A fissure volcano is a type of volcano where the eruption occurs along an elongated crack in the Earth’s surface, rather than through a single, circular central vent. This linear feature, sometimes called a volcanic fissure, functions as the conduit for magma rising from the subsurface. Unlike the familiar, steeply-sloped cone shapes, fissure volcanoes typically do not build tall mountains because their eruptive material spreads out widely. Fissure eruptions result in unique landforms and effusive lava flows.
Distinctive Physical Characteristics
The defining feature of a fissure volcano is its vent geometry, which is a linear fracture that can be several meters wide and extend for many kilometers in length. Magma reaches the surface when a dike, a vertical sheet-like intrusion of molten rock, intersects the ground level. The magma involved in these eruptions is characteristically basaltic, meaning it has a low silica content. This chemical composition gives the lava a very low viscosity, or high fluidity, allowing it to flow easily and rapidly across the landscape. Instead, the repeated eruptions often result in a flat or gently sloped topography.
Geological Formation and Setting
Fissure volcanoes form in areas where the Earth’s crust is being stretched and pulled apart, a process known as crustal extension. This tensional stress allows for the formation of deep, parallel fault lines in the crust, which provide pathways for magma to ascend from deeper reservoirs. The most common setting for this process is at a divergent plate boundary, such as a mid-ocean ridge or a continental rift zone. As the tectonic plates move away from each other, pressure is released on the underlying mantle, which causes solid rock to melt and generate basaltic magma.
Eruption Style and Products
Eruptions from fissures are characterized by an effusive style, meaning they are relatively quiet and non-explosive. The highly fluid basaltic magma rises smoothly, allowing trapped gases to escape easily and preventing the buildup of pressure that causes explosive events. When the magma first emerges, it often creates a spectacular “curtain of fire,” a wall of incandescent lava fountaining along the length of the fissure. The fountaining lava fragments, called spatter, weld together upon landing, building low ridges known as spatter ramparts or small spatter cones directly along the crack. The majority of the erupted material flows away from the vent in broad sheets, traveling long distances, which over time leads to the formation of massive, flat geological features called flood basalts or lava plateaus, such as the Columbia River Basalts.