In part I we found the main bands of a excavation here in SW Iceland:
– dark band of Veidivötn 1477
– double white layer band of Hekla from 1341 (or Öraefajökull 1362) and Hekla 1104
– dark band of Vatnaöldur 870 (called the “Settlement Ash”).
– thick white layer of Hekla 3 (around 1000 BC), one of largest eruptions in Iceland.
But there are many minor layers besides the obvious ones. We will get to them now.
An unknown metalic grey layer
Above the Hekla 3 layer, there is an unknown layer. It has a strange shiny metalic gray color. This is an unknown eruption estimated between 500 AC and 500 BC. Many eruptions happened at that time, not only from Hekla and Katla, but also an eruption at Torfajökull at 150 AC, Hengill at 80 BC (which is only 20km to the west), and also in Vatnafjöll. I don’t think this ash came from Katla or Hekla, unless they erupted a different ash. This type of shiny metalic ash color is notoriously different from every other ash I have seen in Iceland. But I have seen similar shiny lava rocks in Iceland, in a few places, but I can’t remember where. Until then I cannot make a guess about the identity of this layer.
There also seems to be a brown band between the Hekla 3 and the unknown grey layer. It is probably an undated eruption of Grimsvötn, which usually has this color of ash (around 100-500 BC). There is also a light colored band (just above Hekla 3), and that is probably an eruption of Hekla (around 500-1000 BC).
Below the Hekla 3 layer, there are several bands, shown in Fig.2. We first find a thin band of orange material (at 53cm), then a very large band of dark ash (starting at 56cm), and then another broad band of orange material at around 65cm. There is a thin white layer between both broad bands of dark and orange material (not visible in Fig.2).
Around 1500-2000 BC: Torfajökull and Katla?
The first thin orange band is estimated at around 1500 BC. The most likely candidate is the eruption of Torfajökull around 1200 BC, because it tend to erupt such colorful rhyolite ash. The broad grey band is estimated around 2200 BC. Most likely it was one strong eruption from Katla (tephra N4 or N2). What is was, it was big, because this is a thick layer. However around this time, we also had records of an eruptions at Langjökull, dated as ~2050 BC, which was actually nearby, only 40km north (it’s closer than Hekla), in a small shield volcano named Lambahraun. If the eruption started explosively, then its ash might have reached here, but officially there is no known ash from the Langjökull volcanoes and I also don’t expect that even a nearby shield volcano would deposit such a major amount of ash. So we stay with Katla.
Hekla 4 and Grimsnes eruptions (2300 to 4200 BC)
The thin white band is probably the eruption of Hekla4, around 2300 BC, which was a very large eruption. The broad orange material is almost likely from nearby Grimsnes volcano, that erupted several times circa 3500 to 4200 BC. I am actually inside Grimsnes volcanic region; its monogenic cones are just 5-8 km away. During the Grimsnes eruptions, there was some local ash fall. The volcano is just composed of crater rows, with one major explosion crater and the other cones being a deep red. It’s no wonder that the layers from Grimsnes eruptions are of a similar color.
Hekla 5 and Botnahraun/Laki, or Holmsá fires eruptions, or Thjorsáhhraun (5000-6000 BC)?
At around 75cm deep (estimated at 5000 BC) we find what looks like a double layer: white material above and a deep dark brown below. It is easy to assign this white material to Hekla5 (another large Hekla eruption at 5050 BC). The brown material underneath is unknown, but likely Grimsvötn. Possibly the Botnahraun/Laki eruption. Alternatively it might also correspond to another big eruption at this time: the Holmsá fires, another Eldgjá-like fissure that opened to the east of Katla. And still it might also be the Thjorsáhraun lava from Bardarbunga/Veidivötn, around 6600 BC. That lava actually travelled some 200km from Veidivötn towards the southwest, passing only cross 5km east from this location. That is the largest lava field on Earth since the ice age.
