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Geothermal Activity in the Subglacial Katla Caldera, Iceland, 1999–2005, Studied with Radar Altimetry

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Geothermal Activity in the Subglacial Katla Caldera, Iceland, 1999–2005, Studied with Radar Altimetry 66 Annals of Glaciology 45 2007 Geothermal activity in the subglacial Katla caldera, Iceland, 1999–2005, studied with radar altimetry Magnu´s T. GUÐMUNDSSON,1 Þo´rdı´s HO¨ GNADO´ TTIR,1 Arno´r Bergur KRISTINSSON,2 Snæbjo¨rn GUÐBJO¨ RNSSON2 1Institute of Earth Sciences, University of Iceland, Sturlugata 7, IS-101 Reykjavik, Iceland E-mail: [email protected] 2Icelandic Civil Aviation Administration, Reykjavik Airport, IS-101 Reykjavik, Iceland ABSTRACT. The Katla caldera is located under the My´rdalsjo¨kull ice cap and is one of the most hazardous volcanoes in Iceland due to major jo¨kulhlaups that accompany eruptions. Subglacial geothermal activity is manifested in several 10–50 m deep depressions (ice cauldrons) within and at the caldera rim and the total geothermal heat output is of the order of a few hundred megawatts. A short- lived but powerful pulse in geothermal heat output took place in 1999, probably including a minor subglacial eruption, when new ice cauldrons formed in three places and an unexpected jo¨kulhlaup occurred. Following these events, a comprehensive monitoring program was set up for Katla, including ice surface elevation profiling from aircraft, to monitor variations in geothermal heat and detect signs of subglacial water accumulation. A radar altimeter coupled with a kinematic GPS is used, achieving an absolute elevation accuracy of 3 m and internal consistency of 1–2 m. Profiles across the caldera are flown twice a year. An annual accumulation-ablation cycle in surface elevation with amplitude of 5–10 m is observed. By removing this cycle from the data, changes due to subglacial geothermal activity are obtained. After the events in 1999, a decline in geothermal activity was observed. In 2001–03 some ice cauldrons expanded and deepened by 10–15 m, indicating renewed increase in geothermal activity. This trend is also apparent for 2003–05. The increase in geothermal power amounts to a few tens of megawatts. It is likely that the increased thermal output is related to increased seismicity and caused by magma inflow. INTRODUCTION this nature can be a significant hazard to inhabited regions in the vicinity of ice-covered volcanoes. Jo¨kulhlaups caused Geothermal and volcanic activity in glaciated regions is by the release of volcanic or geothermal heat under ice manifested by depressions in the ice surface, formed by en- occur in several regions around the world (e.g. Major and hanced basal melting. Beneath these depressions, water may Newhall, 1989; Pierson and others, 1990). In Iceland, the accumulate, later to be released in jo¨kulhlaups. Flooding of subglacial volcanoes of Grı´msvo¨tn in Vatnajo¨kull and Katla Fig. 1. (a) Map of My´rdalsjo¨kull with the Katla caldera and the neighbouring Eyjafjallajo¨kull ice cap. The radar altimetry survey lines flown in spring and autumn to monitor ice cauldrons are indicated. (b) Map of the Katla caldera with ice cauldrons 1–17. Guðmundsson and others: Geothermal activity in the subglacial Katla caldera 67 Fig. 2. Schematic sections showing the relation between ice caul- drons and underlying geothermal areas. (a) Cauldron with water accumulation, and (b) cauldron with continuous drainage. Fig. 3. Schematic setup of GPS and radar altimeter. The footprint indicated is the width of the first Fresnel zone. in My´rdalsjo¨kull are renowned for being sources of jo¨kul- hlaups of geothermal and volcanic origin (e.g. Thorarinsson, 1974; To´masson, 1996; Larsen, 2000; Bjo¨rnsson, 2002). chasms. Meltwater is accumulated under many cauldrons Katla, with its 100 km2 caldera (Fig. 1) filled with 400– for some periods before it is released in jo¨kulhlaups (Fig. 2a). 700 m thick ice (Bjo¨rnsson and others, 2000), is one of the Some of the smaller cauldrons do not display any obvious most dangerous volcanoes in Iceland because of its proximity changes in surface geometry over extended periods and to inhabited areas. Eruptions produce swift and often seem not to collect water or release it in an episodic manner catastrophic jo¨kulhlaups that over the last 1100 years have (Fig. 2b). Meltwater from these cauldrons apparently seeps drained to the east over the outwash plain of My´rdalssandur away continuously without water accumulation. Both types (e.g. Larsen, 2000). Twenty-two eruptions are known to have occur within the Katla caldera. breached the surface and created tephra layers over the last The total heat output of the geothermal areas within the 1200 years (Elı´asson and others, 2006). In addition, minor, Katla caldera has not been measured but an order of fully subglacial eruptions may have occurred. Such minor magnitude estimate can be obtained from mass balance eruptions are suspected to be responsible for ice cauldron considerations. Limited mass balance