An effusive eruption is a type of volcanic eruption in which lava steadily flows out of a volcano onto the ground. Effusive eruption differs from explosive eruption, wherein magma is violently fragmented when expelled from a volcano. The shape of the lava flows created by effusive eruptions is governed by the type of lava, rate and duration of eruption, and slope of the surrounding areas.
A volcanic eruption is effusive when low-viscosity magma, usually basaltic in composition, is released from the Earth's crust. In an effusive eruption, gas escapes the magma as it erupts and forms lava that flows downhill continuously. This type of lava flow can build shield volcanoes, which are numerous in Hawaii.
Eruptions of basaltic magma often transition between effusive and explosive eruption patterns. The behavior of these eruptions is largely dependent on the permeability of the magma and the magma ascent rate. For an effusive eruption to occur, magma must be permeable enough to allow the expulsion of gas bubbles contained within it. If the magma is not above a certain permeability threshold, it cannot degas and will erupt explosively. Additionally, at a certain threshold, fragmentation within the magma can cause an explosive eruption. This threshold is governed by the Reynolds Number, a dimensionless number in fluid dynamics that is directly proportional to fluid velocity. Eruptions will be effusive if the magma has a low ascent velocity. At higher magma ascent rates, the fragmentation within the magma passes a threshold and results in explosive eruptions. Silicic magma also exhibits this transition between effusive and explosive eruptions, but the fragmentation mechanism differs. The 1912 Novarupta eruption and the 2003 Stromboli eruption both exhibited a transition between explosive and effusive eruption patterns.
Effusive basalt lava flows cool to either of two forms, ʻaʻā or pāhoehoe. Andesite lava typically forms blocky lava flows. Dacite lava flows often form steep-sided mounds, called lava domes, due to their greater viscosity.
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