BACKGROUND:
The surface of the Earth seems to be divided into
water and land. Islands look disconnected, and many children even think that
they are floating on the water. Many books describe plate tectonics as if
the plates are the continents. This is not true. The continents are embedded
in the plates. Many continents occur in the middles of plates, not at their
boundaries or edges. Plates also underlie the Earth’s oceans. A single plate
often includes both continental and oceanic regions. It is important that
students begin to visualize or understand that the plates are a solid rock
shell which includes both dry land and the "land" underneath the oceans.
Plates are composed of the Earth’s crust and upper
mantle, which are collectively called the lithosphere. This layer is like an
eggshell compared to the total thickness of the Earth. Plates do not extend
all the way to the center or the Earth.
All of the plates are moving. They are slow, moving at
speeds of centimeters to tens of centimeters per year. They slide along on
top of an underlying mantle layer called the asthenosphere, which contains a
little magma (molten rock).
The plates are layers of rigid, solid rock. However,
as they move, plates interact at their edges or boundaries. There are three
basic directions or types of boundary interactions. In some places, two
plates move apart from each other; this is called a diverging plate
boundary. Elsewhere two plate move together; this is a converging plate
boundary. Finally plates can also slide past each other horizontally. This
is called a transform plate boundary. Volcanoes and earthquakes help define
the boundaries between the plates. Volcanoes form mostly at converging and
diverging plate boundaries, where much magma is generated. Earthquakes occur
at all three types of boundaries. Because the plates are rigid, they tend to
stick together, even though they are constantly moving. When the strength of
the rocks at the plate boundary is exceeded, they move rapidly, "catching
up" with the rest of the plates. We feel this release of energy as an
earthquake.
PROCEDURE:
- The key concept for the Post Lab is to make sure that the students do
not confuse the continents with the plates. The continents are part of
the plates. Point out the continents to the class on a world map. Have
the students repeat the names of the continents. Keep repeating to them
that the continents have moved by riding on the backs (or tops) of
plates for millions and millions of years.
