1. Rock Density Lab Objectives:
Compare the densities of continental and oceanic crust
Relate rock density to the process of subduction at plate boundaries.

2. Mafic vs. Felsic Magmas: Determine Eruption Pattern at Volcanoes Explosive or Quiet?

3. Differences in density drive sea-floor recycling and plate tectonics:

4. Mafic vs. Felsic Igneous Rocks
Mafic Characteristics
Felsic Characteristics

5. Rock density depends on the mineral or mineral groups that make up a rock
Density depends on mineral composition – not necessarily rock type.

6. Igneous Rocks of the Ocean Crust Mafic ~ 50 % silica High in Magnesium (Mg) and Iron (Fe) Dark color Dense
Basalt & Gabbro Rock of the ocean crust and upper mantle

7. Mafic Igneous Rocks have heavy minerals with IRON & Magnesium
Elements
Iron
Magnesium
Calcium
Minerals
Olivine
Amphiboles
Ca feldspars
make
“MAFIC” and are dark in color due to the iron & magnesium.

8. Igneous Rocks of Continental Crust Felsic ~70% silica Light color Light
Granites, Rhyolites, Andesites
High silica

9. Felsic Igneous Rocks have light minerals with silicon & oxygen
Elements
silicon
oxygen
Minerals
Quartz (silica)
Feldspars
make
“Felsics” and are light in color because they lack iron and are rich in silica.

11. Mafic vs. Felsic Densities: Ocean vs. Continental Crust
Made of…
Mafic/Felsic
Density
Age
Thickness
Ocean
Crust
Continental Crust

12. Differences in density drive sea-floor recycling & plate tectonics:
The ocean floor is made of basalt (mafic magma). Continental crust is made of granite (from felsic magmas).
Ocean crust is denser than continental crust.
When ocean and continental crust collide … the denser ocean crust sinks back into the mantle (Subduction). This recycles the ocean floor.

13. Mafic vs. Felsic Magma: Volcano Shape

14. Mafic vs. Felsic Magma: Volcanic Activity