After the deflation event of 20 January 1977, the Krafla caldera started inflating gain. The rate of uplift near the center was about 10 mm/day. Earthquake activity was at a minimum after the 20 January deflation. During the first week of March the earthquake activity increased again, and during March there have been 100-150 earthquakes per day compared to 100/day prior to the January deflation event, all confined within the caldera. The largest earthquakes are of magnitude 3.6. This increase in seismic activity was predicted two weeks in advance based on the inflation rate of 20 January. One of the fissures monitored during the last few months showed significant widening during the last two inflation periods and contraction during deflation. This fissure is now widening at a slowly increasing rate (Björnsson and others, 1977).

Geological Summary. The Krafla central volcano, located NE of Myvatn lake, is a topographically indistinct 10-km-wide caldera that is cut by a N-S-trending fissure system. Eruption of a rhyolitic welded tuff about 100,000 years ago was associated with formation of the caldera. Krafla has been the source of many rifting and eruptive events during the Holocene, including two in historical time, during 1724-29 and 1975-84. The prominent Hverfjall and Ludent tuff rings east of Myvatn were erupted along the 100-km-long fissure system, which extends as far as the north coast of Iceland. Iceland's renowned Myvatn lake formed during the eruption of the older Laxarhraun lava flow from the Ketildyngja shield volcano of the Fremrinamur volcanic system about 3800 years before present (BP); its present shape is constrained by the roughly 2000 years BP younger Laxarhraun lava flow from the Krafla volcanic system. The abundant pseudocraters that form a prominent part of the Myvatn landscape were created when the younger Laxarhraun lava flow entered the lake.

Information Contacts: K. Grönvald, NVI.