Mount Hekla, located in southern Iceland, is one of the nation’s most active and historically significant volcanoes. Its frequent and often violent eruptions earned it the medieval nickname, the “Gateway to Hell.” Hekla is classified as an active stratovolcano that developed uniquely along a deep, active fissure vent system. This combination results in a distinct, elongated ridge structure unlike the classic cone shape typically associated with stratovolcanoes. The volcano has erupted over twenty times since 1104, reshaping the landscape with both explosive ashfalls and extensive lava flows.
Hekla’s Structural Classification
Hekla is formally categorized as a stratovolcano, or composite volcano, meaning its structure is built up from alternating layers of hardened lava, volcanic ash, and other ejected materials. While traditional stratovolcanoes have steep slopes and a conical profile, Hekla’s shape is modified by its underlying geology. Instead of a single peak, Hekla presents as a prominent, elongated ridge that stretches approximately 10 kilometers.
This distinctive form results from the volcano being constructed directly over the Heklugjá, a long, active fissure vent system. The main fissure is about 5.5 kilometers long and cuts across the summit ridge. During major eruptions, magma surges up through this entire length, creating dramatic “curtains of fire” and depositing material along the ridge. This repeated activity along the linear vent explains why Hekla has an unusual, boat-like shape rather than a standard volcanic cone.
Eruptive Style and Magma Composition
Hekla’s eruptive style is highly variable, often featuring both explosive phases and extensive effusive lava flows within a single event. The most violent eruptions, such as the initial phase of the 1104 event, can be Plinian, characterized by enormous columns of ash and tephra reaching heights of up to 15 kilometers. This explosive behavior is driven by the magma’s intermediate composition, which includes basaltic andesite, dacite, and sometimes rhyolite.
This intermediate magma has a higher silica content (typically above 54% SiO₂), making it more viscous compared to the low-viscosity basalt common in Icelandic rift volcanism. The higher viscosity traps volatile gases, leading to a buildup of pressure released in explosive bursts. The longer the repose period between eruptions, the higher the silica content of the initial magma, resulting in a more explosive opening phase. Eruptions often begin explosively with silica-rich magma before transitioning into more effusive lava flows as deeper, less-evolved magma is tapped.
The Geological Context of Hekla
Hekla’s distinct characteristics result from its complex geological setting in Iceland. The volcano is situated at the southwestern boundary of the East Volcanic Zone (EVZ), part of the divergent Mid-Atlantic Ridge system. Hekla also lies near the intersection with the South Iceland Seismic Zone (SISZ), a transform fault. This tectonic junction is a highly active region, providing abundant pathways for magma to ascend.
The presence of intermediate to silicic magma is unusual for a rift zone typically dominated by basalt. This is explained by the influence of the Iceland mantle plume, or hotspot. The plume supplies magma that undergoes chemical differentiation while pausing in a shallow reservoir beneath Hekla. This fractional crystallization allows the magma to evolve, becoming richer in silica and volatiles, which drives Hekla’s explosive nature. Its location on the EVZ’s flank allows the formation of this central volcano, contrasting with the purely basaltic fissure eruptions common elsewhere on the main rift.