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Effusive Eruptions

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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.

In effusive eruptions, lava flows readily, producing rivers of molten rock

In effusive eruptions, lava flows readily, producing rivers of molten rock.

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 ).

Pictures of A'a, pahoehoe, and pillow lava

(a) A’a lava spread over large areas. (b) Pāhoehoe lava tubes where at the Thurston Lava Tube in Hawai'i Volcanoes National Park. (c) Pāhoehoe lava is less viscous than a'a lava so its surface looks is smooth and ropy. (d) Pillow lava.

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 ).

Road covered by lava from an eruption at Kilauea in Hawaii

A road is overrun by an eruption at Kilauea volcano in Hawaii.

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.

Making Connections

Explore More

Use these resources to answer the questions that follow.

https://www.youtube.com/watch?v=6uFvNJCpl14

  1. What are you viewing early in the video?
  2. What is the source of the thunder-like cracking and booming?
  3. What is causing the rise in the lava lake?
  4. How does a fissure eruption differ from a lava lake eruption?
  5. How does the eruption in this video differ from the eruption in the Mount St. Helens video in the previous concept?

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?

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