Field trip to Mývatn, Tjörnes and Jökulsárgljúfur


Field guide prepared by Kristján Jónasson (kristjan@nattfs.is)
Trip leader: Freysteinn Sigmundsson


On this field trip various geologically interesting places will be visited around lake Mývatn and in the area to the north and east of Mývatn. The trip starts with two stops at lake Mývatn, with a discussion of the geological history of Mývatn and Krafla and a look at some volcanic features. Then we will drive to Tjörnes in the north where there will be one stop with some hiking across a lava field. There we will have a look at a transform fault - rift zone junction. Finally we will visit the Jökulsá canyon (Jökulsárgljúfur) to the east where there will be two stops, showing the canyon, a crater row and its feeder dyke and the largest waterfall in Europe.

Road map, showing the route taken on this field trip (yellow line) and the locations of the stops (circled numbers)

1. stop: Dimmuborgir

Geology and geomorphology of the Mývatn area

Most of the volcanic features of the Mývatn area belong to the Krafla volcanic centre and its associated volcanic system. This volcanic system extends from Bláfjall in the south to Öxarfjörður on the north coast, where it ends at the Tjörnes Fracture Zone.

The Pleistocene

Due to extensive cover by young volcanic products little is known about the early history of the Mývatn area. The Krafla central volcano has been active for 200-300,000 years. It contains a caldera that has been filled with younger products and is, therefore, not a prominent feature in the field. The mapping of the caldera rests mainly on the distribution of a composite welded tuff layer of silicic and basaltic composition. Discontinuous outcrops of this layer (up to 15 m thick) describe the outline of the caldera. The production of this material may have preceded the caldera collapse.

During the last glaciation subglacial volcanic activity created some of the most prominent landmarks of the area. These include the hyaloclastite ridge Námafjall-Dalfjall and Leirhnúkur which together create a 15 km long ridge, 1 km broad at the base. This ridge, probably formed in a single eruptive event, is composed of pillow lavas, pillow breccia and hyaloclastite tuffs. The volume is about 0.4 km3, calculated as dense rock. This hyaloclastite ridge marks the central part of the fissure system, and it dissects the older Krafla caldera.

Subglacial eruptions producing silicic rocks created some outstanding landmarks in the area. Hlíðarfjall, Jörundur, Gæsafjallarani and Hrafntinnuhryggur are elongate rhyolite domes and ridges. The rhyolites exhibit a variety of textures from expanded pumice to dense obsidian and lithic rhyolite. A few, large volume, subglacial basaltic eruptions have produced prominent table mountains in the area, such as Gæsafjöll, Krafla (which the Krafla central volcano is named after) and Bláfjall. They are built up of pillow lavas, pillow breccias and hyaloclastite tuffs, capped by subaerial lava flows.

The postglacial

A time scale for the postglacial history of the Mývatn area is provided by Carbon 14 dated tephra layers. The most important are the easily recognisable tephra layers from Hekla volcano in South Iceland. The postglacial history of the Mývatn area was first pieced together by Thorarinsson (1951), who initiated the use of tephrochronology in Iceland. Later Sæmundsson (1991) has added more detail to the history.

The volcanic activity of the Krafla central volcano and volcanic system appears to have been episodic in postglacial times. Two main episodes are recognised. The Lúdent episode consists of lavas and tuffs which are older than H5 (6500 BP) and probably on the order of 8000-10,000 years old. The Hverfjall episode consists of lavas and tuffs which are younger than H3 (2800-2900 BP) and probably younger than 2700 years old.

The Lúdent episode

Each of the two postglacial volcanic episodes is named after the most prominent landmark created by that episode. In both cases these are tuff rings, formed on long basaltic fissures, where a part of the fissure was submerged below a shallow lake. The tuff ring Lúdent was formed by phreatomagmatic explosive activity. The walls are made of layered tuffs and abundant lithic fragments of basaltic composition. On the rim of the Lúdent tuff ring are younger scoria craters. Several lava flows of basaltic composition were formed during the Lúdent episode, but silicic and intermediate lavas do also occur. The most conspicuous is the Hraunbunga dacite, just north of Lúdent.

