Fagradalsfjall is a volcanic system located on the Reykjanes Peninsula in southwestern Iceland, a region that had been volcanically quiet for nearly eight centuries before activity began in 2021. The volcano’s reawakening, marked by a series of eruptions starting in March 2021 and continuing sporadically, established it as a major focus of geological study and public interest. The classification of Fagradalsfjall is central to understanding the nature of these ongoing eruptions and the landscape they are creating.
The Volcanic Classification of Fagradalsfjall
Fagradalsfjall is primarily classified as a fissure vent system and is part of a wider, recently-activated volcanic complex. This system does not feature a single, large, central volcanic cone, but rather a series of linear fractures in the Earth’s crust from which magma erupts. The eruptions of 2021 and 2022 began with lava emerging from cracks, or fissures, that were initially hundreds of meters long.
This linear venting contrasts with the classic image of a cone-shaped stratovolcano, instead leading to the formation of a broad, gently sloping structure. Geologists suggest that the Fagradalsfjall system is currently in a phase that is characteristic of, or is a precursor to, forming a shield volcano. Shield volcanoes are built almost entirely from highly fluid lava flows that spread out over vast distances, creating a low-profile, dome-like structure resembling a warrior’s shield.
The activity has involved the repeated formation and collapse of small, temporary cones, called spatter cones, which focus the output from the underlying fissure. Over time, the continuous stacking of lava flows from these vents builds the gently sloped mass of a shield volcano.
The Tectonic Setting Driving the Activity
The existence and behavior of the Fagradalsfjall volcanic system are directly controlled by its location on the Mid-Atlantic Ridge. This ridge runs directly through Iceland, effectively splitting the country in two. Fagradalsfjall sits on the Reykjanes Peninsula, which is the onshore extension of this oceanic ridge.
The Mid-Atlantic Ridge is a divergent plate boundary, meaning the two tectonic plates on either side are moving away from each other. This is where the North American and Eurasian tectonic plates are pulling apart at a rate of approximately 18 to 19 millimeters per year. This slow, persistent separation creates immense tension in the crust, forming a rift zone characterized by parallel faults and open fissures.
The rifting process causes the crust to thin and fracture, which reduces the pressure on the underlying mantle rock. This pressure reduction allows the solid rock to melt, generating magma that is less dense than the surrounding material. The magma then exploits the deepest crustal weaknesses, rising through the linear fissure system to erupt at the surface.
Characteristic Eruption Style and Lava Composition
The eruption style at Fagradalsfjall is effusive, meaning the lava flows smoothly onto the surface rather than being violently ejected. This non-explosive behavior is a consequence of the magma’s chemistry and physical properties. The magma is basaltic in composition, which means it is low in silica content.
Low-silica basaltic magma has low viscosity, making it thin and fluid, similar to warm honey. This fluidity allows gas to escape easily and permits the lava to travel long distances over gentle slopes, which is why the eruptions are characterized by lava fountains and rivers of molten rock. The lava emerges at high temperatures, ranging from approximately 1,180 to 1,200 degrees Celsius, and is classified as olivine tholeiite basalt.
As the fluid lava cools, it forms different types of surfaces. The smooth, ropy texture known as pahoehoe often forms on slow-moving flows, while the blockier, rougher surface called a’a develops on faster or cooler flows. The effusive nature and easily flowing lava present a relatively low hazard level, primarily posing a risk from slow-moving lava flows and the release of gases like sulfur dioxide, which tends to accumulate in low-lying areas.