The Volume of the Tunguhraun and Dvergagígahraun Lavas, Central Iceland

Home , Fagradalsfjall

Julia Annina Heilig

Faculty of Earth Sciences University of Iceland 2021

The Volume of the Tunguhraun and Dvergagígahraun Lavas, Central Iceland

Julia Annina Heilig

10 ECTS thesis submitted in partial fulfillment of a Baccalaureus Scientiarum degree in Earth Sciences

Advisor Ásta Rut Hjartardóttir Research specialist, PhD

Faculty of Earth Sciences School of Engineering and Natural Sciences University of Iceland Reykjavík, May 2021

The Volume of the Tunguhraun and Dvergagígahraun Lavas, Central Iceland The Volume of the Tunguhraun and Dvergagígahraun Lavas 10 ECTS thesis submitted in partial fulfillment of a Baccalaureus Scientiarum degree in Earth Sciences

Faculty of Earth Sciences School of Engineering and Natural Sciences University of Iceland Sturlugata 7, 101 Reykjavík

Telephone: 525 4000

Bibliographic information: Julia Annina Heilig, 2021, The Volume of the Tunguhraun and Dvergagígahraun Lavas, Bachelor‘s thesis, Faculty of Earth Sciences, University of Iceland, pp. 38.

Abstract

The Tunguhraun and Dvergagígahraun lavas are two lava fields in the central highland of Iceland. They are the only two eruptions of the Holocene that can be associated with the Tungnafellsjökull central volcano. Therefore, the understanding of Tungnafellsjökull‘s volcanic activity and behavior since the retreat of the glacier depends on research of those lava fields. Here, calculations of the volume and thickness of the Tunguhraun and Dvergagígahraun lavas are presented. Due to lack of data on the topography beneath these prehistoric lavas, an elevation model estimating the topography under the lavas was created, using elevation values around the lava fields and on the kipukas. For these elevation values, as well as those of the lava field, the ArcticDEM was used. Mapped outlines and the interpolated elevation models coupled with an elevation model of the current landscape provided the required data to calculate the approximate volumes of the two lava fields. The Tunguhraun lava is rather small and very thin with a volume of 0.15 km3 and an average thickness of 0.7 m. The Dvergagígahraun lava, on the other hand, is a bit thicker, but one of the smallest lava fields in Iceland, with a volume of 0.36 x 10-2 km3 and an average thickness of 3.1 m. The interpolation for the underlying topography is an estimation and might miss smaller depressions and elevations. As lava tends to flow into depressions, the calculated values might be underestimations. Since the topography in the area has a low relief, big differences are unlikely.

Útdráttur

Tunguhraun og Dvergagígahraun eru hraun á miðhálendi Íslands. Þau eru einu nútímahraunin sem tengd hafa verið megineldstöð Tungnafellsjökuls og eru rannsóknir á þeim því mikilvægar til að skilja virkni og hegðun eldstöðvakerfis Tungnafellsjökuls á nútíma. Í þessari rannsókn var rúmmál og þykkt Tunguhrauns og Dvergagígahrauns metið. Þar sem landslag undir hraununum er óþekkt var undirlag þeirra metið út frá landslagi í kringum þau og áætlað hæðarlíkan fyrir undirlag hraunsins búið til út frá þeim upplýsingum. Hæðargögnin til að áætla líkanið og hæðarupplýsingar fyrir hraunin sjálf voru frá ArcticDEM hæðarlíkaninu. Með því að nota kortlagningu af útlínum hraunanna, áætlaða hæðarlíkanið af undirlagi hraunanna og hæðarlíkan af núverandi yfirborði hraunanna var hægt að meta rúmmál þeirra. Tunguhraun er frekar lítið og hraunlag þess þunnt, það er 0,15 km3 að rúmmáli og meðalþykkt þess um 0,7 m. Dvergagígahraun er hins vegar þykkara að meðaltali en eitt af minnstu gosum á Íslandi, það er 0,36 x 10-2 km3 að rúmmáli og 3,1 m þykkt að meðaltali. Hraun rennur yfirleitt í lægðum og því getur verið að áætluðu hæðarlíkönin nái ekki að meta lægðirnar fyllilega. Niðurstöðurnar eru þ ví líklega frekar vanmat en ofmat. Ólíklegt er að munurinn sé mikill þar sem landslagið á svæðinu er ekki mjög mishæðótt.

Table of Contents

List of Figures ...... vi

Abbreviations ...... x

Acknowledgements ...... xi

1 Introduction ...... 1 1.1 Volcanic Systems ...... 1 1.2 Volcanism in Iceland ...... 1 1.3 Tungnafellsjökull ...... 5 1.4 Lava Volume Estimations ...... 9

2 Methods ...... 11

3 Results ...... 19 3.1 Profiles of the Tunguhraun Lava ...... 23 3.2 Profiles of the Dvergagígahraun Lava ...... 24

4 Discussion ...... 27 4.1 Estimation of Accuracy ...... 27 4.2 Volume Comparison ...... 28 4.3 Flow Behavior of the Tunguhraun Lava ...... 31 4.4 Fissure Orientation and Eruption of the Dvergagígahraun Lava...... 32

5 Conclusion ...... 33

References ...... 35

v List of Figures

Figure 1: The Atlantic Ocean with earthquake epicenters in red (1964-2006), which mark the mid-Atlantic plate boundary. Data are from the epicentral list of the NEIC, US Geological Survey. Figure from Einarsson (2008)...... 3

Figure 2: Volcanic Systems of Iceland in yellow (from Einarsson and Sæmundsson, 1987) and earthquake epicenters from 1994–2007 in red (from the data bank of the Icelandic Meteorological Office). Different volcanic zones are indicated as: RPR Reykjanes Peninsula Rift, WVZ Western Volcanic Zone, SISZ South Iceland Seismic Zone, EVZ Eastern Volcanic Zone, CIVZ Central Iceland Volcanic Zone, NVZ Northern Volcanic Zone, GOR Grímsey Oblique Rift, HFZ Húsavík-Flatey Zone, ER Eyjafjarðaráll Rift, DZ Dalvík Zone, and SIVZ South Iceland Volcanic Zone. The abbreviations Kr, Ka, H, L, V stand for the central volcanoes of Krafla, Katla, Hengill, Langjökull, and Vestmannaeyjar. Figure taken from Einarsson (2008)...... 4

Figure 3: The spreading across the plate boundary in Iceland. Black arrows (ISNET measurements) and red arrows (CGPS stations in Iceland) indicate horizontal GPS station velocities relative to a fixed North American plate. Measurements span over a time interval of 1993-2004 for the ISNET measurements and 1999-2004 for the CGPS stations in Iceland. The green arrows show the predicted velocity of the Eurasian plate relative to a fixed North American plate from the NUVEL-1A plate motion model (DeMets et al., 1994) Figure taken from Árnadóttir et al., (2009)...... 5

