Video Transcript
The image below shows an igneous
rock. A student looking at the rock
notices it contains a lot of small holes in its structure. Is this igneous rock likely to be
plutonic or volcanic?
Igneous rock is rock formed from
the cooling and solidification of molten material from below earth’s crust. When this molten material is below
the surface of the earth, it’s referred to as magma. Magma has a high ambient
temperature. This means that when magma cools
and solidifies, it does so slowly. As magma cools slowly, large
crystals are able to grow. The type of igneous rock formed
when magma cools is called plutonic rock. Granite is an example of a plutonic
rock.
When magma flows to the surface of
the earth, it’s then referred to as lava. Lava cools much more quickly than
magma. As lava cools quickly, there isn’t
sufficient time for large crystals to grow. Thus, small crystals are
formed. Magma contains gases. And when it emerges above the
surface as lava, the gases escape. As the lava quickly cools, these
escaping gases create small holes in the rock. The type of igneous rock formed
when lava cools is called volcanic rock. Basalt is an example of a volcanic
rock.
In this question, the igneous rock
that the student is looking at contains a lot of small holes. The small holes were likely caused
by gases escaping as the rock was formed. Therefore, this igneous rock is
likely to be volcanic.
