Page 79 - Fundamentals of Physical Volcanology

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56 CHAPTER 4

repeated transfer of magma through the system
and the higher temperatures prevailing in the lower
crust mean that the deep plumbing system may
evolve through time to something less dike-like. For
instance, the volcanologist Bruce Marsh suggested
the idea of mush columns. He pictures the plumb-
ing system as a heated zone through which magma
repeatedly passes and in which magma never com-
pletely solidifies so that the zone always contains a
crystal mush through which fresh batches of magma
may ascend. Other scientists have described these
features as heat pipes. Whether magma ascends
continuously or in discrete batches through this
deep plumbing system is not clear, and both situ-
ations may prevail at different volcanoes or at one
volcano at different times. For example, study of
Kilauea volcano over the past 50 years has sug-
gested that supply to its shallow magma cham-
ber is fairly continuous and occurs at a rate of
3 −1
3
∼0.05 km yr −1 or 1.6 m s . Prior to 1950 the rate
of activity was lower and it has been calculated that
3 −1
3
−1
supply rates were only 0.009 km yr or 0.03 m s ,
Fig. 4.17 An example of a dike intruding into the interior suggesting that even if supply is continuous the
of a previously emplaced dike. Erosion has exposed these
rate of supply may vary considerably through time.
dikes in what was the interior of the Ko'olau volcano on
Measurements made at Krafla volcano in Iceland
O'ahu, Hawai’I. The longer dashed lines define the edges of
between 1975 and 1984 suggest continuous supply
the first dike and the shorter dashed lines mark the sides of
3 −1
the later dike. (Photograph by Lionel Wilson.) of magma at a rate of ∼5m s . Patterns of inflation
and deflation during this time were very similar
to those commonly seen at Kilauea. However,
arrives, however, the chances of the sill evolving phases of activity at Krafla such as those of the

into a long-lived chamber improve. This is because 1970s and 1980s are punctuated by long periods
each new pulse of magma heats the walls of both of inactivity, suggesting that magma supply may
the feeder dike and the sill (now a proto-chamber) be continuous for some years during periods of
and so reduces the temperature contrast between activity but is then interrupted or much slower
the magma and country rocks, thus slowing the during the periods of inactivity. Activity at Askja
cooling rate (Fig. 4.16). Once these critical early (also in Iceland) in 1875 also suggests that supply
stages have been overcome then an established to chambers may be intermittent. At Askja, both a
magmatic system is likely to evolve in which magma major rhyolitic Plinian eruption and a substantial
formed at depth has a well-defined pathway link- basaltic fissure eruption seem to have been trig-
ing the zone of magma formation with the shallow gered by a sudden influx of basaltic magma from
magma chamber. Such a magmatic system is thought depth into a shallow magma chamber. Whether the
to underlie Kilauea volcano in Hawai’I (Fig. 4.18). supply is continuous or intermittent may affect the
Even given the above arguments and the issues frequency and character of volcanic eruptions from
discussed in Chapter 3, the nature of the deep a system (see section 4.4.2) but, in terms of survival
plumbing systems feeding magma chambers is of the magma chamber, the important factor is
not well understood. While reuse of dikes provides simply that supply is regular enough to prevent
a mechanism for developing such a system, the solidification of the chamber.