Rock Density Lab
Objectives:
(1)Compare the densities of
continental and oceanic crust
(2)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
or
that make up a rock
Density depends on
necessarily rock type.
– not
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
make
Minerals
•Olivine
•Amphiboles
•Ca feldspars
“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
make
Minerals
•Quartz
(silica)
•Feldspars
“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…
Ocean
Crust
Continental
Crust
Mafic/Felsic
Density
Age
Thickness
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