Ódáðahraun - Askja

From Mývatn the road goes east for about 30 km and then turns south on a mountain track towards the Dyngjufjöll volcanic complex with the Askja caldera. The distance to Askja is about 100 km. A tourist hut at Herðubreiðarlindir is at a distance of 60 km from this crossroads. A stop (10 minutes) will be made to inspect a postglacial pahoehoe lava with well-developed surface features such as lava ropes. The surface has been slightly eroded by flooding in the nearby river Jökulsá á Fjöllum. The next stop (10 minutes) is made on a sandur plane spotted with erratic boulders. This type of deposit will be clearly evident for a large part of the drive to Herðubreiðarlindir. This is a lahar deposit, formed in large glacier bursts caused by volcanic eruptions beneath the Vatnajökull glacier to the south. The lahar or mud flows followed and overflowed the Jökulsá river bed.

The tourist hut is located on a lava flow a short distance from the table-mountain Herðubreið. This lava comes from craters southwest of Herðubreið. The lavas from these craters form a low shield structure, Flatadyngja. They cover a large area and have flowed around Herðubreið towards the Jökulsá river. The age of the lava is about 4000 y. B.P.

The drive towards south continues for another 40 km, first through the remaining part of the Flatadyngja lavas, and then on to lavas coming mainly from the Dyngjufjöll volcanic center. The mountain track swings at one point quite close to the Jökulsá river where the river has cut a section through at least three lava flows. Here is an opportunity to observe features of the lava such as scoriaceous tops and bottoms, variously columned interiors and the arrangement of vesicles, which often are drawn out into pipes.

As the drive continues the track swings to the right towards the mountain ridge Herðubreiðartögl. A discontinuous patchy layer of light colored pumice indicates that here is the northern edge of the pumice sector that formed during the 1875 eruption in Askja. Further south at the southern termination of Herðubreiðartögl, the pumice layer becomes continuous and thickens rapidly, while a view opens to the Dyngjufjöll complex as the road crosses a lava flow from the shield volcano Svartadyngja, partly buried by the tephra from the 1875 Askja eruption.

On leaving the Svartadyngja lava, the track runs across a wide lavafield covered with the 1875 pumice. The lava was erupted from craters within the Askja caldera and its age is around 6000 years BP. The next stop is at Drekagil, a canyon cut into the Dyngjufjöll complex. A walk into the canyon allows the observation of various types of pillow lavas and hyaloclastites.

Time permitting, another nearby canyon can be visited where the hyaloclastite is cut by a dense, columnar dike which can be traced as it turns into pillows at higher level.

From Drekagil the track climbs from about 700 m a.s.l. to the Askja caldera at 1100 m a.s.l. The track follows the lava fan spreading out from a pass, Öskjuop, in the caldera rim. This lava fan represents the overflow from the caldera after it was filled with lava to the elevation of the pass. The youngest and most prominent lava in the landscape is the one formed during the 1961 eruption. Its pitch-black color stands out against the light-colored 1875 pumice which covers the older lavas. The track ends at Öskjuop. From there is a 20 minutes walk to the nested caldera Öskjuvatn which was formed during and after the 1875 eruption. Just inside the Öskjuop are the craters of the 1961 eruption. Visibility permitting, the Askja caldera can be observed from the top of one of the 1961 craters.

The Herðubreið / Herðubreiðartögl

The Dyngjufjöll fissure system extends from the north margin of Vatnajökull some 100 km to the north. The road from Mývatn to Askja first crosses this system and then goes parallel to the system on its eastern side. On this route there are volcanic formations which partly belong to the Dyngjufjöll fissure system but the most conspicuous ones are the result of eruptions from basaltic shields, which have no apparent relation to the tectonic pattern of the fissure systems on either side. Herðubreið is the most impressive volcanic formation en route. Rising 1200 meters above the surroundings this volcano, with its extremely regular table mountain features, dominates the scenery over large areas in this part of the country. A volcanic ridge, Herðubreiðartögl, belonging to this volcanic center extends to the south from Herðubreið.

"Herðubreið and Herðubreiðartögl consist of four main structural, textural and mineralogical units:

Unit I, the southern base area of Herðubreiðartögl (600-820 m.a.s.l.), is dominated by olivine-rich subaerial lava flows forming a relic shield volcano.

