The geological deformation of Iceland is the way that the rocks of the island of Iceland are changing due to tectonic 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.[1] It is an elevated plateau of the sea floor, situated at the crossing of the Mid-Atlantic Ridge and the Greenland-Iceland-Faeroe Ridge.[2] It lies along the oceanic divergent plate boundary of North American Plate and Eurasian Plate. 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.[1]
Iceland is geologically young: all rocks there were formed within the last 25 million years.[3] It started construction in the Early Miocene sub-epoch, but the oldest rocks found at the surface of Iceland are from the Middle Miocene sub-epoch. Nearly half of Iceland was formed from a slow spreading period from 9 to 20 million years ago (Ma).[3]
The geological structures and geomorphology of Iceland are strongly influenced by the spreading plate boundary and the Iceland hotspot. The buoyancy of the deep-seated mantle plume 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.[1]
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, valleys, dikes, volcanoes, grabens and fault scarps.[3]
The geological deformation of Iceland is mainly caused by the active spreading of the mid-oceanic ridge. Extensional cracks and transform faults are found perpendicular to the spreading direction.[1] The transform-fault zones are also known as fracture zones. These fracture zones allow large volumes of lava to be erupted. 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.[3]
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.[1]
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.[1]
In southern Iceland, the block located along the plate boundary is identified as a microplate 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 earthquakes can occur.[1]
Transform fault zones
There are two major and active transform faults zones striking west-northwest in northern and southern Iceland.[4] Two large fracture zones, associated with the transform faults, namely Tjörnes and Reykjanes Fracture Zones are found striking about 75°N to 80°W.[3]
Bookshelf faulting
Fig 2. Bookshelf faulting mechanism: The transform fault is induced by strike-slip motion (sinistral movement) that is transverse to the fault zone. The blocks between the faults are slightly rotated afterwards, moving in a dextral movement.
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".[1]
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.[4] Most earthquake activity in Iceland is focused in the transform faulting zones near the north and south coast.
Transform fault zones
There are two major and active transform faults zones striking west-northwest in northern and southern Iceland.[4] Two large fracture zones, associated with the transform faults, namely Tjörnes and Reykjanes Fracture Zones are found striking about 75°N to 80°W.[3]
Bookshelf faulting
Fig 2. Bookshelf faulting mechanism: The transform fault is induced by strike-slip motion (sinistral movement) that is transverse to the fault zone. The blocks between the faults are slightly rotated afterwards, moving in a dextral movement
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