Plate Tectonic - Volcanoes (6)
Pre Lab 

  • Plotting volcanoes to help locate certain plate boundaries.
  • Comparing the structure of different types of volcanoes.
  • cinder cone
  • composite
  • crater
  • lava
  • magma
  • shield
  • vent

Students review different types of volcanoes.



Mt. St. Augustine in Alaska


The Plate Tectonic Cycle begins with the study of volcanoes. Plate Tectonics is a theory developed by geologists that explains the movements of the Earth’s crust and outer mantle. These two layers make up the Earth lithosphere or the outer shell of solid rock. The lithosphere is about 100 kilometers thick. Plate Tectonics also explains the origin of many geologic phenomena, including volcanoes and earthquakes. Conversely, the occurrence of both volcanoes and earthquakes provide data for understanding more about Plate Tectonics. In the sixth grade labs, students will focus on plotting data and interpreting results.

The theory of plate tectonics helps explain why and where volcanoes occur. The plates move, and interact at their edges, or boundaries. This creates melting, especially at convergent (where two plates come together) and divergent (where two plates move apart) plate boundaries, as shown in figure 1. . Magma is less dense than solid rock, so it will rise towards the Earth’s surface. When it reaches the surface, it causes an eruption.

However, some volcanic action, such as that in the Hawaiian Islands, does not fit the plate tectonics model. Hawaii is in the middle of the Pacific Plate, not at a plate boundary. This and other volcanoes that occur within the plate and not at the edges (intraplate) are probably caused by hotspots, which are magma sources in the Earth’s mantle below the plates. Hotspot magma generation is not fully understood. In the lab, the students will plot the locations of several volcanoes at convergent plate boundary and intraplate settings.

We distinguish three shapes of volcanoes. First, a shield volcano is composed of lava. The name shield describes the low, broad structure of the volcano, like an inverted shield. The Hawaiian volcanoes are shield volcanoes. Second, a cinder cone is a steeper structure and is composed of cinders (or finely pulverized rock) that were explosively erupted from the volcano. Finally, a composite volcano is composed of mixed layers of ash and lava. Composite volcanoes are frequently much higher than shield volcanoes, but not as steep as cinder cones. Mt. Shasta in California, and Mt. Fuji in Japan are examples of composite volcanoes.

  1. Review with the class that earthquakes and volcanoes provide data for geologists that can be used to interpret the Earth's processes, such as plate movement. Have the students locate the following states and countries on their map or globe: Oregon, Washington, California, Italy, Philippines, Japan, Hawaii, Iceland, Alaska, Indonesia, Arizona, Nicaragua, Mexico, and Chile. This will help them locate them quickly when they complete the lab.  You may want to use selected pictures from Volcano (slideshow).
  2. Discuss the 3 types of volcanoes by going over the diagrams shown below. See if students can name the parts of the volcanoes before you label them. Ask them to think about whether there is a distribution pattern of volcanoes or do these different types occur in the same places. In lab, they will discover there is no pattern that can be found. Let them discover this in lab. This lab is an example of a "null" hypothesis, a problem whose answer is no. 

  1. Review with students the different types of eruptions that they may have learned from previous years. The following pictures may be helpful.

Volcanic bombs

Magma with a high gas content will create vesicular volcanic rocks (with holes).

Lava from a quiet eruption.

Mt. St Helens violent eruption releasing ash and volcanic bombs. 

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