The geological deformation of Iceland is mainly caused by the active spreading of the
mid-Atlantic ridge. The Reykjanes Ridge south of Iceland comes ashore at
Reykjanes, where all of the extension of the northern Reykjanes ridge (NRR) is accommodated in the
Reykjanes Peninsula. At
Reykjavík, towards the northern end of this peninsula, the relative movement of the North American Plate away from the Eurasian Plate can be modelled as , but less than 60% of this divergence is accommodated by tectonic structures just to the immediate east of Reykjavík, with most of the rest being absorbed by tectonic structures in the south-east of Iceland. This is as other extensional cracks and
transform faults are found perpendicular to the spreading direction. Two large fracture zones associated with the transform faults, namely the TFZ and SISZ, are found striking about 75°N to 80°W.
Bookshelf faulting Stress is built up during the spreading movements at the plate boundary. The accumulated stress in transform fault zones is released during strike-slip earthquakes. The transform fault is induced by strike-slip motion that is transverse to the fault zone. The blocks between the faults are slightly rotated afterwards. A diagram (fig.2) is shown to illustrate this phenomenon. Since the rotation of the blocks is similar to a line of books leaning on a bookshelf, it is termed
bookshelf faulting. The TFZ is defined seismologically as an approximate triangle with apex about 67°N, sides about , and base of off the north coast of Iceland, connecting the north Iceland volcanic zone (NVZ) and the southern end of the
Kolbeinsey Ridge. This broad fracture zone is characterised by seismic activity,
crustal extension, and transform faulting. The complexity in the TFZ can be generally explained by the magmatic processes and plate motions. The velocity of the divergent plate motion, estimated to be /year, is strongly affected by the Icelandic
mantle plume underneath central Iceland. Volcanic activity can be found in the Dalvík seismic zone and southern tip of the Kolbeinsey Ridge.
South Iceland seismic zone The South Iceland seismic zone (SISZ), also known as the Reykjanes fracture zone(s), is wide and strikes north-east to south-west in south-western Iceland. There are several approximately- right-lateral offsets of the ridge crest. The offsets create a transform fault zone connecting the EVZ and the
Reykjanes volcanic belt. During these events, additional small—scale earthquakes concentrated narrowly and linearly around the transform fault planes. Thus, with the same method, small-scale earthquakes are also used to identify fault planes in the TFZ. These were followed by the slightly smaller
2008 Iceland earthquake.
Volcanic rift zones Many of the volcanoes of Iceland can be grouped by their relationship to rift zones and contribute to the understanding of the deformation that has taken place. Not all the names used to classify the volcanoes into groups are yet standardised, and not all the volcanic and tectonic relationships are well-characterised due to issues such as accessibility or less current activity.
Rift-jump model The evolution of the Icelandic volcanic rift zones can be explained by the rift-jump model.
Synform folding is expected to occur at the active rift axis. However, distinctive reversals in
dip directions are found in southwestern Iceland which indicate an
anticline. It is believed that the relative positions of the Icelandic hot spot and the active rift-spreading axis have changed with time. Assuming the Icelandic mantle plume is stationary, the spreading axis must have changed position. After the active spreading axis has moved away from the plume, the mantle plume would adjust the position of the axis and form a new rift closer to its centre. The migrated axis would gradually become extinct. The EVZ will eventually take over the WVZ according to the rift-jump process. The
Krafla central volcano is not distinctive within the volcanic rift zone. Fissure swarms of the Krafla spread away from the magma chamber and magma flows along the swarms to the north and south of the volcano. Eruptive fissures within the fissure swarms are most common within from the central volcanoes. Fractures within the fissure swarms are common at up to a distance of from the central volcano. North Iceland's last major rift relocation occurred about 6 to 7 million years ago when the northern, now-extinct Snæfellsnes-Húnaflói rift zone (SHRZ) It is the eastern boundary of the Hreppar microplate. Deformed structures, including the dominant northeast-trending eruptive fissure swarms and volcanic structures, Long
hyaloclastite ridges, formed by
subglacial eruptions during the
last glacial period, are distinctive structures in the EVZ. Compared with the WVZ, eruptive fissure swarms and hyaloclastite ridges are generally longer in the EVZ.
Western volcanic zone The Western volcanic zone (WVZ) is located to the north of the SISZ, where its northern end connects to the
Langjökull area. but is now regarded as waning in active rift activity as the hotspot shifts eastward,
Mid-Iceland belt The Mid-Iceland belt (MIB, Central Iceland volcanic zone, CIVZ, Mid-Iceland zone, Hofsjökull zone,
Snæfellsnes volcanic belt The Snæfellsnes volcanic belt (SVB, Snæfellsnes volcanic zone, The SVB erupted through the western aspects of the extinct SHRZ, which is a predecessor to the present MIB. The SHRZ formed when the WVZ had the hotspot directly under it, and existed prior to the last historic rift-zone jump. It is now known that the magma production/maturation timescale, at over 100,000 years, is an order of magnitude or more greater than that in other zones in Iceland, favouring fractional crystallization mechanisms as primary. and is mainly basaltic volcanism from sources such as
monogenetic cinder cones and isolated sub-glacial
tuyas such as Vatnafell rather than from the long fissures found in the rift zones. The most recent eruption in the belt occurred at Rauðhálsahraun in the Ljósufjöll volcanic system about 960.
Öræfi volcanic belt The Öræfi volcanic belt (ÖVB, Öræfajökull-Snæfell volcanic belt, Öræfajökull volcanic system, zone or belt, ÖVZ, There is some evidence from similarities in the compositional studies on Snæfell and the Upptyppingar subglacial volcano in the NVZ for the ÖVB being a flank zone. == See also ==