Page 27 - Caldera Volcanism Analysis, Modelling and Response

2 Fidel Costa

Abstract
This paper reviews the times that silicic magmas related to major caldera systems spend
in the crust prior to eruption. The significance of the time information is evaluated and
combined with magma volumes and temperatures to quantify the mass and thermal
fluxes associated to calderas. The data discussed includes the largest explosive
eruptions on Earth: Taupo Volcanic Zone (New Zealand), the Youngest Toba Tuff
(Indonesia), Yellowstone system (USA), Long Valley (USA), Carter Lake (USA), Valles-
Toledo complex (USA), La Garita caldera (USA), La Pacana (Chile) and Kos (Greece).
Magma residence times are calculated from the difference between the eruption age
and the age obtained by radioactive clocks and minerals that are a closed system at high
magmatic temperatures (e.g., U–Pb system in zircon).
Large ranges of residence times between different systems are found. The shortest
residences (4–19 ky) are those of some magmas from the Taupo Volcanic Zone (Oruanui
and Rotoiti) and Yellowstone (Dry Creek and Lava Creek). There is not a good correlation
between magma volume and residence time, although most eruptions o10 km 3
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have residence times o100 ky, and those W100 km have longer residences, some up to
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300–500 ky (Fish Canyon, La Pacana). The residence times of some small (o10 km ) pre-
and post-caldera magmas indicate that they were extracted from the same reservoir
as the caldera-forming magma (e.g., Long Valley, Taupo). However, the time information
from most small-volume magmas seems to reflect the recycling of crystals from previous
cycles of caldera-forming magmas (Yellowstone), from plutonic rocks of the same
caldera cycle with or without erupted equivalents on the surface (Crater Lake, Taupo,
Long Valley), or from a partially solidified magma reservoir (Taupo). These interpreta-
tions are in agreement with cooling rates and solidification times obtained from simple
thermal models of magma reservoirs.
Magma production rates were calculated from the ratio of erupted volume and
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3 1
residence time, and they vary between o0.001 km y for small deposits (o10 km )
3 1
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and ca. 0.1 km y for the Oruanui eruption (530 km ). Estimates for most eruptions
3 1
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W500 km are within 272 10 2 km y . These high magma production rates are
probably transient and comparable to global eruptive fluxes of basalts (e.g., Hawaii).
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Magma cooling rates for deposits W100 km were calculated from the difference
between the liquidus and pre-eruptive temperatures over their residence times, and
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they vary between 2 10 4 and 3 10 3 Ky . Integration of the calculated residence
times and magma fluxes with a simple rheological model of the crust is not possible and
should be a main topic of research if we are to understand the mechanisms and rates
which permit large amounts of silicic magma to be stored below calderas.
1. Introduction

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Caldera-forming eruptions produce the most voluminous (up to 5,000 km )
explosive eruptions on Earth, and their activity appears to provide clues for
understanding climatic and evolutionary biological changes (e.g., Lipman, 2000a;
Francis and Oppenheimer, 2003). Collapse calderas are among the most
investigated geological objects also because of their association with economic
deposits and geothermal energy. The distinctive feature of caldera-related silica-rich