Rocks and Water
Descriptions of Deposits
Katla hosts a 600 to 750m deep caldera filled by ice. Her caldera rim is breached in three places, to the southeast, northwest and southwest. These gaps in the rim provide outflow paths for the ice in the caldera to feed the main outflow glaciers. Apart from the large flood lava eruption in 934 ad, all historic eruptions of the volcanic system have been located inside the caldera. (11)
Volcanic ProductsKatla’s volcanic products are primarily bimodal in composition, comprising alkali basalt and mildly alkali rhyolites. This bimodal composition is ascribed to rabid segregation of material from a large mantle –source beneath a propagating rift. (11)
TephraAt least 12 silicic tephra layers are known from Katla during the Holocene between 1700 and 660 BP. Similarly, the majority of known outcrops of Katla at the caldera rim and immediately outside it, are silicic. Her tephra products are indicators that Katla has been erupting in phreatomagmatic style throughout the Holocene. (11)
LavaThough eruptions from Katla are usually in the form of jokulhlaups and tephra fallout, In 934 AD, Katla’s fissure system produced one of the largest known Holocene lava flows of about 18 cu km. (4)
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Jökulhlaups
Even though there may not be an eruption seen by a large ash plume, the volcano can produce something just as devastating. When the Ice over the volcano melts because of heat produced by the volcano, it causes water to form a lake under the remaining ice cap. When the ice cap then collapses into the lake or the water breaks through the barriers containing it, a type of lahar called a jökulhlaup rages down the mountain slopes carrying with it ice blocks the size of houses. (6)
*Many sources suggest that the jökulhlaup that followed the eruption of 1755 produced the same outflow as the Nile, Amazon, Mississippi and Yangtze rivers combined. (6) * |
Hydrothermal activity
Katla is covered by the Myrdalsjökull ice cap and because of this, the ice interacting with the magma during an eruption causes eruptions to be phriomagmatic. (5) Katla is known to be seismically active, although seasonal fluctuations have been attributed to her ice cap melting during the summer and the resulting reduction of pressure on the magma chamber.(10)
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Below: Map taken from the Icelandic Institute of Natural History. Shows the geothermal energy of Iceland. (3) *Click to expand*