Unit II, the central part (500 to max. 1000 m.a.s.l.) of both volcanoes is built of pillow lavas, laterally extensive (up to more than 1 km) subaquaeous sheet lavas and several types of hyaloclastites. Above 800-900 m.a.s.l. these rocks are overlain by subaerial sheet lavas, local agglutinates and hyaloclastites.

Unit III in the cliff and plateau area of Herðubreið (above ca. 800-930 m.a.s.l.) consists of a succession of pilow lavas, hyaloclastites, sheet lavas and agglutinates. These differ in structure and texture of hyaloclastites from the central areas and contain more abundant plagioclase phenocrysts (more than 20 vol% compared with <15% vol% plagioclase in the central areas).

Unit IV comprises subaerial lava flows, agglutinates and fallout deposits above ca. 1000 m.a.s.l. at Herðubreiðartögl".

1. In the southern area of Herðubreiðartögl, eruption of primitive olivine tholeiite built a subaerial shield volcano probably during the last interglacial. At the onset of the last glaciation, volcanic activity stopped, perhaps due to increasing lithostatic pressure on the magma reservoir by ice accumulation.

2. Reviving the eruption center of period 1 as part of an eruption fissure, volcanic activity resumed after maximum glaciation and considerable thinning of the ice sheet. Olivine tholeiites of the central areas were erupted during the waning period in a lacustrine environment. Lavas and later voluminous hyaloclastite deposits were produced subaqueously and redeposited by mass flows. Subsequent hydroclastic and effusive eruptions took place close to and above water level. Finally the volcanoes were covered by subaerial lava flows.

3. At Herðubreið, subsequent pillow lava piles overlain by steep-sided hyaloclastite complexes were built in a subglacial environment caused by thickening of the ice sheet approximately 12-15 kA ago as the climate deteriorated. Subaerial lava flows and agglutinates covered these deposits after the volcano rose above the ice sheet. The most highly evolved tholeiites of both volcanoes were erupted during this period.

4. During a postglacial period olivine tholeiitic lava flows and fallout deposits were produced by subaerial eruptions at Herðubreiðartögl."
 

The Dyngjufjöll volcanic complex with the Askja caldera
 

In the following the geological history of the Dyngjufjöll complex is discussed, starting with the Pleistocene formation and ending with volcanic activity in the 19^th and 20^th centuries.

The Pleistocene formations in Dyngjufjöll are the oldest exposed rocks in the area. The age distribution of different formations between respective glaciations during the Pleistocene is, however, not known at present. Clear tillite horizons have not been identified and absolute dating is problematic due to the basaltic composition of the rocks. Recurring stratigraphic sequences, which can be associated with specific environmental factors, degree of alteration of volcanic glass and significant change in magma types erupted, allow a division of the Pleistocene formation into four units.

The oldest of these units is found on the flanks of the volcanic structure as isolated hillocks of hyaloclastite in an advanced state of alteration and partially buried by Postglacial lavas. These hyaloclastites were formed in shallow water with textures ranging from layered Surtseyjan tuffs to mass flow remnants, sometimes mixed with inclusions of laminated water lain sediments.

The southern part of the complex (Thorvaldsfjall), rising 800 m above the base, can be divided into three units, a basal and top units composed of near aphyric olivine tholeiite and a central plagiophyric unit. All units contain the same lithological facies consisting of pillow lavas, air fall Surtseyjan tuffs and mass flows. All three units were formed in a similar shallow water environment on erupting fissures parallel to the strike of the fissure system.

Distribution of the easily recognizable plagiobasalt indicates an erupting fissure close to a water level where some craters on the fissure were totally submerged and producing pillow lavas which flowed downhill into deeper water, while other craters on the same fissure produced fragmented material which is found both below and on the top of the pillow lava flows.

The similarity between the lithologies of the different units and the specific environment required to produce these lithologies constrains the period(s) of formation within a limited timeframe during deglaciation(s) when the volcanic complex had become ice-free but was surrounded by and partly submerged in marginal lakes dammed by the thinning glacier. It is conceivable that glacier unloading triggered a large but short lived production pulse leading to the formation of the units, each unit representing a separate deglaciation process.

The uppermost unit of the sequence was probably formed during the waning stages of the last glaciation 10-12 kA ago. Embedded in the uppermost air fall deposits of this unit is a rhyolitic ash layer which is only found on high ground within the Dyngjufjöll complex but reappears in coastal areas in northern and eastern Iceland directly on top of glacial deposits. The age of this layer is approximately 9800 years B.P. At this time the Dyngjufjöll complex was icefree but since the layer has not been found in the area between Dyngjufjöll and the coast it can be assumed that it fell on the top of a thinning glacier.