Figure 4: InSAR images of the Gjálp eruption period. a) From 3rd of June 1995 to 6th of October 1996, b) from 31st of May 1995 to 3rd of October 1996, c) from 6th of October 1996 to 13th of July 1997, d) from 3rd of October 1996 to 23rd of September 1999. Arrows in figure c) indicate local deformation signals north of the Tungnafellsjökull glacier. The numbers in the lower right corners give the altitude of ambiguity in meters; it indicates the difference in topographic elevation that produces one fringe in an interferogram. The color index in figure a) applies to all four images. Figure taken from Pagli et al. (2007)...... 7

Figure 5: Seismic activity in the Tungnafellsjökull fissure swarm. a) Location map, fissure swarms from Einarsson and Sæmundsson (1987). b) The Bárðarbunga and Tungnafellsjökull volcanoes with earthquake epicenters (15th August 2014 to 10th April 2015) from the Icelandic Meterological Office (2016), seismic stations and GPS stations, eruption sites and modelled ring faults, sill, and dyke. c) Tungnafellsjökull with earthquakes from the 5th October 1996 to 11th April 1998 marked with light grey outlined dots, earthquakes from the 12th April 1998 to 14th August 2014 marked with transparent black outlined dots and earthquakes from the 15th August 2014 to 8th March 2015 marked with dark black outlined dots. The surface fractures are from Björnsdóttir and Einarsson (2013) and the TanDEM-X digital elevation model in the background of figures b) and c)

vi was provided by the German Space Agency (DLR). Figure taken from Parks et al. (2017)...... 8

Figure 6: Mapping of lava flows. The Tunguhraun lava is in green and the Dvergagígahraun lava is in orange. The background image and the image on the inserted map are from Loftmyndir ehf...... 12

Figure 7: Extracted elevation raster on the left for the Tunguhraun lava and on the right for the Dvergagígahraun. Elevation models from ArcticDEM...... 13

Figure 8: Regular points. Top left: Regular points over the Tunguhraun lava. Top right: regular points over the Dvergagígahraun lava. Bottom left: Regular points around the Tunguhraun lava, unnecessary points deleted. Bottom right: regular points around the Dvergagígahraun lava, unnecessary points deleted...... 14

Figure 9: TIN interpolated triangles. On the left the Tunguhraun lava and on the right the Dvergagígahraun lava...... 15

Figure 10: The different elevation rasters. Black is the lowest elevation and white the highest. a) Left: current elevation raster for the Tunguhraun lava. Right: current elevation raster for the Dvergagígahraun lava. b) Left: interpolated elevation raster for the Tunguhraun lava. Right: interpolated elevation raster for the Dvergagígahraun lava. c) Left: elevation difference raster for the Tunguhraun lava. Right: elevation difference raster for the Dvergagígahraun lava...... 17

Figure 11: Mapped outlines of the Tunguhraun lava. The cartographic data is from IS50 database of the National Land Survey of Iceland, the aerial photograph from Loftmyndir Inc and the hillshade in the background is a TanDEM-X digital elevation model from the German Space Agency (DLR)...... 19

Figure 13: The Tunguhraun eruptive vent (Bokki)...... 21

Figure 14: Photos showing a) western Dvergagígahraun lava (darker area), b) eastern Dvergagígahraun lava (darker area), c) westernmost crater of the eastern Dvergagígahraun lava, d) empty lava lake of the eastern Dvergagígahraun lava...... 21

Figure 15: Profile 1. Cross-section from the south (left) to the north (right). Interpolated surface elevation in blue and today’s surface elevation in red. The red line on the map shows the location of the cross-section...... 23

vii Figure 16: Profile 2. Cross-section from the west (left) to the east (right). The profile extends across the crater called Bokki. Interpolated surface elevation in blue and today’s surface elevation in red. The red line on the map shows the location of the cross-section...... 23

Figure 17: Profile 3. Cross-section from west (left) to east (right) in the middle of Tunguhraun lava. Interpolated surface elevation in blue and today’s surface elevation in red. The red line on the map shows the location of the cross-section...... 24

Figure 18: Profile 4. Cross-section from west (left) to east (right) of the northern most part of Tunguhraun lava. Interpolated surface elevation in blue and today’s surface elevation in red. The red line on the map shows the location of the cross-section...... 24

Figure 19: Profile 5. Cross-section from west (left) to east (right) through both parts of the Dvergagígahraun lava. Interpolated surface elevation in blue and today’s surface elevation in red. The red line on the map shows the location of the cross-section...... 25

Figure 20: Profile 6. Cross-section through the western Dvergagígahraun lava field and its crater. Interpolated surface elevation in blue and today’s surface elevation in red. The red line on the map shows the location of the cross- section...... 25

Figure 21: Profile 7. Cross-section through the western most crater of the eastern Dvergagígahraun lava field. Interpolated surface elevation in blue and today’s surface elevation in red. The red line on the map shows the location of the cross-section...... 25

Figure 22: Profile 8. Cross-section through the second crater of the eastern Dvergagígahraun lava field. Interpolated surface elevation in blue and today’s surface elevation in red. The red line on the map shows the location of the cross-section...... 26

Figure 23: Profile 9. Cross-section through the third crater from the west of the eastern Dvergagígahraun lava field. Interpolated surface elevation in blue and today’s surface elevation in red. The red line on the map shows the location of the cross-section...... 26

Figure 24: Profile 10. Cross-section through eastern most crater of the eastern Dvergagígahraun lava field. Interpolated surface elevation in blue and today’s surface elevation in red. The red line on the map shows the location of the cross-section...... 26

Figure 25: Cross-section showing no distinguishable eleveation difference at the lava rim. The green square marks the lava field. The red line on the map shows the location of the cross-section...... 28

Figure 26: Cross-section of the northernmost kipuka. The green square marks the kipuka. The red line on the map shows the location of the cross-section...... 28

viii Figure 27: Area and volume of lava erupted at Fagradalsfjall, southwest Iceland, in early 2021. Dates on x-axis and volume and area on y-axis. Colored dots indicate method of measurement for the values. Figure taken from the website of the Institute of Earth Sciences, University of Iceland (http://jardvis.hi.is/) ...... 30

Figure 28: Flow length and volume. This plot shows flow length in km versus the volume in km3 of Icelandic lava flows with the Tunguhraun lava marked as a green star. Abbreviations stand for: “ls”- pahoehoe (lava shield eruptions), “ph” - pahoehoe (fissure eruptions), “ph+rph” - pahoehoe and rubbly pahoehoe, “rph” - rubbly pahoehoe, “aa-b” - aa lava (mafic), “aa- a” - aa lava (intermediate), “bl” - block lava, “co” - coulee. Figure taken and adapted from Þórðarson and Höskuldsson (2008)...... 31

ix Abbreviations

ISOR Íslenskar orkurannsóknir (Iceland Geosurvey)

