ON THE SUB-GLACIAL RIDGES SURROUNDING HEKLA VOLCANO, ICELAND

Hekla is an active volcano located in south-central Iceland, near the intersection of the Mid-Atlantic Ridge and the South Iceland Seismic Zone. A series of at least 40 Pleistocene sub-glacial volcanic ridges, called mobergs, surround Hekla. The sub-glacial volcanoes strike at an average of ~55° and Hekla strikes 60°, implying that Hekla and the mobergs likely erupted under similar regional stress conditions.

Most mobergs exhibit distinct stratigraphic sequences. The base consists of mostly intact pillow basalts that formed when lava encountered subglacial melt water. The pillows are overlain by hyaloclastite, formed by explosive lava-water interaction, and pillow breccia with clasts ranging in diameter from 1 to 10 cm; normally, the clasts in each moberg's pillow breccia are relatively uniform in size, but the average size varies from one volcanic ridge to another. Some mobergs are capped by intact lavas erupted in a sub-aerial environment near the end of the eruption, but this is rare around Hekla.

The compositions of juvenile lava fragments indicate that they originated from a common parental magma via varying extents of olivine + plagioclase + cpx fractionation. The parental magmas to the mobergs are similar in composition to those responsible for the formation of Hekla lavas by greater amounts of fractionation of an olivine + plagioclase + cpx + titanomagnetite + apatite assemblage. The mobergs have similar compositions to the Pleistocene and historic basaltic flows produced from fissure systems on and around Hekla's flanks, except that the fissure flows have experienced more extensive fractional crystallization prior to eruption. Thus, there appears to be a common magmatic parent supplying the region for at least the past 20,000 years.

We propose that the mobergs, the basaltic fissure flows, and Hekla lavas represent various stages in the evolution of a central shield volcano. Initial volcanism produced mobergs when the area was ice-covered. During this phase, magma chambers were short-lived, limiting the amounts of fractionation that took place. As the volcanic system became more focused and the ice retreated, more evolved fissure flows were produced. Finally, the system evolved to a single, long-lived magma chamber capable of fractionating the highly evolved Hekla lavas erupted in historic times.