This week’s guess the planet image comes from Venus. The Large
circular features are volcanic in origin, and are called “Pancake Domes”. They are
very wide and flat with steep sides and a shallow profile. These domes are found
in Tinatin Planitia, and were imaged by the Magellan spacecraft using radar. The
NASA’s description of this image tells us that “The largest dome is 62 km in diameter. North is up.” Giving a sense of
scale to this site.
Venus is a very volcanic planet. Most of its surface is
covered by lava flows, and there are numerous clusters of shield volcanoes.
These tend not to be as tall as their terrestrial counterparts, but are
generally much wider. Some can be as much as 700 km wide, and so contain far
more erupted material than taller, but steeper volcanoes like Hawaii on Earth. Lava
domes, like the ones in our image also have terrestrial analogues. However, the
pancake domes are 100 times larger than typical lava domes on Earth. Many pancake
domes have cracks in their surfaces. Small central pits are also common.
These domes seem to have formed by the eruption of very viscous
lava. The composition of lava has a massive effect on how it behaves. Different
materials produce different eruption types and result in differently shaped
volcanoes. Viscosity determines how far, and fast lava will flow. The viscosity
of a lava is largely determined by the temperature of eruption, which in turn is
determined by the chemical composition of the rock which has melted to produce it.
There are several broad categories of lava composition, which
depend on the amounts of iron, magnesium, aluminium and silica they contain.
These control the behaviour of the volcanic processes and the morphology of the
lava flows they leave behind.
Mafic lavas have the
most Iron and Magnesium, making them basaltic in nature. Mafic lava flows have
the highest eruption temperatures and thus are not very viscous. They can flow
for very long distances, forming shield volcanoes and flood basalts. The dark
coloured Luna Maria are basaltic plains erupted early in the moon’s history. Large
shield volcanoes on Venus, Earth and Mars were also the result of this sort of
lava.
Felsic lavas are generally
the most viscous. Their composition is dominated by silica and aluminium, with
much less Iron and magnesium. This gives them the lowest eruption temperatures
and they tend to form blocky, fragmented lava fields. Andesitic or intermediate
lavas are somewhere in between. They often form steeper volcanoes, and many of
the iconic cones that we see on earth are of this type. “Ultramafic lavas” can
also be found in some areas.
These categories cover the range of types that are possible,
but the exact composition of a lava flow will depend on the precise blend of
rocks from which it formed. This results in a lot of variation, from one
volcanic area to another. We can infer a lot about the properties of a volcanic
site from its morphology, but we have to be careful that we have multiple lines
of evidence to back up these theories, as other factors can also control the
shape of a lava flow, and weathering can change things dramatically in the
years since the volcano last erupted.
Lava domes like our pancake domes require viscous flows, but
this doesn’t necessarily mean that they had to have been formed by felsic
lavas. Other factors can affect the viscosity of a lava flow, and on Earth we
find lava domes with a range of compositions. Before we can make inferences
about composition we need to look at the surrounding environment, and see which
other forms of volcanism occur in the vicinity of our domes. We can then see
whether patterns emerge in the types of feature which are generally found
together. This ultimately allows us to infer the formation process from the
landscape, and ultimately speculate on things like rock composition and lava
type.
By exploring multiple lines of evidence we can develop a
better understanding of how these features formed, and what this means for the
environment of Venus.
Happy Pancake Day!
Image Credit:
NASA Magellan team
Further Reading:
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