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The geological deformation of Iceland is the way that the rocks of the island of
Iceland Iceland is a Nordic countries, Nordic island country between the Atlantic Ocean, North Atlantic and Arctic Oceans, on the Mid-Atlantic Ridge between North America and Europe. It is culturally and politically linked with Europe and is the regi ...
are changing due to
tectonic Tectonics ( via Latin ) are the processes that result in the structure and properties of the Earth's crust and its evolution through time. The field of ''planetary tectonics'' extends the concept to other planets and moons. These processes ...
forces. The geological deformation help to explain the location of earthquakes, volcanoes, fissures, and the shape of the island. Iceland is the largest landmass () situated on an
oceanic ridge A mid-ocean ridge (MOR) is a seafloor mountain system formed by plate tectonics. It typically has a depth of about and rises about above the deepest portion of an ocean basin. This feature is where seafloor spreading takes place along a div ...
. It is an elevated plateau of the sea floor, situated at the crossing of the
Mid-Atlantic Ridge The Mid-Atlantic Ridge is a mid-ocean ridge (a Divergent boundary, divergent or constructive Plate tectonics, plate boundary) located along the floor of the Atlantic Ocean, and part of the List of longest mountain chains on Earth, longest mountai ...
and the Greenland-Iceland-Scotland ridge. It lies along an oceanic divergent plate boundary: the western part of Iceland sits on the North American Plate and the eastern part sits on the Eurasian Plate. The Reykjanes Ridge of the Mid-Atlantic ridge system in this region crosses the island from southwest and connects to the Kolbeinsey Ridge in the northeast. Iceland is geologically young: all rocks there were formed within the last 25 million years. It started forming in the
Early Miocene The Early Miocene (also known as Lower Miocene) is a sub-epoch of the Miocene epoch (geology), Epoch made up of two faunal stage, stages: the Aquitanian age, Aquitanian and Burdigalian stages. The sub-epoch lasted from 23.03 ± 0.05 annum, Ma to ...
sub-epoch, but the oldest rocks found at the surface of Iceland are from the
Middle Miocene The Middle Miocene is a sub-epoch of the Miocene epoch (geology), epoch made up of two Stage (stratigraphy), stages: the Langhian and Serravallian stages. The Middle Miocene is preceded by the Early Miocene. The sub-epoch lasted from 15.97 ± 0. ...
sub-epoch. Nearly half of Iceland was formed from a slow spreading period from 9 to 20 million years ago (Ma). The geological structures and
geomorphology Geomorphology () is the scientific study of the origin and evolution of topographic and bathymetric features generated by physical, chemical or biological processes operating at or near Earth's surface. Geomorphologists seek to understand wh ...
of Iceland are strongly influenced by the spreading plate boundary and the
Iceland hotspot The Iceland hotspot is a hotspot that is partly responsible for the high volcanic activity that has formed the Iceland Plateau and the island of Iceland. It contributes to understanding the geological deformation of Iceland. Iceland is one ...
. Although some have questioned if a hotspot is necessary to explain the observed deformations of Iceland, this is currently felt to best explain observations of rock composition and age obtained by modern techniques. The buoyancy of the deep-seated
mantle plume A mantle plume is a proposed mechanism of convection within the Earth's mantle, hypothesized to explain anomalous volcanism. Because the plume head partially melts on reaching shallow depths, a plume is often invoked as the cause of volcanic ho ...
underneath has uplifted the Iceland basalt plateau to as high as . The crust over the plume is also up to thick, which is much thicker than elsewhere in Iceland and a contrast to the minimum thickness of , which is a thickness more typical of oceanic divergent plate boundaries. The central part of Iceland is still lifting, but the current rate of crustal lifting of is mainly explained by glacial isostatic adjustment in response to the retreat of ice since 1890, which has removed much weight from the thick ice sheet. The hotspot also produces high volcanic activity on the plate boundary. There are two major geologic and topographic structural trends in Iceland. One strikes north-east in southern Iceland and strikes nearly north in northern Iceland. The other one strikes approximately west to north-west. Altogether they produce a zigzag pattern. The pattern is shown by faults, volcanic fissures,
valley A valley is an elongated low area often running between hills or mountains and typically containing a river or stream running from one end to the other. Most valleys are formed by erosion of the land surface by rivers or streams over ...
s,
dikes Dyke or dike may refer to: General uses * Dyke (slang), a slang word meaning "lesbian" * Dike (geology), formations of magma or sediment that cut through and across the layering of adjacent rocks * Dike (mythology), ''Dikē'', the Greek goddess ...
,
volcano A volcano is commonly defined as a vent or fissure in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface. On Earth, volcanoes are most oft ...
es,
graben In geology, a graben () is a depression (geology), depressed block of the Crust (geology), crust of a planet or moon, bordered by parallel normal faults. Etymology ''Graben'' is a loan word from German language, German, meaning 'ditch' or 't ...
s, and
fault scarp A fault scarp is a small step-like offset of the ground surface in which one side of a fault has shifted vertically in relation to the other. The topographic expression of fault scarps results from the differential erosion of rocks of contrastin ...
s.


