The previous post described what we think we know about the Eldgja eruption. Our knowledge about one of the largest eruptions in Iceland is somewhat limited, surprisingly so given that Iceland was already well populated. One of the few things which seems secure is the date. Eldgja is believed to date to 934 AD, continuing for up to 7 years. But what is the evidence for this proven fact? Is it correct?
Let’s look at how we were able to get such an accurate date on such a poorly observed eruption. There are three strands: Greenland ice cores, written records from Iceland and elsewhere, and volcanic winters.
Ice cores Eldgja left a strong signal in the Greenland ice cores. The sulphate deposition was phenomenal, larger even than Laki. The ice cores are dated just like tree rings, by counting annual layers. The most accurate date comes from the GISP2 core, where the sulphate peak is dated to 934+-2 AD. The Greenland GRIP ice core shows it too, and here it has been dated to 938+-4. Ash found within high sulphate layer in the core has been shown to come from Eldgja: evidence doesn’t come much better than this. The GRIP core shows elevated sulphate over an 8 year period, 933-941 and the authors (Zielinski et al) argue for an extended period of eruptions. But this should be taken with care: a large spike may pollute the annual snow layers just below it, and this would show up as several years of slightly elevated sulphate before the main eruption occurred.
Book of Settlement Attempts have also been made to obtain dates from historical records. The problem is that these records don’t give exact dates, and don’t actually mention Eldgja. There is one indirect reference to Eldgja in the Book of Settlement:
Gnup went to Iceland on account of his own and his brother’s manslaughters and settled land between Kuda-fleet and Isles-river, and all Swans Haunts (Alftaver); there was a great standing-water then and swan catches thereon. Molda Gnup sold from his landtake to many men, and it became thickly peopled, until the earth-fire (i.e. lava) flowed adown there; when they fled westward to Head-Brink (Hofda-brekka), and made there tent-dwellings in the place which now is called Tent-field (Tjaldavöllr). But Vermund, the son of Sigmund Kleykir, would not allow them abidance there, so they went to Horse-garth (Hrossgard) and made a house there and sat there over the winter and quarrels and manslaughters befell there among them. But in the following spring Molda-Gnup and his companions went west to Grind-wick and took up their abode there. They had a scanty store of livestock. By then the sons of Mould (Molda) Gnup, Bjorn and Gnup, Thorstein Hrungnir and Thord Leg-wielder, were of ripe age. (part 4 ch. 12).
It gives a fascinating insight in Viking life! It was a world born of violence, especially so during the bleak and idle winters. Molda-Gnup is believed to have been born around 885 AD and his brother Vermund 890 AD, both in Norway. They came to Iceland for fear of revenge for murders they had committed (there seems to be a bit of a pattern here):
There was a man named Hrolf the Hewing, he dwelt at Nordmæri [Norway], at a place named Mould-Town (Molda-tún); his sons were Vermund and Mould-Gnup; they were men great at manslaughters and smiths in iron.
Their move to Iceland would have been around 910-920 AD. Taking time to have children and growing them to a ‘ripe age’ (presumably ripe for marriage) may have taken another 20 year, so this puts the eruption in the region 930-940, in agreement with the ice record. The sequence of events suggests a spring or summer eruption. Molda-Gnup is mentioned several times in the Book of Settlement: he was an ancestor of a wealthy and powerful family. One such text reads:
There was a man named Avang, an Irishman by race, he first settled in Botn (= Bottom). The wood was at that time so abundant there that he built from it a seagoing ship, and put in her cargo at the place which is now called Hladhamar. His son was Thorleif, the father of Thurid, the wedded wife of Thormod, who was the son of Thjoster at Alftanes, and of his wife Idunn, the daughter of Molda-Gnup. (part 1 ch 14).
Avang came to Iceland one generation before Molda-Gnup (perhaps around 880), when the country was still well forested. The statement that Idunn and Thjoster lived at Alftanes is interesting, as this area was overrun by Eldgja lava. Molda-Gnup had time for at least three grown-up children , and for his daughter (the oldest?) to have her own family before the eruption. This adds a few years to the estimates of when Eldgja occured, perhaps around 935 at the earliest.
