4.3 Igneous Composition

Igneous Rock Composition

Another easy way to tell igneous rocks apart is by their color. The color of igneous rocks can *sometimes* be a strong indicator of the type of magma or lava that produced it, regardless of what texture it has.

Light: Pink, tan, brown, white Intermediate: Light gray or even a mix of black and white grains Dark: Dark gray, black, green, or dark green
Pink to tan, light fine-grained igneous rock.
Figure 4.3.1 CC BY-SA 3.0 By Michael C. Rygel, Wikimedia Commons.
Light gray, porphyritic igneous rock
Figure 4.3.2 CC BY SA 4.0 by Laedri1660 Wikimedia Commons
Porphyritic dark gray to black igneous rock
Figure 4.3.3 CC BY-SA 4.0 by Unknown Author from Wikimedia Commons
Felsic Intermediate (mix of mafic and felsic magmas) Mafic/Ultramafic

 

Classification of igneous rocks by composition. Darker rocks are ultramafic or mafic whereas lighter rocks are intermediate/felsic.
Figure 4.3.4 Classification of igneous rocks by composition. Image by Karla Panchuk, CC BY

Dark-colored rocks are dark gray and result from mafic volcanism.

Intermediate-colored rocks can be gray to light-gray or contain a significant number of dark gray minerals in a light matrix. They result from magmas or lavas that are between felsic and mafic in composition.

Light-colored rocks have fewer dark minerals in their light matrix or can be completely red to tan-colored. These rocks result from felsic volcanism.

**An important exception to this rule is obsidian! Although this rock can sometimes appear completely black, it is actually formed from felsic lavas!**

Let’s Review!

In the assigned reading, you should have learned that the difference between mafic and felsic volcanism is due to chemical composition. What components will you find in mafic rocks and which will you find in felsic rocks?

Let’s examine some of the images of igneous rocks you have seen in the previous Texture exercise (and some new ones, too!), and categorize them according to color!

Decide which category (Light, Dark, or Intermediate) BEST fits the card depicting each of the above mineral samples. Once you have a good grasp of how to categorize the rocks based on their colors, you can take the extra step of assigning each rock a scientific category–felsic , mafic or intermediate. Hint: Look at the color definitions again if you are unsure.

 

Sample 1: Click on the image below to view the sample as an interactive, 3D model!

Black and white (in equal parts) coarse-grained rock
Figure 4.3.5 3D interactive model by rocksandminerals CC BY

Sample 2: Click on the image below to view the sample as an interactive, 3D model!

Rhyolite Interactive Model. Rhyolite is a fine-grained rock with light-tan to pink coloring.
Figure 4.3.6 3D interactive model by Dr. Parvinder Sethi CC BY

Sample 3: Click on the image below to view the sample as an interactive, 3D model!

Scoria interactive model. Dark-colored vesicular rock
Figure 4.3.7 3D interactive model by Sara Carena CC BY-NC.

Sample 4: Click on the image below to view the sample as an interactive, 3D model!

Light tan and pink coarse-grained rock with black flecks
Figure 4.3.8 3D Interactive Model by Digital Atlas of Ancient Life

Sample 5: Click on the image below to view the sample as an interactive, 3D model!

Gray porphyritic rock with gray-white large crystals in dark gray matrix
Figure 4.3.9 3D Interactive Model by WVU Volcanology and Petrology Lab, CC BY

Sample 6: Click on the image below to view the sample as an interactive, 3D model!

Dark gray coarse grained rock
Figure 4.3.10 3D Interactive Model by Sara Carena, CC BY-NC

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