Effusive eruption

An ʻaʻā lava flow from Mauna Loa during its 1984 eruption.

An effusive eruption is a volcanic eruption characterized by the outpouring of lava onto the ground (as opposed to the violent fragmentation of magma by explosive eruptions). Lava flows generated by effusive eruptions vary in shape, thickness, length, and width depending on the type of lava erupted, discharge, slope of the ground over which the lava travels, and duration of eruption.

For example, basalt lava may become ʻaʻā or pāhoehoe. Andesite lava typically forms blocky lava flows, and dacite lava flows often form steep-sided mounds called lava domes due to their viscosity.

Time-lapse video of Kilauea flank vent eruption, 2005

Effusive eruptions occur when hot, (1200°C) runny magmas reach the surface of the Earth's crust. Dissolved gases escape easily as the magma erupts, forming lava that flows downhill quite quickly and easily. Effusive eruptions build up gently-sloping volcanoes like the ones in Hawaii.