Hotspot volcanoes

Why are volcanoes on Mars so different from those on Earth?

This week, NASA announced that it had discovered one of the youngest, possibly the the youngest major volcano on Mars to date.

Arsia Mons – a gigantic 110 kilometer (68 mile) caldera, which is also more than twice the height of Everest – was previously known, but new satellite observations have been used to get a better timeline for its extensive lava flows. The youngest is probably around 50 million years old.

Although this was widely reported to be a dead volcano alongside the dinosaurs, in reality it meant that it had largely survived the heavy beasts of around 16 million years ago. Oddly, this volcano tended to erupt only once every 1-3 million years, whereas shield volcanoes on Earth erupt almost continuously. Hawaii’s Kilauea, for example, has spewed lava continuously since 1983.

Arsia Mons, possibly the youngest major volcano on Mars. (NASA/JPL-Caltech/SSI)

NASA/JPL-Caltech/SSI

You may also have noticed that Earth is covered in volcanoes, while Mars, although pockmarked by volcanic provinces, does not appear to have any active volcanoes anymore. So what’s the problem ? Why is Mars volcanically flawed, but our own pale blue dot alive with the sound of explosions?

First, let’s go over what you need to make a volcano. Essentially all that’s needed is a heat source – and as far as we know there are three powerful enough to make a difference.

When rocky planets are formed by the violent accumulation of space debris, they become incredibly hot. By the time they reach a certain size, the intense gravitational pressure from the celestial body ensures that the planet’s core is at an unfathomable temperature. This huge, ancient cache of thermal energy is known as “primordial heat.”

As the world begins to cool and different parts of the sphere differentiate into a crust, mantle, and core, a temperature and pressure gradient takes place, with both variables decreasing dramatically as you get closer. from the surface. In order to balance the gradient, this primordial heat escapes to the surface, and this heat – in complex ways – is needed to melt rock, produce magma, and fuel volcanoes.

However, this is not the only source of heat. Many elements in rocky worlds are radioactive, and over time they decay, which also releases heat. On Earth, this “radiogenic heat” provides up to 50% of the planet’s internal thermal energy – on Pluto, where there are liquid lakes of nitrogen, radiogenic heat would be entirely responsible for the internal burning of this dwarf planet.

NASA/JPL-Caltech

If you look at Io, Jupiter’s volcanic moon, you will see more volcanoes there than anywhere else in the solar system. The volcanism of this hellish realm is driven by something called “tidal warming”, where Jupiter’s immense gravitational pull – and the amplification provided by the orbits of neighboring moons Europa, Ganymede and Calisto – tears apart the interior of Yo. This generates frictional heating, which melts rock and provides the moon’s many volcanoes with a near-perpetual supply of magma.

So those are our three heat engines. The Earth, being quite large and full of radioactive elements, is still very hot inside. Consequently, we have a huge range of volcanoes, from hotspot monsters fed by fountains of fire rising from the outer core-mantle boundary, to complexes that form when one tectonic plate descends beneath another.

Mars, however, is just over half the size of Earth, and as a result has already cooled so much that there isn’t enough primordial heat left to keep the magma molten. There’s clearly not enough radioactivity inside to run the furnace either, and there’s nothing massive enough nearby to generate significant tidal heating. So today it’s a dead planet.

NASA/JPL-Caltech

In fact, Mars may have always been a failed volcanic planet in some respects. Plate tectonics is necessary to create “evolved” magmatic compositions and a variety of volcanoes – and there’s no solid evidence that it ever worked on our planetary neighbor. Without the movement of the continents, the chemistry of the planet becomes quite dull and your volcanic repertoire is sorely limited.

So what types of volcanoes were active on Mars? Mainly, you might find shield volcanoes – impossibly large mountains that are much wider than they are tall, fueled by colossal plumes from the mantle. Take Olympus Mons, the largest volcano ever discovered, for example: this titan is 25 kilometers (16 miles) tall but it’s also a breathtaking 624 kilometers (374 miles) long. It’s about the same size as Arizona.

Shield volcanoes on Earth are so large because they are fed by mantle plumes for many millennia. The lava that erupts is basic, unevolved, and extremely fluid, meaning it spreads over long distances. Give these shields enough time and they grow to immense sizes.

Mars is no exception, but all of its shield volcanoes are much larger than ours. Why is that?

Well, on Earth, our tectonic plates move gradually and continuously, but the mantle plumes below stand still. Thus, in about 100,000 years, Hawaii’s Kilauea will be gone, and the baby volcano growing off its coast, Loihi, will become the main source of volcanism in the region.

Mars, as mentioned above, has no plate tectonics, meaning underlying mantle plumes have continued to melt rock beneath the same location for millions of years. The shield volcanoes there have grown so large that if they were more massive, they would break through the Martian crust and fall back into the mantle.

There are also a few other types of volcanic activity on Mars. In the past, there was much more water (and ice) available in certain regions of the planet. Under certain mixing conditions, the interaction of water and magma causes a powerful explosion and a very rapid release of heat into the surrounding medium.

When magma is involved, it’s called a phreatomagmatic explosion – when it’s not, and hot rock is the trigger, it’s called a hydrothermal explosion. Based on characteristic glassy deposits spotted by orbiting satellites, scientists know that both types occurred frequently on Mars in the distant past.

Most of the time, however, Mars was once dominated by lava flows, some reaching the size of some Earth continents. It’s absolutely quiet there these days, but it was once a much more impressive world.

In fact, about 3.5 billion years ago, a gargantuan eruption produced so much lava for so long that it effectively emptied part of the mantle on top of the crust. This had the effect of tilting Mars by 20°, which would amount to moving the North Pole towards Paris or Lisbon towards the Sahara desert.

Arsia Mons represented the last significant period of volcanic activity on the Red Planet that we know of. The youngest lava flow anywhere on Mars, however, is no less than 10 million years old – the last gasp of a tiny Martian magma chamber and the last light to go out.