And now we get even older in time… Seydisholar 7000 BC
Below this point, it starts to get complicate to assign the identity of any layer because of a mud deposit underneath. There is some orange material just above it, which I assume it might have been the eruption of Seydisholar at 7000 BC, from the nearest Grimsnes crater row. That was the largest eruption of the Grimsnes system, with an estimated VEI4. And I am just a few kms from it.
Saksunarvatn ash 8000 BC?
At some points, there is a strange thick dark brown band around this depth, at around 70-80cm (see Fig. 4), which could have been the famous Saksunarvatn ash layer (Grimsvötn, 8000 BC): the largest eruption in Iceland in the Holocene. This ash is widespread recorded in northern Europe, and is used as an important marker dating the beginning of the Boreal period (end of the Young Dryas glaciation). Both the double layer (the 5000-6000 BC, referred before) and this deep dark brown layer, seem to ondulate, with one sometimes appearing over the other, and then exchanging positions. Their age is therefore highly uncertain.
The tale of a river bed, nearer sea levels, and also the ice age
Below 90cm we mostly find mud. This might have been a time when glaciers were over this region. At the glaciation peak, the ice sheet must have been at least 400m thick here, because of the nearby tuya Ingolfsfjall. However the peak glaciation must have been short, because we find much more shield volcanoes at this region than tuyas. About 5 km north, there is a large moraine, from where most of the time the large glacier terminated. For most of the ice age I was just at outside of the glacier.
The mud might been also caused by nearby Hvitá river (which drains the now distant Langjökull). The excavation is just next to a waterfall-like valley, thay I know it was the path of Hvitá river now 2km east. Therefore it might been subject to much soil erosion and river deposits sometime before 8000 BC.
In early post-glacial time, the sea level was higher and the coast was actually nearby. There is actually evidende of a coast just 5km south (in the nearby shield volcano Hestfjall). The sea must have been pretty close and again this location was subject to much erosion. Because of all these reasons, we possibly do not have the record for the famous Vedde Ash (Katla 10.000 BC), which is one of the two largest eruptions in Iceland in recent millenia; the other was the Grimsvötn Saksunarvatn ash (both VEI6). In one spot, I did see some white material around 90cm deep, but I am unsure if this was it.
Ancient Lava (from Lyndhalsheidi)
Finally, on the bottom of the excavation, around 1.5m deep, there is a bedrock of lava rock (visible at some spots at lesser depths, such as in Fig.2). They are eroded and rounded (probably by the last glaciation). This is lava from the shield volcano Lyndhalsheidi that is just 8km northwest. Its lava actually flowed where I now stand, but that eruption was on the interglacial before the last glacial, so it was a long ago. However the glaciation continuously exposed and eroded that ancient bedrock.
Layers near the surface
There is no significant ash layer since the 1477 Veidivötn ash. However we can see sometimes faint layers from recent eruptions. One black layer around 5cm is probably the VEI5 Katla 1918. One faint white layer at 8cm is probably Hekla 1845 eruption. And a faint dark layer around 12cm is probably Laki 1783 (but it could have been the eruptions of Katla in 1755 or 1721; not visible in Fig.5).
ICELANDIC ASH RECORDED IN GREENLAND
To finish today I read some papers that described which ash layers appeared in Greenland ice cores. We find there the 1362 Öraefajokull, 1104 Hekla, 870 Vatnaöldur ash, Hekla 3 and Hekla 4 eruptions, the very large ash layers of Saksunarvatn/ Grimsvötn (8000 BC) and Vedde / Katla (10000 BC), followed by many ash layers from Katla, Hekla or Grimsvötn, and finally other two very large ash layers: one from Tindfjallajökull Thorsmörk ignimbrite, 53.000 years ago (that was a VEI6+, and possibly even a VEI7). The volcano is still dormant now and right next to Katla and Eyjafjallajökull); the other big eruption was 300.000 years ago, and hypothesized to be from Krafla or Hofsjökull.
After this lenghty post, please feel free to call me a big ash hole.
Editors note: Do click on the images, then you will see all of the details since they are rather large.
Update by Spica:
Here is the link to part I of the story.