InSAR Interferometric Synthetic Aperture Radar

TIN Triangulated Irregular Network

ArcticDEM Arctic Digital Elevation Model

RPR Reykjanes Peninsula Rift

WVZ Western Volcanic Zone

SISZ South Iceland Seismic Zone

EVZ Eastern Volcanic Zone

CIVZ Central Iceland Volcanic Zone

NVZ Northern Volcanic Zone

GOR Grímsey Oblique Rift

HFZ Húsavík-Flatey Zone

ER Eyjafjarðaráll Rift

DZ Dalvík Zone

SIVZ South Iceland Volcanic Zone

NEIC National Earthquake Information Center

GPS Global Positioning System

CGPS Continuous Global Positioning System

IDW Inverse Distance Weighting (Interpolation)

QGIS Quantum Geoinformation System (Software)

x Acknowledgements

I would like to thank my supervisor Dr. Ásta Rut Hjartardóttir for her helpful guidance and support. Furthermore, I thank Gunnlaugur M. Einarsson from Iceland Geosurvey (ISOR), who authorized me to use their data of the mapped lava flows and their age. Prof. Dr. Páll Einarsson, Dr. Þóra Árnadóttir, Dr. Carolina Pagli, Dr. Michelle Parks, Prof. Dr. Þorvaldur Þórðarson and the Institute of Earth Sciences at the University of Iceland, kindly allowed me to use their images and figures. My appreciation also goes to Dr. Gro Birkefeldt Møller Pedersen and Dr. William Michael Moreland for their support and inspiring conversations. A thank you to the Polar Geospatial Center which provided the ArcticDEM under NSF- OPP awards 1043681, 1559691, and 1542736. The TanDEM-X digital elevation model is from the German Space Agency (DLR), under the project of IDEM_GEOL0123. Finally, a big thank you to Margaret Unger for her input on my thesis and to Finnur Ágúst Ingimundarson and Jarþrúður Ósk Jóhannesdóttir for reading over the Icelandic abstract.

1 Introduction

The Tunguhraun and Dvergagígahraun lavas are two lava fields on the Icelandic highlands (Hjartarson et al., 2019). The volcanic systems in Iceland differ significantly in terms of activity. Information on the volume of lava fields is of major importance for our understanding of a volcano’s activity. The Tunguhraun and Dvergagígahraun lavas are the results of the only two eruptions of the Tungnafellsjökull volcanic system during the Holocene. Therefore, research on them can give essential information on the systems’ activity since the deglaciation. Although the lava fields have previously been mapped by Hjartarson et al., (2019), no volume measurements or estimations have been done on them. This poses the question, how much lava that has erupted in the last ten thousand years can be associated with the Tungnafellsjökull volcanic system? How does this compare to other systems in Iceland? The aim of this thesis is to map the two lava fields with the best possible accuracy and use aerial photographs and digital elevation models to estimate their volume. The first part of the thesis gives some background information on volcanic systems, Iceland, and the study area. Then the methods used in the project are described. In the last part results are presented and set in context.

1.1 Volcanic Systems

A volcanic system can be characterized by central volcanoes and fissure swarms or transecting rift zones (e.g. Sæmundsson, 1979). Generally, a central volcano is a geological structure that forms where most of the magma is discharged in a volcanic system. Originally the term central volcano was defined as an intermediate to the definitions of stratovolcanoes and shield volcanoes. The definition was such that a central volcano is a stratovolcano or shield volcano that features a bimodal composition with both rhyolitic and basaltic rocks. Often central volcanoes also have a geothermal area and one or more calderas (Walker, 1993). In Iceland, many central volcanoes form due to multiple eruptions from the same vent system (e.g. Gudmundsson, 2000).

A fissure swarm is a set of fissures that often lie subparallel to the axis of the volcanic system, propagating from the central volcano (e.g. Sæmundsson, 1979). They typically extend over 10 km in width and can have a length of over 100 km. Fissure swarms usually comprise tensile fractures (no vertical displacement), normal faults and volcanic fissures. They are the visible signs of dyke intrusions (Sæmundsson, 1978).

1.2 Volcanism in Iceland

Iceland is a volcanic island in the northern Atlantic Ocean with a very unique geological background. It lies on the mid-Atlantic ridge (Figure 1), which separates two tectonic plates, the North-American and the Eurasian plates (e.g. Einarsson 1991). In addition to the positioning on the divergent plate boundary, there is also a hot spot under the Icelandic crust (Guðmundsson, 2000). Recent studies suggest that the hot spot is fed by a narrow mantle

1 plume situated underneath Iceland (Wolfe et al., 1997). These two geological features, the divergent plate boundary and the hot spot, are considered to be the reason for the volcanic activity on the island (Guðmundsson, 2000). Due to these unusual circumstances, Iceland is one of the rare places where very diverse volcanism can be found (Thorarinsson & Sæmundsson, 1979). The area of active volcanism on the island is confined to a 15-50 km wide belt (e.g. Gudmundsson, 2000), which can be divided into different volcanic zones (rift zones) (Figure 2): the Reykjanes Peninsula Rift (RPR), Western Volcanic Zone (WVZ), Eastern Volcanic Zone (EVZ), Central Iceland Volcanic Zone (CIVZ), and Northern Volcanic Zone (NVZ) (Einarsson, 2008). There are also transform zones, like the South Iceland Seismic Zone (SISZ) and the Tjörnes Fracture Zone (including the Grímsey oblique rift (GOR), the Húsavík-Flatey fault zone (HFZ), and the Dalvík seismic zone (DZ)) (Einarsson, 1991). As opposed to the rift zones where the plate spreading occurs, the transform zones are generally not volcanically active and are characterized by transform faulting and earthquake activity (Einarsson, 2008). The recently most volcanically active part of Iceland is the axial volcanic zone, following the plate boundary from Reykjanes in the south-east to Öxarfjörður in the north (Thordarson & Larsen, 2006). Volcanic activity in the axial volcanic zone is closely related to the spreading plate boundary. The faster the plate spreads at a particular location, the higher the magnitude of the volcanic activity (Figure 3) (Árnadóttir et al., 2009). The volcanism and fault movements in individual fissure swarms does not happen continuously but in rifting episodes, during which seismicity increases and volcanic eruptions occur (e.g. Larsen et al., 1998; Sigmundsson et al., 2015; Wright et al., 2012). The different volcanic systems enter rifting episodes at different times and can stay active up to decades (Thordarson & Larsen, 2006).

Figure 2: Volcanic Systems of Iceland in yellow (from Einarsson & Sæmundsson, 1987) and earthquake epicenters from 1994-2007 in red (from the data bank of the Icelandic Meteorological Office). Different volcanic zones are indicated as: RPR Reykjanes Peninsula Rift, WVZ Western Volcanic Zone, SISZ South Iceland Seismic Zone, EVZ Eastern Volcanic Zone, CIVZ Central Iceland Volcanic Zone, NVZ Northern Volcanic Zone, GOR Grímsey Oblique Rift, HFZ Húsavík-Flatey Zone, ER Eyjafjarðaráll Rift, DZ Dalvík Zone, and SIVZ South Iceland Volcanic Zone. The abbreviations Kr, Ka, H, L, V stand for the central volcanoes of Krafla, Katla, Hengill, Langjökull, and Vestmannaeyjar. Figure taken from Einarsson (2008).

