Introduction • explosions usually occur in sequences (although
the lengths of the sequences vary widely – from
In Chapter 5 we saw that gas bubbles forming in days to years);
rising magma behave differently depending on the • both types of eruption are characterized by the
rise speed of the magma. At high rise speeds the gas ballistic ejection of large blocks of country rock
bubbles are locked to the magma and steady frag- and volcanic bombs;
mentation of the magma results in the sustained • both types of explosions generate eruption
style of eruption described in Chapter 6. If, how- columns, column height being relatively small
ever, the magma is rising slowly or is stationary, gas except when the explosions are closely spaced
bubbles can rise through a low-viscosity magma in time.
resulting in transient, explosive magmatic eruptions.
The eruptions differ primarily in the violence of
Transient explosions can also occur when external
the events and the composition of the magmatic
surface or subsurface water (often called meteoric
material involved. Strombolian explosions are mild
water since it derives ultimately from rainfall from
events involving basaltic magmas in which almost
the atmosphere) comes into contact with magma.
all of the erupted material is magmatic and is
Such events are best referred to as hydromagmatic
ejected at relatively low velocities (less than
eruptions, although the term phreatomagmatic 200ms ) and deposited close to the vent. By con-
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is used when the water involved is groundwater. trast, Vulcanian explosions generally involve inter-
This chapter discusses the styles of these transient
mediate to evolved magmas and generate some
eruptions and current ideas about the mechanisms
of the highest eruption velocities of any type of
which cause them.
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volcanic eruption (200–400 m s ) and eject large
blocks of magmatic material and country rock to dis-
tances of up to ∼5 km from the vent (see Chapter 1).
7.2 Magmatic explosions
The basic mechanism behind both types of erup-
tion appears to be very similar, the key difference
Initially the two main types of transient explosions
between them being the pressures which are
which can result from the segregation of magmatic
generated in the vent prior to eruption: Vulcanian
gas from its parent magma are considered: Strom-
eruptions are associated with much higher vent
bolian and Vulcanian eruptions. These types of
pressures than Strombolian eruptions.
eruption share many similarities:
The basic mechanism proposed to explain both
• each explosion is short-lived (seconds to minutes types of eruption is that they occur within open
in duration); magmatic systems in which the vent or magma
• repose times between individual explosions are column is capped with relatively cool magma. The
usually short (seconds to hours); rise speed of magma within the system is low or