Geoscience Reference
In-Depth Information
Anatomy of a volcano
Few volcanoes conform to the simple stereotype of a conical hill sitting above a tank-like
magma chamber with lava pouring out of a summit crater. One of the most studied volca-
noes is Etna in Sicily, and it is certainly more complex. It is a very active volcano and is
probably only about a quarter of a million years old. But the rate of activity seen over the
last 30 years cannot have continued for all that time or it would be even bigger. It's unlike
Vesuvius and the island volcanoes of Vulcano and Stromboli to the north. They are strato-
volcanoes fed from the subduction of the Ionian sea floor. Etna, by contrast, probably has
its origins in a mantle plume. But its nature appears to be changing. Measurements of the
composition of lava of different ages show that recently it began to take on more character-
istics of the subduction-fed volcanoes to the north, and the nature of its eruptions do appear
to be changing, becoming more violent and potentially dangerous.
The plumbing beneath a volcano such as Etna can be quite complex. There is not a system
of hollow pipes awaiting the magma; it must force its way up by the route of least resist-
ance. In a mantle plume, that preferred route is probably a lower-density column of mater-
ial that can be easily pushed out of the way by the teardrop-shaped mass of rising magma.
In the harder crust it must find a route through cracks and fissures. A big volcano is very
heavy and can overload the crust it stands on, causing a network of concentric cracks. After
the volcano ceases to be active it can collapse along such cracks, creating a wide caldera.
While magma continues to rise it can force its way into the cracks, forming concentric
swarms of conical sheets or ring dykes. The rise of magma within a volcano will make it
bulge and crack in a series of minor earth tremors. In the case of Etna, the summit crater
shows almost constant activity. I have made the steep climb up the loose cinder cone to
peer in during a quiescent period. Even then, the ground feels warm to the touch and there
is a smell of sulphur in the air. Gusts of steam rise from the vent, accompanied by a sound
not unlike what I might imagine would be made by a snoring giant or a dragon.
Sometimes the dragon awakes and the crater rim is not a safe place to stand. As an eruption
begins, blocks of hot rock up to a metre across can be thrown into the air. In 1979 an erup-
tion started in this way but then fell silent after a period of heavy rain caused a slump inside
the crater. However, the pressure built up and caused an explosion. Unfortunately, many
tourists were standing around the crater rim at the time. Thirty were injured and nine were
killed. Dr John Murray of the Open University recalls another occasion in 1986 when he
was watching what appeared to be a fairly normal eruption with activity slowly building
throughout the afternoon. The volcanic bombs were landing within 200 metres of the vent,
well clear of the geologists. Then suddenly their range increased to more than 2 kilometres.
Huge lumps of rock were whistling over the geologists' heads and landing around them.