4 67˚

RHOF

66˚

65˚

SKRO

REYK HOFN 64˚

VMEY 100 km 63˚ 10 +/− 1 mm/yr

−24˚ −22˚ −20˚ −18˚ −16˚ −14˚ −12˚ Figure 3: The spreading across the plate boundary in Iceland. Black arrows (ISNET measurements) and red arrows (CGPS stations in Iceland) indicate horizontal GPS station velocities relative to a fixed North American plate. Measurements span over a time interval of 1993-2004 for the ISNET measurements and 1999-2004 for the CGPS stations in Iceland. The green arrows show the predicted velocity of the Eurasian plate relative to a fixed North American plate from the NUVEL-1A plate motion model (DeMets et al., 1994). Figure taken from Árnadóttir et al., (2009).

1.3 Tungnafellsjökull

The Tungnafellsjökull volcanic system is one of 30 volcanic systems in the active volcanic region (Thordarson & Larsen, 2006) and is a part of the Central Iceland Volcanic Zone (Björnsdóttir & Einarsson, 2013). It is 55 km long and 15 km wide and has a maximum elevation of 1520 m over sea level. It covers an area of 530 km2 and is a rather small system compared to other volcanic systems in Iceland. This volcanic system is one of four systems that has two central volcanoes, called Tungnafellsjökull and Hágöngur (Thordarson & Larsen, 2006). One of the central volcanoes, Tungnafellsjökull itself, has two calderas. Besides the Tungnafellsjökull caldera, which lies under the glacier, Vonarskarð, just south- east of it, also has a caldera. (Björnsdóttir & Einarsson, 2013). In some sources they are considered to be two separate central volcanoes, leaving the system with three central volcanoes (Friðleifsson & Jóhannesson, 2005). The Tungnafellsjökull central volcano has a radius of about 10 km and is the highest point of the system. The slightly younger Vonarskarð caldera is approximately 8 km wide (Thordarson & Larsen, 2006).

5 The Tungnafellsjökull volcanic system has a moderately mature fissure swarm (Thordarson & Larsen, 2006) which, compared to other fissure swarms in Iceland, is rather short and wide. Usually fissure swarms extend from the central volcano, but because the one at Tungnafellsjökull is so wide, it bypasses the central volcano. The typical graben structure can be detected in the northern part of the swarm. In the south all the normal faults have a downthrow to the west (Björnsdóttir & Einarsson, 2013).

Tungnafellsjökull’s fissure swarm was mapped the first time by Sæmundsson in 1978. Pagli et al. (2007) used InSAR imaging to study deformation in the fissure swarm during the Gjálp eruption in 1996 (Figure 4). Their images showed movement at some faults in the Tungnafellsjökull fissure swarm. This increased scientists’ interest in the Tungnafellsjökull fissure swarm. Gjálp was an eruption underneath the glacier Vatnajökull, about 35 km from Tungnafellsjökull. It was a fissure eruption that lasted 13 days and caused a jökulhlaup. The events during this eruption seemed to have some impact on the Tungnafellsjökull volcanic system, which are described by Gudmundsson et al. (1997). Björnsdóttir and Einarsson (2013) summarized different field studies of the fissure swarm. Observations in the field showed that there had been recent movement on some of the faults. Also, they compared the cumulative seismic moment of the measured earthquakes, which added up to a magnitude 3.4, to the displacement identified with InSAR imaging by Pagli et al. (2007) and its corresponding seismic moment. Those two values did not seem to add up (movement equivalent to a magnitude 5 earthquake). That is why they conclude that the movement in the fissure swarm was not solely triggered by the tectonic stress but might also have been influenced by magma movements at depth (Björnsdóttir & Einarsson, 2013).

The Tungnafellsjökull volcanic system is not very seismically active. There have been on average two earthquakes with magnitude 2 or less per year during the last 26 years. However, studies of the seismicity in the Tungnafellsjökull area show rather surprising increases of seismicity in the years 1996 and 2014 (Figure 5). During these years there was an increase in seismicity of ten times as much in 1996 and 60 times as much in 2014. These increases seemed to occur during the same time as the nearby volcano Bárðarbunga underwent some unrest. In 1996 the Gjálp eruption (described here above) and the 2014 Holuhraun eruption, both eruptions related to the Bárðarbunga volcano, occurred (Parks et al., 2017). At the beginning of the Holuhraun eruption there were dykes leading first towards the east and then to the north, away from the central volcano, and the caldera at Bárðarbunga collapsed. Looking at the timing of the increase in seismicity at Tungnafellsjökull and the events at Bárðarbunga it comes to light that the seismicity started to increase as the caldera at Bárðarbunga began to collapse. As the eruption at Bárðarbunga drew to an end, the seismicity at Tungnafellsjökull also slowed down (Parks et al., 2017). Parks et al. (2017) studied the hypothesis of a possible triggering of the increased seismicity of Tungnafellsjökull by the events at Bárðarbunga in 2014 and 2015. They present results of deformation and stress modelling that suggest that the earthquakes at Tungnafellsjökull were triggered by unclamping faults of the Tungnafellsjökull fissure swarm due to stress transfer from the events at Bárðarbunga (Parks et al., 2017).

In comparison to other active volcanoes in Iceland, the Tungnafellsjökull central volcano is rather quiet. Though the volcanic system is considered to be active, its outbursts during the Holocene have been few and small. Not much is known about the most recent lava flows of the system. There have only been two eruptions during the last approximately 4500 years resulting in the Dvergagígahraun and Tunguhraun lavas. They are both small lava flows and are located to the north-east of the glacier and the central volcanoes (Björnsdóttir &

6 Einarsson, 2013; Hjartarson et al., 2019). Both eruptions occurred outside the central volcanoes and, therefore, no central volcano activity could be detected during the last 9000 years (Sæmundsson, 1982; Friðleifsson & Jóhannesson, 2005). Apart from the mapping of those lava flows (Hjartarson et al., 2019), there have not been many studies focusing on them.

Figure 4: InSAR images of the Gjálp eruption period. a) From 3rd June 1995 to 6th October 1996, b) from 31st May 1995 to 3rd October 1996, c) from 6th October 1996 to 13th July 1997, d) from 3rd October 1996 to 23rd September 1999. Arrows in figure c) indicate local deformation signals north of the Tungnafellsjökull glacier. The numbers in the lower right corners give the altitude of ambiguity in meters; it indicates the difference in topographic elevation that produces one fringe in an interferogram. The color index in figure a) applies to all four images. Figure taken from Pagli et al. (2007).

