Lava-streams vary greatly in size; in some cases the lava, escaping from craters, comes to rest before reaching the base of the slopes of the volcano; in other cases a lava-flow not only reaches the plains below, but extends for many miles over the surrounding country. Hence lava-streams are important geological agents. Let us look at some famous instances. The most stupendous flow on record was that which took place from Skaptar Jökull in Iceland, in the year 1783. In this case a number of streams issued from the volcano, flooding the country far and wide, filling up river gorges which were in some cases six hundred feet deep and two hundred and fifty feet broad, and advancing into the alluvial plains in lakes of molten rock twelve to fifteen miles wide and one hundred feet deep. Two currents of lava which flowed in nearly opposite directions spread out with varying thickness according to the nature of the ground for forty and fifty miles respectively. Had this great eruption taken place in the south of England, all the country from the neighbourhood of London to that of Gloucester might have been covered by a flood of basalt of considerable thickness.
Sometimes, when the lava can only escape at a point low down on the mountain, a fountain of molten rock will spout high into the air. This has happened on Vesuvius and Etna. But in an eruption of Mauna Loa, in the Sandwich Islands, an unbroken fountain of lava, from two hundred to seven hundred feet high and one thousand feet broad, burst out at the base of the mountain; and again in April, 1888, the same thing happened on a still grander scale. In this case four fiery fountains continued to play for several weeks, sometimes throwing the glowing lava to a height of one thousand feet in the air. Surely there can be no more wonderful or awful sight than this in the world.
The volcanoes of Hawaii, the principal island in the Sandwich Islands, often send forth lava-streams covering an area of over one hundred square miles to a depth of one hundred feet or more; but they are discharged quite quietly, like water welling out of a spring. Repeated flows of this kind, however, have in the course of ages built up a great flat cone six miles high from the floor of the ocean, to form this lofty island, which is larger than Surrey; and it is calculated that the great volcanic mountain must contain enough material to cover the whole of the United States with a layer of rock fifty feet deep.
But it is not only on the surface of the land that volcanic eruptions take place; for in some cases the outbreak of a submarine eruption has been witnessed, and it is highly probable that in past geological ages many large eruptions of this nature have taken place. In the year 1783, an eruption took place about thirty miles off the west coast of Iceland. An island was built up from which glowing vapour and smoke came forth; but in a year or less the waves had washed everything away, leaving only a submerged reef. The island of Santorin, in the Greek Archipelago, is a partly submerged volcano.
But in some cases enormous outpourings of lava have taken place, not from volcanoes, but from openings of the ground here and there, and more usually from long fissures or cracks in the rocks lying at the surface. In many cases so much lava has quietly welled out in this way that the old features of the landscape have been completely buried up, and wide plains and plateaux formed over them. Sir A. Geikie says,—
"Some of the most remarkable examples of this type of volcanic structure occur in western North America. Among these that of the Snake River plain in Idaho may be briefly described.
"Surrounded on the north and east by lofty mountains, it stretches westward as an apparently boundless desert of sand and bare sheets of black basalt. A few streams descending into the plain from the hills are soon swallowed up and lost. The Snake River, however, flows across it, and has cut out of its lava bed a series of picturesque gorges and rapids.
"The extent of country which has been flooded with basalt in this and adjoining regions of Oregon and Washington has not yet been accurately surveyed, but has been estimated to cover a larger area than France and Great Britain combined. Looked at from any point on its surface, one of these lava plains appears as a vast level surface, like that of a lake bottom. This uniformity has been produced either by the lava rolling over a plain or lake bottom, or by the complete effacement of an original, undulating contour of the ground under hundreds of feet of lava in successive sheets. The lava, rolling up to the base of the mountains, has followed the sinuosities of their margin, as the waters of a lake follow its promontories and bays."
A few further examples of mud-lavas may be mentioned here. Cotopaxi, a great volcano in Ecuador, South America, with a height of 17,900 feet, reaches so high into the atmosphere that the higher parts are capped with snow. In June, 1877, a great eruption took place, during which the melting of snow and ice gave rise to torrents of mud and water, which rushed down the steep sides of the mountain, so that large blocks of ice were hurried along. The villages around to a distance of about seventy miles were buried under a deposit of mud, mixed with blocks of lava, ashes, pieces of wood, etc.
Sometimes a volcano discharges large quantities of mud directly from the crater. In this case the mud is not manufactured by the volcano itself, but finds its way through fissures and cracks from the bed of the neighbouring sea or rivers to the crater. Thus, in the year 1691, Imbaburu, one of the Andes of Quito, sent out floods of mud containing dead fish, the decay of which caused fever in the neighbourhood. In the same way the volcanoes of Java have often buried large tracts of fertile country under a covering of volcanic mud, thus causing great devastation.