Deformation of Iceland

The geological deformation of Iceland is mainly caused by the active spreading of the
mid-Atlantic ridge The Mid-Atlantic Ridge is a mid-ocean ridge (a Divergent boundary, divergent or constructive Plate tectonics, plate boundary) located along the floor of the Atlantic Ocean, and part of the List of longest mountain chains on Earth, longest mountai ...
. The Reykjanes Ridge south of Iceland comes ashore at
Reykjanes Reykjanes () is a small headland on the south-western end of the Reykjanes Peninsula in Iceland, giving the main peninsula its name. Volcanic action is responsible for forming the entire peninsula. The nearest town is Keflavik. The name, , trans ...
, where all of the extension of the northern Reykjanes ridge (NRR) is accommodated in the
Reykjanes Peninsula Southern Peninsula (, ) is an administrative unit and part of Reykjanesskagi (pronounced ), or Reykjanes Peninsula, a region in southwest Iceland. It was named after Reykjanes, the southwestern tip of Reykjanesskagi. The region has a populatio ...
. At
Reykjavík Reykjavík is the Capital city, capital and largest city in Iceland. It is located in southwestern Iceland on the southern shore of Faxaflói, the Faxaflói Bay. With a latitude of 64°08′ N, the city is List of northernmost items, the worl ...
, 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 fault A transform fault or transform boundary, is a fault (geology), fault along a plate boundary where the motion (physics), motion is predominantly Horizontal plane, horizontal. It ends abruptly where it connects to another plate boundary, either an ...
s are found perpendicular to the spreading direction. The transform-fault zones are also known as
fracture zone A fracture zone is a linear feature on the ocean floor—often hundreds, even thousands of kilometers long—resulting from the action of offset mid-ocean ridge axis segments. They are a consequence of plate tectonics. Lithospheric plates on eit ...
s. These fracture zones allow large volumes of
lava Lava is molten or partially molten rock (magma) that has been expelled from the interior of a terrestrial planet (such as Earth) or a Natural satellite, moon onto its surface. Lava may be erupted at a volcano or through a Fissure vent, fractu ...
to be erupted. The most productive volcanic region is located under the
Vatnajökull Vatnajökull ( Icelandic pronunciation: , literally "Glacier of Lakes"; sometimes translated as Vatna Glacier in English) is the largest and most voluminous ice cap in Iceland, and the second largest in area in Europe after the Severny Island i ...
glacier in the mid-east of Iceland, where all of about of extension is being accommodated near a plate
triple junction A triple junction is the point where the boundaries of three tectonic plates meet. At the triple junction each of the three boundaries will be one of three types – a ridge (R), trench (T) or transform fault (F) – and triple junctions can be ...
. On the surface of Iceland, linear volcanic fissures formed along the rifts and appear in a swarm-like pattern. They are connected by fracture zones, forming the volcanic zones. The Kolbeinsey Ridge assumes 100% of the divergence rate of measured near Akureki on the north coast of Iceland, which compared to the
vector Vector most often refers to: * Euclidean vector, a quantity with a magnitude and a direction * Disease vector, an agent that carries and transmits an infectious pathogen into another living organism Vector may also refer to: Mathematics a ...
in the south-east of Iceland is less and slightly more pointing to the north. Accordingly, in between, Iceland is being twisted slightly, and the tectonic structures are diverging more at the south than at the north. Deformation has also been influenced by the context of glaciation and its retreat from around 3.3 million years ago. The historic subglacial volcanic eruptions result on the exposure, after the recent ice retreat, of distinctive flat-topped landforms such as
tuya A tuya is a flat-topped, steep-sided volcano formed when lava erupts through a thick glacier or ice sheet. They are rare worldwide, being confined to regions which were covered by glaciers and had active volcanism during the same period. As lava ...
s, and rebound effects need to be accounted for in the seismic interpretations outlined below.