Figure 5: Seismic activity in the Tungnafellsjökull fissure swarm. a) Location map, fissure swarms from Einarsson and Sæmundsson (1987), b) The Bárðarbunga and Tungnafellsjökull volcanoes with earthquake epicenters (15th August 2014 to 10th April 2015) from the Icelandic Meterological Office (2016), seismic stations and GPS stations, eruption sites and modelled ring faults, sill, and dyke, c) Tungnafellsjökull with earthquakes from the 5th October 1996 to 11th April 1998 marked with light grey outlined dots, earthquakes from the 12th April 1998 to 14th August 2014 marked with transparent black outlined dots and earthquakes from the 15th August 2014 to 8th March 2015 marked with dark black outlined dots. The surface fractures are from Björnsdóttir and Einarsson (2013) and the TanDEM-X digital elevation model in the background of figures b) and c) was provided by the German Space Agency (DLR). Figure taken from Parks et al. (2017).

8 1.4 Lava Volume Estimations

Information about the volume and thickness of lava flows can give insights on the volcano’s history and its behavior. Stevens et al. (1999) describe two general techniques on how thicknesses of lava fields can be estimated. First, the planimetric method is rather simple and was the one applied most of all at that time. The approximate volume of a lava field is calculated by measuring the area the lava field covers and multiplying it by an estimated mean thickness. The irregular morphology as well as the underlying topography can make it difficult to derive an exact value of lava thickness, making the planimetric method rather inaccurate.

The second general method, or topographic approach, described by Stevens et al. (1999) takes a three-dimensional approach. Here, the topography of the area is measured before and after the eruption. The data is then compared, and the elevation change between the two measurements gives a much more precise volume estimation than does the planimetric method. Though this method is favored to the planimetric method, it is not always possible to make use of it. It requires information on the underlying bed, which is not always available.

A study in Iceland used a more advanced approach to calculate volumes of the last five eruptions of the volcano Hekla. To calculate the volume, digital elevation models were made from historical stereo photographs, recording the surface elevation pre- and post-eruption. By doing this, high-precision estimates of lava volumes and effusion rates for Hekla eruptions were derived (Pedersen et al., 2018).

Two other studies, one on the Fogo Volcano in the Atlantic Ocean (Bagnardi et al., 2016) and another on the Nyamulagira Volcano in the Democratic Republic of the Congo (Albino et al., 2015), use similar methods. High-resolution tri-stereo optical imagery and TerraSAR-X add-on for Digital Elevation Measurement respectively were used to generate high-resolution Digital Elevation Models. These were then used to find the elevation difference and the volume of eruptions.

As aforementioned, all these methods require some kind of record of the topography of the area pre-eruption. Since this study includes two lava fields from eruptions that occurred over 4500 years ago, no such records are available. This problem is addressed here by creating an interpolated elevation model of the underlying topography, using the elevation data surrounding the lavas.

10 2 Methods

The goal of this thesis was to estimate the volumes of the Tunguhraun and Dvergagígahraun lava fields in the best possible way. This was done with remote sensing techniques. To be able to calculate a possible volume of the two lavas, both the areas (determined by mapping the outlines of the lava field) and the thicknesses need to be known. To determine the areas, aerial photographs from Loftmyndir Inc. were used. The thickness of the lava was established by subtracting the elevation of the underlying bedrock from the elevation of the lava field. The fact that these lavas are over 4500 years old makes it more difficult to calculate the volume. Since no data about the landscape underneath the lava fields exist, it needed to be modelled. This was done by TIN (Triangulated Irregular Networks) interpolation. It is a process where points with missing data are estimated based on surrounding known points. TIN interpolation is often used for elevation modelling whereas others (e.g., Inverse Distance Weighting – IDW interpolation) are used for mineral concentration or biological population studies, for example (Gandhi, 2019). The interpolated elevation model made it then possible to calculate the thickness of the lava, and in combination with the outlines of the lava flow, derive the lava volume. To determine elevation data, digital elevation models from ArcticDEM (Porter et al., 2018), with a resolution of 2 m per pixel, were used. On the next pages a more detailed description follows.

First, the outlines of the two lavas needed to be mapped. The Iceland Geosurvey (ISOR) had already mapped them and gave permission to use their data (Hjartarson et al., 2019). On the basis of these outlines, the lava flows were mapped in greater detail (Figure 6) in the ArcMap software by using digital aerial photographs from Loftmyndir Inc. The mapping was done in the scale of 1:500 or smaller using photographs that were taken in 2016 by Loftmyndir Inc.

Figure 6: Mapping of lava flows. The Tunguhraun lava is in green and the Dvergagígahraun lava is in orange. The background image and the image on the inserted map are from Loftmyndir ehf.

12 To calculate the volume of the lavas, the surface elevation underneath the lavas needed to be known. Such information does not exist; instead, an approximation of how the landscape might have looked before the eruptions is generated. By adding elevation data to points drawn on the area around the lava field, a possible surface level underneath can be calculated. This was done with the QGIS software. Each step is described in detail in the following paragraphs.

Digital elevation models from ArcticDEM (with a resolution of 2 m per pixel) (Porter et al., 2018) were used to determine elevation data. To make it easier to work with the vector layer, part of the layer was extracted. This minimized the program workload. To extract the necessary segment, a polygon was drawn around the lavas with a 1.3-3 km distance to the edge of the lava flows. In the QGIS software, the tool Clip Raster by Mask Layer was used to extract the raster in the shape of the polygon (Figure 7).

919.8 m 1010.4 m 731.7 m 918.7 m

Figure 7: Extracted elevation raster on the left for the Tunguhraun lava and on the right for the Dvergagígahraun. Elevation models from ArcticDEM. Next, a grid of points over the clipped elevation model was created with the tool Regular points (Figure 8). The tool draws points in a rectangle over the selected layer, therefore all the points outside of the elevation model and all the ones inside and on the outline of the lava needed to be deleted (Figure 8). To add an elevation attribute to the points the tool Add raster values to points was used. This saved the elevation value for the corresponding location from the ArcticDEM elevation models to each point. Here, the nearest neighbor interpolation was applied. This method finds a missing value at a specific spot – the nearest neighbor value, – and adds it instead. This is the simplest solution, while others, for example, calculate a possible value using all neighboring values.

Figure 8: Regular points. On the top left: regular points over the Tunguhraun lava. On the top right: regular points over the Dvergagígahraun lava. On the bottom left: regular points around the Tunguhraun lava, unnecessary points deleted. On the bottom right: regular points around the Dvergagígahraun lava, unnecessary points deleted.

14 The main step was then to create an estimated landscape with TIN interpolation. It is used to estimate the unknown elevation values by interpolating them from the known values in the surrounding area. The method then uses these estimated values to create a model of the underlying landscape in the form of triangles (Figure 9). In the TIN interpolation for the landscape under the Tunguhraun and Dvergagígahraun lavas, the accuracy of 4 pixels was used. Here it was important to ensure that all involved layers have the same coordinate system.

