Mýrdalsjökull, the glacier of Mýrdal, is the fourth largest and southern most glacier in Iceland. The glacier lies in the second biggest caldera of the country, Katla Caldera (110 km²) The average thickness of the glacier is 250 metres, but in some places it is as thick as 750 m. Its surface is of about 600 km², which represents around 130 cubic kilometers of ice!
Goðabunga is the highest peak of the glacier at 1515 m, followed by Hábunga at 1505m, but both are snow covered peaks and their height can vary a lot depending on the importance of the snowfall on the glacier. The humid air which comes over the highlands from the coast south of the glacier provides it with snow which in time changes into ice.
Side glaciers flow down to the lowlands from the main glacier. The largest of them is Kötlujökull which flows down a cleft into the caldera eastwards towards the black lava Sands of Mýrdal. The glacial tongue Sólheimajökull flows southwards down a narrow and deep valley to about 100 m above sea level, while Entujökull flows towards the northwest. Under all three of these glaciers, glacial rivers flow, whose beds also serve as channels for the glacial floods which usually follow eruptions in Katla.
The most common floods flow down the flats/Sands of Mýrdal, though floods under the Sólheimajökull glacier down the Sólheima Sands and Skógar Sands have also been heard of. Glacial floods have also run westwards under Entujökull to Markarfljótsaura, the last time about 1200 years ago. The history of Mýrdalsjökull is intimately related to that of the vocano Katla, which has shaped the landscape of the area through the ages.
The Sólheimajökull glacier flows south from Mýrdalsjökull glacier in an U-shaped valley. The glacier is about 15 km long, 1-2 km wide. Under the ice cap of Mýrdalsjökull is a 650-700 m deep caldera which extends down to 650 m altitude. Sólheimajökull flows down from one of the passes in the caldera at about 1050 m altitude, between Hábungu and Goðabungu.
The head of the glacier is about 1000 m altitude and the glacier margin goes down to the lowlands around 100 m above sea level. The sole of the glacier lies 50 m below sea level. Sólheimajökull glacier is very sensitive to changes in climate. From Sólheimajökull flows the river Jökulsá on Sólheimasandi. East of the river is the Sólheimasandur (outwash sand plain) and west of the river Skógasandur (outwash sand plain).
The glacier is a "spill-over" from the high ice cap. Glacier ice flows, rather rapidly, through a breach in the caldera rim. The surface has a typical crevasse pattern and the ice thickness varies from a few dozen metres at the snout to maybe 200-300 m at the valley head.
There are dark tephra bands in the ice but much dirt is also present at the margins, sediments of glacial origins, carried from the inside bottom layers of ice to the glacier surface by ice flow. As in all glaciers, the flow is a combination of basal sliding and internal deformation if the ice mass. The lagoon and flat ground moraine in front of the glacier is very recent.
Katla Volcanic System
Katla Volcanic system is one of the most active and productive in the country. The central volcano of Katla is set with a large caldera. The Mýrdalsjökull ice cap largely covers the volcano (560 km2) and 1,510 m a.s.l. A catastrophic explosive eruption occurred about 12,000 years ago, when part of the caldera may have formed. At least 20 eruptions have occurred in Katla in historical times, the most recent in 1918 and possibly in 1955. A major eruption (Eldgjá eruption), partly outside the glacier occurred around 934 AD.
The prehistoric eruptions of Katla produced dozens of ash layers found in the soil in Southern Iceland. Among them is evidence of silica-rich magma being erupted, instead of the common basalt, including a thick layer to the south of Katla that dates back 12,000 years, from a cataclysmic event, possibly related to the formation or sudden deepening of the caldera.
Katla is said to have erupted around 20 times since the settlement of Iceland, the last time being in 1918. The glacial floods which occur as a consequence of eruptions in Katla are formidable and remarkable natural disasters since an estimated 100.000-300.000 cubic metres of water per second flow during a Katla flood.
Katla has both shaped the magnificent landscape of the area and caused severe damage to those who inhabit it. At the time of the settlement, an estimated 40 farms existed on the Sands of Mýrdal, but they were destroyed by the damage caused by the Katla floods. One could say that the mighty Katla who lived in Þykkvabæjarklaustri gave the locals a hard time with her magic.
Bedrock aged from 0.78 million years back to 3.2 million years stems from volcanic activity during the many cold glacial and warm interglacial periods within the Plio-Pleistocene and Lower Pleistocene periods (until the Bruhnes magnetic reversal, 0.78 million years ago).
This bedrock is composed of brownish or dark grey tuffs or so-called hyaloclastites from glacial periods, interspersed with lava flows from interglacial periods. Hyaloclastites are former ash, pumice and lava bits cemented together by secondary silicates, made into tuff and breccia. The dark basalt glass in the tephra (sideromelan) has been thermally altered at low temperature, to form brownish palagonite.
Pillow lava is common. Rocks from the Upper Pleistocene period (younger than 0.78 million years) mainly form rough hyaloclastite (móberg) mountains, often along with pillow lava, that were formed during eruptions below glacier ice or in the sea. These formations may be classified as hyaloclastic cones and hills, higher table mountains (tuyas or Icel. stapi) and multi-summit ridges composed of pillow lavas and hyaloclastites.
The term Móberg or Palagonite Formation is commonly used for the areas with bedrock of this kind. The bedrock of Mýrdalur and vicinity belong to the Móberg Formation of Southern Iceland. Some of it is from the Upper Pleistocene period and the bedrock as a whole is eroded by Ice Age glaciers and flowing water.
Many of the more prominent peaks and hills can be looked at as individual volcanic vents plus the associated pile of eruptive rocks above the vent, like Hatta mountain behind Vík or Reynisfjall, Hafursey or Pétursey.
Thick lava flows are exposed in number of the tuff and breccia series. Some of the lava flows display beautifully arranged basalt columns, formed during the cooling of the lava. It contracts while cooling along horizontal cleavage planes. One of the best examples is found at Reynisfjara.