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.
Effusive eruptions occur when hot, (1200°C) low-viscosity 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 shield volcanoes like the ones in Hawaii.
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