Iceland is a landmass formed entirely by volcanic forces and continues to be shaped by frequent activity. The island sits at the nexus of two powerful geological phenomena, resulting in a highly diverse and constantly evolving volcanic landscape. Its unique position has led to the development of distinct volcanic structures, ranging from linear cracks that spill vast amounts of lava to towering, ice-capped mountains capable of explosive eruptions.
The Geological Foundation of Icelandic Volcanoes
Iceland’s intense volcanic activity is a direct consequence of its location on a major boundary between tectonic plates. The island straddles the Mid-Atlantic Ridge, a massive underwater mountain range where the North American and Eurasian plates are slowly pulling apart. This process of divergence, known as seafloor spreading, creates a continuous rift in the crust, providing a pathway for magma to rise toward the surface.
This rifting mechanism alone would produce volcanic activity, but Iceland’s exceptional output is supercharged by a second feature: a mantle plume, or hotspot, located beneath the island. The mantle plume is an upwelling of abnormally hot rock from deep within the Earth, supplying a vast, sustained reservoir of magma. This dual geological setting—a spreading plate boundary combined with a robust magma source—ensures the continuous and voluminous supply of molten rock needed to form and sustain a wide variety of volcanic systems.
The plate divergence causes the crust to stretch and thin, which favors the formation of long, parallel fractures and fissures. Magma rising from the mantle plume exploits these weaknesses, creating entire volcanic systems that are linear rather than circular. This process allows volcanism to occur over a broad zone across the island, rather than being confined to a single point.
Fissure Systems: The Defining Icelandic Structure
The most characteristic volcanic feature of Iceland is the volcanic system, which is an elongated zone of rifting and magma accumulation. These systems are dominated by fissure vents, which are long, linear cracks in the Earth’s surface through which lava erupts. Magma usually rises through these fissures, creating a “curtain of fire” effect during the initial eruption phase.
Eruptions from these fissures are overwhelmingly effusive, producing highly fluid, basaltic lava that flows easily and rapidly across the landscape. The low viscosity of the magma allows it to travel great distances, building up extensive lava fields rather than steep cones. This is the mechanism behind flood basalts, where massive volumes of lava are extruded over a short period.
One of the most famous examples of this type is the Laki eruption of 1783–1784, which extruded an immense volume of lava from a fissure approximately 27 kilometers long. More recently, the eruptions on the Reykjanes Peninsula, such as the activity near Fagradalsfjall, have been characterized by effusive lava flows emerging from newly opened fissures. This linear style of volcanism is fundamentally tied to the ongoing rifting of the tectonic plates.
Volcanic systems are often composed of an entire swarm of parallel fissures, sometimes stretching for dozens of kilometers. The vast majority of historical volcanism in Iceland, and the creation of its extensive basaltic plains, is a result of these fissure eruptions along the rift zones. They represent the most direct expression of the divergent plate boundary.
Central Volcanoes: Shield and Composite Structures
While fissure vents dominate the rift zones, many of Iceland’s volcanic systems also contain a central volcano, which takes the form of a mountain. These central structures are classified as either shield volcanoes or composite volcanoes, depending on their shape and the composition of their erupted material.
Shield Volcanoes
Shield volcanoes are characterized by a very broad, gently sloping profile, resembling a warrior’s shield lying on the ground. This shape results from eruptions of highly fluid basaltic lava that travels far from the vent before cooling. These volcanoes are built up slowly over thousands of years by countless non-explosive lava flows originating from a central vent or short fissures on the flanks.
Composite Volcanoes
Composite volcanoes, also known as stratovolcanoes, are steeper, conical structures built from alternating layers of lava flows, ash, and fragmented rock (tephra). The magma feeding these volcanoes is often more viscous, containing higher gas content which leads to more explosive eruptions. Examples like Hekla and Eyjafjallajökull are prominent composite volcanoes.
The presence of glacial ice on many of Iceland’s composite volcanoes, such as Katla, increases their explosive potential. When hot magma interacts with the overlying ice or meltwater, it causes a violent steam explosion. This results in phreatomagmatic eruptions that produce large volumes of fine ash.