A fissure volcano, or fissure vent eruption, occurs when lava erupts through an elongated break in the rock at the surface of the earth. Fissure vent eruptions are common in the mid-ocean ranges. Also known as shield volcanoes, they produce pillow basalt formations such as those that make up the Hawaiian Islands.
Shield volcanoes form little ash. The lava they release flows in all directions from the vent. The heat from the lava cause fires if it encounters vegetation. If the pressure builds in a fissure volcano, a wall of lava spews into the air. In volcanoes with less pressure, the lava bubbles to the surface and flows slowly to form new rock.
Magma forms deep in the earth. Because it is lighter than the materials at the earth's surface, it tends to rise. Underground, magma works its way through interconnected chambers toward the surface, seeking a weak spot in the earth's crust to escape. If magma finds a central vent, it escapes to create conical volcanoes. Small central vent volcanoes spew lava high into the air where it solidifies before hitting the ground, forming a cinder cone. Larger central vent volcanoes form lava cones that are eventually blown away by a volcanic explosion, such as what occurred with Mount St. Helens.