Diagrammatic section of an active volcano.
a, Central pipe or funnel. b, b, Walls of the crater or cup. c, c, Dark turbid cloud formed by the ascending globular clouds d, d. e, Rain-shower from escaped vapour. f, f, Shower of blocks, cooled bombs, stones, and ashes falling back on to the cone. g, Lava escaping through a fissure, and pouring out of a cone opened in the mountain side.
In the funnel a which passes down from the crater or cup b, b, white-hot lava was surging up, having a large quantity of water and steam entangled in it. The lava, or melted rock, would be in much the same state as melted iron-slag is, in the huge blast-furnaces in which iron-rock is fused, only it would have floating in it great blocks of solid rock, and rounded stones called bombs which have been formed from pieces of half-melted rock whirled in air and falling back into the crater, together with clinkers or scoriæ, dust and sand, all torn off and ground down from the walls of the funnel up which the rush was coming. And in the pipe of melted rock, forcing the lava upwards, enormous bubbles of steam and gas d, d would be rising up one after another as bubbles rise in any thick boiling substances, such as boiling sugar or tar.
In the morning before the eruption, when only a little smoke was issuing from the crater, these bubbles rose very slowly through the loaded funnel and the half-cooled lava in the basin, and the booming noise, like that of heavy cannon, heard from time to time, was caused by the bursting of one of these globes of steam at the top of the funnel, as it brought up with it a feeble shower of stones, dust, and scoriæ. Meanwhile the lava surging below was forcing a passage g for itself in a weak part of the mountain-side and, just at the time when our attention was called to Vesuvius, the violent pressure from below rent open a mouth or crater at h, so that the lava began to flow down the mountain in a steady stream. This, relieving the funnel, enabled the huge steam bubbles d, d to rise more quickly, and to form the large whitish-grey cloud c, into which from time to time the red-hot blocks, scoriæ, and pumice were thrown up by the escaping steam and gases. These blocks and fragments then fell back again in a fiery shower f, f either into the cup, to be thrown up again by the bursting of the next bubble, or on to the sides of the cone, making it both broader and higher.
Only one feature in the diagram was fortunately absent the evening we went up, namely, the rain-shower e. The night, as I said, was calm, and the air dry, and the steam floated peacefully away. The next night, however, when many people hurried down from Rome to see the sight they were woefully disappointed, for rain-showers fell heavily from the cloud, bringing down with them the dust and ashes, which covered the unfortunate sight-seers.
This was what happened during the eruption, and the result after a few days was that the cone was a little higher, with a fresh layer of rough slaggy scoriæ on its slopes, and that on the side of the mountain behind the Hermitage a new lava stream was added to the many which have flowed there of late years. What then can we learn from this stream about the materials which come up out of the depths of the earth, and of the manner in which volcanic rocks are formed?
The lava as I saw it when coming first out of the newly-opened crater is, as I have said, like white-hot iron slag, but very soon the top becomes black and solid, a hard cindery mass full of holes and cavities with rough edges, caused by the steam and sulphur and other gases breaking through it.[1] In fact, there are so many holes and bubbles in it that it is very light and floats on the top of the heavier lava below, falling over it on to the mountain-side when it comes to the end of the stream. Still, however, the great mass moves on, so that the stream slides over these fallen clinkers or scoriæ. Thus after an eruption a new flow consists of three layers; at the top the cooled and broken crust of clinkers, then the more solid lava, which often remains hot for years, and lastly another cindery layer beneath, formed of the scoriæ which have fallen from above (see Fig. 40).
Fig. 40.
Section of a lava-flow. (J. Geikie.)