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

Effusive eruptions are a volcanic phenomenon; in some ways the opposite of explosive eruptions. An effusive eruption is characterized by an outpouring of low viscosity lava which has a fairly low volatile content. Usually, shield volcanoes have effusive eruptions. Basaltic magma erupted in an effusive manner may become an ʻaʻā or a Pāhoehoe and silicic magma blocky lavas and domes.