| Summary: | The Northern Volcanic Rift Zone, Iceland, is a ~200 km long segment of the Mid-Atlantic plate boundary, where the North American and the Eurasian plates are diverging. The rift zone consists of about 5-6 volcanic systems with central volcanoes and fissure swarms, in addition to the Tungnafellsjökull Volcanic System at the border of the rift zone. The volcanic systems are the locus of eruptive activity in the Northern Volcanic Rift Zone. The central volcanoes consist of elevated massifs, high temperature geothermal areas, calderas and silicic formations. Fissure swarms with eruptive fissures and high density of fractures extend in opposite directions from the central volcanoes. In the Northern Volcanic Rift Zone, the fissure swarms are between 0.5 and 15 km wide and between 30 and ~125 km long. In this study, fractures and eruptive fissures within the fissure swarms of the Northern Volcanic Rift Zone were mapped in detail from aerial photographs. The results of the study indicate that eruptions are less common at the distal parts of the fissure swarms than closer to the central volcanoes. The proximal parts of the fissure swarms also generally show higher fracture density, even when the effect of the age of the lava flows has been taken into account. Older lava flows in the Krafla and Askja Fissure Swarms have usually higher fracture densities, suggesting repeated dike intrusions into the same parts of the fissure swarms during Postglacial times. Fractures in the fissure swarms of the Northern Volcanic Rift Zone are characteristically oriented towards north or NNE, i.e. more or less perpendicular to the spreading direction. However, deviations from this pattern occur in certain areas. These areas include the caldera volcanoes in the Northern Volcanic Rift Zone, Krafla and Askja, where some fractures and eruptive fissures are concentric to, or radiate from the calderas. Second example involves east-west oriented fractures and eruptive fissures near the Vatnajökull glacier, and third example eroded WNW-oriented ...
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