BACKGROUND:
There are two types of igneous rocks, based on rates of cooling.
Volcanic
rocks cool quickly, and form on the Earth’s surface around volcanoes.
These are sometimes referred to as "lava rocks." Lava refers to
the molten rock, or magma, which cools to make volcanic rock.
The second
type of igneous rock is plutonic rock, which forms from the slow cooling of
magma within the crust of the earth.
PROCEDURE:
- In your kit you have samples of the volcanic rock rhyolite and the
plutonic rock granite. Show the rocks to the children and have them
describe them. Both rocks are light in color, but one of the rocks has
large gray, white, and black minerals. This is granite. The other rock
has very small minerals that cannot be seen with the naked eye; this is
rhyolite. Under a microscope the children may be able to see very small
minerals, but not as large as those in the granite. Use the
magnifier to look at the specimens closely.
The reason that one rock has large minerals (granite) is that it
cooled much slower (by thousands of years) than the other (rhyolite).
Volcanic rocks tend to be fine grained (minerals are small) and plutonic
rocks tend to be coarse grained (large minerals). This is a
generalization, but is basically correct.
- Have students color the worksheet. Tell students that there are many
igneous rocks being formed in currently erupting volcanoes.
- Glue some of the large grained sand in the
plutonic area of the volcano they colored. The small grained sand should
be glued in the volcanic area. Use the magnifier to look at
the grains of sand.
If you do not have sand, just use
confetti or similar art material, with two different sizes. The larger
size should be glued in the plutonic area, the smaller should be glued
in the volcanic area. This exercise emphasizes that plutonic rocks have
large minerals and volcanic rocks have smaller crystals.
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