This presentation will explain how geologists figure out the conditions responsible for
turning hot magma into solid rocks and how we can make some simple observations to identify
and classify some common igneous rock types.
There are three specific things we want you to learn. The first is to figure out if a
rock formed near Earth’s surface or deep underground. Next, we want to further classify
these rocks on the basis of their chemical composition, and finally, we would like you
to be able to name some common types of igneous rocks
We can divide all igneous rocks into two groups. Volcanic igneous rocks form on or near Earth’s
surface. An example would be a lava flow. In contrast, when magma cools and solidifies
below ground, it forms a plutonic igneous rock.
Both volcanic and plutonic igneous rocks form when magma solidifies. The big difference
is in how rapidly that process takes place. Magma that starts at temperatures in excess
of a thousand degrees Celsius will cool rapidly in the relatively cold conditions of Earth’s
surface. This rapid cooling produces millions of tiny crystals of different minerals that
can only be observed using a microscope.
In contrast, plutonic igneous rocks cool slowly under much hotter conditions, allowing some
minerals to grow into relatively large crystals that can be readily distinguished by the naked
eye.
The same type of magma can produce either plutonic or volcanic rock, depending upon
where it comes to rest.
Geologists can use the texture of an igneous rock - that is the size of the crystals or
minerals within the rock - to determine if it had a plutonic or volcanic origin. Visible
grains indicate a plutonic igneous rock, but if you can’t see any individual minerals,
it must be a volcanic igneous rock.
For example, is this rock volcanic or plutonic?
How about this one?
We can’t see any individual crystals or mineral grains in the first rock, so we would
classify it as volcanic. We can clearly see light and dark minerals in the other sample,
so we would identify it as plutonic.
How about these examples?
We can differentiate volcanic and plutonic rocks on the basis of their texture, but if
we want to classify igneous rocks further, we have to divide them up by composition.
We will use the silica-content of the igneous rocks to divide them into three groups. Fortunately,
we don’t have to do any fancy chemical analysis to determine composition as the color of the
igneous rocks serves as a proxy for its silica content. Light-colored rocks are mainly composed
of silica-rich minerals. In contrast, dark-colored rocks are made up of silica-poor minerals.
We use the terms felsic, mafic, and intermediate to label igneous rocks with high, low or medium
silica contents. Felsic igneous rocks are silica-rich and are dominated by light-colored
minerals. Mafic rocks are darker, and intermediate igneous rocks contain roughly equal measures
of light and dark minerals.
The chemical composition of these rocks ranges from about half-silica for the mafic rocks
to more than two-thirds silica in the felsic rocks.
Let’s try classifying a few igneous rocks by composition. What about these examples,
can you identify the felsic, mafic and intermediate varieties?
Here are the answers.
Now that we know how to identify the texture and composition of igneous rocks, we are ready
to learn how to apply this information and match it with the names of some common igneous
rocks.
We can separate igneous rocks into volcanic and plutonic varieties on the basis of texture.
And there is a felsic, mafic and intermediate example of each, giving us six possible rock
names.
Felsic volcanic and plutonic rocks are known as Rhyolite and Granite respectively. Their
intermediate equivalents are Andesite and Diorite, and the darker mafic volcanic igneous
rock is Basalt, and the plutonic version is Gabbro.
Try creating this table on your own as a useful way of remembering the classification scheme.
Now, let’s see if you can apply this new-found knowledge to identify some igneous rock samples.
We see visible light-colored grains, so this must be a plutonic, felsic igneous rock, and that would be Granite.
You are on your own now for the rest. Good luck!
So, to summarize, we can use the texture of igneous rocks to tell us something about where
they formed relative to Earth’s surface and their color to indicate their chemical
composition. In a future lesson we will discuss how composition is linked to the plate tectonic
setting where the magma originally formed. On the basis of texture and color, geologists
can classify igneous rocks and use this information to decipher the geologic history of the region
where the rocks were found.
We had three learning objectives for this lesson. How confident are you that you could
readily complete these tasks?