Article,
Geophysical evidence for the locations, shapes and sizes, and internal structures of magma chambers beneath regions of Quaternary volcanism
Philosophical Transactions of the Royal Society of London. Series A, Mathematical and Physical Sciences, 310 (1514): 473--510 (Apr 27, 1984)
DOI: 10.1098/rsta.1984.0005
Abstract
This paper is a review of seismic, gravity, magnetic and electromagnetic
techniques to detect and delineate magma chambers of a few cubic
kilometres to several thousand cubic kilometres volume. A dramatic
decrease in density and seismic velocity, and an increase in seismic
attenuation and electrical conductivity occurs at the onset of partial
melting in rocks. The geophysical techniques are based on detecting
these differences in physical properties between solid and partially
molten rock. Although seismic refraction techniques, with sophisticated
instrumentation and analytical procedures, are routinely used for
detailed studies of crustal structure in volcanic regions, their
application for magma detection has been quite limited. In one study,
in Yellowstone National Park, U.S.A., fan-shooting and time-term
techniques have been used to detect an upper-crustal magma chamber.
Attenuation and velocity changes in seismic waves from explosions
and earthquakes diffracted around magma chambers are observed near
some volcanoes in Kamchatka. Strong attenuation of shear waves from
regional earthquakes, interpreted as a diffraction effect, has been
used to model magma chambers in Alaska, Kamchatka, Iceland, and New
Zealand. One of the most powerful techniques in modern seismology,
the seismic reflection technique with vibrators, was used to confirm
the existence of a strong reflector in the crust near Socorro, New
Mexico, in the Rio Grande Rift. This reflector, discovered earlier
from data from local earthquakes, is interpreted as a sill-like magma
body. In the Kilauea volcano, Hawaii, mapping seismicity patterns
in the upper crust has enabled the modelling of the complex magma
conduits in the crust and upper mantle. On the other hand, in the
Usu volcano, Japan, the magma conduits are delineated by zones of
seismic quiescence. Three-dimensional modelling of laterally varying
structures using teleseismic residuals is proving to be a very promising
technique for detecting and delineating magma chambers with minimum
horizontal and vertical dimensions of about 6 km. This technique
has been used successfully to detect low-velocity anomalies, interpreted
as magma bodies in the volume range 10^3-10^6 km3, in several volcanic
centres in the U.S.A. and in Mt Etna, Sicily. Velocity models developed
using teleseismic residuals of the Cascades volcanoes of Oregon and
California, and Kilauea volcano, Hawaii, do not show appreciable
storage of magma in the crust. However, regional models imply that
large volumes of parental magma may be present in the upper mantle
of these regions. In some volcanic centres, teleseismic delays are
accompanied by P-wave attenuation, and linear inversion of spectral
data have enabled computation of three-dimensional Q-models for these
areas. The use of gravity data for magma chamber studies is illustrated
by a study in the Geysers-Clear Lake volcanic field in California,
where a strong gravity low has been modelled as a low-density body
in the upper crust. This body is approximately in the same location
as the low-velocity body delineated with teleseismic delays, and
is interpreted as a magma body. In Yellowstone National Park, magnetic
field data have been used to map the depth to the Curie isotherm,
and the results show that high temperatures may be present at shallow
depths beneath the Yellowstone caldera. The main application of electrical
techniques in magma-related studies has been to understand the deep
structure of continental rifts. Electromagnetic studies in several
rift zones of the world provide constraints on the thermal structure
and magma storage beneath these regions. Geophysical tools commonly
used in resource exploration and earth-structure studies are also
suited for the detection of magma chambers. Active seismic techniques,
with controlled sources, and passive seismic techniques, with local
and regional earthquakes and teleseisms, can be used to detect the
drastic changes in velocity and attenuation that occur at the onset
of melting of rocks and to delineate in three dimensions the shape
of the partly melted zone. Similarly, decreases in density and electrical
resistivity in rocks during melting, can be detected. Seismic refraction
and reflection are not yet used extensively in magma chamber studies.
In a study, in the Yellowstone region, seismic delays occurring in
a fan-shooting configuration and time-term modelling show the presence
of an intense molten zone in the upper crust. Deep seismic sounding
(a combination of seismic refraction and reflection) and modelling
amplitude and velocity changes of diffracted seismic waves from explosions
and earthquakes, have enabled mapping of small and large magma chambers
beneath many volcanoes in Kamchatka, U.S.S.R. Teleseismic P-wave
residuals have been used to model low-velocity bodies, interpreted
as magma chambers, in several Quaternary volcanic centres in the
U.S.A. The results show that magma chambers with volumes of a few
hundred to a few thousand cubic kilometres volume seem to be confined
to regions of silicic volcanism. Many of the magma bodies seem to
have upper-mantle roots implying that they are not isolated pockets
of partial melt, but may be deriving their magma supplies from deeper
parental sources. Medium or large crustal magma chambers are absent
in the andesitic volcanoes of western United States and the basaltic
Kilauea volcano, Hawaii. However, regional velocity models of the
Oregon Cascades and Hawaii show evidence for the presence of magma
reservoirs in the upper mantle. The transport of magma to the upper
crust in these regions probably occurs rapidly through narrow conduits,
with transient storage occurring in small chambers of a few cubic
kilometres volume. Very little use has been made of the gravity and
magnetic maps to model magma chambers. The number of available case
histories, though few, indicate that these data can be very useful
to give constraints on the density and temperature in magma chambers.
Seismic, gravity, and electromagnetic techniques have been used to
model regional structure in several rift zones of the world. Together
the data indicate lithospheric thinning under the rifts with possible
subcrustal storage of magma and diapiric intrusions into the crust.
The current status of the use of geophysical techniques in magma
chamber studies can be summarized as follows. Though powerful experimental
methods for data collection and mathematical and computational techniques
for modelling are available, the two dozen or so available case histories
seem to represent isolated, technique-oriented studies. Only in a
few regions, such as Kamchatka, U.S.S.R., and Yellowstone and Socorro,
U.S.A., are data from multiple geophysical techniques becoming available.
Several studies in different tectonic and volcanic environments,
which use a suite of geophysical experiments capable of measuring
different physical properties of rocks and having a wide range of
resolutions, are needed to understand the problems of magma generation,
migration, and storage. Many figures, data and results presented
in this paper are from several different publications. I am indebted
to the authors and publishers for permitting their use. I am very
grateful to some of the authors who supplied photo prints of figures.
Tim Hitchcock's help in preparing and organizing the figures was
invaluable. Dr F. W. Klein, Dr W. D. Mooney, and Dr R. S. J. Sparks
reviewed the manuscript and made useful suggestions. 10.1098/rsta.1984.0005