The Ketildyngja lava shield

Towards southeast from the Krafla volcanic system a lava shield, Ketildyngja, formed about 3500 years ago. A lava flow from this shield volcano flooded the Mývatn area and went through Laxárdalur towards the north coast. This lava is referred to as The Older Laxárlava. This lava flow dammed up a shallow depression which was subsequently occupied by the Mývatn lake.

The Hverfjall episode

After a repose period of more than 5000 years, activity started again in the Krafla volcanic system. The activity can be separated into 5 or 6 distinct "Fires", each consisting of several fissure eruptions occurring within a limited time frame.

Hverfjall Fires (2600-2800 BP): The first volcanic eruption to occur during the Hverfjall Fires created the Hverfjall tuff ring. Hverfjall is the southernmost part of an eruptive fissure which extended to Hrossadalur in the north, but Jarðbaðshólar, a prominent row of craters, just south of the diatomite factory, were formed in this same eruption. The fissure is 7.5 km long. A lake covered the southernmost part of the fissure, causing the eruption to be phreatomagmatic and building the impressive Hverfjall tuff ring. Tephras from the Hverfjall crater were carried towards northeast, but the original thickness of the deposit is unknown due to subsequent wind erosion. The H3 Hekla tephra is found below the Hverfjall tuff, separated by a thin layer of soil. This puts the age of the Hverfjall eruption at approximately 2700 years. Lavas were also erupted in the central and northern parts of the Krafla system during the Hverfjall Fires

Hóll Fires (1900-2300 BP) and the Younger Laxárlava (2300 BP): During the Hóll Fires several eruptions occurred in the eastern part of the Krafla central volcano. At about the same time a large eruption occurred on a 10 km long fissure in the southern part of the area. This eruption produced the so-called Younger Laxárlava, which is the most voluminous lava known from the Krafla system. The eruption left two conspicuous crater rows, Þrengslaborgir and Lúdentsborgir. The lava flowed to the west covering most of Mývatn, and further through Laxárdalur and spread out in Aðaldalur. The area covered by the lava is 170 km2 and its volume is 2 km3.

The distribution of the Younger Laxárlava around Mývatn. The crater rows, the Dimmuborgir lava blister and the pseudocraters are shown. the time of the eruption.
 

The Younger Laxárlava has significantly contributed to the present scenery around lake Mývatn. The most outstanding features are the crater groups, which formed in the lava and are now found on the lake shores or as islands in the lake. These craters are referred to as pseudocraters, or secondary craters, because they are not true volcanic craters but form in the lava by steam explosions where the lava flows over wet ground or shallow water.

Dimmuborgir is another peculiar and picturesque formation in this lava to the southwest of Hverfjall. Here an elevated shield like blister, 1-2 km across, has formed in the lava. The central part of the blister foundered, as lava escaped through a breach to the west, leaving a remarkable depression surrounded by vertical walls of lava. Within the depression a number of lava pillars stand as rugged peaks 10-20 m high, their tops being in level with the surrounding lava surface. The sides of the cylindrical pillars are marked by slickensides formed when the consolidated surface of the lava subsided and scraped the sides of the pillars as the liquid below was drained away.

Dal Fires (1100 BP): During the Dal Fires an eruption occurred to the west of Námafjall and created the crater row Svörtuborgir. Lava from this 500 m long fissure, and another fissure further to the south, flowed around Hverfjall and on top of the Younger Laxárlava. Some minor craters further to the north are believed to have formed in this event. The road to the Krafla power plant lies on top of another lava from the Dal Fires.

The Mývatn Fires (AD 1724-1729) and the Krafla Fires (AD 1975-1984) are the subject of another field trip.

2. stop: Skútustaðir

One of the most prominent landscape features in the Mývatn area are the so-called pseudocraters, or rootless craters, formed when lava flows over wet ground or a shallow lake. Steam explosions cause spattering of the lava which results in the formation of scoria cones. A good view of some of the most regular crater forms is at Skútustaðir.

