The geological deformation of Iceland is the way that the rocks of the island of

Iceland Iceland ( is, Ísland; ) is a Nordic island country in the North Atlantic Ocean and in the Arctic Ocean. Iceland is the most sparsely populated country in Europe. Iceland's capital and largest city is Reykjavík, which (along with its s ...

are changing due to

tectonic Tectonics (; ) are the processes that control the structure and properties of the Earth's crust and its evolution through time. These include the processes of mountain building, the growth and behavior of the strong, old cores of continents k ...

forces. The geological deformation explains the location of earthquakes, volcanoes, fissures, and the shape of the island. Iceland is the largest landmass (102,775 km²) 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 diverg ...

. 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 or constructive plate boundary) located along the floor of the Atlantic Ocean, and part of the longest mountain range in the world. In the North Atlantic, the ridge separates the North Ame ...

and the Greenland-Iceland-Faeroe Ridge. It lies along the oceanic divergent plate boundary of

North American Plate The North American Plate is a tectonic plate covering most of North America, Cuba, the Bahamas, extreme northeastern Asia, and parts of Iceland and the Azores. With an area of , it is the Earth's second largest tectonic plate, behind the Pacific ...

and

Eurasian Plate The Eurasian Plate is a tectonic plate that includes most of the continent of Eurasia (a landmass consisting of the traditional continents of Europe and Asia), with the notable exceptions of the Indian subcontinent, the Arabian subcontinent and ...

. The western part of Iceland sits on the

and the eastern part sits on the

. The

Reykjanes Ridge The Mid-Atlantic Ridge is a mid-ocean ridge (a divergent or constructive plate boundary) located along the floor of the Atlantic Ocean, and part of the longest mountain range in the world. In the North Atlantic, the ridge separates the North Am ...

of the Mid-Atlantic ridge system in this region crosses the island from southwest and connects to the

Kolbeinsey Ridge The Kolbeinsey Ridge is a segment of the Mid-Atlantic Ridge located to the north of Iceland in the Arctic Ocean. It is bounded to the south by the Tjörnes Fracture Zone, which connects the submarine ridge to the on-shore Northern Volcanic Zone ri ...

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 made up of two stages: the Aquitanian and Burdigalian stages. The sub-epoch lasted from 23.03 ± 0.05 Ma to 15.97 ± 0.05 Ma (million years ago). It was prece ...

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 made up of two stages: the Langhian and Serravallian stages. The Middle Miocene is preceded by the Early Miocene. The sub-epoch lasted from 15.97 ± 0.05 Ma to 11.608 ± 0.005 Ma (million y ...

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 (from Ancient Greek: , ', "earth"; , ', "form"; and , ', "study") is the scientific study of the origin and evolution of topographic and bathymetric features created by physical, chemical or biological processes operating at or n ...

of Iceland are strongly influenced by the spreading plate boundary and the

Iceland hotspot The Iceland hotspot is a hotspot which is partly responsible for the high volcanic activity which has formed the Iceland Plateau and the island of Iceland. Iceland is one of the most active volcanic regions in the world, with eruptions occur ...

. 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 hot ...

underneath has uplifted the Iceland Basalt Plateau to as high as 3000 meters. The hot spot also produces high volcanic activity on the plate boundary. There are two major geologic and topographic structural trends in Iceland. One strikes northeast in Southern Iceland and strikes nearly north in northern Iceland. The other one strikes approximately west-northwest. 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 Hill, hills or Mountain, mountains, which will typically contain a river or stream running from one end to the other. Most valleys are formed by erosion of the land surface by rivers ...

dikes Dyke (UK) or dike (US) may refer to: General uses * Dyke (slang), a slang word meaning "lesbian" * Dike (geology), a subvertical sheet-like intrusion of magma or sediment * Dike (mythology), ''Dikē'', the Greek goddess of moral justice * Dikes ...

volcano A volcano is a rupture 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 often found where tectonic plates are ...

es,

graben In geology, a graben () is a depressed block of the crust of a planet or moon, bordered by parallel normal faults. Etymology ''Graben'' is a loan word from German, meaning 'ditch' or 'trench'. The word was first used in the geologic contex ...

s and

fault scarp A fault scarp is a small step or offset on the ground surface where one side of a fault has moved vertically with respect to the other. It is the topographic expression of faulting attributed to the displacement of the land surface by movement al ...

Deformation of Iceland

The geological deformation of Iceland is mainly caused by the active spreading of the

mid-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 diverge ...

. Extensional cracks and

transform fault A transform fault or transform boundary, is a fault along a plate boundary where the motion is predominantly horizontal. It ends abruptly where it connects to another plate boundary, either another transform, a spreading ridge, or a subductio ...

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 moon onto its surface. Lava may be erupted at a volcano or through a fracture in the crust, on land or un ...

to be erupted. On the surface of Iceland, linear

volcanic fissure A fissure vent, also known as a volcanic fissure, eruption fissure or simply a fissure, is a linear volcanic vent through which lava erupts, usually without any explosive activity. The vent is often a few metres wide and may be many kilom ...

s formed along the rifts and appear in a swarm-like pattern. They are connected by fracture zones, forming the volcanic zones.

