• en

The Krafla eruption 1975 began in the morning of December 20 near Leirhnjúkur in the Krafla caldera, near Mývatn. This place is a well known volcanic area, a part of the active volcanic zone of Iceland. This long lasting Krafla eruption from 1975 to 1984, with some shorter or longer intervals between single eruptions, is very similar to the so called Mývatn-fires which took place in the same area 1724-1746, also with several single eruptions and shorter or longer intervals between. The Mývatn period of eruptions started with a great explosive eruption on the slopes of the mountain Krafla. Then a big explosive crater, Víti, was formed. 

The Mývatn Eruptions 1724-1729 and 1746.

Large clusters of fissures are characteristic of the Mývatn/ Krafla area. Such fissure swarms extend in a north-south direction, consisting of open fissures, dislocation and crater rows. Since the ice age, there have been three main periods of eruptions in the Mývatn area. The first is called the Ludent period, which ended about 6000 B.C. The Hverfjall period of eruptions lasted from 2000 to 1500 B.C.

The beginning of the third period of eruptions came with the so-called ‘Mývatn Fires’ from 1724 to 1729, and a single eruption in 1746. The recent series of eruptions, called Kröflueldar, or ‘Krafla Fires’, which began in 1975, are con­sidered to be a continuation of the Mývatn eruptions, and the sequence of events has been very similar.

The following is a short history of the Mývatn Fires as their comparison with the Krafla Fires is of great interest. There are contemporary sources of information about the events in the writings of Jón Sæmundsson, the parish priest at Reykjahlíð. The period of eruptions began with a violent explosive eruption on the slopes of Mt. Krafla in the morning of May 17 1724. An explosive crater, later known as Viti, came into being. There were also several smaller craters, including Litla Víti. The canyon now known as Hveragil is believed to have been formed at the same time. It is a large canyon which still contains bubbling mud pots (fumaroles). There were several earthquakes in the Mývatn area before and after the explosive eruption which created Viti. They then decreased somewhat in intensity, but at the beginning of 1725, they began to increase in magnitude, and on January 11, the hill Leirhnjúkur split open in an eruption. On April 19 there was another at Bjarnarflag, where the diatomic plant stands to day. On August 21 1727 a large flow of lava began to issue from Leirhnjúkur. It increased greatly in magnitude on April 18 1728, when it began to well out in several places from the Leirhnjúkur fissure. On that same day, an eruption began north of Bjarnarflag and a new eruption began at Bjarnarflag itself about the same time. After a short pause, a major eruption began along the fissure south of Leirhnjtikur on December 18 1728. From there lava flowed in the direction of the populated Reykjahlíð area. On January 30 1729 the biggest eruption began at Leirhnjtikur. On July 6, 1729 lava flowed like a river in spate down into inhabited areas, engulfing the farms of Gröf and Fagranes, and ending up in lake of Mývatn. On August 7 it overwhelmed Reykjahlíð Farm, and by August 27 it had almost encircled the church, which stood on a small hill.

The Mývatn Fires came to an end in September 1729, after having lasted almost continuously for five years. Another eruption, however, began in Leirhnjtikur on July 10 1746. It was a very powerful eruption, but of short duration.

krafla-eruption-1975-84-2

Above is a view from Leirhnjúkur in the Krafla caldera towards the north, showing a cluster of fissures and craters created by several Krafla eruptions. The crater in the foreground has just been formed. It goes by the name of Hóffell.

 

Sometimes steam vents and mud pots are formed in volcanic craters after an eruption has come to an end. The mud pots in the picture below were formed at Leirhnjúkur after the end of the eruption there on September8 1977. Steam vents and fumaroles are also frequently forerunners of volcanic eruptions, emerging on spots where eruptions may materialize later.

krafla-eruption-1975-84-3

The Krafla Eruptions 1975-1981 and 1984.

In the morning of December 20 1975, there was a severe earthquake near Krafla, and at 11:08 a.m. that day, an eruption began along a 3 km long fissure produced by the Mývatn eruptions, north of Leirhnjúkur. The flow of lava stopped at 11:35, and it was all over within 12 hours. That brief eruption marked the beginning of the Krafla eruptions.

There was no eruption in 1976, but earthquakes and rising and sinking of the surface of the earth indicated that volcanic activity was continuing. Swelling of the earth was created in the area within the Krafla caldera. The cause of such swellings is molten lava (magma) which steadily moves upwards into a magma chamber at about 3 km depth. Sometimes the earth’s crust breaks and lava flows up through the cracks, but at other times the lava flows northward or southward underground along the cluster of rifts (fissure swarms).

The second Krafla eruption occurred on April 27 1977, when there was an eruption 4 km north of Leirhnjúkur. This was a very small eruption, even smaller than the first.