3. stop: Tjörnes

Tjörnes fracture zone and transform-rift intersection

On the north coast of Iceland, the rift zone in North Iceland is shifted about 120 km to the west where it meets with, and joins, the mid-ocean Kolbeinsey ridge. This shift occurs along the Tjörnes fracture zone, an 80 km wide zone of high seismicity, which is an oblique (non-perpendicular) transform fault. There are two main seismic lineaments within the Tjörnes fracture zone, one of which continues on land as a 25 km long WNW trending strike-slip fault. This fault, referred to as the Húsavík fault, is marked by land surface depressions occupied by lakes, that is, sag ponds. The fault scarp exceeds 200 m at several locations, suggesting a significant dip-slip component. In fact, there are some indications that its vertical displacement may be as much as 1400 m. The right-lateral displacement is at least 5-10 km, but may be as much as 60 km.

Tectonic map of the Tjörnes peninsula.

The Húsavík fault meets with, and joins, north trending normal faults of the Þeistareykir fissure swarm in the axial rift zone. The most clear-cut of these junctions occurs in a basaltic pahoehoe lava flow, of Holocene age, where the Húsavík fault joins a large normal fault called Guðfinnugjá. At this junction, the Húsavík fault strikes N55°W, whereas Guðfinnugjá strikes N5°E, so that they meet at an angle of 60°. The direction of the spreading vector in North Iceland is about N73°W, which is neither parallel with the strike of the Húsavík fault nor perpendicular to the strike of the Guðfinnugjá fault. During rifting episodes there is thus a slight opening on the Húsavík fault as well as aconsiderable dextral strike-slip movement along the Guðfinnugjá fault. Consequently, in the Holocene lava flow, there are tension fractures, collapse structures and pressure ridges along the Húsavík fault, and pressure ridges and dextral pull-apart structures subparallel with the Guðfinnugjá fault. The 60° angle between the Húsavík strike-slip fault and the Guðfinnugjá normal fault is the same as the angle between the Tjörnes fracture zone transform fault and the adjacent axial rift zones of North Iceland and the Kolbeinsey ridge. The junction between the faults of Húsavík and Guðfinnugjá may thus be viewed as a smaller-scale analogy to the junction between this transform fault and the nearby ridge segments.

4. stop: Jökulsárgljúfur

Jökulsá canyon (Jökulsárgljúfur)

The Jökulsá canyon was formed by one or more catastrophic floods in the river Jökulsá á Fjöllum. The total erosion of rock is estimated to have been 0.6 - 0.7 km3. The oldest ash layers within the canyon are about 2000 years old. The 2900 year old Hekla layer H3 is missing within the canyon, but is found outside it. This gives an age for the last catastrophic flooding of approximately 2500 years. The peak flow has been estimated as 400,000-500,000 m3/s and the total volume of water at least 10 km3, possibly several times more. By comparison the flood in Skeiðará in 1996 peaked at 45,000 m3/s and the total volume of water was 4 km3. The origin of the floodwater in Jökulsá has been somewhere in the northern part of Vatnajökull

Sveinar-Randarhólar crater row and feeder-dyke

By far the best example of a feeder dyke in Iceland is the feeder to the 6000-8000 year old Sveinar-Randarhólar crater row, which is the longest crater row in Iceland (70 km). It is well exposed in the eastern wall of a 100 m deep Jökulsárgljúfur canyon. The dyke is very variable in thickness. In the eastern wall of the canyon next to the river a segment of the dyke, offset 85 m to the east, is only 2 m thick. The segment that connects with the crater of the Randarhólar crater row, however is 4.5 m thick next to the river, but 6.7 m thick at the foot of the vertical cliffs below the crater of Randarhólar. On approaching the crater (and the surface) the dyke becomes thicker and is probably around 10 m where it connects with the crater and associated lava flow. In the western wall of the canyon there is an offset 2 m thick segment of this same dyke. The dyke strike is N4°W, the dip is from 82°W to about 90° and the dyke rock is very fine grained, with well-developed columnar jointing but no visible vesicles.

