An effusive eruption is an eruption dominated by the outpouring of lava onto the ground. 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. An effusive eruption is distinct from the violent fragmentation of magma by an explosive eruption.

For example, basalt lava may become a'a or pahohoe, and flow in deep narrow channels or in thin wide sheets. Andesite lava typically forms thick stubby flows, and dacite lava often forms steep-sided mounds called lava domes.