The third eruption began at 18:00 on September 8 1977, 3 km north of Leirhnjfikur. It was over by 01:00 a.m. There were no eruptions in 1978 and 1979, but the land continued to heave and sink in tune with the subterranean magma movements.

The fourth Krafla eruption began on March 16 1980, at 16:20, and was over by 22:30 same day. The fifth eruption began at 12:45 on July 10 1980. This eruption took place in an area called Gjástykki, a cluster of fissures about 4-500 m wide and 3.5-4 km long. By early morning on July 18 it was all over. The resulting lava field, which is known as Snagahraun, is about 5-6 km2 in area.

The sixth Krafla eruption began at 22:04 on October 18 1980 with lava emerging from a 7 km long fissure about 2.3 km north of Leirhnjúkur. This eruption was over by October 23, leaving a lava field of about 12 km2.

The seventh Krafla eruption began at 14:10 on January 30 1981 from a fissure which opened across Ethólaborgir, an old crater row. The lava flowed mainly in a northerly direc­tion. The eruption ended February 4 after lasting for 5 days. The eighth eruption began at 01:52 in the morning of November 18 1981 north of Leirhnjúkur and came to an end on November 23, having lasted for about 5 days. The area of the lava field created by this eruption was about 17.5 km2, partly covering earlier Krafla lava.

The total area of these eight Krafla lava fields is 30.0 km. The map on the right shows the different lava flows, but often older lava flows were partially overrun by later ones.

The ninth Krafla eruption began at 23:49 on September 4 1984, and was overby September 18.  The total area of the nine Krafla lava fields is 36 km2 and the total volume is about 0.25 km3.

krafla-eruption-1975-84-5

As mentioned above, steam vents and mud pots sometimes emerge in volcanic craters when an eruption has come to an end. They may also be forerunners of eruptions, appearing on the spot where an eruption will subsequently start. The photo above was taken at Leirhnjúkur in the Krafla caldera where a large steam vent and fumarole of this type (called Nina) cropped up in an explosion on a hill on August 2 1977. Soon afterwards (on September 8 1977) the third Krafla eruption began in a fissure north of Leirhnjúkur.

The picture at the below is of a recently solidified ropy lava stream from that eruption, 3 km north of Leirhnjúkur.

krafla-eruption-1975-84-7

The picture above shows rifts in the process of opening on the surface of the Krafla caldera. When originally positioned there, the pipe ends joined in the middle. Now there is a considerable gap.

krafla-eruption-1975-84-9

The fifth Krafla eruption began on July 10 1980 in the Snagaborgir craters in Gjástykki. By July 18 it was all over. The lava field from this eruption covered an area of about 5-6 km2. It was called’ Snagahraun. because it encircled a hill by the name of Snagi. Above is a picture of sunrise over Gjástykki at that time, viewed through a cloud of smoke from the erupting Snagaborgir.

krafla-eruption-1975-84-10

Above is an aerial view of the lava river issuing from the Snagaborgir craters during the eruption of July 1980. ‘The crater row is on the far left in the picture. The lava flows in a northerly direction along Gjástykki. Large pieces of solidified lava float on the surface of the lava stream like chunks of ice on a river.

krafla-eruption-1975-84-15

 

 

‘The picture above shows the craters of the seventh Krafla eruption, which began on January 30, 1981. A fissure opened across Elhóla- borgir, an old crater row. The bottom picture left, taken on February 1, shows that a part of the fissure has closed, and a central crater cone is being built up. The eruption came to an end on February 4 after lasting for 5 days.

 

Below is a view of the Krafla eruption of January 1981 from the campsite just above the dislocation bordering the lavaflow. The weather was fine at the time so that sleeping outside the tents was possible as ample radiant heal emanated from the lava fountain.

However, there were instances of slight ash fall and gas emission so it was inadvisable to sleep too close to the eruption zone. Besides, few people were inclined to sleep in the presence of such spectacular phenomena – the constantly shifting lava fountains and the golden stream of molten lava just a few yards away.

krafla-eruption-1975-84-18

Extensive research work was done and measurements taken in the Krafla area during the eruption period. Precision clinometers and seismographs were positioned at severed points in the area, and radio messages were transmitted automatically from the equipment to the research station of the Nordic Volcanic Research Centre at Reykjahlu) near Lake Mývatn.

Gas specimens were taken regularly at the volcanic fissures of Leirhnjúkur, even in a mid-winter snowstorm (above right), in order to monitor chemical changes in the escaping gas. They might indicate magma movements, which in turn might signed an approaching eruption. If earth tremors intensified and the measuring instruments indicated an imminent eruption, emergency management teams were alerted so that they could make appropriate arrangements for the safety of the local residents.