Figure 9: TIN interpolated triangles. On the left the Tunguhraun lava and on the right the Dvergagígahraun lava. To calculate the volume of the lava fields two layers were used: the point raster with the current elevation values (curel) and the interpolated raster (intel). Both these layers first needed to be cropped to the outlines of the lava fields (Figure 10 a) and b)). This was done with the Clip Raster by Mask Layer tool. From the two layers another layer with the elevation difference was created (Figure 10 c)). This is possible with the Raster calculator tool. The command curel@1 - intel@1 gives the desired results. The last step in computing the volume of the lava fields is to use the layer with the elevation difference values and the tool raster volume. To avoid influence of the negative elevation difference (see Figure 10 c) at some points caused by error of the interpolated elevation model, the option count only above base level was chosen. This ensures that the negative values are not subtracted from the volume estimations.

15 a)

922.6 m 986.8 m 734.8 m 923.6 m

876.2 m 969.4 m 737.4 m 924.1 m

16 c)

57.4 m 14.5 m -11.5 m -5.3 m

Figure 10: The different elevation rasters. Black is the lowest elevation and white the highest. a) On the left the current elevation raster for the Tunguhraun lava. On the right the current elevation raster for the Dvergagígahraun lava. b) On the left the interpolated elevation raster for the Tunguhraun lava. On the right the interpolated elevation raster for the Dvergagígahraun lava. c) On the left the elevation difference raster for the Tunguhraun lava. On the right the elevation difference raster for the Dvergagígahraun lava. In addition to the volume calculations, different profiles across the lava fields were drawn with the profile tool, giving more insight into the topography of the lava.

18 3 Results

The following figures shows the mapping of the Tunguhraun lava in the north of the Tungnafellsjökull glacier and the Dvergagígahraun to the north-east of it (Figure 11 and Figure 12). The Tunguhraun lava field (Figure 11) is an elongated area, with the crater (Figure 13) in the very south and many kipukas. It covers an area of 210.95 km2. The Dvergagígahraun lava (Figure 12 and Figure 14) has two parts, one smaller one in the west (Figure 14 a) and a bigger one in the east (Figure 14 b). They are separated by a hill. The western part features one crater in the south and the eastern part features 4 small craters (Figure 14 c). All together the Dvergagígahraun covers an area of 1.18 km2.

Figure 11: Mapped outlines of the Tunguhraun lava. The cartographic data is from IS50 database of the National Land Survey of Iceland, the aerial photograph from Loftmyndir Inc. and the hillshade in the background is a TanDEM-X digital elevation model from the German Space Agency (DLR).

Figure 12: Mapped outlines of the Dvergagígahraun lava. The cartographic data is from IS50 database of the National Land Survey of Iceland, the aerial photograph from Loftmyndir Inc. and the hillshade in the background is a TanDEM-X digital elevation model from the German Space Agency (DLR). Observations in the field gave an insight into the eruption style. While the Tunguhraun lava erupted from a single crater, with a relatively high elevation and surrounded by a rather flat lava field, the Dvergagígahraun lava surfaced at different locations, building up five different craters and seemingly also created lava lakes that drained before they solidified completely (Figure 14 d).

Figure 13: The Tunguhraun eruptive vent (Bokki).

a) b)

c) d)

21 By examining the digital elevation models from ArcticDEM (Porter et al., 2018) the highest and lowest elevation of the lava fields can be determined. The highest point of the Tunguhraun lava is 922.6 m above sea level and the lowest point at 734.8 m. For the Dvergagígahraun lava these values are 986.8 m and 923.6 m above sea level respectively (Figure 10 a). In comparison, the interpolated elevation models show maximal and minimal elevations of 876.2 m and 737.4 m above sea level for the surface under Tunguhraun and 969.4 m and 924.1 m above sea level for the surface under Dvergagígahraun (Figure 10 b).

The software calculations reveal that the Tunguhraun lava has a volume of 0.15 km3 and the Dvergagígahraun lava has a volume of 0.36 x 10-2 km3. For the Dvergagígahraun lava two different volumes were calculated. Firstly, the interpolated elevations were derived from all the surrounding elevations (0.30 x10-2 km3). This method might not give an accurate value for the underlying elevation since the high elevation of the hill in between the two lava parts can falsify the values. For the second calculation, therefore, the elevation points on the hill between the lavas were deleted and not included in the calculations (0.36 x10-2 km3). The difference between these two results is 0.06 x10-2 km3 or 16%.

Figure 10 c) shows that the largest elevation difference between today’s surface and the interpolated surface under the lava fields are 57.4 m for the Tunguhraun lava and 14.5 m for the Dvergagígahraun lava. Using the mapped areas and volume calculations an average thickness of about 0.7 m for the Tunguhraun lava and about 3.1 m for the Dvergagígahraun lava can be derived.

The different profiles give a better understanding of the distribution of the lava over the area. The profiles show the interpolated surface in blue and today’s surface elevation in red. The red line on the map shows the location of the cross-section.

22 3.1 Profiles of the Tunguhraun Lava

Profile 1 (Figure 15): This first profile shows a cross-section of the Tunguhraun lava from south to north. It indicates that the biggest difference between the two surfaces is in the south, where the crater is situated. The differences are, in general, rather small. In some places the elevation line of the lava goes below the ground line; this occurs as the ground line was interpolated and not measured, which can cause inaccuracies. This method was used with the assumption that the landscape in the area is fairly even and, therefore, some deviations can occur. The fact that the lava field is in places quite thin can also be an explanation as to why there may be inaccuracies. The maximum thickness of this cross- section is approximately 44 m.

Elevation(m)

Distance (m) Figure 15: Profile 1. Cross-section from the south (left) to the north (right). Interpolated surface elevation in blue and today’s surface elevation in red. The red line on the map shows the location of the cross-section.

Profile 2 (Figure 16): Shows a cross-section from the west to the east of the southernmost part of the Tunguhraun lava. The cross-section goes through the crater Bokki. Here the lava field is thickest. The maximum thickness of this cross-section is approximately 53 m.

Elevation(m)

Distance (m) Figure 16: Profile 2. Cross-section from the west (left) to the east (right). The profile extends across the crater Bokki. Interpolated surface elevation in blue and today’s surface elevation in red. The red line on the map shows the location of the cross-section.

23 Profile 3 (Figure 17) and Profile 4 (Figure 18): Additional west-east cross-sections. These are situated further north than Profile 2 and indicate that the thickness of the lava field is much less here. Both the kipukas in the lava flow and the fact that the red line (today’s surface elevation) on the profile is in some places below the interpolated surface elevation (blue line) shows how thin the lava is here and how the calculations may have a larger degree of error because of it. The maximum thicknesses of cross-sections 3 and 4 are approximately 5 m.

Elevation(m)

Distance (m) Figure 17: Profile 3. Cross-section from west (left) to east (right) in the middle of the Tunguhraun lava. Interpolated surface elevation in blue and today’s surface elevation in red. The red line on the map shows the location of the cross-section.