Plate boundary deformation zone

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 100 km wide. It accumulates strain which come from rifting episodes and larger earthquakes. In southern Iceland, the block located along the plate boundary is identified as a

microplate A microplate, also known as a microtiter plate (''Microtiter'' is a registered trademark in the United States, therefore it should not be used generically without attribution), microwell plate or multiwell, is a flat plate with multiple "wells" ...

and is named the Hreppar Block. The deformation zone is relatively small since it has no significant evidence of active deformation, earthquakes or volcanism. The northern boundary of the block is linked to the Central Iceland Volcanic Zone (CIVZ), where diffuse volcanism occur. The southern boundary of the block is termed the South Iceland Seismic Zone, 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 ...

s can occur.

Transform fault zones

There are two major and active

s zones striking west-northwest in northern and southern Iceland. Two large fracture zones, associated with the transform faults, namely Tjörnes and Reykjanes Fracture Zones 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 Reykjanes Fracture Zones.

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) is a tectonically complicated area. It connects the North Iceland Volcanic Zone (NVZ) and the southern end of the

. This 50 km wide fracture zone is characterised by seismic activity, crustal extension and transform faulting. The volcanic fissure swarms of the Northern Volcanic Zone are connected to the southern end of Tjörnes Fracture Zone. For example, its southeast end is connected to the

Krafla Krafla () is a volcanic caldera of about 10 km in diameter with a 90 km 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 ...

fissure swarm. The main structural components of the Tjörnes Fracture Zone can be divided into three parts which trend from northwest to southeast, the Grímsey seismic zone, the Húsavík-Flatey fault zone and the Dalvík seismic zone. The Tjörnes Fracture Zone shows a huge spatial difference in seismic activity. For example, the westernmost part of the Tjörnes Fracture Zone shows seismic activity, but a few larger earthquakes (>M=5.5) also appear in the zone. The complexity in the Tjörnes Fracture Zone can be generally explained by the magmatic processes and plate motions. The velocity of the divergent plate motion, estimated to be 18.9 mm/year ±0.5 mm/year, is strongly affected by the Icelandic

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 (or Zones), is 75 to 100 km wide, and strikes northeast to southwest in southwestern Iceland. There are several approximately 40 km right lateral offsets of the ridge crest. The offsets create a transform fault zone connecting the Eastern Volcanic Zone and the

Reykjanes Reykjanes () is a small headland on the southwestern tip of the Reykjanes Peninsula in Iceland, giving the main peninsula its name. The region is about from Iceland's international airport. As the name means "smoking peninsula" connected to volc ...

. 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 South Iceland Seismic Zone. Since the transform motion in the South Iceland Seismic Zone is left-lateral, right-lateral faulting would occur and rotation of blocks would appear counterclockwise. The sequential occurrence of major earthquakes in the South Iceland Seismic Zone provided evidence of bookshelf faulting. Within a single event, earthquakes begin in the eastern part of the South Iceland Seismic Zone 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 piezometers. ...

. 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 ...

(~10 km) to the lithostatic boundary at 3 km 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, a large earthquake (M=6.6) occurred in the South Iceland Seismic Zone. During this event, 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 Tjörnes Fracture Zone.

Volcanic rift zones

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 superimpose ...

folding is expected to occur at the active rift axis. However, distinctive reversals in dip directions are found in south-western Iceland which indicate an

anticline In structural geology, an anticline is a type of fold that is an arch-like shape and has its oldest beds at its core, whereas a syncline is the inverse of an anticline. A typical anticline is convex up in which the hinge or crest is the ...

. 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. The spreading axis migrates westward at a rate of 0.3 cm/year. After the active spreading axis has moved away from the plume, the mantle plume would adjust the position of the axis and forms 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), and all of which are currently active. The volcanic rift zones cross the island from southwest to northeast. Each zone consists of 20–50 km wide belts and is characterised by active volcanoes, numerous normal faults, a high temperature geothermal field and fissure swarms. The Eastern Volcanic Zone will eventually take over the Western Volcanic Zone according to the rift jump process.

Northern Volcanic Zone

The 50 km wide 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

central volcano and its associated fissure swarms. The

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 20–30 km distance from the central volcanoes. Fractures within the fissure swarms are common at up to a distance of 70–90 km 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.

Eastern Volcanic Zone

The Eastern Volcanic Zone (EVZ) is located in south-east Iceland. It connects to the South Iceland Seismic Zone and NVC in its western and northern end 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 ice ...

Glacier area which is the accepted location of the Icelandic hot spot. Deformed structures, including northeast trending eruptive fissure swarms and normal faults, can be found in Eastern Volcanic Zone. Long hyaloclastite 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 volcan ...

s during the last glacial period, are distinctive structures in the Eastern Volcanic Zone. Compared with Western Volcanic Zone, eruptive fissure swarms and

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 margin ...

ridges are generally longer in the Eastern Volcanic Zone. 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 Eastern Volcanic Zone is geologically young, as mentioned above, the Eastern Volcanic Zone will eventually take over the Western Volcanic Zone according to the rift jump process model.

Western Volcanic Zone (WVZ)

The Western Volcanic Zone is located to the north of the South Iceland Seismic Zone, where its northern end connects to the Langjökull area. It has been the active propagating rift in the last 7 million years. Volcanic fissures and normal faulting are common features in the southern part of the Western Volcanic Zone. In the northern part of the Western Volcanic Zone, normal faulting is still common but volcanic fissures become less dominant. Shield volcanoes are also observed in this zone. Þingvellir Graben is clear evidence as proof of divergent plate movement in Iceland. It shows a clear extensional feature.

References

{{Iceland topics Geology of Iceland