Ljósufjöll

Ljósufjöll peaks. The name - Light mountains - is derived from the light-coloured rhyolite volcanics The Ljósufjöll volcanic system on the Snæfellsnes Volcanic Zone has been moderately active in the Holocene. The last eruption was a small basaltic eruption in the 10th century CE. The Ljósufjöll system is composed of a central volcano and a fissure swarm. The central volcano is a ridge of sub-glacially erupted rhyolitic mountains (1063 m a.s.l., no glacial cover) but the Holocene volcanism is confined to small basaltic eruptions. The fissure swarm is ~90 km long and reaches 20 km width east of the central volcano. The fissure swarm has produced numerous small basaltic lava flows during the Holocene, the largest covering 33 km2, and small basaltic tephra deposits. Eruption frequency during the last 10,000 years is about 1 eruption every 400 years on average. The Holocene volcanic activity occurs on short WNW-ESE trending fissures and most of the lava flows are small in volume. Open fissures or faults which have been active during the Holocene are relatively few and the movements are small. The dominant magma type during the Holocene is alkali basalt and the chemistry seems to suggest a deep-seated source. Warm springs and numerous cold carbon dioxide springs are found within the Ljósufjöll system. The northwestern half of the Ljósufjöll volcanic system is a 3-6 km wide, partly mountainous terrain of hyaloclastites and lava flows. The central volcano is ridge-like, forming a row of peaks rising to 1063 m a.s.l. with relatively shallow passes in between. Holocene lava flows have emanated from short fissures marked by scoria cones. In this part of the volcanic system the old Neogene (Tertiary) basement rocks are extensively covered by the younger volcanic products. The fissure swarm is up to 20 km wide south-east of the central volcano. There the younger basaltic formations dot the landscape which is dominated by extensively faulted Neogene (Tertiary) rocks. Most of the Holocene activity seems to be confined to lows in the regional topography. The lava flows emanate from short fissures marked by relatively large scoria-rich cones. Nothing is known about the plumbing system but the geographical change in chemistry of the products seems to point to local ascent of basaltic magma from depth rather than lateral flow of magma from centre along the fissure swarm. The magma appears to ascend along reactivated old faults in the upper crust. Within the Ljósufjöll volcanic system about 17 basaltic lava flows younger than 11,500 years have been identified, the youngest less than 1100 years old. A few relatively small basaltic fissure eruptions have occurred during the Holocene, two of them younger than 4000 years but none during historical times. The longest lava flow extends 5-6 km from source. Rhyolitic eruptions occurred mainly during glacial periods and have been dated from 100,000 years back to 400,000 years ago. Most of the eruptions were subglacial. The volume of erupted material is unknown. One interglacial rhyolitic lava has flowed south from the centre of the volcanic system and reaches ~5 km in length. One basaltic eruption has occurred during historical times, in the first half of the 10th century CE. The small lava Rauðhálsahraun, about 13 km2, emanated from a single crater that also produced a tiny scoria layer covering about 50 km2. Other Holocene lava flows are older. At the NW end of the fissure swarm three lava flows, commonly known as Berserkjahraun, are thought to be younger than 4000 years. Other lava flows younger than 4000 years include Rauðamelskúlur and Grábrókarhraun, ~2600 and ~3600 years old, respectively. Nothing is known about the duration of eruptions but from the volume of eruptives it is likely that eruptions are short lived on the time scale of days or few weeks. Little knowledge is about the eruption frequency but it seems likely that it is periodic (time scale unknown). The average frequency is