Figure 1 shows annual seismic strain release from 1970-September 1977 in the Myrdalsjökull area. After 1977, the annual strain release was similar to that of 1970-74.

Figure 1. Annual seismic strain release in the Myrdalsjökull area, February 1970 through September 1977. From Skjálftabref (published by the Icelandic Meteorological Office, no. 26, October 1977. Courtesy of Ragnar Stefánsson.

Geological Summary. Katla volcano, located near the southern end of Iceland's eastern volcanic zone, is hidden beneath the Myrdalsjökull icecap. The subglacial basaltic-to-rhyolitic volcano is one of Iceland's most active and is a frequent producer of damaging jökulhlaups, or glacier-outburst floods. A large 10 x 14 km subglacial caldera with a long axis in a NW-SE direction is up to 750 m deep. Its high point reaches 1380 m, and three major outlet glaciers have breached its rim. Although most recorded eruptions have taken place from fissures inside the caldera, the Eldgjá fissure system, which extends about 60 km to the NE from the current ice margin towards Grímsvötn volcano, has been the source of major Holocene eruptions. An eruption from the Eldgjá fissure system about 934 CE produced a voluminous lava flow of about 18 km3, one of the world's largest known Holocene lava flows. Katla has been the source of frequent subglacial basaltic explosive eruptions that have been among the largest tephra-producers in Iceland during historical time and has also produced numerous dacitic explosive eruptions during the Holocene.

Information Contacts: R. Stefánsson, Icelandic Meteorological Office.