Iceland is one of the most volcanically active places on Earth, with nearly thirty systems having erupted within the Holocene epoch. Determining the “type” of volcano Iceland represents is complex because the island is not a single geological structure but a massive volcanic province. Its dynamic geography arises from a continuous interaction between plate tectonics and a powerful magmatic source. Consequently, Iceland hosts a diverse range of volcanic forms and eruption styles, meaning it does not fit neatly into one simple classification.
Iceland’s Unique Tectonic Foundation
Iceland’s volcanism stems from a rare dual geological setting where a divergent plate boundary intersects with a deep-seated mantle plume. The island sits directly atop the Mid-Atlantic Ridge, where the North American and Eurasian tectonic plates are pulling apart. This continuous rifting causes decompression melting in the mantle, allowing magma to rise and fill the space created by the separating plates. The plates are currently diverging at a rate of approximately 2.5 centimeters per year, constantly widening the Atlantic Ocean and splitting the island.
Adding to this extensional environment is a powerful mantle plume, often called the Iceland hotspot, which channels a large volume of hot rock toward the surface. This deep thermal source explains why Iceland is a large landmass emerging from the ocean, unlike most submerged segments of the Mid-Atlantic Ridge. The combined effect of rifting and the plume’s excessive magma supply results in the copious quantities of basaltic material that have built the island. This mechanism provides the high magma flux necessary to sustain the island’s varied and frequent volcanic activity.
Dominant Volcanic Structures and Lava Types
The immense volume of magma generated by Iceland’s foundation manifests primarily through effusive eruptions, where material flows out rather than exploding violently. This behavior is dictated by the magma’s composition, which is overwhelmingly low-viscosity, iron-and-magnesium-rich basalt. The fluid nature of this lava allows it to travel long distances, creating characteristic broad, low-relief landforms.
A defining characteristic of Icelandic volcanism is the prevalence of fissure eruptions, where magma reaches the surface along linear cracks rather than a single central vent. These fissures can extend for many kilometers and produce spectacular “curtains of fire,” forming long, elongated volcanic ridges. Recent activity on the Reykjanes Peninsula, for instance, has been characterized by these systems, where fresh lava flows continuously from a crack in the ground.
Iceland hosts numerous shield volcanoes, which are broad, gently sloping mountains built up over repeated effusive eruptions of fluid basalt. These structures, like Skjaldbreiður, resemble a warrior’s shield lying on the ground because the lava spreads out easily. The island also features central volcanoes, such as Hekla and Katla, which are more cone-shaped stratovolcanoes developing over a long-lived magma plumbing system. While these central volcanoes can produce more varied and explosive eruptions, their foundational material remains largely basaltic.
Subglacial Volcanism and Hydrological Interactions
A distinct factor in Icelandic volcanism is the extensive interaction between magma and the island’s significant ice cover, including major ice caps like Vatnajökull. When an eruption occurs beneath a glacier, the intense heat rapidly melts vast quantities of ice, drastically changing the eruption style from effusive to explosive. This process results in phreatomagmatic eruptions, where the sudden cooling and fragmentation of the lava by water creates large amounts of ash and fragmented rock known as hyaloclastite.
This interaction also generates unique landforms, most notably the tuyas, which are flat-topped, steep-sided mountains. Tuyas form when lava erupts into an ice-bound cavity or lake. The meltwater prevents the lava from spreading, causing it to pile up and consolidate into a distinctive table-like shape. These structures, along with hyaloclastite ridges, are common features across the island, marking locations of past subglacial activity.
The most immediate hazard resulting from this ice-magma interaction is the jökulhlaup, or glacial outburst flood. The heat from a subglacial eruption can melt several cubic kilometers of ice, releasing torrents of water that accumulate under the ice cap. When this meltwater breaks free, it surges across the landscape, transporting enormous volumes of sediment and reshaping river plains. These floods are a frequent occurrence, with historical records showing jökulhlaups transporting sediment loads of up to 100 million tons in a single event.