8.6 Effusive Eruptions

Is Stromboli just a rolled-up pizza?

For most people a stromboli is a rolled sandwich of dough, cheeses, and meats. For volcanologists, Stromboli is a volcano for which a type of eruption was named. Strombolian eruptions spew lava into the air but do not explode as massively as in the plinian eruptions in the previous concept. Still, the power of a volcano is easily seen in this eruption on Mt. Stromboli in Italy.

Effusive Eruptions

Mafic magma creates gentler effusive eruptions. Although the pressure builds enough for the magma to erupt, it does not erupt with the same explosive force as felsic magma. Magma pushes toward the surface through fissures. Eventually, the magma reaches the surface and erupts through a vent (Figure below). Effusive eruptions are common in Hawaii, where lavas are mafic.

• A Quicktime movie with thermal camera of a lava stream within the vent of a Hawaiian volcano is seen here: http://hvo.wr.usgs.gov/kilauea/update/archive/2009/Nov/OverflightFLIR_13Jan2010.mov.

Types of Lava

Low-viscosity lava flows down mountainsides. Differences in composition and where the lavas erupt result in three types of lava flow coming from effusive eruptions. A’a lava forms a thick and brittle crust that is torn into rough and jagged pieces. A’a lava can spread over large areas as the lava continues to flow underneath the crust's surface. Pāhoehoe lava forms lava tubes where fluid lava flows through the outer cooled rock crust. Pāhoehoe lava is less viscous than a'a lava, so its surface looks is smooth and ropy. Mafic lava that erupts underwater creates pillow lava. The lava cools very quickly, forming roughly spherical rocks. Pillow lava is common at mid-ocean ridges (Figure below).

Effusive Eruptions Damage

People can usually be evacuated before an effusive eruption, so they are much less deadly. Although effusive eruptions rarely kill anyone, they can be destructive. Even when people know that a lava flow is approaching, there is not much anyone can do to stop it from destroying a building or road (Figure below).

Summary

• Mafic magma creates effusive eruptions. The pressure builds but the lava does not explode so violently from the vent.
• Effusive eruptions cause damage but usually people can be evacuated, so there are few or no fatalities.
• Mafic magma cools into different types of flows like a'a, pāhoehoe, and pillow lava.

Review

1. Why do mafic lavas flow rather than explode?
2. Compare and contrast a'a and pāhoehoe lavas.
3. How do pillow lavas form?

Explore More

1. Why is Kilauea considered one of the most active volcanoes in the world?
2. What does the height of Kilauea’s lava lakes mean regarding upcoming eruptions?
3. Where does the lava go?
4. What do shield volcano eruptions look like?
5. Why is it difficult to predict a volcanic eruption from a volcano like Kilauea?
6. What would scientists need to know to better be able to predict eruptions from Kilauea?

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