TSK 11 Göttingen 2006 Andrew & Gudmundsson Figure 1: Map of Iceland with shield volca- noes and fissure marked as circles and lines respectively. The Neovolcanic Zone is also shown as the darker shaded area, and the main ice caps are marked. Holocene shield volcanoes in Iceland Poster Ruth Andrew1 Agust Gudmundsson1 Introduction Holocene shield volcanoes (lava shields) are common in Iceland, but they are restricted in space and time. As re- gards space, most of the shield volca- noes in Iceland occur within two bands in the West and North Volcanic Zones (Fig. 1). There are no shields in the East Volcanic Zone apart from the is- land of Surtsey. The shields are mostly at the margins of or outside the volcanic systems (Fig. 2). As regards time, many Holocene shield volcanoes formed some 5000–10000 years ago during early post- glacial time. Apart from the shield on top of the island of Surtsey, there are no known shields in Iceland younger than about 3500B.P. 1 Dept. Structural Geology and Geodynamics, University of Göttingen, Germany Formation The formation of shield volcanoes fol- lows a general pattern. The eruption begins with a fissure that subsequently becomes concentrated at several vents, which generate overlapping shields. A single vent, the last one to remain ac- tive, generates a main shield which buries the earlier overlapping smaller shields. The shield volcano itself con- sists of two main morphological units; a central cone and a lava apron. The cone is generally roughly symmetrical, and the apron roughly a circular shape, though both are affected by topogra- phy (Rossi, 1996). There is no caldera collapse associated with Icelandic shield volcanoes, since they are supplied with magma from elongated reservoirs at the boundary between the crust and upper mantle. Composition In Iceland, the shield volcanoes are mainly of two compositions, olivine- tholeiite basalt and picrite basalt, both Figure 2: The Reykjanes Peninsula with its volcanic systems and their postglacial shields and fissures shown. Note the loca- tion of many shield volcanoes on the out- skirts or outside of volcanic systems. Ke- flavik Airport is also shown along with the main settlements of the peninsula. 1 Andrew & Gudmundsson TSK 11 Göttingen 2006 Figure 3: Skjaldbreidur shield volcano in the West Volcanic Zone. of which are primitive basalts. The magma composition is determined by the local pressure and temperature con- ditions, as well as the composition of the rock undergoing partial melting. Rossi (1996), who documented 40 to 50 Holocene shield volcanoes in Iceland and made a detailed study of 31 found all the shield volcanoes to be mono- genetic. Of the 31 shields studied in detail, 24 are olivine-tholeiite basalts and 7 picrite basalts. This indicates that the conditions for the formation of olivine-tholeiite shields are more com- monly satisfied than the conditions for picrite shields. Picrite magma is a primary magma from the mantle. During the waning stages of the last glaciation and the early post- glacial period, magma of this compo- sition migrated up under the volcanic zones of the Reykjanes Peninsula at ex- ceptionally high rates. This high rate of migration is attributable to the de- crease in lithostatic pressure as a re- sult of the rapid melting of the glaciers. Later in the postglacial period, the pi- crite magma is thought to have been trapped by the lighter olivine-tholeiite magma that subsequently accumulated in the upper parts of the magma reser- voirs (Gudmundsson, 1986). Shields and Fissures The volumes of shield volcanoes differ from those of fissure eruptions within the same volcanic system. The aver- age lava volume of Holocene shield erup- tions is much larger than the average lava volume of Holocene fissure erup- tions. For example, on the Reykjanes Peninsula the mean eruptive volume of 101 volcanic fissures is about 0.1 cubic kilometers, whereas that of 26 shields is about 1 cubic kilometre. A typical large Holocene shield volcano on the Reyk- janes Peninsula is Skjaldbreidur, which has a volume of around 15 cubic kilo- meters (Fig. 3). There is also a corre- lation between the compositions of the two types of eruption; the fissures pro- duce more-evolved basalts, commonly tholeiite or quartz-tholeiite. Conclusions Here we present conceptual and numer- ical models as to how the stress changes related to the deglaciation may, partly at least, explain four related volcano- tectonic features in Iceland. First, how high-density basaltic magmas were able 2 TSK 11 Göttingen 2006 Andrew & Gudmundsson to reach the surface to form the shields. Second, why most of the shields formed in the early part of the postglacial pe- riod, with hardly any formed during the past 3500 years. Third, why many of the shields formed at the margins of the vol- canic systems to which they are associ- ated rather than at their centres. And, fourth, why the shields became confined to the West and North Volcanic Zones, with essentially no shields in the East Volcanic Zone. References Gudmundsson A (1986) Mechanical aspects of postglacial volcanism and tectonics of the Reykjanes Peninsula, Southwest Iceland. Journal of Geophysical Research 91, 12711– 12712. Rossi MJ (1996) Morphology and mechanism of eruption of postglacial shield volcanoes in Iceland. Bulletin of Volcanology 57, 530–540. 3