Iceland Hotspot and Powerful Geological Events Iceland sits atop the Mid-Atlantic Ridge tectonic plate boundary, which separates the Eurasian and North American plates. The ridge is an underwater mountain chain stretching for about 16,000 km along the north-south axis of the Atlantic Ocean. A rift valley running along its spine is formed by plate tectonics and is the center of new crust formation. Molten lava constantly rising from beneath the Earth's crust cools and is pushed out from the ridge's edges, widening the gap between continents.

Iceland was formed by the coincidence of the spreading boundary of the North American and Eurasian plates with a hotspot or mantle plume—an unusually hot upwelling of rock in the Earth's mantle. As the plates separate, excessive eruptions created volcanoes and filled rift valleys. Subsequent movements cut through these later lava fields, forming long, linear valleys bounded by parallel faults. The separation of the ridge began around 150 million years ago in the north and 90 million years ago in the south. These movements continue today with earthquakes, the reactivation of old volcanoes, and the formation of new ones. Iceland, due to extra volcanism from the hotspot underneath—the Iceland plume—is the largest island atop the ridge and is slowly moving northwest. Other islands in the Atlantic Ocean formed by volcanism along the Mid-Atlantic Ridge include the Azores, Bermuda, Madeira, Canary Islands, Ascension, St Helena, and Tristan da Cunha.

Volcanism in Iceland Because Iceland is on the Mid-Atlantic Ridge, it is divided by the movements of shifting tectonic plates. As the plates move, one to the east and the other to the west, both the North American and Eurasian systems drift northwest over the hotspot. Hotspots usually feature a layer of 20–100% molten rock at depths of 5–20 km, providing enough material for eruptions. Iceland is home to more than 100 volcanoes, about 35 of which have erupted in recent history. Volcanism in Iceland is attributed to the combination of Mid-Atlantic Ridge activity and hotspot activity. Eruptions occur roughly every 5–10 years and typically consist of basaltic lava and tephra. A few long-lived centers, such as Hekla, erupt more silicic magmas. The hotspot triggers eruptions in the southern volcanic zone, which includes volcanic systems such as Mt. Hekla, Vestmannaeyjar (Westman Islands), Katla caldera, Eyjafjallajökull, the Laki region, and the western subglacial part of Vatnajökull, where the Grímsvötn volcano is the most active.

Some of the most active new crust formation areas in southwest Iceland are open to tourists. The journey from Keflavík Airport to Reykjavík takes you along the edge where the North American plate meets the Eurasian plate. A drive to the nearby Þingvellir valley reveals an older part of the rift system, where the world's first parliament was located, and offers a wide panorama encompassing both sides of the plate boundary. A flight to Heimaey Island shows how new land is formed and illustrates the dangers of living in the path of a spreading rift.

Landforms Iceland can be divided into three regions by the age of its basaltic rocks. Tertiary flood basalts make up most of the northwest quarter of the island. The accumulation of these lava flows is at least 3,000 m thick. Quaternary flood basalts and hyaloclastites are exposed in the central, southwest, and eastern parts of the island. Quaternary rocks are cut by the neovolcanic zone, which contains active rifts where most of the active volcanoes are found. The rifts are topographic depressions bounded and filled by many faults. Fissure swarms constitute most of the neovolcanic zone. The swarms are 5–10 km wide and 30–100 km long. Rift regions have opened by about 30 m in the last 3,000–5,000 years. The neovolcanic zone covers about one-third of Iceland’s surface area. Nearly 60% of all regional fissure eruptions worldwide have occurred in Iceland.

Volcanic Eruptions Iceland is one of the world’s most volcanically active areas. It is estimated that one-third of the lava erupted worldwide since 1500 AD has been produced in Iceland. In the past 10,000 years, there have been 35 volcanoes that have erupted. Between 1900 and 1998, 11 volcanoes erupted: Krafla, Askja, Grimsvotn, Loki–Fögrufjöll, Bárðarbunga, Kverkfjöll, Esjufjöll, Hekla, Katla, Surtsey, and Heimaey. Most eruptions have come from fissures or shield volcanoes and featured basaltic lava flows.

During the last Ice Age, Iceland was covered by ice, and all eruptions were subglacial. Parts of the ice caps remain, and Iceland continues to experience many subglacial eruptions. Eighty-three percent of known subglacial eruptions worldwide occurred in Iceland. The eruption at Grimsvotn is one example. Subglacial eruptions produce a special type of volcano known as a table mountain or moberg mountain. The massive volumes of meltwater produced by subglacial eruptions can exit from beneath glaciers as gigantic glacial floods called jokulhlaups. The discharge can be up to 20 times the flow of the Amazon River.

The 1783 Laki eruption was the largest single basaltic lava eruption in history (14.7 cubic km). Recent eruptions include the Krafla eruptions of 1974–1984, a brief eruption at Hekla in 1991 and again on February 26, 2000, the 2010 Eyjafjallajökull eruption, and four Grímsvötn eruptions: in 1996, 1998, 2004, and 2011.