Plate boundary deformation zones

Crustal movements have created two plate-boundary deformation zones between the major plates, the North American Plate and the Eurasian Plate. In northern Iceland, the width of the deformation zone is about wide. It accumulates strain which comes from rifting episodes and larger earthquakes. This manifests as the Tjörnes fracture zone (TFZ) off the northern coast.


Hreppar microplate

In southern Iceland, the block located along the plate boundary is identified as a
microplate A microplate, also known as a microtiter plate, microwell plate or multiwell, is a flat plate with multiple "wells" used as small test tubes. The microplate has become a standard tool in analytical research and clinical diagnostic testing lab ...
, and is named the Hreppar block or Hreppar microplate. Its current independent motion relative to the major plates was confirmed by
GPS The Global Positioning System (GPS) is a satellite-based hyperbolic navigation system owned by the United States Space Force and operated by Mission Delta 31. It is one of the global navigation satellite systems (GNSS) that provide geol ...
measurement. Internal block deformation is negligible since it has no significant evidence of active deformation, earthquakes, or volcanism; a propagating rift origin from the eastern volcanic zone (EVZ) is proposed with the western volcanic zone (WVZ) being the receding rift. The northern boundary of the block is linked to the Mid-Iceland belt (MIB), where diffuse volcanism occurs. The southern boundary of the block is termed the South Iceland seismic zone (SISZ), where
strike-slip earthquake In geology, a fault is a planar fracture or discontinuity in a volume of rock across which there has been significant displacement as a result of rock-mass movements. Large faults within Earth's crust result from the action of plate tectonic f ...
s can occur.


Transform fault zones

There are two major and active
transform fault A transform fault or transform boundary, is a fault (geology), fault along a plate boundary where the motion (physics), motion is predominantly Horizontal plane, horizontal. It ends abruptly where it connects to another plate boundary, either an ...
zones striking west to north-west in northern and southern Iceland. 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''. Bookshelf faulting is an indicator of the young geological history of the fault zones. It is common in the SISZ and Reykjanes area.


Other evidence

Besides bookshelf faulting, the presence of the Icelandic fault zones are supported by seismological evidence. In Iceland, deformation usually concentrates over a zone of finite width. Thus, earthquakes usually occur along the active fracture zones between ridge crests. Most earthquake activity in Iceland is focused in the transform faulting zones near the north and south coast.


