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MAGMA STORAGE 45

rated to low-pressure conditions prior to eruption. plied with fresh magma, earthquakes will tend
(Typically eruption temperatures at Kilauea vol- to occur around the edges of the chamber because
cano are ∼1150°C, for example.) In other words, the influx of magma causes stress at the chamber
igneous petrology suggests that it is very common walls. Similarly when magma is withdrawn from the
for mantle-derived magma to experience crustal chamber during an eruption or intrusion, changes
storage prior to eruption. Furthermore, detailed in stress and the emplacement of feeder dikes gen-
petrological study of the properties of the various erate more earthquakes. However, no earthquakes
minerals in volcanic rocks can indicate the actual are generated within the magma itself, so when the
depth at which storage occurred. locations of earthquakes are plotted in a diagram, a
region (the seismic gap) may appear in which there
is a scarcity of earthquakes and which coincides
4.2.3 Geophysical observations
with the zone in which magma is stored (Fig. 4.3).
During the past 100 years scientists have developed The time taken for a seismic wave to travel
increasingly sophisticated methods to monitor through the ground to a detector depends on the
the activity of volcanic systems (see Chapter 11). seismic velocity of the material through which
Various geophysical techniques have been devel- the wave passes. Compressional seismic waves (p
oped which can be used to “look inside” active vol- waves) will pass through magma but their speed in
canoes and these indicate the presence of stored magma is much slower than in solid rock. So seis-
magma beneath the summits of many active vol- mologists can use delays in the passage of seismic
canoes. The two most widely used techniques waves through the ground to detect zones of low
employ seismic and deformation methods. seismic velocity which correspond to areas of magma
storage. In some cases they may do this by using the
natural seismic waves generated by earthquakes
SEISMIC TECHNIQUES
within the volcanic system, but they may also carry
A range of seismic techniques can be used to det- out seismic surveys in which the seismic waves are
ect the presence and estimate the size of magma generated artificially using controlled explosions.
chambers. One method is to look for a seismic gap A further technique involves locating the source
beneath the summit region of the volcano. Earth- of volcanic tremor within a volcanic system.
quakes occur continuously within active volcanic Volcanic earthquakes are recorded as discrete
systems. The majority of these earthquakes are too events by seismometers (trace A in Fig. 4.4). In

small to be “felt” but they are readily detectable contrast, volcanic tremor is a continuous type of
using seismometers and their source or focus can seismic signal which can last minutes, days or even
be located. When a magma chamber is being sup- longer (trace B in Fig. 4.4). Tremor is a very com-

Caldera
NW SE
Fig. 4.3 Distribution with depth,
and with position in a vertical plane
oriented NW–SE, of the sources of Deformation
centre
some earthquakes around the summit
magma chamber of Kilauea volcano.
The earthquakes are generated by Magma 5 km
chamber
brittle fractures in response to stresses
in rocks, and cannot be generated in
the molten or partially-molten magma
in the chamber. (Modified from fig. 14 10 km
in Decker et al. (1983) Seismicity and
surface deformation of Mauna Loa
volcano, Hawai’I. EOS, Trans. Am. 0 10 20 km
Geophys. Union, 37, 545–547.)