Iceland

• are they more explosive than basaltic eruptions under ice?
• are they capable of piercing ice caps, and if so, what do subaerial rhyolites look like?
• do subsidence cauldrons form during such eruptions?
• do englacial lakes develop?
• how does the eruption rate affect the types of rhyolite body formed?

The Torfajökull volcanic complex, Iceland is an ideal place to study subglacial rhyolites. Measuring 18 by 12km and active for about the last 1 Ma, Torfajökull is the largest silicic centre in Iceland. It is an active volcanic complex, the last eruption being in 1477. Numerous spectacular subglacial rhyolites have been emplaced during the last glacial period (c.115-10ka ago).

See below for some images from this fascinating and beautiful region.

Fissure eruptions which have melted through the ice

Looking NW from Illihnúkur towards flat-topped Kirkjufell, Torfajökull volcanic complex. Kirkjufell and the lower-lying ridges were formed by subglacial rhyolitic fissure eruptions around 50 to 80 thousand years ago. The prominent flat top is formed by a steeply ramped rhyolite lava flow which was probably emplaced subaerially. Photograph by Dave McGarvie.

Evidence for complex magma-ice interaction

20m thick hot mass flow deposit in Grænagil, on the flanks of Bláhnúkur, an englacial rhyolite volcano overlooking Landmannalaugar campsite. Prominent black pods are obsidian 'stringers' set in greenish altered breccias. These poorly sorted breccias contain pumice clasts set in a vesiculated ashy matrix. There is little or no textural gradation from the core to the rim of the obsidian stringers, hence it appears that phreatomagmatic fragmentation did not occur. However, there is evidence that they were emplaced whilst hot: a centimetric zone of baked ash is found at many margins and some stringers show ductile deformation. Irregular pale veins, conspicuous at the top left of the outcrop, are filled with vesiculated ash. They locally truncate obsidian stringers and are thought to indicate vapour mobility and instability of the deposit both during and shortly after emplacement.
The pale body in the bottom right hand corner is the microcrystalline interior of an intrusive rhyolite lobe.

Different types of rhyolite lava body Here are two examples of the varied rhyolite facies found at Torfajökull. The photo on the left shows a columnar jointed obsidian flow on the flanks of Rauðufossafjöll. Jointing patterns suggest emplacement in a subglacial cavity, with chilling of the upper margin against the ice roof. The photo below is of a 150m thick subhorizontal rhyolitic body south of Kirkjufellsvätn. Its lateral extent and geometry suggest subaerial emplacement, perhaps within a subsidence cauldron. Such facies have been investigated in detail this summer - more details to be posted soon. Photographs by Dave McGarvie.