When is a volcano dead? You never can tell. A volcano goes off when it wants to, quite regardless of the fact that it has had the reputation for a thousand years of being dead. And the worst of it is volcanoes are like guns—only more so. A gun doesn't shoot any harder because it wasn't supposed to be loaded; but the volcano, if it breaks out unexpectedly, is violent in proportion to the length of time it has been apparently dead. This is the reason. The original vent becomes plugged up with the cooled lava. This plug being harder than the rest of the mountain, the next outbreak is forced to take a new course, and the longer the forces of explosion are held back the greater the accumulation of energy and the more violent the discharge.
But why do volcanoes go off at all? Why can't they be quiet and well-behaved like other mountains? Nobody knows for sure. On one thing all scientific men seem to be now agreed; namely, that while the rocks inside the earth are hot enough to melt they are hard as steel, owing to the tremendous pressure of the rocks above them, and one theory about volcanic eruptions is that they are caused by the release of the pressure on this rock in one place and a pressing down in another, as the earth's crust settles and crumples around the centre. Some of this rock—that on which the pressure is released—melts and rises under the folds of rising rock, and so makes the granite hearts of the greater mountains. Some of it wells up through the cracks in the rock and spreads in lava fields, while some of it gushes up and explodes at the points where cracks cross and so make volcanoes.
This is one theory, but there are others. The latest is so big that we will have to take it into the mind in sections.
THE LATEST THEORY OF ERUPTIONS
1. Imagine the interior of the earth divided into three zones. The central zone, of course, is the hottest. Between this central zone and the zone reaching down forty miles or so from the surface is a middle zone. (Think of a doughnut ball inside a doughnut ring, with space between the ball and ring. That will give you the idea.)
2. From what is known of the laws of heat it is assumed that the flow of heat from the central to the middle zone is greater than the loss of heat from the central to the outer zone. Thus the heat income of the middle zone would constantly exceed its outlay, and so it would get hotter and hotter.
THE MYSTERIOUS SHAFT OF MOUNT PELÉE
In 1902, after the first explosion, Mount Pelée continued its eruptions for several months, and in the late stages there slowly rose, through the crater, this strange shaft of red-hot lava, like a great iron beam forged by giant hammers in Vulcan's famous blacksmith-shop. As it rose it crumbled and finally fell to pieces. It was forced up by the gases beneath and shaped by the crater through which it came; but can you conceive of anything more weird and awesome?
3. This middle zone is made up of different kinds of rock that require different degrees of heat to melt them. So some parts of this zone would melt and form pockets of liquid rock, while other parts were still unmelted.