There is one other oblique reference to the eruption in the Book of Settlement (part 4 ch 12):
Hrafn Haven-Key was a great Viking, he came to Iceland to settled land between Holm’s-river and Isle’s-river and dwelt at Din-Shaws (Dynskogar). He foretold a volcanic eruption, and moved his dwelling to Low-isle (Lágey): his son was Aslak ‘orgodi’ and from him the Lowislanders are descended.
This suggests there were precursors to the eruption. A significant earthquake is most likely, doing enough damage (but also leaving enough time) to make people move away.
Non-Icelandic records Strothers has identified one possible reference to Eldgja in European records, by the 10th century Saxon chronicler Widukind of Corvey:
Indeed before the death of King Henry many prodigies occurred, such as: The brightness of the Sun outdoors in a cloudless sky appeared almost nil, but it streamed indoors, red as blood, through the windows of houses. Likewise for the mountain where the overlord of the states was buried, according to report, because the mountain erupted flames in many places.
King Henry of Saxony died in July 936. The text is difficult to understand but as there are no ‘flaming mountains’ in Saxony, Strothers interprets this as a reference to Iceland. The ‘overlord of the states’ was a title of the leader of the Icelandic Parliament (the Althingi), and the first leader Úlfljótr, disappeared from the records in 934, at the start of his three-year term. He would have traveled to the parliament through the area covered by Eldgja lava. Does Widukind imply that he died in or shortly before this eruption? If so, the eruption can be dated to 934 AD or shortly after, in spring as the Althingi convened in mid-June. But one has to be careful. ‘Flames’ was often used in the old records to describe northern lights, in which case this may have nothing to do with Iceland, or volcanoes. There are some anachronisms: the ‘states’ did not come into existence until 964, and the actual title of the Althingi leader was ‘Law speaker’. Úlfljótr wrote the first law (parts of which survive in the Books) but there is no evidence he was the leader. Histories can be open to wishful interpretations, and here the evidence that Widukind talked about Iceland is not that strong.
Elsewhere in Iceland To complicate things further, there was another eruption, almost simultaneous with Eldgja, which left tephra over much of north-east Iceland. It is dated to 938+-6. We know little about it apart from the general location. These hardy Vikings just didn’t think large eruptions as worth noting. Such were the pre-VolcanoCafe days.
Date and duration
Based on the ice core dates and these historical records, Eldgja is normally said to have started in 934. The duration would have been at least 2 years, based on 8 episodes lasting 2-4 months each (the latter is derived from the structure of two lava lobes). The strong sulphur peak covers 3 years, which is one year longer than Laki. This gives a duration of 2 years (the extra year is needed for the sulphate to drop out of the atmosphere). However, the fact that the ice cores show two sulphur peaks dated 7 years apart is often taken as evidence that Eldgja lasted for 6-8 years.
There are some holes in these arguments. Can we do better?
Let’s return to those ice cores. They show two events, separated by about 7 years, with the first one much stronger than the second. Ash found in the first event clearly identifies it with Katla, and thus with Eldgja. But the dates derived from the GISP2 and the NEEM ice cores for this event differ by 4 or 5 year.
Michael Sigl, in a Nature article last year, has tried to fix this dating problem. In the 7th and 10th century, twice there was a strong jump in 14C in the atmosphere, which left a record in trees that were growing at the time. The tree rings give precise dates for these jumps, which may have been due to major solar flares. Such flares would also have produced 10Be. Sigl managed to identify the beryllium isotope in the ice cores, but the date differed from those given by tree rings by 5 years. He attributes this to an error in the established time line for the ice cores. People had miscounted. Shifting the ice core dates to the ones from the tree records changes the time line for these cores: the date for the large sulphate spike in 934 now moved to 939 AD, for both ice cores.
This is an intriguing proposal. There are some lose ends: they don’t state why or when the error or miscounting in the ice core dating occurred (dates more recent than 1200 seem fine), and in the revised time line the sulphate spike around 1104 AD has moved to 1108 and no longer aligns with the 1104 Hekla eruption it was assigned to. The peak in 77 AD has moved to 88 AD and can no longer be attributed to Vesuvius. But in light of the other evidence above, the case for Eldgja to have started in 939 has certainly strengthened.
