Page 231 - Fundamentals of Physical Volcanology

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208 CHAPTER 13

oxygen, sodium, and other volatile elements accom-
panies Io in its orbit around Jupiter. The ease with
which gases are lost from an atmosphere depends
on their molecular weight, and the values for water,
carbon dioxide, diatomic sulfur (the commonest
form), and sulfur dioxide are close to 18, 44, 64,
and 64, respectively. The inference is that over the
course of geological time Io has erupted and lost
essentially all of the water and carbon dioxide from
its mantle. It may well have erupted most of the
sulfur-related compounds onto the surface, but
they have not yet been completely lost but instead
are recycled through volcanic activity. The recy- Fig. 13.19 Near the left edge of this 250 km wide image a
mildly explosive eruption is taking place in one of the chain
cling is not in the form of subduction back into the
of calderas forming the Tvashtar Catena on Io. The image is
mantle, for there are no signs of plate tectonics on
a composite of data obtained through near-infrared, clear,
Io, but instead consists of sulfur and sulfur dioxide
and violet filters on the Galileo spacecraft. (Image courtesy
collecting on the surface as solids, being buried of NASA/JPL/University of Arizona.)
under layers of pyroclasts and lavas, being heated
by the geothermal gradient, and melting to form
“aquifers” in the crust. The dikes feeding new erup- whether these magma reservoirs have formed at
tions sometimes cut through these aquifers and the neutral buoyancy levels in the crust, but this is hin-
volatile-poor magmas in the dikes absorb the aquifer dered by uncertainties about the volatile content, if
liquids. The liquids then boil to vapor and drive the any, of the ascending magmas and also the density
violently explosive eruptions just as if they were structure of the crust. The low value of the acceler-
expanding magmatic volatiles. The heights of the ation due to gravity on Io would lead us to expect
largest plumes, commonly up to at least 300 km, that levels of neutral buoyancy might be at depths
and their widths, up to 1200 km, imply eruption of order 15 km and that magma reservoirs might
−1
speeds of more than 1000 m s , and the analysis in have large vertical extents.
Chapter 6 shows that this could be achieved only if Many calderas are clearly the sites of eruptions
the magma contained about 30 wt% of volatiles. from fissures, both explosive and effusive (Fig. 13.19).

This is vastly more than could be dissolved in any Attempts have been made to estimate eruption rates
possible juvenile magma from the mantle. by analyzing the sizes and dynamics of plumes, by
Some plumes seem to be caused not by dikes measuring the heat release rate from eruption sites,
absorbing liquid volatiles but by lava flows advanc- and by measuring the increases in areas of lava
ing over a surface rich in solid volatiles. A finite time flows with time. The latter is not easy because the
is taken for enough heat to be conducted down- Galileo spacecraft only made close approaches to
ward to melt and evaporate the solids, and so jets of Io every few months. Also, the resolution of the
gas punch holes through many places in the lava images was not good enough to allow flow thick-
flow after its front has gone by and combine to form nesses to be estimated accurately. Nevertheless,
a plume. This is somewhat akin to the formation of there is some convergence on eruption rates up
3 −1
5
3 −1
6
pseudocraters around rootless vents in terrestrial to 10 m s with occasional bursts up to 10 m s
3
lava flows crossing marshy ground, and also can and erupted volumes up to 100 km . The high erup-
be thought of as a kind of hydromagmatic activity, tion rates are consistent with the high temperatures
although the volatile involved is not water. and low viscosities of the magmas and, if the lava
There are many calderas on Io (Fig. 13.19), typ- flows are being fed from magma reservoirs beha-
ically with diameters of several tens of kilometers, ving elastically, the arguments in Chapter 4 imply
implying that beneath them are magma reservoirs that the reservoirs should have volumes of up to
3
of similar size. Attempts have been made to infer 30,000 km . This is possible with ∼50 km diameter