Syn-emplacement fracturing in the Sandfell laccolith

By Tobias Mattsson, Uppsala University

The rhyolitic Sandfell laccolith is located in the East fjords of Iceland, two fjords north of the classical Breiddalur central volcano.  The volume of the laccolith is about 0.6 km³ and its intrusion depth (540 m) can be determined with exceptional accuracy by measuring the thickness of the rock domed by the intrusion of the rhyolite magma into the younger banking lavas. During field work we discovered concentric fracture layers in more than one third of the laccolith. By looking at the rhyolite groundmass next to and in the fracture layers we determined that they were related to flow banding and strain-localisation in the magma during laccolith inflation and induced degassing that led to rapid crystallisation. A thick shell in the magma body thus deformed in a brittle fashion and solidified during intrusion emplacement. We think that these kind of self-freezing processes are important during the emplacement of viscous magma bodies in the shallow crust and promote inflation of magmatic intrusions as opposed to lateral magma propagation and/or eruption.

Tobias Mattsson is a Ph. D. student at Uppsala University (tobias.mattsson@geo.uu.se), Sweden, supervised by Steffi Burchardt and Karen Mair. His research is focused on the emplacement of granitic magma bodies in the shallow crust.  You can read Tobias’s paper on the fracturing during the emplacement of the Sandfell laccolith here: https://www.frontiersin.org/articles/10.3389/feart.2018.00005/full