Postglacial volcanic activity in the Dyngjufjöll complex is associated with two fissure swarms tangential to the east and west boarders of the present Askja caldera. In addition large amount of lava has erupted in close vicinity to Dyngjufjöll fissure system producing many lava shields of different sizes. To the north of the main massif are Kollóttadyngja, Flatadyngja, Litladyngja and Svartadyngja, (dyngja in this context is synonymous with shield) and Trölladyngja to the south. Together, the lavas from Dyngjufjöll and from these various shields form the lava desert known as Ódádahraun.

Large amounts of lava were produced from the western branch of the fissure system across the Dyngjufjöll complex before the deposition of Hekla layer H4. These lavas flowed mainly towards NW and SW but also towards E into a small caldera, of early Postglacial age, situated north of the present Askja caldera. This caldera became filled with lavas from the western fissures and overflow occurred towards NE and NW.

Two shield volcanoes, Kollóttadyngja and Svartadyngja, were formed during this pre-H4 period. Kollóttadyngja is presently surrounded by younger lavas and the distribution of its lavas is therefore unknown. Svartadyngja is also surrounded by younger lavas except towards SE where lavas from this volcano can be traced south of Herðubreiðartögl and further east until they disappear beneath fluvial sediments from Jökulsá á Fjöllum.

The Askja caldera collapse dates back to this period. Postglacial age of the Askja caldera is indicated by the subaerial lavas filling the older caldera subsidence to the north which were subsequently cut by the younger Askja collapse. Due to topographic changes caused by the Askja subsidence, the older caldera was detached from vent areas which might otherwise have poured lavas onto its surface. Therefore, the Askja subsidence postdates the surface lavas in the older caldera. These surface lavas appear to be older than H4 so that the Askja subsidence occurred sometime between complete deglaciation (ca 6000-7000 years ago) and the deposition of H4, 4500 years ago. The formation of Askja must have occurred in the earlier part of this period since the time before H4 was sufficiently long to fill the Askja caldera to the Öskjuop threshold and subsequently to deposit lavas in a fan reaching past Herðubreiðartögl and to the river Jökulsá á Fjöllum.

Two lavas belonging to the Dyngjufjöll fissure system erupted in the area north of Dyngjufjöll during the period between the deposition of Hekla 4 and the "unknown" layer. In this period the lava shield Flatadyngja, SE of Kollóttadyngja, was formed. Lavas from this shield flowed to the north of Herðubreið and through the pass between Herðubreið and Herðubreiðartögl.

In the period between the "unknown" layer and Hekla 3 activity in the Dyngjufjöll volcanic system was concentrated within the Askja caldera. Lavas covered the caldera bottom and escaped through Öskjuop. These lavas did, however, not reach as far as the lavas older than Hekla 4. Shortly before the deposition of Hekla 3 the youngest lava shield in the area, Litladyngja, formed slightly to the west of the eastern branch of the Dyngjufjöll volcanic system, between Kollóttadyngja and Dyngjufjöll.

In the period between Hekla 3 and Hekla 1158 lavas may have issued inside the caldera and covered by later lavas. No lava flow has been found outside Öskjuop with Hekla 1158 as the oldest layer.

In the period between Hekla 1158 and Öræfajökull 1362 activity was limited to craters inside the Askja caldera. Lavas on the caldera floor from this period appear to have originated from both the eastern and western branch of the volcanic system.

In the period between Ö 1362 and layer "a" 1477 all the activity was within the caldera. It appears that a large part of the caldera floor was covered with lava in a single eruption from the western branch of the volcanic system. This eruption occurred shortly before 1477 AD, because the "a" layer is deposited immediately on top of the lava. This lava extends out through Öskjuop. Although the eruptions since the Hekla 1158 layer was deposited fall within historical time in Iceland, written records are not specific about volcanic activity in Dyngjufjöll.

A rough esimate of lava volumes produced in and around the Dyngjufjöll complex within the time periods defined by the tephra layers allows an order of magnitude determination of volcanic productivity in the area during Holocene (Sigvaldason et al. 1992). It appears that volcanic productivity in the period from the end of glacial rebound (ca. 8000 years B.P) until the deposition of Hekla tephra H4 at 4500 years B.P. was higher by a factor of 30 than the productivity since the deposition of Hekla tephra H3 2900 years B.P. This change in productivity has been explained as a melting anomaly in the mantle caused by pressure changes by loading and deloading of glacier ice, which in this central part of Iceland reaches a thickness of up to 2000 meters during peak glaciation.

