The viscosity of the magma, however, is also an important factor in determining whether an eruption will be explosive or nonexplosive. A low-viscosity magma, like basalt, will allow the escaping gases to migrate rapidly through the magma and escape to the surface.

Basaltic eruptions Basaltic composition magmas are the most common effusive eruptions because they are not water saturated and have low viscosity. Most people know them from the classic pictures of rivers of lava in Hawaii. Eruptions of basaltic magma often transition between effusive and explosive eruption patterns.

Effusive eruptions occur when hot, (1200oC) runny basalt magmas reach the surface. Dissolved gases escape easily as the magma erupts, forming lava that flows downhill quite easily. Effusive eruptions build up gently-sloping Shield Volcanoes like Hawaii.

Basalt lava erupts from Pu’u ‘Ō’ō cone at Kīlauea Volcano, Hawai’i. Lava spilling from the cone has formed a series of effusive ‘A’ā lava channels and flows. The largest known effusive eruptions on Earth have paved hundreds to thousands of square kilometers of its surface with basaltic lava.

Effusive eruptions When the magma is viscous (or sticky), it can’t flow when it reaches the surface, so it builds up forming a lava dome. Aerial view of the Holuhraun lava field, Iceland September 2014. This eruption produced around 1.4 km3 of lava, over a period of six months, which covered an area of 84 km2.

The most active effusive volcano in the world today is Kilauea in Hawaii. Its most recent eruption is called the Pu’u ‘Ō’ō eruption. It lasted 35 years, starting on the 3 January, 1983. It ended in spectacular fashion in June 2018 with a collapse of the crater floor and the lava lake contained within it.

Part of Hall of Planet Earth. Most explosive eruptions occur in volcanoes above subduction zones, where one tectonic plate dives beneath the other. Eighty to 120 kilometers below the surface, magma forms when the rocks of the mantle melt just above the subducting plate.

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