Volcanos in space - Skywatching

Volcanoes are important things.

They play a part in recycling the surface rocks of a world, and in building new land. For example, all the Hawaiian Islands are actually active or extinct volcanoes.

Here on Earth, volcanoes come in two main types: shield and plinian.

Shield volcanoes are formed from lava that comes from the Earth's mantle. It is mainly basalt, which runs smoothly, like molasses. It forms flat, mound-shaped volcanoes, looking like a shield in cross section. They rarely erupt explosively.

Plinian volcanoes form over subduction zones, where an oceanic plate is being pushed down and under a continental one. The descending plate material takes with it part of the continental shelf, seabed rocks and a lot of seawater. The two ingredients that make plinian volcanoes behave very differently from shield volcanoes are silica (sand) and water. The material carried down gets mixed with molten basalt and other rocks, producing new, less dense rocks, which then bubble upward to the surface, building volcanoes along the line of the subduction zone. The silica makes the lava very thick and viscous, so it tends to plug up its vents. The water, rising along with the lava, is in the form of superheated steam. The result is a colossal build up of pressure in plugged vents, causing explosive eruptions.

Mount Vesuvius is one of these plinian volcanoes. Its eruption in 79 AD, which buried Pompeii and Herculaneum, was observed by Pliny the Elder, who made the mistake of being too curious and getting too close, and described by his surviving son Pliny the Younger. That is why we refer to volcanoes that erupt explosively as "plinian." 

The volcanoes making up the "Ring of Fire" around the Pacific Ocean are plinian, and associated with subduction zones. This includes Mount St. Helens and the other volcanoes along the West Coast.

Venus is dotted with volcanoes and enormous lava flows. The volcanoes are almost all round, flat mounds – shield volcanoes. There are no signs of plinian volcanoes, which is consistent with our having as yet seen no signs of plate tectonics on that world. This is really interesting, because Venus is almost the same size as the Earth, formed from the same stuff, but so different geologically.

Maybe oceans are an important factor. Venus is a dry furnace of a world. Mars has the biggest known shield volcano in the solar system, Olympus Mons (Mount Olympus). It is almost 22 kilometres high. There are other shield volcanoes, together with large lava flows and plains, but again, no plinian volcanoes. Once Mars had oceans, but as yet no signs have been found of that world having plate motions and subduction.

The most volcanically active world in the Solar System is Io, Jupiter's closest moon. The continuous tidal "kneading," due to being close to a giant planet, is producing huge amounts of heat, leading to almost continuous eruptions of sulphur-laden magma onto the surface of the moon.

Lava is molten rock. In the outer reaches of the solar system, water is just another rock mineral. In fact, ice is a major component of those rocks. The result is mud volcanoes. When tidal distortions or other things cause the ice to melt, mud erupts onto the surface and flows across the surface as a lukewarm lava, which then freezes.

Even further out, on worlds like Pluto, where it is even colder, there are eruptions of liquid nitrogen lava.

It's likely that at some point in their lives, all worlds with solid surfaces had volcanoes. Therefore, why is it that despite the huge lava flows and lava plains on the moon, there are no large volcanoes?

• Mercury, the closest planet to the Sun, hides low in the sunset glow.
• Before dawn, Jupiter and Saturn are close together in the south and Mars in the southeast.
• The moon will be full on the 5th.