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Volcanoes on Earth have fascinated and terrified people for millennia, extinct volcanoes are seen all over the solar system; on Mars & Mercury and the Moon to name just a few places. Venus and Earth both have active volcanoes on their surface. However, the most volcanically active place in the solar system is Jupiter’s moon Io.

Io is kept volcanically active by the tidal forces of Jupiter and its moons pulling at it, the changes in gravity cause it to flex and bulge which generates heat inside it and causing parts of its to melt. If it was not for this tidal heating then it would no longer be volcanically active, being small and so cooling quickly.

Ongoing Volcanic Eruption at Tvashtar Catena, Io

Volcanism on Io, (NASA/JPL/UoA)

Whilst there are some other geological features on Io, such as mountains, there are few impact craters observed, which shows that it has a relatively young surface. It does have many volcanic craters. The volcanoes are dominated by basalt lavas (like those on Hawaii) these can form lava flows which flow over hundreds of kilometres. In addition to this, the volcanoes erupt lava flows of sulphur and sulphur dioxide,  The crater-like depressions which are seen on the surface, look a lot like calderas. On Earth, which form when a magma chamber is emptied and a volcano collapse in on itself (it is not known if this same mechanism occurs on Io).

The first evidence of volcanic activity was plumes of material spotted rising above the surface like a fountain. These plumes create a huge amount of material mainly sulphur dioxide which rains back onto the surface.

The highly volcanically active nature of Io means that each year the equivalent of around 1-1.5 cm of material over the whole moon is produced, a staggeringly fast rate of depositing material over a large area in geological terms. This leads to craters on the surface being hidden giving it one of the youngest surfaces of all the bodies in the solar system.


Volcanic plume on Io as seen from Voyager (Nasa/JPL/USGS)

Volcanoes dominate much of the landscape of Io, and perhaps provide some insights into what was happing other bodies in the solar system before the cooled down and became volcanically inactive. It shows how the appearance bodies can be dominated by the external gravitation influence and interaction with of other bodies.



Giant ticks on Venus

Venus is probably the only other planet still volcanically active other than Earth, it has lava flows. Venus express has shown sulphur dioxide changes and hot spots on the surface which suggest that it is still volcanically active.


Heat map of Idunn mons, showing a recent lava flow (ESA/Nasa/JPL)

The volcanoes on Venus should be similar to hotspot volcanoes on Earth like Hawaii. On Venus, they are mostly similar but on much larger scale.


Pancake domes on venus (Nasa/JPL)

Other volcanoes seen on the planet are called pancake volcanoes. 15 km diameter but only 1 km high these are an artefacts of higher viscosity lavas, which on Earth would build up to steep-walled stratovolcanos  atmospheric pressures (90 times that of Earth’s atmosphere) prevents more viscous lavas from building up vertically and the high surface temperature of Venus which allows the lava to flow for longer before setting forming pancake shapes.

Sometimes these volcanic structures can undergo collapse along the edges of them with radiating valleys around the outside, creating the appearance on an insect with legs. these are known as scalloped margin domes or Tick – Like structures.


The Tick – a scalloped dome volcano on venus (NASA/JPL)

The volcanoes of Venus show what a difference that the conditions on a planet can make to the shape and style of surface features even if the processes seem similar processes to those on Earth.

The marks on Mars

Looking at Earth, Venus, and Mercury so far has shown different systems of tectonics. The last terrestrial planet Mars has some familiar features but all is not as simple as it seems.


Mars Magnetic crust as seen by Mars Global surveyor (Nasa/JPL)

Magnetic images of Mars show band like repetitions of positive and negative anomalies at its southern hemisphere. These bands look similar to magnetic bands seen in Earth’s oceanic crust.


Bands of magnetic anomalies in the crust of the Atlantic ocean

The bands on Earth’s ocean floors represent the gradual formation of crust over time; as the plates move apart and magma rises up to form new crust it records the magnetic field as it forms, over time the periodic flipping of the Earth’s magnetic field leads to an alternating pattern.

Mars doesn’t currently have an active magnetic field, and crater dating shows that the south is older than the northern 1/3 of the planet which is topographically 3-6 km lower than the southern portion of the planet. This northern lowland does not show magnetic banding. Theories suggested for this lowland include proto-tectonics like that on Earth but which did not occur until after the planet’s magnetic field stopped and/or large impacts causing the substantial topographic difference between the two regions.

Another feature which provides clues about the nature of Martian tectonics is Tharsis Plateau, this is a vast volcanic system close to the equator and includes the 22 km high Olympus Mons, the highest volcano in the solar system. This volcano was able to grow much bigger than the hotspot volcanoes seen on Earth due to a lack of movement, if the plates had been moving then the volcano would not have developed to the same extent. This shows that Mars the plates were not moving unlike on Earth.


Olympus Mons (JPL)

The huge weight of the plateau put a lot of stress on the crust around it and led to the formation of  Valles Marineris. The extra load on the crust mean that large valleys formed close to the edge of Tharsis Plateau as the weight caused the crust to buckle and break and shear.


Valles Mariners, a crack caused by the weight of nearby volcanic plateau (NASA)

The magnetic bands suggest that early in Martian history it may have had some form of spreading ridge generating crust in a magnetic field. Both of these processes stopped as Mars cooled. These bands and the north-south topographic divide hint at an early active tectonic history, however, there are few signs of large-scale subduction which would allow recycling of plates so it seems unlikely that Mars ever developed a fully functioning multiple plate tectonic system, though an understanding of its history is far from complete.