Elevation(m)

Distance (m) Figure 18: Profile 4. Cross-section from west (left) to east (right) of the northern most part of the Tunguhraun lava. Interpolated surface elevation in blue and today’s surface elevation in red. The red line on the map shows the location of the cross-section.

3.2 Profiles of the Dvergagígahraun Lava

Profile 5 (Figure 19): A cross-section through both parts of the Dvergagígahraun lava, from west to east. The maximum thickness of this cross-section is approximately 12 m.

Elevation (m)

Distance (m) Figure 19: Profile 5. Cross-section from west (left) to east (right) through both parts of the Dvergagígahraun lava. Interpolated surface elevation in blue and today’s surface elevation in red. The red line on the map shows the location of the cross-section.

Profiles 6, 7, 8, 9 and 10 (Figures 20, 21, 22, 23 and 24): Cross-sections through each of the individual patches of lava with its own crater. Here maximal thicknesses are approximately: 7 m (Profile 6), 6 m (Profile 7), 11 m (Profile 8), 8 m (Profile 9) and 12 m (Profile 10).

Elevation (m)

Distance (m) Figure 20: Profile 6. Cross-section through the western Dvergagígahraun lava field and its crater. Interpolated surface elevation in blue and today’s surface elevation in red. The red line on the map shows the location of the cross-section.

Elevation (m)

Distance (m) Figure 21: Profile 7. Cross-section through the westernmost crater of the eastern Dvergagígahraun lava field. Interpolated surface elevation in blue and today’s surface elevation in red. The red line on the map shows the location of the cross-section.

Elevation (m)

Distance (m) Figure 22: Profile 8. Cross-section through the second crater of the eastern Dvergagígahraun lava field. Interpolated surface elevation in blue and today’s surface elevation in red. The red line on the map shows the location of the cross-section.

Elevation (m)

Distance (m) Figure 23: Profile 9. Cross-section through the third crater from the west of the eastern Dvergagígahraun lava field. Interpolated surface elevation in blue and today’s surface elevation in red. The red line on the map shows the location of the cross-section.

Elevation (m)

Distance (m) Figure 24: Profile 10. Cross-section through the easternmost crater of the eastern Dvergagígahraun lava field. Interpolated surface elevation in blue and today’s surface elevation in red. The red line on the map shows the location of the cross-section.

26 4 Discussion

4.1 Estimation of Accuracy

The accuracy of the volume estimations is debatable. Creating and using interpolated elevation models implies that the calculated surface elevation underneath the lava flows is not 100% correct. There is a chance of missing small depressions and elevations that existed before the eruption. Due to gravitational forces lavas tend to flow into depressions, filling them up and hiding them from view. That might cause some error in the lava volume estimations. However, taking into account the surrounding area, which is in general rather flat, large elevation differences underneath the lava seem unlikely. The kipukas of the Tunguhraun lava can give some information about the landscape underneath. Their elevation is not much higher than the lava or the landscape surrounding the lava field. Another indicator for a rather smooth underlying surface is that the lava does not lie on top of another lava field but on a topography that had been scoured by a glacier. The error caused by unexpected topography is, therefore, thought to be minor. It could be, however, that the volume estimations are a little too low due to undetected, filled depressions.

As calculations were performed, the mean thickness of the Tunguhraun lava seemed very small, which led to the decision of calculating the volume a second time. For the first calculation, the elevation values of the kipukas were taken in account for the interpolated elevation model. For the second calculation, only the elevation values around the lava field were used. The calculation with elevation values of the kipukas resulted in a volume of 0.15 km3 and that without of 0.19 km3. Estimations including the kipuka values thus result in a 20% smaller volume than the estimations where those values are omitted. However, the values are both in the same order of magnitude and, therefore, the difference is not very relevant.

Drawing cross-sections over the Tunguhraun lava that reach over the lava rim and into the surrounding landscape was thought to give an idea of the thickness of the lava in some places and suggest whether the calculated estimations were reasonable. The cross-sections showed, however, no clear elevation differences at the lava edge (Figure 25). Moreover, most of the kipukas do not show a distinguished increase of elevation. The northernmost kipuka is the only one that can be distinguished as having an elevation (of about 6 m) to the surrounding lava (Figure 26). Some of the kipukas are even depressions into which lava did not flow. These observations did not give the anticipated second estimation on the lava thickness to compare to, but they confirmed that the lava must be very thin in some places, which explains the very low average thickness despite the stacked-up vent, Bokki.

Elevation (m)

Distance (m) Figure 25: Cross-section showing no distinguishable elevation difference at the lava rim. The green square marks the lava field. The red line on the map shows the location of the cross-section.

Elevation (m)

Distance (m) Figure 26: Cross-section of the northernmost kipuka. The green square marks the kipuka. The red line on the map shows the location of the cross-section.

In conclusion, the volume estimations presented in this thesis might be somewhat underestimated. Nevertheless, they give a good estimate of the size of these two eruptions.

4.2 Volume Comparison

The results of this study present lava volumes of 0.15 km3 for the Tunguhraun lava and 0.36 × 10-2 km3 for the Dvergagígahraun lava. Their average thicknesses are about 0.7 m (Tunguhraun lava) and about 3.1 m (Dvergagígahraun lava). Both these Holocene eruptions are very small in volume. Below are a few volume estimations from other volcanic systems with which to compare these numbers.

Hekla erupted about 23 times since the settlement of Iceland (874 AD). For the most recent five eruptions it was possible to create elevation models of the area both before and after the events, because of existing aerial photographs of the area before the eruptions. All five eruptions produced a volume of at least one order of magnitude larger than the Dvergagígahraun lava and three of the five also a larger volume than the Tunguhraun lava flow. In the eruption of 1947 to 1948 Hekla produced approximately 0.742 ± 0.138 km3. In 1970 it produced about 0.205 ± 0.012 km3, in 1980, about 0.169 ± 0.016 km3, in 1991, about 0.241 ± 0.019 km3, and in 2000, 0.095 ± 0.005 km3 (Pedersen et al., 2018).

28 Compared to the eruption at Holuhraun in 2014-2015, the largest eruption in Iceland since the Laki eruption in 1783-1784, the Tunguhraun and Dvergagígahraun eruptions are both tiny. During Holuhraun about 1.2 km3 of lava erupted (Bonny et al., 2018). That is about eight times more than the Tunguhraun lava and over 300 times more than the Dvergagígahraun lava. In this context it can be said that the name “Dvergagígahraun”, which translates to “Dwarf-Crater-Lava”, suits the size of the eruption very well. Even in comparison with some intrusions, the Dvergagígahraun lava is still small. In 2007-2008 there was an intrusion under Upptyppingar, east of the Askja volcano, that was estimated to be about 0.042-0.047 km3 (Hooper et al. 2011). The intrusion under Eyjafjallajökull in 1999 added up to a volume of about 0.03 km3, according to the deformation seen on InSAR images (Pedersen & Sigmundsson, 2005). Even these intrusions are still one order of magnitude larger than the Dvergagígahraun lava. Of course, in the volume estimation of the Dvergagígahraun lava, the magma that might have solidified in the crust without erupting is not included.

