Plate Tectonic - Volcanoes (3)

  • Comparing rocks from different volcanoes.
  • Determining if volcanoes produce different types of rocks.
  • basalt
  • composite volcano
  • obsidian
  • pumice

Students look at rocks from different volcanoes.

Mono Craters


Volcanoes produce different types of volcanic rocks. The creation of volcanic rocks depends on the type of eruption and the source and composition of the magma erupted. The variety of volcanic rocks can range from ash to rocks with small minerals in them. Geologists describe volcanic rocks by their appearance and mineral composition. They examine both hand specimens, and usually make "thin sections", very thin slices of rock that they can examine under the microscope. They then name rocks based on their observations. For example, basalt (a dark gray to black rock) contains more minerals of olivine and hornblende than andesite (a gray rock). Andesite may contain quartz but basalt will not. A volcanic rock that had gas discharged with the eruption may have "holes" within the rock (called vesicles) while a quiet lava flow would not have such gas bubble holes. Some volcanic rocks start cooling inside the magma chamber, forming some larger minerals. When the volcano erupts, it brings up these larger minerals, as well as other lava which then cools into smaller crystals. In a hand sample of this kind of rock, you would see large minerals embedded in a dark matrix of smaller minerals that would require a microscope to see. Obviously, the naming of igneous rocks is complex. This lab begins to expose students to naming igneous rocks.

Mount Lassen in northern California

Clear Lake near the city of Clearlake

Although volcanoes can be close to each other, they may have different types of rocks. In this lab the students look at volcanics from Mt. Lassen, Mt. Shasta, Mono Craters, and Clearlake, which are all located in California.

  1. Discuss with students all the different types of volcanic rocks that can form from volcanoes.
  2. Locate the following areas where California volcanic rocks can be found: Mt. Lassen, Mt. Shasta, Clearlake, and Mono Lake (all in northern California). Explain that the Mt. Lassen, Mt. Shasta and Mono Craters volcanoes are sleeping (dormant volcanoes), and that the Clearlake volcanics are no longer active (extinct volcano).
  3. Instruct students to look at their igneous rock samples and describe them. Tell the students to use words like "black, gray, heavy, glassy, can see minerals", or any other descriptive terms. All of the specimens form a specific locality may not look exactly the same. The students' descriptions should reflect what they observe in the individual samples. Have them record their descriptions on the worksheet.
  4. Make a composite class description of the rocks. The descriptions should reflect a difference between the rock types. For example, samples from Mt. Lassen and Mt. Shasta should be heavier rocks and have many more visible minerals than those from Clear Lake or Mono Craters. The Clear Lake sample is obsidian and Mono Crater sample is a light pumice. Compare the samples as follows:
Clearlake Mono Craters Mt. Lassen Mt. Shasta
light color
can see minerals
  1. Next, have the students make a small model of Mt. Lassen using clay. Mt. Lassen is a composite volcano, composed of lava and ash layers. Use the presentation to help them construct the clay model. The diagram on the lab sheet gives the students clues for making the model. Make sure that they include a crater and a slope which is appropriate for a normal composite volcano. Students will use this model in the HAZARDS lab.

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