It would appear from Mr. Helland's observations that the same fissure has sometimes been made use of at more than one period of eruption. He describes some old craters on the line of the Laki fissure, which had been active long before the outbreak of 1783.[281]
When the lava issues from fissures it is in such a condition of plasticity that it can be drawn out into threads and spun into ropes. When the slope over which it flows is steep it often splits up into blocks on the surface. Where the ground is flat the lava spreads out uniformly on all sides, forming wide plains as level as a floor. Thus the vast lava-desert of Odádahraun covers a plain 3640 square kilometres in area, or, if the small-lava-streams north from Vatnajökull be included, 4390 square kilometres. This vast flood of lava (about 1700 English square miles in extent) would, according to Mr. Thoroddsen, cover Denmark to a depth of 16 feet. The whole of this enormous discharge has been given forth from more than twenty vents situated for the most part on parallel fissures.
Not less striking is the picture of fissure-eruption to be met with at Laki—the scene of the great lava-floods of 1783. "Conceive now," says Mr. Helland, "these hundreds of craters, or, as they are called by the Icelanders, 'borge,' lying one behind another in a long row; every one of them having sent out two or more streams of lava, now to the one side, now to the other. Understand further that these streams merge into each other, so as to flow wholly round the cones and form fields of lava miles in width, which, like vast frozen floods, flow down to the country districts, and you may form some idea of this remarkable region."[282]
[282] Op. cit. [p. 24]. Mr. Helland allows an average thickness of 30 metres for the mass of lava which issued in two streams, one 80 kilometres (nearly 50 miles), the other 45 kilometres (about 28 miles) long. He estimates the total volume of lava discharged in the 1783 eruption at 27 milliards of cubic metres, equal to a block 10 kilometres (6 miles 376 yards) long, 5 kilometres (3 miles 188 yards) broad, and 540 metres (1771 feet) high; op. cit. [p. 31]. Mr. Thoroddsen remarks that the older estimates of the volume of lava discharged by this eruption have been greatly exaggerated. He puts the area covered by lava at 565 square kilometres and the contents at 121/3 cubic kilometres. Verhand. Gesell. Erdkunde Berlin, 1894, p. 296.
The basaltic lavas have issued in a comparatively liquid state, form thin sheets and reach to great distances. The western stream from the Laki eruption of 1783 flowed for upwards of 40 miles; a prehistoric lava from Trölladyngjá in Odádahraun flowed for more than 60 miles.
In the course of time the successive streams of lava poured out upon one of these wide volcanic plains gradually increase the height of the ground, while preserving its generally level aspect. The loose slag-cones of earlier eruptions are effaced or swallowed up, as one lava-stream follows another. Eventually, when, by the operation of running water or by fissure and subsidence, transverse sections are cut through these lava-sheets, the observer can generally notice only horizontal beds of lava piled one above another, including the dykes connected with them and intercalated masses of loose slag, that remain as relics of the old craters.
In some places the lava has gradually built up enormous domes, like those of Hawaii, having a gentle inclination in every direction, as may be seen especially in the district between Floderne Skjalfanafljot and Jökulsà Most of the large volcanic piles of North Iceland are of this nature. The highest of them are 1209 and 1491 metres high by from 6 to 15 kilometres in diameter. The elliptical crater of the highest of these eminences measures 1100 by 380 metres.[283]
[283] Mr. Thoroddsen, op. cit. xiv. ii. No. 5, pp. 10, 23.
Large conical volcanoes of the Vesuvian type built up of alternating lavas and tuffs are not common in Iceland, but some occur and rise into lofty glacier-covered mountains, such as Öræfajökull (6241 feet), Eyjafjallajökull (5432), and Snaefellsjökull (4577). Hekla (4961) also is similarly composed of sheets of lava and tuffs, but has not been built as a cone. It forms an oblong ridge which has been fissured in the direction of its length and bears a row of craters along the fissure.[284]