From chapter 3, exercise 8 “In Greater Depth”.

Because fine-grained textures make classification difficult, extrusive rocks are often classified based on their chemical compositions.

Extrusive rocks from the Cascade Range, Oregon (% wt.)
Rock 1 Rock 2 Rock 3 Rock 4 Rock 5 Rock 6 Rock 7 Rock 8
SiO2 48.6 50.68 63.81 59.85 53.5 69.36 74.12 55.92
FeO 10.31 9.27 4.05 5.39 8.31 2.32 1.26 7.32
MgO 6.73 6.12 2.28 3.58 5.42 1.14 0.16 4.04
CaO 10.65 9.67 4.87 5.95 7.63 3.07 1.23 6.77
Na2O 2.47 2.7 3.72 3.65 3.18 3.92 4.08 3.49
K2O 0.25 0.48 1.96 1.28 0.73 3.02 4.47 0.97
Other 20.99 21.08 19.31 20.3 21.23 17.17 14.68 21.46

Read exercise 8a, complete exercise 8b (plot the data)

Plot the data from the table.
First, plot SiO2 (X-axis) versus FeO (Y-axis) data for each rock sample. Connect the points and label the line at one end as “FeO.”
Then do the same with MgO, CaO, Na2O, and K2O.
The result will be five curves showing how the oxide concentrations change as SiO2 increases.

Use this plot or make one in Excel

Exercise 8d: Which elements decrease with increasing abundance of SiO2 and which elements increase with increasing abundance of SiO2?

Decreasing oxides Increasing oxides

Exercise 8f
Finally, let’s see how the rock compositions might reflect changes in the mineralogy of the volcanic rocks. According to Bowen’s Reaction Series diagram (Fig. 3.18), the mafic, intermediate, and felsic extrusive rocks should contain the following minerals (either as phenocrysts or in the groundmass):

1. Which of the oxides (FeO, MgO CaO, Na2O, K2O) are abundant in the minerals making up mafic rocks?
2. Which of the oxides are the least abundant in minerals making up mafic rocks?
3. Which of the oxides are abundant in the minerals making up felsic rocks?
4. Which are the least abundant in minerals making up felsic rocks?

Sample Solution