The majority of the volcanic mountains inland each mark a weak spot above a radiating channel, or where one was blocked by a core of congealed lava, and were formed in the same way as those near the coast, surface water from the snows and rains finding its way into the channels in place of water from the sea.
I believe that the molten rock from the central vent, even when not forced into the channels by the pressure of confined gases, slowly 1 eats' its way by redissolving the igneous rock lying in them, and that when it lies any length of time in a large channel this is enlarged by the melting of its walls and roof, which yield as readily to the intense heat of a large quantity of molten rock as a rod of soder would if inserted into a crucible of molten lead : therefore at this very hour it is likely that in many of the channels radiating from the vent of which Askja is the outlet, molten matter is slowly and steadily redissolving the lava with which they are clogged and inclosed. It is fearful to think that there are 70,000 people living upon an island honeycombed with channels charged with molten rock that may at any moment burst forth beneath a homestead, and consume it and its inhabitants. Equally alarming is the fact that in the crater above the great central volcanic vent is a large lake of recent formation, whose water-year after year increasing in volume, and now five miles in circumference and two hundred feet in depth-will assuredly, should a slight explosion loosen the lava-floor of that crater, find its way below, come in contact with molten matter, and cause an explosion to which those in 1872 and 1875, which opened rifts from ten to thirty miles in length, will be comparatively insignificant, an explosion that in all probability will send another molten flood coursing over the island-and who can tell where? it may be in the very streets of Reykjavik itself!
The history of Askja, subsequently to the building up of the inland plateau and the uniting thereby of the miocene fragments into one island, may be told in a few lines. Let us imagine that the deposits immediately around the now greatly narrowed in and sealed central vent have been several times ruptured by explosions accompanying the eruptions which built up the volcanic mountains now standing in every part of Iceland of post-tertiary formation; and that the lava-flows now forming the widely extending base of Askja!'s encircling mountain wall have issued from the fissures around the immense core of congealed lava plugging the central vent, whose diameter is that of the present Askja crater ; that the radiating channels are mostly hermetically sealed by the congealed remains of former molten floods, and that the masses of tuff and other strata now forming the highest part of Askja's encircling mountain have yet to be upheaved. A tide of molten rock again sets in the direction of the Icelandic outlet; and ere it can ' eat' its way into the radiating channels, the disturbed strata surrounding the core are partly redissolved by the intense heat, fissured asunder, and torn therefrom by the enormous pressure below, and the core, being no longer supported by their cohesion, sinks bodily to some depth into the abyss, in much the same manner as the tract now beneath the lake did in 1875, and the deep cauldron-shaped hollow was formed in which the lava-floods bared in the cliffs bordering the lake on the north were to be subsequently deposited. The eminence formed by the summit of the core and the surrounding-strata was doubtless, owing to its altitude, covered with snow, and this melting, the water came into contact with the molten matter, adding by the generation of an immense volume of steam to the violence of the terrible convulsion that took place when the huge masses of the tuff sub-strata lying far below were upheaved through the innumerable sheets of lava lying above them, and Askja's mountainous periphery was built up above the base formed of these deposits, by the masses of strata upheaved, and the amorphous matter which issued during the eruption. Possibly, all around the core as it subsided, lava welled up and spread over its summit within the cauldron-shaped hollow, the first stratum to be deposited therein. In course of time it is likely the core would be gradually redissolved by the intense heat of the molten matter by which it was surrounded, and ejected in the form of lava, as only by such action can the existence be accounted for of the abyss into which the mass of strata, five miles in circumference, disrupted in 1875 subsided. Lava-flood after lava-flood has subsequently burst forth in the crater, and in the course of ages it has been filled up to its present level, that of the two gaps in its mountainous periphery, through the eastern of which, at any rate, the later lava-floods that have here issued have found an outlet and flowed over the plain around.