The Jupiter system - Introduction to Planetary Geomorphology - page 153

Horizons flyby and are likely to continue erupting for
years to come.
With one or two exceptions, the volcanoes on Io are not
classic cone-shaped mountains like Mount Fuji. Rather,
they are wide, low-pro le features composed of lava ows
that spread great distances from their sources and are
termed paterae. In some cases individual ows can be
traced more than 500 km, suggesting that they must have
been extremely high-temperature, low-viscosity lavas.
This suspicion was con rmed when temperatures as
high as 1,340 °C were recorded, higher than those of
most lavas erupted on Earth today. However, very early
in Earth ' s history, a type of lava called komatiite was
produced, and it is estimated that its eruptive temperatures
were somewhat similar to those observed on Io.
Moreover, komatiite is very rich in iron and magnesium,
which is consistent with estimates of the compositions of
some of the lava flows on Io. Planetary volcanologists
conclude that we might be seeing a type of volcanic
activity on Io comparable to some eruptions on Earth ~3
Ga ago.
Williams and Howell ( 2007 ) classi ed three styles of
eruptions on Io: (1) flow-dominated (effusive) eruptions,
(2) explosion-dominated eruptions, and (3) intra-patera
volcanism. Flow-dominated eruptions, typi ed by the
Amirani lavas (Fig. 8.7) , produce large ow fields that
emanate from central vents or ssures over months to
years, similar to eruptions in Hawaii. The colors of the
lavas and temperature measurements suggest that they
are basaltic. These eruptions produce low-pro le shield
volcanoes such as Maasaw Patera (Fig. 8.8) and pancake-
shaped features such as Apis Tholus (Fig. 8.9) . Some
flows have associated bright deposits that appear to be
generated from the lavas flowing over sulfur dioxide
deposits (Fig. 8.10) . Flow-dominated eruptions can also
include explosions, producing Prometheus-type plumes.
Named for their type locality on Io, such explosions
produce umbrella-shaped plumes of sulfur dioxide
100 - 200 km high that form circular ash deposits around
their vents.
Explosion-dominated eruptions produce enormous
plumes that can exceed heights of 500 km, as well as
large ow fields of lava, but they are short-duration,
sporadic, high-energy events called “ outbursts. ”
Typi ed by Pele (Fig. 8.5) , these eruptions eject mate-
rials at speeds as high as 1 km/s, and rain huge quan-
tities of reddish sulfur and silicate ash onto the surface
(Fig. 8.11) . The associated lava flows involve very
high rates of effusion; for example, the emplacement

Figure 8.7. The vent for the Amirani lava ow- eld is in the
lower center of this picture of Io; compound lava ows extend
250 km north, toward the top of the image. The smaller ow
extending west from the vent was found to have a high temperature,
suggesting that it was active at the time of the observations; some
flows extending to the south were tube- and channel-fed (NASA
Galileo PIA02506).

of the Pillan lava eld (Fig. 8.11) was estimated to
involve flow rates of 1,740 to 7,450m 3 /s, comparable
to the largest rates on Earth for basaltic lavas in recent
time.
Intra-patera eruptions occur within calderas and can
accompany some explosive eruptions. For example, Loki
volcano (Fig. 8.12) repeatedly changes temperature and
shape with time, while its caldera contains con ned lava
lakes with surface crusts that are disrupted by magma
circulation in the lava lake.