VOLCANOES OF FIRE AND ICE

Volcanoes can blow hot and cold, but what’s the difference between the two?

Two main types of volcanoes exist in space. The first and most familiar is the type that spews out molten rocks, typically at high temperatures of at least 700 degrees Celsius (1,292 degrees Fahrenheit). These exist on terrestrial planets and moons that are composed primarily of metals or silicate rocks – in our Solar System they tend to be closest to the Sun. When they erupt, magma leaves the volcano and reaches the surface; it then becomes known as lava. The volcanoes on Io, Venus and Mars – both active and extinct – are of this type. Our Moon has also had such volcanism. “Recently, volcanism that was around 100 million years old was discovered on the Moon,” says Khan, pointing to NASA's Lunar Reconnaissance Orbiter, which showed the Moon's volcanic activity gradually slowed over time.

The second type of volcano is very different. Cryovolcanoes, which are colloquially known as ice volcanoes, still have a heated interior, but they spew water mixed with ammonia or methane rather than molten rock. Cryovolcanoes exist on icy moons such as Enceladus and Titan, which circle Saturn. “The water that comes up from the liquid ocean beneath the icy crust of these moons behaves very similarly to lava,” says Lopes. “It’s defined as volcanism because it's a process of bringing material to the surface.”

1 Tidal heating

When there is tidal friction, the interior of the moon starts to become very hot.

2 Melting ice

It heats a pressurised H2O pocket that melts ices. Because the heat has to somehow escape, it begins to push upwards to the body’s surface.

3 Eruption

When it breaks through, it sends a water vapour plume and ice particles into the air. The friction heats nitrogen, which builds pressure and erupts.

4 Main vent

A composite volcano’s magma escapes through a large main vent at the top.

5 Shield volcano

Wider than composites, they have gentle, sloping sides; lava is able to flow out easily.

6 Multiple eruptions

Layers of hardened lava build up. A conical appearance appears as a result.

7 The crust

Magma breaks through the crust on its route up to the surface.

8 Secondary vents

The magma seeks other outlets and can also escape through secondary vents.

9 Shield magma

The magma chamber of a shield volcano is spread over a wider subsurface area.

10 Lava channels

Venus has lava channels. Its longest is 6,800 kilometres (4,200 miles) long.

11 Rising magma

As magma rises, pressure builds. The buildup is more intense under a composite volcano.

12 Chamber

Beneath the surface is molten liquid rock, pooled in what is called a magma chamber.