measurements formation accompanied with sudden jo¨kulhlaups in 1955 (Eythorsson, 1945; Rist, 1957; Institute of Earth Sciences, and 1999 (e.g. Tryggvason, 1960; Guðmundsson, 2005). unpublished data, 2001) indicate that the winter mass Following an unexpected and sudden jo¨kulhlaup in July balance in the caldera is on average 5–7 m a–1 water 1999 and signs of increased geothermal activity, a equivalent (w.e.). Observations from 2001 (Institute of Earth comprehensive monitoring system was set up around Katla, Sciences, unpublished data) and comparison with measure- since these signs were interpreted as an increased risk of ments for the same elevation span at Vatnajo¨kull (Bjo¨rnsson eruption. The monitoring system is jointly run by the and others, 2002) indicate a negative summer balance of Icelandic Meteorological Office, the Hydrology Department 2–3 m a–1 w.e. Thus, the average net balance for the Katla of the National Energy Authority and the Institute of Earth caldera is considered to be in the range 2–4 m a–1 w.e. and Sciences, University of Iceland. It consists of enhanced the total annual balance in the 100 km2 caldera is expected seismic surveillance, continuous GPS monitoring of ground to be 0.2–0.4 km3 a–1 w.e. Most of this ice flows out of the deformation, automatic gauging stations in rivers and caldera down the outlet glaciers. The order of magnitude of regular ice surface profiling to monitor ice cauldrons and ice melted underneath the ice cauldrons is about 10% of the their potential for subglacial water storage. In this paper we 0.2–0.4 km3 a–1 w.e. Using calorimetry, with the latent heat ¼ Â 5 –1 outline the methods used to monitor the ice cauldrons, of fusion for ice Li 3.35 10 Jkg and water density describe the geothermal activity within the Katla caldera ¼ 1000 kg m–3, the annual heat output required to melt and discuss the implications of these results for Katla. 0.02–0.04 km3 a–1 w.e. would be 200–400 MW. This value is highly uncertain but indicates that the long-term heat output of the geothermal areas within the Katla caldera is of the ICE CAULDRONS order of few hundred megawatts. The term ice cauldron has been used for depressions on the surface of glaciers caused by enhanced and localized heat flux from the bedrock (Bjo¨rnsson, 1975). The size of ice SURVEYING SYSTEM cauldrons varies but most cauldrons are roughly circular or The successful use of a radar altimeter coupled with GPS elliptic in shape. Cauldrons, sustained by subglacial hydro- positioning of aircraft during the Gja´lp eruption in Vatna- thermal upwelling may be semi-permanent or they may be jo¨kull, Iceland in 1996 (Guðmundsson and others, 1997; transient features that persist for a few years. The largest 2004) revealed the potential of this relatively simple and cauldrons in Iceland are the Skafta´r Cauldrons in the western inexpensive method to obtain quantitative data on ice part of Vatnajo¨kull with diameters of 2–3 km (Bjo¨rnsson, surface elevation changes. In the present system, the ice 2002). Most cauldrons are smaller, including several within surface profiling is carried out by recording simultaneously the drainage area of the Grı´msvo¨tn caldera in Vatnajo¨kull the kinematic GPS position and flight elevation using a (e.g. Bjo¨rnsson and Guðmundsson, 1993), or the cauldrons radar-altimeter (Fig. 3) aboard the aircraft of the Icelandic within the Katla caldera. In common usage the term Civil Aviation Administration. cauldron covers the spectrum from shallow, uncrevassed The radar-altimeter is an analogue Collins ALT-50, oper- depressions to deep, steep-sided and highly crevassed ating at 4300 MHz with an error of 2%. This radar altimeter 68 Guðmundsson and others: Geothermal activity in the subglacial Katla caldera estimate of just under 1 m. The GPS gives a position once every second, while the ground clearance is measured 4 times a second. Typical aircraft velocity is 70–80 m s–1 implying that a sounding of the glacier surface occurs at 17– 20 m intervals along the flight line. Accurate time, based on GPS satellite constellation time from the GPS receiver, together with the altimeter readings (digitized through an AD converter) are sampled onto a laptop PC. After post- processing of the GPS positions, the coordinates of the three intermediate elevation sounding points are obtained by linear interpolation of the 1 second GPS coordinates. The internal consistency of the airborne elevation sound- ings was tested on analysis of the amount of intersection in 54 crossover points on My´rdalsjo¨kull. The mean waypoint error obtained was 1.2 m. Traverses of the same lines between waypoints with the airborne radar altimeter system have been compared to DGPS positions collected by a snowmobile travelling the same lines on the glacier surface 1 in September 2000 (Fig. 4). The Trimble Pathfinder sub- metre DGPS system used for the snowmobile traverses has been shown to yield a horizontal error less than 1 m and a vertical error less than 2 m for baseline lengths of 50–100 km, Fig.
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