Comparing the Chemical Composition of Hawaiian Basalt and Mount Saint
Helens Dacite (Grades 7-8)
Activity 8 uses a bar graph to compare and contrast the chemical
compositions of Hawaiian basalt and Mount St. Helens dacite. Composition
is also related to violence of eruption and the color of the rock. The
dacite contains more silica. In general, the greater the silica content
of a magma, the more violent the eruption. The low silica content of
Hawaiian magmas allows gas to escape, resulting in gentle eruptions. The
high silica content and viscosity of dacitic magmas traps the gas and
results in explosive eruptions. The Hawaiian basalt has the greatest
amount of iron and magnesium. The greater the iron and magnesium content
of a rock, the darker its color.
Activity 8. Comparing the Chemical Composition of Hawaiian Basalt and
Mount St. Helens Dacite
SiO2 = silica, Al2O3 = Aluminum oxide, FeO = total iron
oxide,
MgO = magnesium oxide, CaO = calcium oxide, Na2O =
sodium oxide, K2O =
potassium oxide, TiO2 = titanium oxide, Other = other
elements. Values are weight percent oxide.
- Which rock has the greatest amount of silica?
- How is the amount of silica related to the explosivity of volcanic
eruptions?
- Which rock has the greatest amount of iron and magnesium?
- How does the amount of iron and magnesium influence the color of the
rock?
To VolcanoWorld