Name: _________________________________ Date:_______________ Period:____
Ms. Rabinowitz ICS 1A
EARTH SCIENCE
Subduction and Density: Basalt and Granite
Which has greater density: Basalt or Granite?
BACKGROUND
A plate boundary is where two plates meet. When two plates move towards each other they form what is
called a convergent boundary.
Ocean crust is made mostly of iron-rich rock known as basalt. Continental crust is mostly composed of
silica-rich rock called granite.
When an ocean plate and a continental plate meet at a convergent boundary, one plate moves under the
other in a process called subduction. At a subduction zone, the plate that sinks returns to the mantle
where it heats, melts, and forms magma.
In this lab we will test the density of the rock that makes up the ocean crust and the continental crust. The
denser crust moves beneath and melts at a subduction zone.
INSTRUCTIONS
1. Use the scale to find the mass of each rock then record it in the Mass of Rock row.
2. Pour water from the beaker into the graduated cylinder to the 50 mL line.
3. Slide the rock into the graduated cylinder
4. Record the volume of rock + water in the Volume in Total row.
5. Subtract 50 mL from the Volume in Total mL and record in Volume of Rock row.
6. Pour the water back into the beaker and remove the rock.
7. Find the Density by dividing the Mass of Rock by the Volume of Rock.
EXAMPLE
Mass of Rock
45 g
Volume (Rock + Water) = Total Volume
65 mL
Volume of Rock = Total Volume – 50 mL
15 mL
Density = Mass of Rock/Volume of Rock
3 g/ mL
OCEAN FLOOR
Basalt 1
Basalt 2
Basalt 3
Granite 1
Granite 2
Granite 3
Mass (g) of Rock
Total Volume (mL)
Volume (mL) of Rock
Density (g/mL)
CONTINENTAL FLOOR
Mass (g) of Rock
Total Volume (mL)
Volume (mL) of Rock
Density (g/mL)
CLASS DATA:
Trial #
1
2
3
average
Group 1
OCEAN CRUST -- BASALT
Group 2
Group 3
Group 4
Group 5
Group 6
Trial #
1
2
3
average
Group 1
CONTINENTAL CRUST -- QUARTZ
Group 2
Group 3
Group 4
Group 5
Group 6
QUESTIONS
1. What is the average density of basalt, which makes up the ocean plate? ______________
2. What is the average density of granite, which makes up the continental plate? __________
3. What, if any, possible sources of error could account for unusual results?
____________________________________________________________________________
____________________________________________________________________________
4. By comparing the class averages, I found that _________________ had a greater density than
_________________.
5. By carrying out experiments similar to the one you just performed, scientists have determined the
density of basalt (ocean crust) to be 3.0 g/mL and the density of granite (continental crust) to be
about 2.7 g/mL.
Based on this information, which plate would sink at a convergent boundary where an ocean plate
meets a continental plate? _________________________________________________________