Tjörnes fracture zone

The Tjörnes fracture zone (TFZ, Tjörnes Volcanic Zone, TVZ) is a tectonically complicated area. It is perhaps best regarded as a trans-tensional zone with transform faulting and spreading. 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 Extensional tectonics is concerned with the structures formed by, and the tectonic processes associated with, the stretching of a planetary body's crust or lithosphere. Deformation styles The types of structure and the geometries formed depend on ...
, and transform faulting. The volcanic fissure swarms of the NVZ are connected to the southern end of TFZ. For example, its south-east end is connected to the
Krafla Krafla () is a volcanic caldera of about in diameter with a long fissure zone. It is located in the north of Iceland in the Mývatn region and is situated on the Iceland hotspot atop the Mid-Atlantic Ridge, which forms the divergent boundary ...
fissure swarm. The main structural components of the TFZ can be divided into three parts which trend from north-west to south-east: the
Grímsey Grímsey () is a small Icelandic island, off the north coast of the main island of Iceland, where it straddles the Arctic Circle. Grímsey is also known for the puffins and other sea birds which visit the island for breeding. The island is a ...
seismic zone, the Húsavík-Flatey fault zone and the Dalvík seismic zone. The TFZ shows a huge spatial difference in seismic activity. For example, the westernmost part of the TFZ shows seismic activity, but a few larger earthquakes (> M5.5) also appear in the zone. earthquakes have occurred in the Dalvík zone. Shear stress accumulated along the Húsavík-Flatey fault zone caused localized, tectonic rotation as evidenced by field and paleomagnetic observations. 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 A mantle plume is a proposed mechanism of convection within the Earth's mantle, hypothesized to explain anomalous volcanism. Because the plume head partially melts on reaching shallow depths, a plume is often invoked as the cause of volcanic ho ...
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 The Reykjanes Peninsula ( ) in southwest Iceland is the continuation of the mostly submarine Reykjanes Ridge, a part of the Mid-Atlantic Ridge, on land and reaching from Esjan, Esja in the north and Hengill in the east to Reykjanestá in the ...
. There is a significant change in the age and
lithology The lithology of a rock unit is a description of its physical characteristics visible at outcrop, in hand or core samples, or with low magnification microscopy. Physical characteristics include colour, texture, grain size, and composition. Lit ...
of the volcanoes in a north-south direction near Reykjanes Peninsula due to bookshelf faulting. Bookshelf faulting is common in the SISZ. Since the transform motion in the SISZ is left-lateral, right-lateral faulting would occur and rotation of blocks would appear counter-clockwise. The sequential occurrence of major earthquakes in the SISZ provided evidence of bookshelf faulting. Within a single event, earthquakes begin in the eastern part of the SISZ with larger magnitudes and end up with smaller magnitudes in western part of the zone. In the transform fault zones of Iceland, earthquakes usually occur on small scales (micro-earthquakes) due to plate straining and
pore fluid pressure Pore water pressure (sometimes abbreviated to pwp) refers to the pressure of groundwater held within a soil or rock, in gaps between particles ( pores). Pore water pressures below the phreatic level of the groundwater are measured with piezometer ...
. Pore pressure increase can induce seismicity. A large amount of pore fluid pressure migrates from the
brittle–ductile transition zone __NOTOC__ The brittle-ductile transition zone (hereafter the "transition zone") is the zone of the Earth's crust that marks the transition from the upper, more brittle crust to the lower, more ductile crust. For quartz and feldspar-rich rocks in c ...
at about to the at depth. Large-scale seismic activity is triggered if the pressure cannot pass through the transition zone. Small-scale earthquakes are also released locally in or above the migration path. In 2000, two large earthquakes at occurred in the SISZ. During these events, additional small—scale earthquakes concentrated narrowly and linearly around the transform fault planes.Stefánsson, R., Guðmundsson, G. B., & Roberts, M. J. (2006). ''Long-term and short-term earthquake warnings based on seismic information in the SISZ''. Veðurstofa Íslands. 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 The 2008 Iceland earthquake was a doublet earthquake that struck on 29 May at 15:46 UTC in southwestern Iceland.. The recorded magnitudes of the two main quakes were 5.9 and 5.8 , respectively, giving a composite magnitude of 6.1 . There were no ...
.


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 In structural geology, a syncline is a fold with younger layers closer to the center of the structure, whereas an anticline is the inverse of a syncline. A synclinorium (plural synclinoriums or synclinoria) is a large syncline with superimposed ...
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 In structural geology, an anticline is a type of Fold (geology), fold that is an arch-like shape and has its oldest Bed (geology), beds at its core, whereas a syncline is the inverse of an anticline. A typical anticline is convex curve, c ...
. 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. At least part of the spreading axis migrates at a rate of about . 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. There are three major volcanic zones in Iceland, which are the Northern, Eastern, and Western volcanic zones (NVZ, EVZ, WVZ), all of which are currently active. The volcanic rift zones cross the island from southwest to northeast. Each zone consists of belts and is characterised by active volcanoes, numerous normal faults, a high-temperature geothermal field, and fissure swarms. The EVZ will eventually take over the WVZ according to the rift-jump process.


