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5.24: Explosive Eruptions

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What would cause a giant explosion like this one?

The type of eruption a volcano has depends on the composition of the magma. Silica-rich, felsic magma can trap a lot of gas. So these magmas produce explosive eruptions. Silica-poor mafic magmas trap little gas. These magmas produce quiet eruptions. Which type of eruption do you think is safest to be near?

Eruptions

All volcanoes share the same basic features. First, mantle rock melts. The molten rock collects in magma chambers that can be 160 kilometers (100 miles) beneath the surface. As the rock heats, it expands. The hot rock is less dense than the surrounding rock. The magma rises toward the surface through cracks in the crust. A volcanic eruption occurs when the magma reaches the surface. Lava can reach the surface gently or explosively.

Explosive Eruptions

An explosive eruption produces huge clouds of volcanic ash. Chunks of the volcano fly high into the atmosphere. Explosive eruptions can be 10,000 times as powerful as an atomic bomb ( Figure below ). Hot magma beneath the surface mixes with water. This forms gases. The gas pressure grows until it must be released. The volcano erupts in an enormous explosion.

Eyjafjallajökull volcano in Iceland spewed ash into the atmosphere in 2010. This was a fairly small eruption, but it disrupted air travel across Europe for six days.

Ash and particles shoot many kilometers into the sky. The material may form a mushroom cloud, just like a nuclear explosion. Hot fragments of rock, called pyroclasts , fly up into the air at very high speeds. The pyroclasts cool in the atmosphere. Some ash may stay in the atmosphere for years. The ash may block out sunlight. This changes weather patterns and affects the temperature of the Earth. For a year or two after a large eruption, sunsets may be especially beautiful worldwide.

Volcanic gases can form poisonous, invisible clouds. The poisonous gases may be toxic close to the eruption. The gases may cause environmental problems like acid rain and ozone destruction.

Mt. St. Helens

In 1980, Mount St. Helens, located between Portland, Oregon and Seattle, Washington, erupted explosively. The eruption killed 57 people, destroyed 250 homes, and swept away 47 bridges. The volcano blew off its top so that it lost over 400 meters (1,300 feet) of height. Mt. St. Helens is still active ( Figure below ). Within the crater, a new lava dome formed.

(A) Mount St. Helens eruption on May 18, 1980. Mt. Adams is in the background on the right. (B) The eruption of Mt. St. Helens blew down acres of trees like they were toothpicks.

Much of the damage in the eruption was due to pyroclastic flows. Pyroclastic flows of pyroclasts and ash knock down everything in their path. The temperature inside a pyroclastic flow may be as high as 1,000 o C (1,800 o F). A pyroclastic flow at Montserrat volcano is seen in this video: http://faculty.gg.uwyo.edu/heller/SedMovs/Sed%20Movie%20files/PyroclasticFlow.MOV .

Lahars are mixtures of ash and water that travel down river valleys. On Mount St. Helens, the eruption melted glaciers. The water mixede with ash from the eruption and created tremendous lahars. The lahars raced down valleys and knocked out many bridges.

Mt St. Helens was not a very large eruption for the Cascades. Mt. Mazama blew itself apart in an eruption about 42 times more powerful than Mount St. Helens in 1980. Today all that remains of that huge stratovolcano is Crater Lake ( Figure below ).

Crater Lake fills the caldera of the collapsed Mt. Mazama. The bathymetry of the lake shows volcanic features such as cinder cones.

Crater Lake, located in Crater Lake National Park in southern Oregon.

Vocabulary

  • eruption : The release of magma and gases onto Earth's surface.
  • explosive eruption : A potentially devastating eruption of rock, lava, ash, and gas exploding from a volcano.
  • lahar : A volcanic mudflow containing ash, rock, and water from melting snow or rainfall that races down river valleys during an eruption.
  • pyroclast : Fire rock. A rock fragment that is blown from a volcano during an eruption.
  • pyroclastic flow : Hot ash, gas, and rock that race down a volcano’s slopes during an eruption.

Summary

  • Felsic magmas erupt explosively, creating pyroclastic eruptions.
  • Pyroclastic eruption types include tephra, ash, and lahars.
  • Mt. Mazama blew its top off and then collapsed, creating Crater Lake in Oregon.

Practice

Use this resource to answer the questions that follow.

Understanding Volcanoes: Mt. Saint Helens at http://dsc.discovery.com/tv-shows/discovery-presents/videos/understanding-volcanoes-mt-saint-helens.htm (1:40)

  1. What was the force of Mt. Saint Helens' eruption?
  2. How many people died in this eruption?
  3. How much ash was sent into the air?
  4. What type of eruption was it?
  5. What is the deadliest part of an eruption?

Review

  1. Why do convergent plate boundaries have explosive eruptions?
  2. Why do felsic magmas erupt explosively?
  3. How do volcanic gases affect the atmosphere?

Vocabulary

eruption

eruption

Release of magma and gases onto Earth's surface.
explosive eruption

explosive eruption

Potentially devastating eruption of rock, lava, ash, and gas exploding from a volcano.
lahar

lahar

Volcanic mudflow containing ash, rock, and water from melting snow or rainfall that races down river valleys during an eruption.
pyroclast

pyroclast

Fire rock; a rock fragment that is blown from a volcano during an eruption.
pyroclastic flow

pyroclastic flow

Hot ash, gas, and rock that race down a volcano’s slopes during an eruption.

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Difficulty Level:

Basic

Grades:

6 , 7

Date Created:

Jan 04, 2013

Last Modified:

Dec 15, 2014
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\n Front Matter\n

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\n Foreword\n

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\n The study of Earth Science is the study of Planet Earth. Middle School Earth Science Concepts presents Earth Science as a set of concepts, with each concept centered around a specific category, such as the Earth's crust or plate tectonics. Each concept is comprised of a series of lessons, with each lesson focusing on one specific topic.\n

\n

\n Preface\n

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\n Dedication\n

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\n Chapters\n

\n

\n \"\"\n

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\n \n
\n

\n Back Matter\n

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Original text