BACKGROUND:
Volcanic eruptions provide clues for understanding
what is happening to the outer part of the Earth. Volcanoes are evidence
that the Earth is restless, especially within the crust and upper mantle.
The source of the molten rock, which geologists call magma, is actually not
the center of the Earth, but primarily the top 100 km (crust and upper
mantle) of the planet. Since we cannot drill holes this deep, volcanic rocks
provide important information about rocks and processes on the outer portion
of the Earth.
Most volcanoes help geologist define the edges of
plate boundaries. The plates are the outer, rigid, solid rock skin of the
Earth. They are composed of the Earth’s crust and upper mantle. The plates
move through time. Plates have moved together, away, and slide past each
other. These motions generate volcanoes and earthquakes. Volcanoes are most
common where plates come together (converging) and pull apart (diverging).
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 one or more vents. The magma moves upward
through the vent because the magma is less dense than the surrounding rock.
It breaks through the surface of the Earth at the volcano’s crater. If the
lava is liquid enough it will cause a lava flow.
There are many different types of volcanoes,
reflecting the different compositions of magma and types of eruptions.
Different vulcanologists (people who study volcanoes) sometimes classify
them differently. In our scheme we use the simplified U.S. Geological Survey
classification.
A volcano that is composed completely of lava is
called a shield volcano. Kilauea in Hawaii is an example. A composite
volcano, like Mt. St. Helens in Washington is composed of lava and other
volcanic rocks like scoria, ash, and volcanic breccia that are formed by
explosive eruptions. A cinder volcano is composed of just scoria (cinder is
an old term for scoria) and ash.
PROCEDURE:
- Inform students that this lesson will start a unit on Plate Tectonics,
or the study of earthquakes, volcanoes, and moving plates. Ask the
students if they remember what a plate is. If they do not know, tell
them that over the next few weeks they will learn! Explain that a plate
is a part of the crust and upper mantle that "floats" on a
layer of more liquid rock (asthenosphere). Remind students that the
plates move, and that plate motion causes volcanoes, earthquakes, and
mountain ranges. This happens most often at the edges of plates, at
plate boundaries.
Draw
the picture on the right. Explain the different parts of a volcano to
the class. You can also refer to parts of the Volcano (slideshow) to
show appropriate pictures.
Have the students copy the words onto the picture on their worksheet.
Have them color their pictures. Ask them to use their imaginations. Hang
their completed pictures around the classroom.
- The main concept for the students to learn is that hot molten rock is
called magma when it is inside the Earth, and that when the hot molten
rock reaches the surface of the Earth it is called lava. They should
also learn the other parts of a volcano described above.
- Ask the children if there are any major chemical differences between
magma and lava from the same volcano. The answer is "no."
Geologists give magma and lava separate names to signify whether the
molten rock is located inside or outside of the earth. The difference
between magma and lava is apparent when the molten rock cools. When
magma cools inside the Earth, it has large minerals, because it cools
slowly. Lava has very small minerals because it cools quickly.
- To help the students understand the difference in appearance between
lava and magma, use an analog that they can relate to. For example, an
unfrosted chocolate cake looks different on the inside than on the
outside. The inside of the cake is airy and full of holes, this can be
related to magma. The outside of the cake is solid and dry, this can be
related to lava. However, even though the inside and outside appear
different, they both come from the same recipe, as did both the magma
and lava.