The first eruption in historical time which is well documented is the explosive eruption of 1875, which resulted in the formation of the nested caldera Öskjuvatn. At the time, however, Askja was practically unknown. Located within a large desert it had never been described. All early observations were made from a distance of about 100 km until shortly before the plinian eruption when a group of young farmers from the Mývatn district visited Askja on February 15, 1875 and gave a short description of what was happening.

Precursors to the eruption of 1875 were reported as early as February 1874. Unusually dense steamclouds were observed in Dyngjufjöll from afar. During the two last weeks of December 1874 strong and frequent earthquakes were felt in North Iceland. On 1 January 1875 a column of smoke rose in a southerly direction from the Mývatn district and on the following day frequent earthquakes were felt in that area, but cloudy weather resulted in poor visibility soon afterwards. Minor ash fall was observed in Northern Iceland. Earthquake activity came to an end a little later.

On 18 February 1875 a basaltic fissure eruption started in Sveinagjá, 40 to 70 km north of Askja, on the continuation of the Dyngjufjöll volcanic system. This eruption lasted, with short quiet intervals, for several months and produced 0.2-0.3 km3 of lava. It has been suggested that these lavas came from a basaltic holding chamber beneath the Askja caldera, travelling up to 70 km by lateral diking. This, however, is speculative and difficlt to constrain.

On 15 February 1875 a subsidence was observed in the SE corner of the caldera. An area of 3-4 hectares had subsided as much as 10 m. To the west of the subsidence there was an active crater, but the description indicates no production of lava or tephra, only violent boiling in mud pots at the time of observation. Pumice and ash had been deposited on the mountains to the northeast of the active area.

A plinian outbreak started in the early morning of 29 March 1875. Tephra fell at 3:30 a.m. in Eastern Iceland, 70 km to the east of Askja, carried by strong winds. The tephra was grayish in color, fine grained and so wet, that it could be formed like clay. The first phase lasted about one hour. At 5:30 in the morning the air had cleared somewhat, but shortly afterwards the main eruption started. Light-brown pumice, with ever increasing grain size, continued to be produced until noon. Although tephra-fall stopped in populated areas at noon, the eruption continued until the morning of the following day. The tephra contained a number of white and black fragments of volcanic glass, as well as basaltic sand, which was much heavier than the pumice and remained in place when everything else was removed by wind and water.

When visited in 1876 a triangular area of subsidence had formed in the southeast corner of the Askja caldera. It measured 4580 m in east-west direction and about 2500 m north-south. The deepest part lay 234 m below the bottom of the caldera. The bottom of the subsidence was occupied by a lake 1255 m in diameter. Around the lake were numerous concentric step-like faults which had cut the northern rim of the subsidence. At that time the lake level was 150 m below the caldera floor. The subsidence was practically filled with water to the present level in 1907.

The volume of the lake is 1.231 km3 and the area of the lake is 10.7 km2. The lake surface lies 50 m below the Askja caldera floor and the minimum volume above the lake surface is 0.54 km3. However, the subsidence cuts the mountainsides to the east and south of the lake, which are up to 400 m higher than the Askja caldera bottom. It is therefore difficult to give an exact estimate of the total subsidence. The absolute minimum value is 1.77 km3, but 2-2.5 km3 is probably a more realistic figure. The volume of freshly fallen tephra is 1.8 km3 or 0.43 km3 calculated as solid rock. The volume of the Sveinagjá basaltic lava is 0.2 to 0.3 km3. A dyke going from Askja to Sveinagjá 1 km high and 4 m wide has a volume of 0.16 km3. Adding these figures together gives a total volume of known and implied material to leave a magma chamber beneath Askja of 0.89 km3. This is less than half the volume of the subsidence.

The first phase of the 1875 Askja eruption was the production of basaltic tephra on January 1. This tephra is found inside the caldera but also on the outer flank of the caldera rim where it is sandwiched between layers of snow which fell during the winter 1874-1875. Within the Askja caldera the basaltic tephra is covered by thick layers of phreatic mud deposits which in turn are overlain by the products of the main silicic eruption of March 29.