Northern volcanic zone

The Northern volcanic zone (NVZ) is composed of five volcanic systems arranged zigzag-like along the mid-Atlantic plate boundary. It shows quite low seismic activity. The volcanic activity is confined to the
Krafla Krafla () is a volcanic caldera of about in diameter with a long fissure zone. It is located in the north of Iceland in the Mývatn region and is situated on the Iceland hotspot atop the Mid-Atlantic Ridge, which forms the divergent boundary ...
central volcano A central volcano is a type of volcano formed by basalts and silica-rich volcanic rocks. They contain very few or no volcanic rocks of intermediate composition, such that they are chemically bimodal volcanism, bimodal. Large silicic eruptions at c ...
and its associated fissure swarms. It accommodates the entire rifting of North Iceland and can be regarded at the present time to be in a steady state of spreading rate. There is a greater predominance of lava shields than in the other active rifting areas. The Icelandic
shield volcano A shield volcano is a type of volcano named for its low profile, resembling a shield lying on the ground. It is formed by the eruption of highly fluid (low viscosity) lava, which travels farther and forms thinner flows than the more viscous lava ...
es that produced these large lava fields here and in the WVZ did so in a single, almost-continuous eruptive process that is distinct from the repeated, discontinuous eruptions often seen in shield volcanoes elsewhere in the world and that would earn them the classification as a central volcano in the Iceland geological context. The
Krafla Krafla () is a volcanic caldera of about in diameter with a long fissure zone. It is located in the north of Iceland in the Mývatn region and is situated on the Iceland hotspot atop the Mid-Atlantic Ridge, which forms the divergent boundary ...
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. Fractures within the fissure swarms are generally subparallel to each other. Irregular fracture patterns are found where the Húsavík transform fault meets the fissure swarms, which indicates interaction between the fissure swarms and the strike-slip faults. The division between the NVZ and the EVZ is arbitrary, as the rifting structures are a single continuous structural identity. A potential boundary exists given the change in strike direction of the fissure swarms formed in the last 10 million years at latitude 64.7°, which was also followed by the 2014 dyke
intrusion In geology, an igneous intrusion (or intrusive body or simply intrusion) is a body of intrusive igneous rock that forms by crystallization of magma slowly cooling below the surface of the Earth. Intrusions have a wide variety of forms and com ...
from
Bárðarbunga Bárðarbunga (, alternative name Veiðivötn), is an active and productive stratovolcano located under Vatnajökull in Vatnajökull National Park which is Iceland's most extensive glacier. The second highest mountain in Iceland, above sea le ...
towards the north, but this might be crossed by volcanic activity originating either to the south or north. 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) shifted eastward to the new rift axis in the NVZ. There is with new dating techniques accurate understanding of SHRZ history from the northwestern region of Iceland to the northeastern that was not available when the SHRZ was first described.


Eastern volcanic zone

The Eastern volcanic zone (EVZ) is located in south-east Iceland and has high volcanic activity. It connects to the SISZ and NVC in its western and northern ends respectively. Seismic activity focuses in the
Vatnajökull Vatnajökull ( Icelandic pronunciation: , literally "Glacier of Lakes"; sometimes translated as Vatna Glacier in English) is the largest and most voluminous ice cap in Iceland, and the second largest in area in Europe after the Severny Island i ...
Glacier area which is the accepted location of the Icelandic hot spot. The EVZ started forming between 1.5 and 3 million years ago as a result of NVZ southward propagation. It is the eastern boundary of the Hreppar microplate. Deformed structures, including the dominant northeast-trending eruptive fissure swarms and volcanic structures, and some normal fault structures, can be found in the EVZ. Long
hyaloclastite Hyaloclastite is a volcanoclastic accumulation or breccia consisting of glass (from the Greek ''hyalus'') fragments (clasts) formed by quench fragmentation of lava flow surfaces during submarine or subglacial extrusion. It occurs as thin marg ...
ridges, formed by
subglacial eruption Subglacial eruptions, those of ice-covered volcanoes, result in the interaction of magma with ice and snow, leading to meltwater formation, jökulhlaups, and lahars. Flooding associated with meltwater is a significant hazard in some volcanic a ...
s 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. During the past
glacial period A glacial period (alternatively glacial or glaciation) is an interval of time (thousands of years) within an ice age that is marked by colder temperatures and glacier advances. Interglacials, on the other hand, are periods of warmer climate betw ...
, a huge volume of basaltic eruptions occurred, producing the long volcanic fissure swarms. The EVZ is geologically young; as mentioned above, the EVZ will eventually take over the WVZ according to the rift-jump model. The southern EVZ is propagating to the southwest through older Eurasian Plate crust. This results in active, sometimes explosive volcanism, and rifting in an off-rift region called the South Iceland Volcanic Zone (SIVZ).


