Katla is one of the largest central volcanoes in Iceland, covered by the Mýrdalsjökull ice cap. The volcanic system, including Mýrdalsjökull area and the fissure swarm, Eldgjá, northeast of it, is about 30 km wide in its south-western part, narrowing gradually to the northeast and reaches a length of 78 km. The caldera, Katla, is located under the ice cap and is about 100 km2 and 700 m deep, filled with ice.
The Katla volcanic system is famous for numerous subglacial eruptions. In the Katla volcanic system, 21 eruptions are known in historical time. The last eruption in Katla occurred in 1918. The total amount of tephra produced in that eruption has been estimated to have been around 700 million m3 and the glacial outburst flood (jökulhlaup) about 8 km3
The Eyjafjöll volcanic system is a stratovolcano with well developed 2,5 km wide caldera at the top. The complex is covered by an ice cap above 1.000 m altitude and the highest point was at 1.666 m a.s.l. before the eruption 2010. Eruptions in Eyjafjallajökull are rare and relatively smaller than in Katla. Four eruptions in Eyjafjallajökull are known in historical times, in 920, 1612, 1821-23 and 2010. Before the eruption in 2010 there had been earthquake swarms there in 1994 and 1999 and the volcano was therefore under a close watch as an eruption under the ice cap would cause immediate danger to farms in close proximity to the mountain.
The first eruption, at Fimmvörðuháls an ice-free ridge between Eyjafjallajökull and Mýrdalsjökull, lasted from 20th March to 12th April 2010. This small eruption created the craters of Magni and Móði on the north side of Fimmvörðuháls, directly across the popular hiking trail between Skógar, south of the pass, and Þórsmörk, immediately to the north. The eruption produced limited amounts of basaltic lava and attracted a lot of attention from tourists that could get close to the spectacular lava fountains and lava streams.
The second eruption started on 14th of April 2010 within the ice-capped caldera, with an explosive summit eruption amplified by magma-ice interaction. Jökulhlaups (glacier outburst floods) are more common in Iceland than elsewhere in the world because of the interaction of volcanoes and glaciers. A warning system is operated by the Icelandic Meteorological Office that informs Civil Protection Authorities of impending floods or jökulhlaups. An important test was put to the system in the Eyjafjallajökull volcanic eruption on 14th April 2010 and the evacuation plans worked out well and no lives were lost due to the flooding.
Grímsvötn is a central volcano below Vatnajökull ice cap. The volcanic system consists of the Grímsvötn central volcano, about 100 km long and 15 km wide volcanic fissure swarm including the Laki cone-row. The subglacier central volcano has developed a large composite caldera in the glacier Vatnajökull with a high temperature geothermal area. Grímsvötn is the most active volcano in Iceland. The number of eruptions since the time of settlement is uncertain but it is believed to be at least 60 (compared to about 20 in Katla). The most recent ones (at the time of writing) are those in 2011, 2004, 1998 and 1996.
Glacial bursts (jökulhlaup) originate in Grímsvötn and charge down the River Skeiðará. They were the main reason that the Ring Road around Iceland was not completed until 1974. The caldera contains a lake which is covered by a 200-300 meters thick ice slab. Creeping ice and continuous melting caused by geothermal heat cause water to accumulate and the water level of Grímsvötn Lake rises. The ice slab also rises. When the water reaches a critical level it seeks an outlet north-east of Grímsfjall. The water begins to gush from the foot of the glacier into the bed of the River Skeiðará. A glacial burst begins, slowly at first, increasing in volume until peak flow is reached. The flow then suddenly decreases and the exit channel closes.
Toxic ash ruined pastures so livestock got sick with a disease called gaddur (dental fluorosis), and starved. The weather cooled due to the mist, and sea ice reached the coast. When winter arrived in 1783–1784 livestock collapsed from starvation and disease due to the toxic volcanic material. People died of hunger. During the hardship one in five Icelanders (about ten thousand people) died, and around 75% of the farm animals were lost. In Fljótshverfi, Meðalland and Síða, about 40% of the population died, 20 farms were covered with lava and another 30 were badly damaged and had to be temporarily abandoned.
Katla Geopark is home to two of the largest basaltic flood lava eruptions in historical times in Iceland, Laki in 1783-1784 and Eldgjá around 938 AD. The Eldgjá fissure is at least 50 km long, extending from the Mýrdalsjökull ice cap in the southwest (Katla), into the Vatnajökull ice cap in the northeast. The most spectacular part of Eldgjá is a 8 km long part of it in the
southwest where the fissure is about 400 m wide and 150 m deep. The fissure is a complex volcanic structure of a graben, an eruptive fissure and an explosive crater row. In the southernmost part the eruption was subglacial, and caused a large glacial outburst flood in association with the eruption, jökulhlaup.
The Eldgjá and Laki lavaflows are not only big on an Icelandic scale, they are also some of the largest lava flows on earth since the end of the ice age about 10.000
years ago. The two lava streams have very different appearance, the Laki lava is covered with thick moss while the Eldgjá lava is often covered with younger material but also characterized by countless number of pseudocraters or rootless vents (Landbrotshólar and Álftavershólar). These have been compared to similar features observed on the planet Mars.
Tindfjallajökull is the oldest and most mature volcanic system in Katla Geopark. No eruptions have been recorded there since the time of settlement but some small eruptions are believed to have taken place in early postglacial time. A well-known ignimbrite layer in the Þórsmörk area is believed to have originated from a large explosive eruption in the Tindfjöll volcanic system about 55.000 years ago when a 5 km wide caldera is believed to have been formed.
Tindfjallajökull is one of the smaller icecaps in Iceland but the peaks of Ýmir and Ýma, that take their names from Norse mythology, provide for excellent all-around view, reaching a height of 1.462 m above sea level. The area is also a popular training ground for rescue teams.