The glacier is roughly parallel to the direction of the country's active volcanic zone: north-east to south-west. It is about long and wide, and has a slightly narrower point roughly between the lake Hvítárvatn on the Kjölur mountain road to the east and the Þrístapajökull glacier to the west, near another smaller glacier, Eiríksjökull, which is not quite connected to Langjökull. It is the nearest large glacier to Reykjavík. The area of the glacier includes some mountains, e.g. Jarlhettur on the east side of Langjökull, a palagonitic mountain range, which originated in a fissure vent under a glacier during the Ice Age. The mountain Skríðufell is situated on the east, above lake Hvítárvatn. Other mountains on the eastern side of Langjökull are Fjallkirkja, Þursaborg and Péturshorn. A little to the east of Fjallkirkja is the hut of the Icelandic Glacier Research Society, which includes scientists as well as interested amateurs.
Glaciers in the area
A number of outlet glaciers reach down from Langjökull to the valleys and plains below. These include Norður- and Suðurjökull to the east; Vestri- and Eystri-Hagafellsjökull at the southern end of Langjökull which are separated by the mountain Hagafell; and Þrístapajökull to the west. Geitlandsjökull is an outpost to the southwest, a glacier covering a tuya which is connected to Langjökull. Research shows that the outlet glaciers Norður- and Suðurjökull reached as far as lake Hvítárvatn until about 1900 but have retreated rapidly since then.
Despite the size of Langjökull, not many rivers directly originate there. However research has shown that large quantities of water flow in sub-surface streams to lake Þingvallavatn and reappear in springs in and around the lake, while the same happens on the west side, giving rise to various tributaries of the Hvítá River as well as some rivers flowing north towards Húnaflói. Thus many of the hot springs in the Borgarfjörður region, such as Deildartunguhver, receive ground water from Langjökull. The same is true for the lake Hvítárvatn to the east: ⅔ of its waters appear to come from Langjökull by underwater channels. Langjökull is shrinking quite fast, and some researchers believe that it will disappear in about 150 years if climate change continues at its recent pace.
Volcanism
Under Langjökull there are two or more volcanic systems. The calderas can easily be seen from the air. The best known of these is Hveravellir with its hot spring and high temperature area to the east of the glacier. During an Ice Age some shield volcanoes of this system covered the plains with lava in the region of today's Kjölur mountain road: the Kjalhraun. The lava field is about 7,800 years old. A smaller volcanic system lies to the north-west of the glacier in the Arnarvatnsheiði, north of the glacier Eiríksjökull. About 1,000 years ago, the craters of this system produced the lava field Hallmundarhraun which extends some 50 km westward into the valley of the Hvítá, and is traversed by underground streams which emerge at the beautiful waterfalls of Hraunfossar not far from Húsafell. Another volcanic system to the south-west of Langjökull is Presthnúkur, named after its central volcano, a Rhyolitedome with a high temperature area at its foot. Its fissures extend under Langjökull. Volcanically, the region is relatively quiet, compared to other regions in Iceland, with just 32 eruptions in the last 10,000 years.
Highland roads
Two highland tracks, open in the summer months, lead alongside this glacier: the Kaldidalur and Kjölur tracks. The former runs along the west of Langjökull, between it and the small Ok glacier. To the south it leads towards Þingvellir and Reykjavík, while to the north it leads to the Hvítá valley which leads down towards Borgarnes. Kaldidalur is no longer classified by the Icelandic road authorities as an F-road. Nevertheless, rental car companies forbid their clients to drive on the road with other cars than four-wheel drives. The latter runs along the east side of Langjökull, between it and Hofsjökull. It is the main route across the interior between the main population centres in the SW and the N of the country, but nevertheless traffic is extremely sparse.