Western volcanic zone

The Western volcanic zone (WVZ) is located to the north of the SISZ, where its northern end connects to the
Langjökull Langjökull (, Icelandic for "long glacier") is the second largest ice cap in Iceland (), after Vatnajökull. It is situated in the west of the Icelandic interior or Highlands of Iceland and can be seen clearly from Haukadalur. It covers the ...
area. It has been the active propagating rift in the last 7 million years, but is now regarded as waning in active rift activity as the hotspot shifts eastward, and with the re-classification of the active volcanoes of the Reykjanes Peninsula from the WVZ to the
Reykjanes volcanic belt The Reykjanes Peninsula ( ) in southwest Iceland is the continuation of the mostly submarine Reykjanes Ridge, a part of the Mid-Atlantic Ridge, on land and reaching from Esjan, Esja in the north and Hengill in the east to Reykjanestá in the ...
(RVB), which is now regarded as a trans-tensional zone with transform faulting and spreading. Structures related to normal faulting are much more apparent than in the younger EVZ. The WVZ initially took over as the principle plate boundary in Iceland about 6 million years ago from its predecessor the Snæfellsnes Rift, which then connected western Iceland to north Iceland. During this period of activity, it was known as the Reykjanes-Langjökull rift zone, which propagated to the southwest. Until 2 million years ago, the seafloor spreading around Iceland was accommodated by this rift zone, an ill-defined transverse connecting zone between it and the NVZ. As the EVZ became active, the Reykjanes-Langjökull rift zone became today's WVZ. The WVZ remains active despite being an ultra-slow spreading center with extension rates of , which is 20–30% of the total opening across south Iceland. The discontinuous failing of the WVZ compared to other described failing rifts is evidenced by, for example, the middle and northern parts of the WVZ having more recent volcanic activity than the southern parts, which is not as expected for a rift failing from the north. In the northern part of the WVZ, normal faulting is still common, but volcanic fissures become less dominant and active. The WVZ is the western boundary of the Hreppar microplate. Shield volcanoes are also observed in this zone. The
Þingvellir Þingvellir (, anglicised as ThingvellirThe spelling ''Pingvellir'' is sometimes seen, although the letter "p" is unrelated to the letter thorn (letter), "þ" (thorn), which is pronounced as "th".) was the site of the Althing, Alþing, the annual ...
Graben is evidence of divergent plate movement in Iceland. It shows a clear extensional feature. It is located to the north of the plate triple junction manifest as the intersection of the WVZ, RVB, and SISZ near the
Hengill Hengill () is a volcanic table mountain situated in the south-west of Iceland, to the south of Þingvellir. The volcano is still active, as evidenced by its numerous hot springs and fumaroles, but the last eruption occurred approximately 2,000&n ...
volcano.


Mid-Iceland belt

The Mid-Iceland belt (MIB, Central Iceland volcanic zone, CIVZ, Mid-Iceland zone, Hofsjökull zone, Hofsjökull volcanic zone, HVZ) of volcanoes connects the WVZ to the intersection region of the NVZ and EVZ. It is parallel to the north of the SISZ and has a relationship to its transform nature. There is a small component of extension, that induces local volcanism but has no obvious strike-slip movement. The extension component has been proposed to be caused by the opposite sense of rotation of crustal blocks to the north, which is in the Eurasian Plate, and the Hreppar microplate to the south. It is thus the northern boundary of the Hreppar microplate as already mentioned. Both volcanic and seismic activity has been low in the
Holocene The Holocene () is the current geologic time scale, geological epoch, beginning approximately 11,700 years ago. It follows the Last Glacial Period, which concluded with the Holocene glacial retreat. The Holocene and the preceding Pleistocene to ...
.


Intra-plate deformation

This is manifest as volcanic belts separate to the rift associated zones above. In the case of the Öræfi volcanic belt, it is considered to be an embryonic rift, a process likely to be repeating past events with the southern part of the NVZ as an embryonic rift millions of years ago, given rift-zone jumping.


