While Earth has more than 1300 active volcanoes, it is not the only celestial body within the
solar system which has active volcanism.
For example, Jupiter's sulfur covered moon Io contains several hundred shield volcanoes,
sometimes producing eruptions which send lava fountains and ash into space.
In contrast, Mars from a volcanic perspective is traditionally thought of as a dead planet.
The most notable volcanic feature on Mars is Olympus Mons, which covers an area the
size of France and stands at 25 kilometers or 16 miles in height.
Its last caldera forming eruption occurred 140 million years ago.
However, more recent evidence has suggested that this volcano produced another smaller
eruption a mere 2.4 million years ago.
Then, in 2020 a far younger volcanic deposit was spotted.
In a different section of the planet, a large ash deposit was spotted adjacent to a fissure
system which was dated as young as 53,000 years in age.
This deposit covers 217 square kilometers of ground, containing two discernable layers.
The inner and darker layer represents pyroclastic flow deposits formed when the eruption column
collapsed downwards, releasing energetic currents of gas and volcanic particles.
The outer lighter layer represents an ashfall deposit, where particles of volcanic ash fell
after drifting from the prolonged eruption.
Using a series of methods, scientists were able to estimate the total volume of these
ash and pyroclastic flow deposits which gave a figure of between 0.011 and 0.028 cubic
kilometers.
This means that on the 0 to 8 volcanic explosivity index the eruption had a rating of a 3, and
was near identical in size to the 3.5 month long explosive eruption of Mount Redoubt during
2009 in Alaska.
Very little lava was ejected during this Martian eruption, which was presumed to have sent
a column of ash to a height of 10 kilometers or 6 miles, while pyroclastic flows reached
up to 6.5 kilometers or 4 miles distant.
You might see these figures and think "wait, that was only a VEI 3 eruption?".
The reason why these numbers sound high relates to both the low gravity on Mars which is about
1/3rd that of Earth, and the planet's overall thin atmosphere which is about 0.6% that on
Earth.
As a result, pyroclastic flows travel 3 times as far and eruption columns reach 5 times
higher into the atmosphere.
Converting these figures to what would have occurred if the same eruption happened on
Earth, the numbers seem much more like those expected in a VEI 3 eruption.
For context, the 36 kilometer long fissure which produced this explosive eruption is
part of a much larger system of fissures and faults which is referred to as Cerberus Fossae.
In this section of Mars, the crust is inferred to be slowly spreading apart to the north
and south.
Any deep magma within the crust can thus potentially use these fissures as a path of least resistance,
resulting in a fissure eruption on the surface.
Although the exact reason why these lengthy faults exist, one model suggests that the
presence of the Tharsis volcanoes to the east to be a cause.
However, it is equally as likely that these fissures were created in part by a series
of ancient massive dike intrusions at depth.
There is additional evidence that magmatic dikes still may be present underneath a section
of these faults.
Cerberus Fossae is one of the most seismically active regions on Mars, and may marsquakes
behave in a manner to that a volcano tectonic earthquake would on Earth.
Although it is unknown what triggered the basaltic fissure eruption at Cerberus Fossae
sometime between 210,000 and 53,000 years ago, this age range has an interesting overlap
with a prominent nearby impact crater.
The Zunil Crater is located 11 kilometers to the southeast of the young eruption deposit,
and based on the low levels of erosion and other impact craters in its center may have
formed at the same time that the volcanic eruption occurred.
Although this cannot be yet proven, it is possible that the force of the impact which
created the 10 kilometer wide impact crater jolted deep magma, causing it to rise to the
surface and produce a new volcanic eruption.
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