Plate Tectonic - Volcanoes (3)
Pre Lab 

  • Comparing volcanic and plutonic igneous rocks.
  • Explaining why there are many kinds of volcanic rocks.
  • basalt
  • granite
  • igneous
  • minerals
  • obsidian
  • volcano

Students look at different volcanic rocks

A lava fountain eruption in Hawaii


Volcanoes are structural evidence of plate tectonics. They occur because the Earth's crust is moving, allowing molten rock, or magma, from below to rise through cracks in the crust to the Earth's surface. The reason why magma rises is simple. It is less dense than the surrounding rock, so it moves upward, just like warm air rises toward the ceiling in a room. Magma is molten rock that has not reached the surface of the Earth. "Lava" is molten rock that has reached the surface. When lava cools down it forms volcanic rocks, when magma cools (without reaching the surface) it forms plutonic rocks. Volcanic and plutonic rocks are both types of igneous rocks.

There are many different types of volcanic and plutonic igneous rocks. However, they all have in common that they were once molten, and have since cooled down and become solid. Igneous rocks look different because of two factors: (1) they have cooled at different rates and (2) the "Mother" Magma (original melted rock) was a different composition. Geologists then use these factors to name igneous rocks. For example, magmas cooling at different rates develop different sized crystals. Quick cooling (hours to years) igneous rocks have small crystals. For example, basalt has small crystals that can be seen under a microscope, inferring that basalt cooled quickly. Obsidian (volcanic glass) cooled so quickly that has virtually no crystals. Magma that cools slowly (thousands to millions of years) creates rocks with large minerals, like granite.

The parts of a volcano include a reservoir of magma inside the Earth, called a magma chamber. The magma chamber is connected to the surface of the Earth by a vent. The magma moves upward through the vent because it is less dense than the surrounding rock. It breaks through the surface of the Earth at the volcano’s crater, and may flow down its side as a lava flow. Note that magma generally does not form in the magma chamber. This magma is generated deeper in the mantle of the Earth, and ascends and collects in the magma chamber.

There are several stages in the life of a volcano. An volcano is active when lava is being extruded or the magma is moving upward from the magma chamber. An eruption is possible in an active volcano, although it may not have erupted recently. A dormant volcano is "sleeping". It can become active again, when new magma rises upward within it. An extinct volcano has no signs of becoming active. Usually the magma chamber has not been refreshed for thousands to millions of years. Volcanoes may repeatedly move between dormant and active phases.

  1. Introduce the Plate Tectonic Cycle to the students. You may want to use selected pictures from the Volcano (slideshow). Tell them that the outermost portion of the Earth moves causing stress within the Earth's crust. Preview the unit with them, telling them that in the upcoming weeks they will study rocks from volcanoes, how energy waves cause damage during an earthquake, how stress builds up in the Earth, and how volcanic hazards can harm people.
  2. Draw the diagram to the right on the board. Explain the parts of a volcano. Emphasize that plutonic rocks form underground, while volcanic rocks cool on the surface
  3. Show the students samples of basalt, obsidian and granite. Basalt and obsidian are volcanic rocks; granite is plutonic. Ask students how they can determine this. The answer is: plutonic rocks (such as granite) cool slowly in a relatively undisturbed environment permitting the growth of large mineral crystals which can easily be seen by the unaided eye. Volcanic rocks cool quickly, so the minerals do not have as much time to form large crystals, hence you cannot see minerals in a hand specimen of such rocks as basalt. Obsidian is essentially frozen magma; it has cooled so fast that almost no crystals have formed. It thus looks like dark glass. Tell the students that only basalt and obsidian come from a volcano, granite is formed deep within the crust and upper mantle of the Earth.

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