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摘要
Advancing our knowledge of caldera volcanoes enables better assessment of hazard and more efficient harnessing of resources. In this paper we review developments in concepts of magma storage and transport during the life cycle of caldera plumbing systems. We draw together: a) geological, geochemical and petrological data from intrusions and eruption deposits; b) geophysical and geochemical data from modern restless calderas; and c) geological and structural evidence from ancient calderas as well as insights from numerical and analogue models. Overall, magma plumbing systems beneath calderas develop incrementally as magma rises, intrudes and rejuvenates. Eventually accumulation and eruption of a sufficient magma volume drives subsidence of the plumbing system roof to form a caldera. The magma plumbing system may then reside relatively unchanged or continue to re-intrude on a variety of scales. Consequences include continued eruptions, crustal resurgence, or new cycles of caldera formation.
Large magma volumes characteristic of calderas may evolve as a single progressively-enlarging reservoir or through the rapid amalgamation of small, initially-independent magma pockets. Eruptible magmas may reside at depths of up to 17 km, but typically lie at shallower depths as a caldera system evolves. Timescales of subcaldera magma residence reveal two remarkable concepts: (1) portions of melt within a magma may remain molten for > 106 years, and (2) melt can be created and mobilized in a few thousand years or less. Geophysical and geochemical data illustrate the present state of active sub-caldera plumbing systems and their development on timescales of hours to years. These studies commonly reveal aseismic, low-velocity zones at depths > 6 km with spatial extents that can be larger than the caldera. The seismic attributes are consistent with rock hosting magma bodies of variable volume and melt content. These are commonly overlain by shallower low-velocity zones linked with ground deformation. The exact nature of these shallower zones is unclear, but interpretations often include shallow sills and laccoliths, and hydrothermal circulation is likely a key process as well.
Seismicity and geodetic data record the interplay between magma movement and crustal deformation at calderas. Together with evidence from field studies, numerical simulations and analogue models, such data show that magma migration at calderas may involve considerable lateral transport through dykes or sills to a site of eruption. While caldera-related magma intrusions commonly exploit structures produced by caldera subsidence, they may also follow regional tectonic structures that extend well beyond the border of the caldera. The increased structural complexity that occurs as a caldera evolves increases the permeability of the crust. This may promote small volume eruptions and shallow storage of magma in the post-collapse phase.
Our review highlights the significant progress made in understanding the range of intrusion styles and timescales that define the magma plumbing beneath calderas. Yet there are still key questions that limit the application of this understanding to directly benefit humanity: (1) What are the limits to the detection of shallow magma bodies, and can we assess the eruption risks associated with magma bodies of different sizes, depths, compositions and crystal contents? (2) Is it viable to generate electricity by extracting heat directly from magma through the sub-caldera plumbing system?
Geophysical and drillhole data from Krafla caldera, Iceland, show that rhyolite can exist undetected at shallow levels within the caldera and may not represent a hazard even when intersected by a borehole. Current work at Krafla is assessing the possibility of extracting geothermal energy from shallow magma bodies and/or superheated steam zones directly above. However, the extent and connectivity of this magma to larger volumes in time and space, as well as its applicability to other systems, may only be answered with continued focused magma drilling, geophysical experimentation and geological exploration.
- 出版日期2018-2
