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

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

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

Mafic vs. Felsic Igneous Rocks Mafic Characteristics Felsic Characteristics

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

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

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. http://whatonearth.olehnielsen.dk/rocks/volcanicrocks.asp

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

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. http://whatonearth.olehnielsen.dk/rocks/volcanicrocks.asp

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

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.

Mafic vs. Felsic Magma: Volcano Shape

Mafic vs. Felsic Magma: Volcanic Activity