Plate Tectonic - Hazards (5)
Pre Lab 

  • Comparing volcanic and earthquake hazards.
  • Exploring why some volcanoes are more dangerous than others.
  • ash
  • lava
  • mudslide
  • viscous
  • volcanic hazard

Students learn how mudflows form.

The 1980 eruption of Mt. St. Helens, which triggered many mudflows.


Earthquakes and volcanoes cause damage not only to humans and their structures, but to the environment and wildlife. Earthquakes can cause damage by shaking,  tsunamiís, and landslides. Volcanoes can cause damage by ash flows, release of gases, mudflows, lava flows, and landslides.

Earthquakes may be associated with volcanic hazards, especially as a precursor to an eruption. However, earthquakes can cause disasters without the help of a volcano. Earthquakes generate seismic waves that can release great energy. But if the earthquake occurs in an area that is not populated, it is not a human disaster.

Volcanic damage occurs because of the products generated by an eruption. The type of eruption is important. For instance, a quiet lava eruption gives human enough time to evacuate an area. Violent eruptions that occur with little warning are sometimes difficult to avoid. Many people do not believe that volcanoes can cause much damage, and refuse to move.

Volcanoes erupt differently, depending on the composition and thickness of the erupting lava, the amount of gas in the parent magma, and force of the eruption. Volcanoes that erupt lava that is low in silica and gases tend to be "quiet," mostly pouring out streams of fairly fluid lava. Kilaeua volcano in Hawaii is a good example. Volcanoes that erupt silica-rich magmas, and that have a lot of gas, tend to be explosive. This produces tremendous clouds of volcanic ash, ash flows, and gases. The loose material produced by these eruptions often becomes the raw material for landslides or mudflows. can cause dangerous explosions as well as gentle lava flows.

  1. With the class, review that earthquakes and volcanoes are dangerous. Explain that every year, they cause damage not only to humans, but to the environment and wildlife as well. Earthquakes cause damage to structures that cannot withstand shaking. Volcanoes can cause dangerous explosions as well as gentle lava flows. Explain that this unit will concentrate on volcanic hazards. Remind the students that after an eruption, life will come back in due time.
  2. Show the class the slides of the aftermath of the 1980 Mt. St. Helens eruption and other disasters in the Volcano Slideshow. The remaining slides show different types of hazards, ranging from flowing lava to violent eruptions causing great plumes of ash and debris to explode from a volcano.
  3. Show the students the image below, or draw it on your board. Make sure that the students understand that water (melted snow) mixes with ash to form mudflows. Ask students if the shape of a volcano determines if the mud moves slowly or fast. The steeper the slopes of a volcano, the faster the mudflow will "ride down" its flanks. In the lab, students will learn that the more viscous (thicker) the material, the faster it will flow.

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