Journal of Conference Abstracts

Persistently Active Volcanoes of East Java, Indonesia

Simon A. Carn¹ (sac23@esc.cam.ac.uk), David M. Pyle¹ (dmp11@cam.ac.uk) & Clive Oppenheimer² (C.Oppenheimer@open.ac.uk)

¹ Dept of Earth Sciences, Cambridge University, Cambridge CB2 3EQ, UK.

² Dept of Geography, Cambridge University, UK.

Eastern Java, part of the Sunda volcanic arc, contains several volcanoes that have undergone phases of persistent activity. These include the presently dormant Lamongan (alt. 1671m; 07^o 58'57"S 113^o 20'26"E); Semeru (alt. 3676m; 08^o 6'36"S 112^o 55'12"E), erupting continuously since 1967; and Bromo (alt. 2329m; 07^o 31"S 112^o 57'00"E), which erupted in March 1995. All these volcanoes present a considerable hazard to the local populace, and are as yet very poorly understood.

Lamongan was one of the most active Indonesian volcanoes during the 19th Century, with around 40 recorded phases of strombolian-type activity and the eruption of 15 basalt to basaltic-andesite lava flows. There are also many (>80) older cinder cones and maars on its lower flanks, whose distribution reflects the occurrence of fissure-style eruptions, probably controlled by radial dyking, and the alternation of magmatic and phreatomagmatic activity. The picrobasaltic-basaltic cinder cone lavas are generally less evolved than the 19th Century flows. Structural considerations indicate that Lamongan may be in the initial stages of caldera growth, in contrast to the adjacent long-lived Bromo-Tengger-Semeru complex.

Geochemical data obtained from Lamongan lavas will be presented, in conjunction with satellite imagery (SAR, Landsat TM, SPOT) which is being used to study the structure of the volcanoes, and the volumes of erupted products.

Elastic Instabilities in Minerals

Michael A. Carpenter (mc43@esc.cam.ac.uk)

Dept. of Earth Sciences, University of Cambridge, Cambridge, U.K.

Any mineral which displays a change in lattice parameters due to a structural phase transition will also show anomalies in its elastic constants. Unlike some properties associated with phase transitions, elastic anomalies may extend to many hundreds of degrees on either side of the transition point. Every elastic constant of the mineral is likely to be affected, though the precise variations are determined both by symmetry and by the strength of coupling between individual spontaneous strain components and the driving order parameter for the transition. Of all phase transitions which can occur in crystalline materials, those with significant strain and elastic variations are most likely to conform closely to the precepts of Landau theory. Furthermore, if the correct Landau free energy expansion can be generated for a given phase transition, the variation of all the elastic constants can be predicted. There are still experimental difficulties associated with measuring elastic properties on small crystals, and this is an area where theory currently precedes experiment. An example of how sensitive elastic properties can be to transition mechanism will be provided by a hypothetical orthorhombic to monoclinic transition. More concrete examples of elastic instabilities and anomalies in minerals are provided by albite, quartz and perovskite. In the limit of pressure and temperature approaching a transition point, extreme elastic softening may occur.