Snæfellsnes volcanic belt

The Snæfellsnes volcanic belt (SVB, Snæfellsnes volcanic zone, SVZ) is an area of renewed intra-plate volcanism (North American Plate), less than 1.5 million years old. 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. The SHRZ had produced underlying crustal
tholeiitic The tholeiitic magma series () is one of two main magma series in subalkaline igneous rocks, the other being the calc-alkaline series. A magma series is a chemically distinct range of magma compositions that describes the evolution of a mafic ...
flood basalt A flood basalt (or plateau basalt) is the result of a giant volcanic eruption or series of eruptions that covers large stretches of land or the ocean floor with basalt lava. Many flood basalts have been attributed to the onset of a hotspot (geolo ...
s that are over 5 million years old. It is unknown if the SHRZ or hotspot interactions are the reason for the SVB, and this continues to be an area of study. Proposed mechanisms of magma production invoke
partial melting Partial melting is the phenomenon that occurs when a rock is subjected to temperatures high enough to cause certain minerals to melt, but not all of them. Partial melting is an important part of the formation of all igneous rocks and some metamorp ...
of hydrothermally altered, dominantly basaltic crust or
fractional crystallization Fractional crystallization may refer to: * Fractional crystallization (chemistry), a process to separate different solutes from a solution * Fractional crystallization (geology) Fractional crystallization, or crystal fractionation, is one of the ...
of primary basaltic magma, or both mechanisms. 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. The SVB comprises the stratovolcanoes of
Snæfellsjökull Snæfellsjökull (, ''snow-fell glacier'') is a 700,000-year-old glacier-capped stratovolcano in western Iceland. It is situated on the westernmost part of the Snæfellsnes peninsula. Sometimes it may be seen from the city of Reykjavík over Faxa ...
,
Helgrindur Helgrindur (, also known as Lýsuskarð, Lysuhóll or Lysukard) is a volcanic mountain range or massif in the middle of the Snæfellsnes peninsula that provides a backdrop to the port of Grundarfjörður. The volcano can be regarded as potentiall ...
(Lýsuskarð), and
Ljósufjöll Ljósufjöll () is a fissure vent system and central volcano on the Snæfellsnes Peninsula in Iceland. The name derives from the central volcano and translates into English as "Mountains of the Light". Geography The volcanic system has a lengt ...
in a peninsular east-to-west lineament, and is mainly basaltic volcanism from sources such as monogenetic
cinder cone A cinder cone or scoria cone is a steep, volcanic cone, conical landform of loose pyroclastic rock, pyroclastic fragments, such as volcanic ash, clinkers, or scoria that has been built around a volcanic vent. The pyroclastic fragments are forme ...
s and isolated sub-glacial
tuya A tuya is a flat-topped, steep-sided volcano formed when lava erupts through a thick glacier or ice sheet. They are rare worldwide, being confined to regions which were covered by glaciers and had active volcanism during the same period. As lava ...
s such as Vatnafell rather than from the long fissures found in the rift zones. These volcanoes have recently erupted small volumes of transitional to alkaline magmas relative to the less-evolved magmas and larger volumes from the rift zones. The underlying crust is thicker at about than in the active rift zone areas. Magma storage, in the studied regions of the belt, occurs just above the Moho, at about in the lower-to-mid crust, which is not usually the situation in rift zones, where magma chambers occur in the mid to shallow crust at about . The oldest volcanic rocks formed since the belt erupted transitional lavas through the basement tholeiitic magma series are at the mountain of Setberg to the north east of Grundarfjörður and near Elliðatindar, on each side of the present Helgrindur volcanic system. 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, Eastern flank zone) is to the east and parallel to the EVZ and NVZ. Its three component central volcanoes of
Öræfajökull Öræfajökull (; 'Öræfi glacier' or 'wasteland glacier') is an ice-covered volcano in south-east Iceland. The largest active volcano and the highest peak in Iceland at , it lies within the Vatnajökull National Park and is covered by part of ...
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Esjufjöll The subglacial Esjufjöll () volcano is located in the south-east part of the Vatnajökull icecap. Esjufjöll is a strict nature reserve (IUCN category Ia). Geography The recognisable mountains associated with the volcano are nunataks within Vatn ...
, and Snæfell are in a southwest-to-northeast trending lineament and have
rhyolite Rhyolite ( ) is the most silica-rich of volcanic rocks. It is generally glassy or fine-grained (aphanitic) in texture (geology), texture, but may be porphyritic, containing larger mineral crystals (phenocrysts) in an otherwise fine-grained matri ...
through to alkalic erupted basalts. 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

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Geology of Iceland The geology of Iceland is unique and of particular interest to geologists. Iceland lies on the divergent boundary between the Eurasian plate and the North American plate. It also lies above a hotspot, the Iceland plume. The plume is believed ...
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Volcanism of Iceland :''The volcano system in Iceland that started activity on August 17, 2014, and ended on February 27, 2015, is Bárðarbunga.'' :''The volcano in Iceland that erupted in May 2011 is Grímsvötn.'' Iceland experiences frequent volcanic activity, ...
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Geology of Reykjanes Peninsula The Reykjanes Peninsula ( ) in southwest Iceland is the continuation of the mostly submarine Reykjanes Ridge, a part of the Mid-Atlantic Ridge, on land and reaching from Esja in the north and Hengill in the east to Reykjanestá in the west. ...
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List of volcanic eruptions in Iceland This is an incomplete list of volcanic eruptions in Iceland. Please see External links below for databases of Icelandic eruptions which include over 530 events. ''For latest information about the current/ongoing series of eruptions near Grindav ...


References

{{Iceland topics Geology of Iceland