Effusive eruption

A volcanic eruption is effusive when the erupting magma is volatile poor (water, carbon dioxide, sulfur dioxide, hydrogen chloride, and hydrogen fluoride), which suppresses fragmentation, creating an oozing magma which spills out of the volcanic vent and out into the surrounding area.

[8] This type of lava flow builds shield volcanoes, which are, for example, numerous in Hawaii,[9] and is how the island was and currently is being formed.

Degassing prior to eruption, through fractures in the country rock surrounding the magma chamber,[12] plays an important role.

For silicic magmas to erupt effusively, the ascent rate must be 10−5 to 10−2 m/s, with permeable conduit walls,[4] so that gas has time to exsolve and dissipate into the surrounding rock.

If the flow rate is too fast, even if the conduit is permeable, it will act as though it is impermeable[4] and will result in an explosive eruption.

[16] If a dome forms and crystallizes enough early in an eruption, it acts as a plug on the system,[16] denying the main mechanism of degassing.

If this happens, it is common that the eruption will change from effusive to explosive, due to pressure build up below the lava dome.