Náttúrufræðingurinn

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Náttúrufræðingurinn - 1979, Side 137

Náttúrufræðingurinn - 1979, Side 137
Aboul 6 km furtlier west there is a big crater simply named Eldgigur (= volcanic crater). It is about 800 m in diameter and rises 80 m above the surroundings. Eldgígur is taken to be the easternmost part of the eruptive fissure Rauðhólar which accordingly is about 29 km long. The eruptive fissure is supposed origin- ally to have continued between Rauðhól- ar and Eldgígur but a part of the íissure is now covered by the glacier. Eldgígur and Rauðhólar are petrographically iden- tical, but Eldgígur seems to liave been active somewhat later than the main fis- sure. The Rauðhólar eruptive lissure is situated within a narrow graben which cuts the Miklafell mountain. The vertical displacement is in places 8—10 m. A tec- tonic lissure cuts along the craters indicat- ing sonie tectonic movement after the eruption. The area ol the Rauðhólar lava flow is 228 km2 and the volume is calcu- lated about 6,84 knt3. Rauðhóll and Bunuhólar at Hervararstaðir. Rauðhóll at Hervararstaðir is a specta- cular crater built up of volcanic cinder, bombs and blocks but without a lava flow. It is on the northern end of an eruptive fissure, the Bunuhólar, at the upper end of the Holtsdalur valley (dalur = valley). From there a lava flow went down the narrow Holtsá canyon but disappears under sediment at the lower end of the valley. The extension of this ílow is not known. Petrographically it is similar to the above mentioned flows. The Hálsagigir west of Skálarfjall is an eruplive fissure with imposing lava and cinder formations. The lava flow clisap- pears under the sediments of the Skaltá river and under the Skaftáreldahraun (Laki lava) of 1783. Accordingly nothing is known of the extension ol the lava flow. The Hálsar lava is similar to tlte L;iki lava, but somewhat richer in olivine. The age of the eruptions. Two light-coloured (acidic) tephra layers give important information con- cerning the age of eruptions under dis- cussion. The age of the lowermost of tltem, H-4 (cf Thorarinsson 1971 and Jónsson 1975) is according to C14 age determina- tion about 3800 C14 years old. The same tephra layer is about 25 cnt above the Rauðhólar lava flow at Miklafell and 15—20 cm above the cinder of the Bunu- hólar craters. Accordingly Eldgígur- Rauð- hólar and Rauðhóll-Bunuhólar have erup- ted at approximately the same time. The second acidic tephra layer (H-3?) which has been dated 3520 C14 years, as well as H-4, is underneath the 120 cm thick pumice from the Hálsar eruption which accordingly can hardly be more than about 2000 years. The material used for these age determinations is stems of birch iu the tephra itsell'. It has been stated by Thoroddsen (1894), Sapper (1908) and several other authors that the disastrous eruption of the Laki eruptive lissure in 1783 is not the first eruption on the fissure in post- glacial time. Several older craters can be recogni/.ed along the eruptive fissure of 1783. M hc lavas of these two eruptions are identical. ln the Meðalland district an olcl lava l'low is now mostly covered by sand de- posits. This tlow, here named Botnar lava, is superimposed by the Eldgjá (Lantl- brot) lava, and a sand deposit may be seen belween the two lava flows at Botnar. M’his lava llow is first mentioned by Rob- son (1952). It may be followed from west of the Botnar larm along tlie Eldvatn river to Gamlibær east ol' Steinsmýri. The presents author is of the opinion that this old lava flow is derived front the í'irst eruption of the Laki fissure in post- glacial time, and that it is perhaps 6000— 7000 years old or more. The ages of the mentioned eruptions are considered to be appoximately:
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