Iceland’s location on a mid-ocean ridge and above a mantle plume makes it a very active volcanic island. Eruptions take place every 3–6 years on average.
A volcano is considered active if it has erupted during the Holocene Epoch, i.e., within the last 11,000–12,000 years. It is thought that around 41 volcanic systems in Iceland remain active today (including submarine systems), collectively forming the active volcanic zones of Iceland. Iceland’s most active volcanic system is the Grímsvötn system in Southeast Iceland, which has erupted every decade on average for the past thousand years. Iceland’s largest system is the Bárðarbunga system, to the northwest of Grímsvötn. Many of the country’s largest lava fields originate from Bárðarbunga.
A volcanic system is a group of volcanoes that is believed to be connected to a shared magma chamber or store of magma in the Earth’s crust. Most volcanic systems consist of a central volcano, which is often the most active part of the system, and a fissure swarm that cuts through the central volcano. High-temperature geothermal fields are also often associated with central volcanoes. Central volcanoes erupt periodically over their lifetime, which can span millions of years. Volcanic systems in Iceland are often named after the central volcano within the volcanic system, such as Hekla and Katla. Volcanic systems with no clear central volcano usually bear the name of the associated geothermal system, such as Svartsengi and Reykjanes.
Volcanism in Iceland is diverse. Most eruptions involve either basaltic magma of low viscosity or viscous rhyolitic magma. Basaltic magma is more common and can erupt at any location within the volcanic system; rhyolitic magmas develop only in a central volcano, in connection with repeated magma intrusions.
The active volcanic systems of Iceland are often classified on the basis of rock series: tholeiitic, transitional alkalic, and alkalic (see map). A rock series is a series of distinct types of rock that share a common origin, which is reflected in their composition. A volcanic system produces only rock that belongs to a single rock series. Tholeiitic systems are found along the Mid-Atlantic Ridge, where the North American Plate and the Eurasian Plate are diverging. Alkalic and transitional alkalic systems develop in volcanic zones outside the active spreading ridge.
The chemical composition of the magma affects an eruption’s behaviour, as does the volcano’s environment. Eruptions in Iceland may be subglacial or submarine, or they may occur on land as shield, effusive, and phreatic eruptions. Basaltic eruptions on dry land tend to erupt passively: lava flows steadily from the volcano, eventually forming lava fields. Basaltic magma erupts explosively in wet environments (e.g., under a glacier or in water), forming tephra. The same is true of rhyolitic magma, but the high gas content and viscosity of rhyolitic magma can produce explosive eruptions even on dry land. Underwater and subglacial eruptions produce ridges and piles of tephra that can consolidate into hyaloclastites. Eruptions tend to evolve from one type to another. The Surtsey eruption in 1963–1967 is an example of an eruption that evolved from an explosive eruption to an effusive one.