It is believed by some geologists that a subterranean channel connects the volcanic vent of Hecla with the great central one of Askja. This theory is based on the fact that a number of lava floods have burst forth simultaneously at different times[p. 154] at great distances from the volcanoes, leading to the supposition that innumerable subterranean channels exist in the neighbourhood.
The eruptions attributed to the volcano of Hecla vary much in number, some authorities saying there have been 40. Mrs Somerville quotes them at 23, and Mr Locke, in his 'Guide to Iceland,' at 17 in number. In the latter's work is given a table of most of its principal eruptions. One of these was of a singular nature; huge chasms opened in the earth, and for three days the wells and fountains became as white as milk, and new hot springs sprang into existence.
The twelfth eruption of this mountain was also of unusual violence. It occurred in January 1597. For twelve days previously to the outbreak loud reports were heard all over the Island, while no less than eighteen columns of fire were seen ascending from it during its eruption. The ashes it threw out covered half the Island.
The seventeenth eruption commenced on the 2d September 1845, and continued for seven months. On this occasion the ashes were carried over to Shetland, and the columns of smoke rising from the mountain reached a height of 14,000 Danish feet.
Such is a brief description of the tremendous forces which dominate Iceland. Here Nature works in silence for long periods beneath the crust of the earth, and then, with little or no forewarning, bursts forth in uncontrollable fury, ruthlessly devastating with its fiery streams whatever impedes its course.[p. 155]
Who can wonder that, under such existing terrors, the scanty inhabitants of the Island are a sad and dejected race. A people with death and terror continually at their doors can hardly be otherwise; whilst competitive industry, energy, and hopeful prosperity are alike suppressed by the constant devastations which occur.
With respect to the Thermal Springs, these must be considered as products of the same underground fires, and which form a second characteristic of Iceland.
These Springs may be divided into three kinds, viz., those of unceasing ebullition, those which are only sometimes eruptive, and wells which merely contain tepid water, though supposed to have been formerly eruptive.
Professor Bunsen, who passed eleven days by the side of the Great Geyser in Iceland, attributes the phenomenon to the molecular changes which take place in water after being subjected to heat. In such circumstances, water loses much of the air condensed in it, and the cohesion of the molecules is thereby increased, and a higher temperature required to boil it. In this state, when boiled, the production of vapour is so instantaneous as to cause an explosion.
Professor Bunsen found that the water at the bottom of the great Icelandic Geyser had a higher temperature than that of boiling water, and that this temperature increasing, finally caused its eruption.