Below we can see geologists and technicians at the Nordic Volcanic Research Centre in Reykjahlið), discussing measurment results and possible implications with regard to magma movements.

krafla-eruption-1975-84-22

After the eight Krafla eruption in November 1981 had come to an end the earth’s surface near the Leirhnjúkur began to swell, as it had done before after the end of each eruption. In early 1982 measurements indicated, that the pressure in the magma cambers had. reached a maximum level, The rise of the earth’s surface had slowed down, which was the usual sign of a decrease in the afflux of magma from below. Then there was a period of very little or no surface swelling, broken by periods of a slow rise. In August 1982 there was an increase in earth tremors with a simultaneous slight land rise. This activity diminished again in September. This was the first sign of that the earth’s crust above the magma chamber was about to split open.

Similar land rise and earthquake periods occurred in October-December 1982, then in June and September-October 1983,again in February and in August 1984. These were all signs that volcanic activity in the Krafla caldera had not come to an end, the course and timing of subsequent events was quite uncertain. It was evident, however, that the pressure in the main magma chamber was sufficient to force open a passage up to the surface so that only a minor development one way or the other might trigger off an eruption,

At 20:25 on September 4 1 984 sinking of the land surface stalled in the Krafla caldera. Shortly afterwards local seismographs registered some earth tremors. The earthquakes persisted, and at 23:49 the first glow from erupting lava was spotted. The first fissure to appear was 2 km long, but an hour later the total length of the numerous separate eruption fissures was more than 8 km, most of them in similar places as earlier fissures in the Krafla caldera. Large amounts of lava flowed from the rifts during the first few hours of the eruption, but soon the lava flow diminished with part of the fissures closing.

By September 7 the lava eruption near Leirhnjúkur had come to an end, leaving a big steam vent which ejected mud and ash.

In the days that followed the lava eruption in the northern pari of the fissure grew in intensity with lava flowing mostly in a northerly direction along the Gjástykki.

krafla-eruption-1975-84-24

The picture above was taken from the air on September 9, 1984.

There is a view into the very active crater at Ethólaborgir, where a sizable cone had built up. The lava flowed northwards, spreading over a wide area, part of which had not. been covered by lava from this Krafla eruptions before (see picture above). It was interesting to note the way the lava flow, like a bulldozer, pushed large amounts of sand before its advancing thrust when engulfing sandy terrain. Conversely, areas covered with vegetation were marked by gas explosions and flashing gas flames before succumbing to the advancing lava. The lava frequently followed old rifts, widening them as it surged forward. After entering and filling up one rift, the lava sometimes reappeared in another rift nearby with the deceptive look of a new eruption.

krafla-eruption-1975-84-25

Above is an aerial view from the south over the Krafla caldera. In the foreground ( left) is a large steam vent, ejecting a mixture of steam, mud and ash. This eruption emerged near Leirhnjúkur after the lava eruption there had come to an end on September 7. ‘The active volcanic crater-cone at Ethólaborgir, and the northward flowing lava from there are in the background.

Below, a part of the Krafla caldera is seen from the north. The lava is flowing from the Ethólaborgir craters on September 9, 1984, over a sandy terrain, filling all hollows on its way.

krafla-eruption-1975-84-26

The ninth eruption of the Krafla fires came finally to an end in the afternoon of September 18, 1984.

At the beginning of this chapter it was mentioned that the course of the 1975-84 Krafla eruption, was very similar to that of the 1724- 1746 ‘Mývatn Fires’, which occurred in the same area. In both cases there were several single eruptions with internals of varying length.

During the Mývatn Fires there were many eruptions in the 6 years of 1724-1729, after which there was a lull of 17 years before the fined short but fierce eruption of 1746.

In the recent Krafla eruptions the corresponding intervening peri­ods were first the 7 years of 1975-1981, then an interval of only 2 years before the September 1984 eruption started. From that time and until the time of writing in 1991 some rising and sinking movements of the surface in the Krafla caldera continued. No one can tell what the future holds, of course.

It is well known that the Mid-Atlantic Ridge cuts through the volcanic zone of Iceland, where the edges of the plates on each side of the ridge meet. These plates, and with them the two parts of Iceland on each side of the ridge, are moving apart by about 1 cm a year. This means that during the nearly 250 years between the Mývatn and Krafla eruptions, Iceland has been widened by nearly 5 in. This movement is not even, however, as it occurs during the so-called rifting periods only. The total opening of rifts across the fissure swarm during the Krafla eruptions was not far from 4-5 in. If the Krafla events have come to an end now, there may be a similar development around the year 2225. If such events commence earlier, the opening up of the fissure swarm, would not need to be more than 2 cm for each year from the last year of the Krafla eruption, provided the plates on each side of the Atlantic Ridge continue to move at the same pace as they do now.