The Western territory of Vatnajökull National Park
The western part of the Vatnajökull National Park is influenced by four central volcanoes and their fissure swarms. The volcano Katla lies to the southwest, below the Mýrdalsjökull ice cap, with the Eldgjá fissure running to the northeast. Grímsvötn, a central volcano in the Vatnajökull ice cap, sends a fissure system to the southwest where the Lakagígar crater row is.
On the western rim of the Vatnajökull ice cap lies the volcano Hamarinn, which is probably connected to the hyaloclastite ridges between Langisjór and Tungnaá. Furthest to the north is Bárðarbunga which sends a fissure swarm southwest to the Veiðivötn lakes, all the way south to Torfajökull.
Long hyaloclastite ridges are typical features in the area, and unique to Iceland. They are most prominent between the Tungnaá and Skaftá rivers, e.g. Grænifjallgarður and Tungnaárfjöll. The hyaloclastite ridges formed in eruptions below Ice Age glaciers. Lake Langisjór is a long mountain lake between two ridges. In the south of the area are a few postglacial (Holocene) crater rows. The most prominent are Eldgjá and Lakagígar. Hyaloclastite mountains alternate with postglacial lavas in a wide strip of land between Þórisvatn and Tungnaá, stretching to Vatnajökull.
Lakes and rivers
Glacial melt water has a large influence on the area. In the north there is a big river, Tungnaá, which flows from under the Tungnaárjökull and Sylgjujökull glaciers, and runs along a wide valley between hyaloclastite ridges. It spreads itself as braided channels and is a very typical example of an Icelandic glacial river. In this area there are many crater lakes which formed in the Vatnaöldur and Veiðivötn eruptions. The lakes are beautiful, and popular fishing spots.
The river Skaftá now flows south of Fögrufjöll. Until the middle of the last century it flowed into, and out of, Langisjór. On the lower ground, the river Skaftá splits in three. The last fifty years or so, Skaftá has flooded roughly every two years. These glacier outbursts originate in two cauldrons in the Vatnajökull ice cap, Eystri- and Vestari-Skaftárketill. Below them lies a geothermal area which melts the glacier. The melt water collects until it is enough to float the glacier, allowing a flood to burst forth. The glacier outbursts carry a lot of sediment.
Soil and erosion
In the Skaftárhreppur district precipitation is high, which is ideal for the vegetation. On the other hand, the volcanoes pour ash over the area and rivers deposit vast amounts of sand and silt. The soil is therefore very sandy, and sensitive to disturbance; it can also be very thick, 10–15 m. The volcanic history of the last 10,000 years is recorded by the many ash layers in the soil. Soil erosion is common in the whole area but the vegetation has better growing conditions than in many of Iceland's other hyaloclastite ridge areas, because of the high precipitation and mild climate. In the highland area the land is mostly bare sand with virtually no vegetation.
Plants and wildlife
The ecosystem of the area is shaped by volcanic eruptions, high precipitation, and a relatively warm climate. Although the precipitation is high, the water quickly trickles away through the porous lava and pumice, so vascular plants with a root system have difficulty making use of it. The vegetation is, therefore, typically mosses and lichens which absorb the precipitation directly through leaves and thalli. The thick hummocks of fringe moss on the Skaftáreldahraun lava are typical for the area. They prevent higher plants from growing, except where a lot of soil has blown onto the lava. Probably nowhere else in the country is moss as prevalent in nature as it is in the uplands of Skaftárhreppur district, where it forms 90% or more of the plant coverage. In the Tungnaáröræfi region there is a large area of desert where less than 5% of the land is vegetated. The lavas are, however, often covered by lichen (Stereocaulon species).
Two specialised and rare habitat types occur in this volcanic area: the breiskuhraunavist (dominated by Stereocaulon species lichen) in the Skaftáreldahraun lava; and vikravist (named after pumice) elsewhere in the area.
The breiskjuhraunavist habitat type takes its name from the lichen which covers the rougher parts of the lava, colouring it grey. Woolly fringe moss (grey) and dense fringe moss (yellow-green) are also prominent. Woolly fringe moss grows where the substrate is drier, on hills or ridges, and dense fringe moss dominates in hollows which collect snow and are damper.
The vikravist habitat type is named after the pumice and ash which are common in the soil. Vegetation is very sparse and low-growing, shaped by the constant movement of the pumice. Some high order plants survive there, such as moss campion, sea campion, thrift, and arctic fescue.
Wildlife is sparse. A few species of bird nest in the area. Common species are snow bunting, northern wheatear, golden plover, purple sandpiper, and meadow pipit. The great northern diver can be seen on lakes which have trout. Foxes and field mice are the only wild mammals.
In historical times there have been two enormous eruptions in the area west of Vatnajökull. In about 934 Katla erupted and threw ash over the surrounding countryside. A large fissure, Eldgjá, opened to the northeast. It is 75 km long, reaching almost to the Vatnajökull ice cap. Many sections of the fissure were volcanically active, although the southern section was most productive. Lava flowed to Álftaver on the Mýrdalssandur sand plain, along the Skaftá river course and down to Meðalland (the Landbrotshraun lava). This was undoubtedly the largest eruption in the history of Iceland. The other eruption was in 1783–84 when the Laki Fires occurred. A 25 km long eruptive fissure opened where the Laki crater row now stands. The second largest lava flow in Icelandic history ran from the craters at Laki.
The area north of Tungnaá is all volcanic in origin, and many of the largest postglacial lavas came from there, e.g. the great Þjórsárhraun lava from about 9000 years ago, which ran all the way to the sea between Þjórsá and Ölfusá. In historical times there have been at least three eruptions in this area. In 871 there was a big eruption which produced an ash layer which is called landnámslag, or the settlement layer. The Vatnaöldur ash cones formed in that eruption. In 1477 there was an eruption at Veiðivötn which formed the present landscape of small lakes. The third eruption was 1862–1864 when a fairly large lava flow was produced. All of these eruptions are believed to have been part of the Bárðarbunga volcanic system.