The position of the main vents and the succession of events can be pieced together by studying the distribution of the eruption products. The siicic products can be divided into groups according to their physical appearance:

1. Tephra ranging in colour and grain size from nonvesicular greyish-white, fine-grained tephra to vesicular pumice of light-brown to black color and with increasing grain size upward in the pumice deposit.
2. Welded agglutinate, reddish to black in colour.
3. Highly vesicular black lava.
4. Less vesicular dikes of the same chemical composition as the lava and the tephra.
5. Chunks of obsidian of various color shades from black to greenish, often partly vesiculated.
6. Fragments of silicic rocks ranging from granophyric nodules to flint-like angular glasses.

Distribution of airfall tephra around the Askja lake has thickness maxima indicating proximity to producing craters which where arranged along a fissure extending across the present lake from ENE to SWS

Craters on this fissure erupted the bulk of the tephra produced in the plinian eruption. The first product was the nonvesicular grayish-white, fine-grained tephra. The prevailing strong westerly wind blew the material to the east. A layer of fine-grained irregularly bedded tephra was deposited on the east side of the vent or vents, but it thins rapidly further east. On the windward side of the subsidence in the southwestern corner of the Askja caldera, the tephra layer is maximally 3 m thick, but thins out to zero within a few hundreds meters from the 1875 subsidence. A sharp discontinuity in the tephra layer between the fine-grained tephra and the overlying brown to black tephra represents the lull in activity from about 4:30 to 5:30 in the morning of 29 March 1875. This highly vesicular pumice was ejected with great vigor, and a larger proportion of the material was carried outside the caldera than during the earlier phase. The distribution of the material within the caldera is very distinct. A line drawn through the thickness maxima for the two phases indicates a parallel shift of the eruption fissure towards southeast.

A pronounced difference is between the material deposited to the east of the subsidence and that deposited on the windward side to the west. In the southwestern corner of the caldera the material, deposited directly on the grayish white tephra from the first phase, is a welded agglutinate without any pumiceous material, neither at the top nor the bottom of the layer. The welded agglutinate, erupted from the southernmost vent on the fissure, was deposited on the sloping wall of the subsidence and flowed back into the depression.

Towards the end of the eruption, vents located close to the northeastern end of the fissure produced material hot enough to weld into an agglutinate. This is covered by a layer of large pumice clasts, representing the waning phase of the eruption.

In summary the sequence of events was as follows:

1. Injection of basaltic magma to shallow depth beneath the Askja caldera as a result of a rifting event affecting a large part of the 100 km long Dyngjufjöll fissure system. The rifting caused intense seismic activity and opening of fissures in the northern part of the system. Beneath Askja the basaltic intrusions were quenched by a large ground water body confined within the structural boundaries of the caldera with a resulting increase in geothermal activity as observed already in the beginning of 1874. Boiling of the ground water body continued for 13 months gradually exhausting the water reserve.
2. By January 1^st 1875 continued basaltic injection finally broke through the ground water barrier resulting in a short lived eruption of Surtseyjan tephra.
3. On February 15^th a significant subsidence was observed et the location of the present Askja lake. This subsidence was caused by the depletion of the water reservoir, a phenomena commonly observed in heavily exploited geothermal areas.
4. On February 18 basaltic eruptions started on the fissure system 40 km to the North of Askja.
5. The basltic intrusions beneath Askja engulfed a small body of silicic magma without intruding it. A slight heating of the silicic magma (not detectable in mineral equilibria) upset the volatile balance and initiated partial degassing and shift in oxygen fugacities of the magma which is displayed in the color stratification of the tephra..
6. With the water barrier removed the silicic magma was mobilised first by injection of dikes along the eastern caldera wall, which served as feeders to a dacitic lava flow.
7. This led to pressure release and explosive boiling of the silicic magma with opening of a ENE-WSW striking fissure(s) and production of fine-grained nonvesicular tephra for about one hour in a plinian eruption. After this initial phase the intensity of the eruption was somewhat reduced for about one hour before the beginning of a main plinian phase, which lasted for about 6 hours.
8. The phreatic explosion crater Víti was formed after the plinian eruption came to an end and phreatic activity continued for several months or years after the eruption.

In the period between 1920 and 1930 four basaltic eruption occurrred in and around the Askja lake subsidence producing small lava fields and building an island in the Askja lake. A fifth eruption occured on the southern flank of the Dyngjufjöll complex during this period.

The latest volcanic event in Dyngjufjöll occurred in 1961

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