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4 Magma storage

4.1 Introduction they form, processes operating within them, and
how they regulate volcanic activity.
The previous chapter dealt with how magma moves
within the mantle and crust. Although some magma
4.2 Evidence for magma storage
may ascend directly to the surface, most magma
within the crust
experiences a period of storage during its ascent.
This storage may be permanent, i.e., the magma
cools and solidifies forming an intrusive body, or We start by looking at the various lines of evidence
may represent only a temporary halt en route to which show that magma is commonly stored
the surface. Magma storage during ascent has a within the crust prior to eruption.
profound impact on the nature of magmatic and
volcanic activity. Amongst other things it influences
4.2.1 Calderas and magma chambers
the composition of the erupting magma, the phys-
ical properties of the magma (such as its viscosity), Probably the most obvious evidence for magma
and the size and frequency of volcanic eruptions. storage within the crust comes from the morphol-
This chapter examines what is known about the ogy of volcanoes. Many volcanoes have a summit
geometry and size of crustal storage zones, how crater called a caldera. Figure 4.1 shows the sum-

Fig. 4.1 The summit caldera complex
of Kilauea volcano, Hawai’I, seen from
the southwest. The large pit crater,
Halema’uma’u, on the floor of the
main caldera is ∼1000 m in diameter.
(Photograph by Pete Mouginis-Mark,
University of Hawai’I.)