Fig. 293.—Cones on the great Laki fissure, Iceland. (From a photograph by Dr. Tempest Anderson.)
In some instances, according to Mr. Thoroddsen, lava wells out from the whole length of a fissure without giving rise to the formation of cones, the molten material issuing either from one or from both sides and flowing out tranquilly. Thus from three points on the great Eldgjá chasm lava spread out quietly without giving rise to any craters, though at the southern prolongation of the fissure, where it becomes narrower, a row of low slag-cones was formed. The three lava-streams flooded the low ground over an area of 693 square kilometres, or 270 English square miles. In the great majority of cases, however, the lava as it ascends in the fissure gives rise to long ramparts of slags and blocks of lava piled up on either side, or to a row of cones along the line of the open chasm. Thus, on the Laki fissure, which runs for about 20 miles in a north-east direction, the cones amount to some hundreds in number.
Fig. 293a.—Plan of small craters along the line of great Laki fissure, Iceland. (After Mr. Helland, reduced.)
The cones consist generally of slags, cinders, and blocks of lava. They are on the whole not quite circular but oblong, their major axis coinciding with the line of the chasm on which they have been piled up, as along the marvellous line of the Laki fissure. In many places they are exceedingly irregular in form, changes in the direction of outflow of lava or of escape of steam having caused the cones partially to efface each other.
As regards their size, the cones present a wide range. Some of them are only a few yards in diameter, others several hundred yards. Generally they are comparatively low mounds. On a fissure hardly 30 feet long, Mr. Thoroddsen found a row of twelve small cones built exactly like those of largest size, but with craters less than three feet in diameter. On the Laki fissure some are only a couple of yards high; the majority are much less than 50 yards in height, and hardly one is as much as 100 yards.[278] And yet these little monticules, as Mr. Helland remarks, represent the pipes from which milliards of cubic metres of lava have issued. While other European volcanoes form conspicuous features in the landscape, the Icelandic volcanoes of the Laki district, from which the vastest floods of lava have issued in modern times, are so low that they might escape notice unless they were actually sought for.[279]
[278] Mr. Thoroddsen, however, states that there are about 100 ranging between 20 and 100 metres in height.
As they have generally arisen along lines of fissure, the cones are, for the most part, grouped in rows. The hundreds of cones that mark the line of the Laki fissure present an extraordinary picture of volcanic energy of this type. In other instances the cones occur in groups, though this distribution may have arisen from the irregular uprise of scattered vents along a series of parallel fissures. Thus to the north-east of Laki a series of old cones entirely surrounded by the lavas of 1783 lie in groups, the most northerly of which consists of about 100 exceedingly small craters that have sent out streams of lava towards the N.N.E.[280]
[280] Op. cit. [p. 25]. The great lava-fields of Iceland are likewise dotted over with secondary craters or "hornitos" which have no direct connection with the magma below, but arise from local causes affecting the outflowing lava. They are grouped in hundreds over a small space.