5. stop: Dettifoss

Dettifoss waterfall

The Dettifoss waterfall is the largest (i.e. most powerful) waterfall in Europe. In the 1970's there were plans to build a 161 MW hydroelectric power plant at Dettifoss. This has now been abandoned and the waterfalls and the canyon are now protected. There are, however, still plans for damming the river in the highlands and diverting large parts of it towards the east as part of a huge hydroelectric project.


Selected references:

Calderone, G.M., K. Grönvold & N. Oskarsson, 1990. The welded air-fall tuff layer at Krafla, northern Iceland: a composite eruption triggered by injection of basaltic magma. J. Volcanol. Geotherm. Res., 44, 303-314.

Elíasson, S., 1977. Molar um Jökulsárhlaup og Ásbyrgi. Náttúrufræðingurinn, 47.

Fjäder, K., A. Gudmundsson and T. Forslund, 1994. Dikes, minor faults and mineral veins associated with a transform fault in North Iceland. J. Struct. Geol., 16, 109-119.

Grönvold, K., 1976. Variation and origin of magma types in the Namafjall area, North Iceland. Bull. Soc. géol. Fr., XVIII, 869-870.

Gudmundsson, A., S. Brynjolfsson and M. Th. Jonsson, 1993. Structural analysis of a transform fault-rift zone junction in North Iceland. Tectonophysics, 220, 205-221.

Gudmundsson, A., 1995. Ocean-ridge discontinuities in Iceland. J. Geol. Soc. London., 152, 1011-1015.

Jónasson, K., 1994. Rhyolite volcanism in the Krafla central volcano, north-east Iceland. Bull. Volcanol., 56, 516-528.

Nicholson, H., M. Condomines, J.G. Fitton, A.E. Fallick, K. Grönvold & G. Rogers, 1991. Geochemical and isotopic evidence for crustal assimilation beneath Krafla, Iceland. J. Petrol., 32, 1005-1020.

Nicholson, H. & D. Latin, 1992. Olivine tholeiites from Krafla, Iceland: Evidence for variations in melt fraction within a plume. J. Petrol., 33.

Noll, H., 1967. Maare und maar-aehnliche Explosionskrater in Island. Sonderveröffentlichung des Geol. Inst. d. Univ. Koeln, Heft 11, 117s.

Schiellerup, H., 1995. Generation and equilibration of olivine tholeiites in the northern rift zone of Iceland. A petrogenetic study of the Bláfjall table mountain. J. Volcanol. Geotherm. Res., 65, 161-179.

Sigurðsson, O., S. Zóphóníasson, L. Hannesdóttir and S. Þorgrímsson, 1975. Dettifossvirkjun, Jarðfræðiskýrsla. Orkustofnun ROD-7526.

Sveinbjörnsdóttir, A.E., M.L. Coleman & B.W.D. Yardley, 1986. Origin and history of hydrothermal fluids of the Reykjanes and Krafla geothermal fields, Iceland. Contrib. Mineral. Petrol., 94, pp. 99-109.

Sveinbjörnsdóttir, Á.E., 1992. Composition of geothermal minerals from saline and dilute fluids - Krafla and Reykjanes, Iceland. Lithos, 27, 301-315.

Sæmundsson, K., 1973. Straumrákaðar klappir í kringum Ásbyrgi. Náttúrufræðingurinn, 43, 52-60.

Sæmundsson, K., 1974. Evolution of the axial rifting zone in Northern Iceland and the Tjörnes fracture zone. Geol. Soc. Am. Bull., 85, 495-504.

Sæmundsson, K., 1991. Jarðfræði Kröflukerfisins. In: Garðarsson, A. et al. (eds.) Náttúra Mývatns, p. 24-95. Hið íslenska náttúrufræðifélag, Reykjavík.

Thorarinsson, S., 1951. Laxárgljúfur and Laxárhraun. Geogr. Ann. Stockholm, 33, 1-89.

Thorarinsson, S., 1960. The postglacial history of the Mývatn area and the area between Mývatn and Jökulsá á Fjöllum. Internat. Geolog. Congress 21. Session, Norden 1960. On the Geology and Geophysics of Iceland.

Tómasson, H., 1973. Hamfarahlaup í Jökulsá á Fjöllum. Náttúrufræðingurinn, 43, 12-34.