How studying solidified, exposed magma chambers helps to interpret volcano deformation and pre-eruptive unrest

The remnants of kilometre-sized solidified magma bodies exposed in volcanic areas are the product of magma accumulation beneath active volcanoes. These magma bodies can have formed over time spans ranging from months to hundreds of thousands of years, and some have triggered unrest and fed eruptions at the volcano surface. Here, we focus on melt-dominated magma bodies in the upper crust, which represents a sub-volcanic magma-storage level overlying a deeper, likely mush-dominated, igneous plumbing system. Based on several examples in eastern Iceland, we present field observations, structural analyses, 3D reconstructions, and petrological and fabric analyses that shed light on (1) the growth of magma chambers during single, fast, or multiple, long-term, magma injection events and (2) the deformation of the surrounding host rock as a result of different styles of magma emplacement. Moreover, we present evidence for syn-emplacement eruptions from one of the field examples.

We then discuss how field studies of solidified upper crustal magma chambers can inform the interpretation of volcanic unrest signals at active volcanoes. For instance, certain styles of magma emplacement create pronounced surface deformation and seismicity, while others may show initial seismicity that resembles dyke and/or sill emplacement but then allows for the emplacement of vast amounts of magma at shallow depth. This emplacement can likely happen without any significant surface deformation and with very little seismicity. Hence, solidified, exposed magma chambers that formed in the upper crust can provide valuable clues to improve eruption risk and volcano hazard assessment.