Page 351 - Caldera Volcanism Analysis, Modelling and Response

326 Agust Gudmundsson

Figure 11 Multiple and nested calderas are common in many volcanic regions. Here is the triple
caldera of the Dyngjufjo º ll-Askja central volcano in central Iceland.The northernmost caldera has
no name and is of unknown age, but the other two are the Askja caldera formed at 5000--6000
BP, and the Lake O º skuvatn caldera, which formed following the AD 1875 Askja eruption.The
vertical displacement along the ring fault of the Askja caldera is at least 180 m, while that on the
ring fault of the Lake O º skuvatn caldera is 250 m (modi¢ed fromThorarinsson,1963).

And, third, why many calderas remain inactive during the subsequent evolution of
the associated magma chamber. This last point, elaborated below, follows from the
shape of the chamber becoming unfavourable to ring-fault slip while being
favourable to dyke and sheet injections and eruptions.

4. Behaviour of Crustal Rocks

The mechanical behaviour of crustal rocks in the laboratory has been
studied extensively and is now comparatively well known ( Jaeger and Cook, 1979;