Volcano geodesy and magma dynamics in Iceland

Here we review the achievements of volcano geodesy in Iceland during the last 15 years. Extensive measurements of crustal deformation have been conducted using a variety of geodetic techniques, including leveling, electronic distance measurements, campaign and continuous Global Positioning System (GPS) geodesy, and interferometric analysis of synthetic aperture radar images (InSAR). Results from these measurements provide a comprehensive view of the behavior of Icelandic volcanoes. Between inflation, intrusion, and eruption episodes, volcanoes are likely to deflate or show no sign of seismic activity. Subsidence rates are often in the range of a few millimeters to a few centimeters a year, reducing progressively with time since the last eruption or intrusion at the volcano. Subsidence can be caused by cooling and contraction of magma, outflow of magma, it can be related to plate spreading. Volcano subsidence or lack of deformation is often interrupted by episodic magma flow towards near-surface locations. Such magma recharge has been observed geodetically at Hengill, Hekla, Eyjafjallajökull, Katla, Grímsvötn, and Krafla volcanoes, with inflow inferred to last from a few months up to two decades. In the last 15 years, five volcanic eruptions, three intrusive events and two >M6 earthquakes have occurred. In recent years, the Grímsvötn and Katla volcanoes have exhibited continuous inflation of a few centimeters per year, which at Grímsvötn culminated in an eruption on 1 November 2004. Hekla and Torfajökull volcanoes have inflated at rates an order-of-magnitude less. Subsidence is occurring presently at the Askja and Krafla volcanoes. Within the period of geodetic measurement, signals consistent with no deformation are typical for most of the 35 active volcanoes in Iceland.