On March 19th, 2021 the latest volcanic eruption to date started in Iceland. It is situated in the Reykjanes Peninsula, in the southwest of Iceland. It is a fissure eruption that began in one fissure, in which the flow focused and built up two craters. About three weeks after the start of the eruption, another fissure opened, just a few hundred meters away from the first one. Following, another two fissures opened within the first four weeks of activity. The flow rate at the beginning of this outburst was estimated to be approximately 7 to 8 m3 per second. Researchers have been surveying the eruption and estimating the volume of erupted lava over the last months (Figure 27). Figure 27 shows the growth of the lava flow area and volume. On March 25th the lava flow reached the Dvergagígahraun volume and has since been constantly increasing. The opening of the second fissure, especially, initiated a sudden increase in the lava field area. The eruption is still ongoing, but latest measurements (May 18th, 2021) give a lava volume of 38.3 million m3 (Institute of Earth Sciences at the University of Iceland et al., 2021). This information and Figure 27 were extracted from the website of the Institute of Earth Sciences (http://jardvis.hi.is/). The main work was done by a collaboration of scientists at the Institute of Earth Sciences at the University of Iceland, the National Land Survey of Iceland (https://kortasja.lmi.is/) and the Icelandic Institute of Natural History (3D-Models) with participation from several other institutes, both national and international.

Figure 27: Area and volume of lava erupted at Fagradalsfjall, southwest Iceland, in early 2021. Dates on x-axis and volume and area on y-axis. Colored dots indicate method of measurement for the values. Figure taken from the website of the Institute of Earth Sciences, University of Iceland (http://jardvis.hi.is/) Þórðarsson and Höskuldsson (2008) compared the volume of Icelandic lava flows with their length and plotted them in a diagram (Figure 28). The volume of the Dvergagígahraun lava is so small that it cannot be plotted on this diagram. The Tunguhraun lava plot (marked with a light green star in Figure 28) indicates that its volume of 0.15 km3 and the longest distance from the crater (flow length) of about 13.3 km is amongst the smaller of lava fields but with a rather long flow length.

Figure 28: Flow length and volume. This plot shows flow length in km versus the volume in km3 of Icelandic lava flows with the Tunguhraun lava marked as a green star. Abbreviations stand for: “ls”- pahoehoe (lava shield eruptions), “ph” - pahoehoe (fissure eruptions), “ph+rph” - pahoehoe and rubbly pahoehoe, “rph” - rubbly pahoehoe, “aa-b” - aa lava (mafic), “aa-a” - aa lava (intermediate), “bl” - block lava, “co” - coulee. Figure taken and adapted from Þórðarson and Höskuldsson (2008).

4.3 Flow Behavior of the Tunguhraun Lava

The Tunguhraun lava field shows a rather uneven distribution. The majority of the area of the lava field is very thin but at its highest point, the vent, the lava is around 40 m thicker. This topography could give us information about the behavior of the lava flow. The distribution itself is not unusual. It indicates that there were two or more kinds of different mechanisms of emplacement occurring, e.g., Strombolian activity, that splatters magma up into the air where it starts to cool slightly. As it lands on the rim of the vent, it is by then more granular and sticks to it. That way an elevation can build up around the vent. There is usually also an overflow of magma. According to this mechanism the magma does not cool

31 down as quickly and can, therefore, flow away from the vent (Francis, 2004). The crater of the Tunguhraun lava has a maximum width of about 150 m and the lava thickness at the vent is estimated at a maximum of about 50 m. Compared to the craters building up at Fagradalsfjall (the eruption which started March 19th, 2021 and is ongoing) the height of the vent compared to the medium thickness of the lava field seems normal. The first three craters at Fagradalsfjall have the roughly-estimated dimensions (on April 18th, 2021) of 40 m diameter and 20 m height for “vent 1”, 40 m diameter and 10 m height for “vent 2” and 20 m diameter and 30-50 m height for “vent 3” (estimations based on differential DEM and orthophotographs from April 18th, 2021 by Gro Pedersen, personal communication).

4.4 Fissure Orientation and Eruption of the Dvergagígahraun Lava

The fissures in the north-east of Tungnafellsjökull glacier vary in direction. From west to east, they turn more towards the east and fan out that way (Björnsdóttir, 2012). The fissure underlying the Dvergagígahraun craters has an east-north-east direction (66°) and is compared to the other fissures the one pointing most to the east. The different orientation of the fissures is likely connected to the stress field of the central volcano. Often fissure swarms are subparallel to the regional stress field, but sometimes they fan out or radiate from the plutonic center (central volcano) (Ernst et al., 1993). Although at first glance the eruptive fissure of the Dvergagígahraun lava shows an unusual direction, observations at other dyke intrusions like the one at Bárðarbunga in 2014 shows that dyke propagation can take unexpected paths which can be understood with a thorough analysis of the volcanic and regional stress fields (Sigmundsson, 2015).

The Dvergagígahraun lava flow created 5 different craters aligning to the same fissure. One of the craters is separated from the other four by a small hill. This indicates that the magma could not reach the surface through the hill and instead surfaced on the other side of it. The lava fields from the four vents east of the hill are all connected. Some of them have signs of lava lakes. The nature of the Dvergagígahraun activity could perhaps be comparable to the one occurring at the Reykjanes peninsula at the time of writing (the eruption at Mt. Fagradalsfjall, 2021).

32 5 Conclusion

The outlines of the Tunguhraun and Dvergagígahraun lavas were mapped on the basis of aerial photographs and with that their area was estimated to be 210.95 km2 (Tunguhraun lava) and 1.18 km2 (Dvergagígahraun lava). Due to lack of information on the underlying landscape, an elevation model was created with the TIN interpolation technique of the Qgis software, showing a possible landscape before the eruptions. The newly-created elevation model made it possible to compare today’s elevations with the approximated elevations before the eruptions. By using these data, estimations of lava volumes could be derived. The Tunguhraun lava has a volume of about 0.15 km3 and the Dvergagígahraun lava has a volume of about 0.36 x 10-2 km3. This amounts to an average thickness of approximately 0.7 m (Tunguhraun lava) and 3.1 m (Dvergagígahraun lava).

In comparison with other lava fields in Iceland the Tunguhraun lava and especially the Dvergagígahraun lava are small lava fields. While the Tunguhraun lava only has one crater, the Dvergagígahraun has five and seems to have erupted out of one fissure. The eruptive fissure has an east-north-east direction and seems to be part of a radiating pattern of fissures connected to the stress field of the center volcano. The large discrepancies of lava thickness throughout the Tunguhraun lava indicate two kinds of flow mechanisms, Strombolian activity, that allowed the crater to be formed, and overflow, hence allowing the lava to travel farther from the vent.

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