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44 CHAPTER 4

Table 4.1 The size of calderas formed during a selection of volcanic eruptions.
3
Eruption date Caldera Caldera diameter (km) Erupted volume* (km )
1991 Pinatubo, Philippines 2.5 4–5
1968 Fernandina, Galapagos 5 × 6 0.1
1912 Katmai, Alaska 2.5 × 4 12
1883 Krakatau, Indonesia 8 10
1.4 ka Rabaul, Papua New Guinea 10 × 15 11
1.8 ka Taupo, New Zealand 35 35
3.6 ka Santorini, Greece 7 × 10 25
75 ka Toba, Indonesia 30 × 80 1500
600 ka Yellowstone, USA 40 × 70 1000–2000
27.8 Ma La Garita, USA 35 × 75 5000

Data taken from Lipman (2000) Calderas. In Encyclopedia of Volcanoes, pp. 643–662. Academic Press.
*Dense rock equivalent.

mit caldera of Kilauea volcano which is ∼3 × 5km 3.5
across and up to ∼120 m deep. This is a relatively
small caldera. Observations of modern eruptions, 3
and geological studies of older volcanoes, show
that calderas often form during large-volume erup-
2.5
tions. In general, the larger the erupted volume
Pressure (GPa)
the larger the size of the caldera which is formed 2
(Table 4.1). Calderas are thought to form by the col- Olivine Tholeiite
lapse of surface layers into an underlying magma
reservoir as magma is erupted from it. 1.5 Peridotite

1
4.2.2 Petrological evidence for
magma storage
0.5

Study of the petrology of igneous rocks allows us
to distinguish between rocks formed from magma 0
erupted directly from the mantle and those formed 1000 1200 1400 1600
from magma which has been stored within the
Temperature (°C)
crust prior to eruption. A melt formed within the
mantle has a composition and temperature which Fig. 4.2 Variation of liquidus temperatures of two melts
reflects the depth (and, therefore, pressure) at with pressure. In each case the liquidus temperature
which it formed. Typically a mantle melt will form increases with pressure and hence with depth beneath the
at temperatures of 1200–1400°C. The liquidus tem- surface. (After fig. 3 in Lambert, I.B. and Wyllie, P.J. (1972)
Melting of gabbro (quartz eclogite) with excess water to 35
peratures of melts depend strongly on the pressure
kilobars, with geological applications. Journal of Geology,
conditions, and decrease with decreasing pressure
80, 692–708. Copyright University of Chicago Press.)
(Fig. 4.2). For this reason, if a magma generated in
the mantle is erupted without a period of shallow
storage occurring, the magma will have a tempera- lower pressures change the composition and tem-
ture and composition which reflect its origin in the perature of the erupting magma. Study of igneous
mantle. If, however, the magma is stored prior to rocks shows that mantle-derived magmas are usu-
eruption then it has the opportunity to cool and ally close to their low-pressure liquidus temperature
begin to crystallize. Cooling and crystallization at upon eruption and thus that they have equilib-