Náttúrufræðingurinn - 1989, Blaðsíða 59

Náttúrufræðingurinn - 1989, Blaðsíða 59
Walker, G.P.L. 1974b. Eruptive mechan- isms in Iceland. I Geodynamics of Ice- land and the North Atlantic area (ritstj. Leó Kristjánsson). Reidel, Dor- dreclit. Bls. 189-201. Wickman, F.E. 1976. Markov models of repose-period patterns of volcanoes. I Random processes in geology (ritstj. D.F. Merriam). Springer-Verlag, New York. Bls. 135-161. SUMMARY Intrusion frequencies of magma chambers and extrusion frequen- cies of volcanic systems by Ágúst Guðmundsson Nordic Volcanological lnstitute University of Iceland IS-101 REYKJAVIK ICELAND This paper reviews and extends the au- thor’s earlier model on the intrusion and extrusion frequencies associated with vol- canoes at divergent plate boundaries (Ágúst Guðmundsson 1986a, 1988). Here the model is applied to two specific re- gions in Iceland. The long-term dike in- trusion frequency (i.e., of the order of 104 years or more), as predicted bv the model, is applied to the Álftafjörður dike swarm, of Tertiary age, in Eastern Iceland (Ágúst Guðmundsson 1983). The short-term ex- trusion frequency (i.e., of the order of 103 years or less), as predicted by the model, is applied to Holocene volcanism on the Reykjanes Peninsula in Southwest Iceland (Ágúst Guðmundsson 1986d). The main conclusions of the paper may be summa- rized as follows. 1) At divergent plate boundaries, the dike intrusion frequency of a deep-seated magma reservoir, i.e., the frequency of magma flow from the reservoir, is largely determined by the associated spreading rate as well as the aspect (height/width) ratio of the reservoir. High spreading rates and/or aspect ratios increase the as- sociated dike intrusion frequency. 2) The dike intrusion frequency of a shallow magma chamber, i.e., the fre- quency of flow of magma out of the cham- ber, is normally controlled by that of the magma reservoir that feeds the chamber. Because the magma content of a typical reservoir is from five to one hundred times the magma content of a typical chamber (Fig. 2), a single magma flow from a reservoir may lead to the injection of up to several dozen dikes and inclined (cone) sheets from the associated cham- ber. This may explain the enormous num- ber of sheets that occur near to some large plutons (extinct magma chambers) in Ice- Iand (Figs. 2 and 4). 3) According to the model, the short- term extrusion frequency of a volcanic system may be similar to the dike intru- sion frequency of the source reservoir, in which case most or all of the dikes would be feeders. However, the long-term extru- sion frequency of a volcanic system is nor- mally only about 10% of the associated dike intrusion frequency. 4) The Grímsvötn volcano beneath the Vatnajökull ice sheet has erupted 7-11 times every century during the past 400 years. The Grímsvötn volcano is fed by a double chamber, i.e., a chamber associ- ated with a reservoir (Fig. 2). The model explains such a high extrusion frequency by two factors. First, it is proposed that the aspect ratio of the source reservoir is high (Fig. 3) compared with other reser- voirs at the divergent plate boundary in Iceland, which leads to a high dike intru- sion frequency. Second, because the chamber is much smaller than the source reservoir, each magma flow from the res- ervoir may trigger injection of up to sever- al dozen dikes (or inclined sheets) from the associated chamber. In contrast to the current activity in the Krafla volcano in north Iceland, where less than 50% of dikes from the chamber liave reached to the surface (led to an extrusion), most or all the dikes or sheets in Grímsvötn have, 53
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