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Effusive eruption differs from explosive eruption, wherein magma is violently fragmented and rapidly expelled from a volcano. Effusive eruptions are most common in basaltic magmas, but they also occur in intermediate and felsic magmas.
Mafic magma creates gentler, effusive eruptions. There is little or no gas. The lava is thin and fluid so that it flows over the ground like a river. Mafic magma pushes toward the surface through fissures. Eventually, the magma reaches the surface and erupts through a vent (Figure below).
Volcanic eruptions can be explosive, sending ash, gas and lava high up into the atmosphere, or the magma can form lava flows, known as effusive eruptions. Whether an eruption is explosive or effusive largely depends upon the amount of gas in the magma.
In general, eruptions can be categorized as either effusive or explosive. Effusive eruptions involve the outpouring of basaltic magma that is relatively low in viscosity and in gas content. Explosive eruptions generally involve magma that is more viscous and has a higher gas content.
An eruption dominated by the outpouring of lava onto the ground is often referred to as an effusive eruption (as opposed to the violent fragmentation of magma by explosive eruptions).
On the one extreme there are effusive Hawaiian eruptions, which are characterized by lava fountains and fluid lava flows, which are typically not very dangerous. On the other extreme, Plinian eruptions are large, violent, and highly dangerous explosive events.
A second type of volcanic eruption is a non-explosive or effusive eruption (Figure 8.11). Because the composition of magma is different in different volcanoes, the properties of the lava are different. In effusive eruptions, lava flows are relatively calm and do not explode out of the volcano.
The most common type of volcanic eruption occurs when magma (the term for lava when it is below the Earth's surface) is released from a volcanic vent. Eruptions can be effusive, where lava flows like a thick, sticky liquid, or explosive, where fragmented lava explodes out of a vent.
Effusive eruptions occur when hot, (1200 o C) 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.
An effusive eruption is a volcanic eruption characterized by the relatively gentle flow of lava onto the Earth's surface, as opposed to explosive eruptions that violently expel ash, gas, and pyroclastic materials.