Remember the dimming of the Sun described by Widukind? There are two similar records for this. The Spanish historian Juan de Mariana (17th century) states that there was a darkening of the Sun on two days of different months of the same year around this time, 19 July and 15 October. The first is easily identified as the total solar eclipse of 19 July 939, when the path of totality was near Madrid and Lisbon. For the second darkening, 15 Oct 939, the light of the Sun changed to a pale colour. The Irish Annals of Astronomy, 939, mention The colour of blood on the sun from the break of day until the middle of the next day.
Is the revised chronology correct? The authors make a good case (although the evidence for dimming of the Sun is more dubious because of the inconsistent date of Widukind, plus the fact that major peat fires can have similar effects). There are two concerns. First, the GISP2 core which gives the later data has a larger uncertainty of +-4 yr, so it does not disagree with a 934 date. Second, the re-dating is based on tree ring records. If there is an error in these, the rug is pulled from underneath their method. That is not entirely impossible. If a year is so cold that the selected trees (which are normally from the edge of their distribution, to maximise the effect of climate on the growth rings) do not grow at all, that year will be missed and all older dates shift forward. The main risk for this comes from the extreme climate event of 1258, caused by Rinjani. This may be less likely but more evidence is needed.
The final piece of evidence is from the weather. The limited records from this period sadly do not include daily weather reports. Only extreme weather events would have been described. The winter of 933/934 was cold: the Black Sea and Bosporus were reportedly frozen (presumably only along the edges, not fully!). But an even colder winter had happened in 928/929 when the Thames was frozen for 13 weeks, so this was not completely exceptional. After that, no extreme weather is reported until 939/940, when a severe winter happened, listed for instance in the Annals of Ulster. This cold weather affected much of Europe and Asia and lasted three years. Famine followed in Germany and France, first in 940 and again in 942. Records of Chinese weather (Fei and Zhou, 2006, Fig. 2) tell us that the winters of the years following AD 934 were not unusually cold. But extreme cold came in mid 939 with snow in July. The following winter was the coldest in the 30 years between 923 and 954, and the next two winters again were cold. To make things worse, this was followed by two years of exceptional drought. In a recent paper the same authors hesitantly suggest that this was caused by Eldgja, although that presumably started 8 years earlier. In the revised dates, the cold and drought follows Eldgja much more closely.
A similar point has been made regarding the Nile. Its water level is sensitive to major eruptions, which tend to affect the rainfall in its source region. Climate models have confirmed there is a relation. The year after Laki was extreme, with the lowest flow rate over 400 year. A lesser low point happened after Katmai. The flow around 934 was normal, bit it was extremely low during and after 939. Luke Oman (2006) has used this to argue already a decade ago that Eldgja may have been misdated.
While 934 was cold only in Europe, the period 939-942 was cold across the northern hemisphere. This is similar to what happened after Laki. Laki has been blamed for hastening the French revolution. How about Eldgja? What is the evidence for political upheaval? Actually, surprisingly limited. In China, the Later Jin Dynasty collapsed shortly after and this was clearly infuenced by climate. The Abbasid rulers in Baghdad crumbled and the Buyid Dynasty took over, but this process had been on-going for a decade or more. The Viking wars continued across Europe and the Middle East but did not change much. There may have been a fortuitous element: Eldgja happened at the start of the Medieval Warm Period, when European climate was benign. Laki happened during the Little Ice Age and this may have worsened its effects.
As an aside, the Icelandic fires affect climate much more than may be expected from there volume. They emit much more sulphur than normal volcanoes, and this has climatic damage beyond their lava size. The distribution is not world-wide because Iceland is so far north. Icelandic sulphate stays mainly at mid-to-high northern latitudes. But this makes things worse because with a much smaller area to affect, it is much less diluted. The concentration may be 5 times or more higher than that of an equivalent tropical eruption. Iceland may be small, but it packs a punch.
From all this, it seems plausible, but not fully proven, that the Eldgja eruption started in spring 939 AD, five years later than generally accepted. It lasted perhaps two years (not 6 or 8 years: this came from attempts to reconcile the different dates from different ice cores), and caused a volcanic winter which affected Europe, China and Africa for three years.
Next time We now know a plausible date. There are some questions that still need to be discussed: what actually caused the fissure eruption? Where did the magma come from? Why did the eruption stop? And when will the next one be? Come back for part 3 of the Eldgja saga!
updated to add the bit about the Nile
To be continued