5.4: Activity 5C- Plate Densities

An important property of geological plates is their density. Remember the asthenosphere has fluid-like properties, such that tectonic plates ‘float’ relative to their density. This property is called isostasy and represents the equilibrium between crustal height and relative density. This is similar to buoyancy in water. For example, if a cargo ship has a full load of goods it will appear lower than if it were empty because the density of the ship is on average higher. Therefore, the relative density of two plates can control how they interact at a boundary and the types of geological features found along the border between the two plates.

Recall the Earth’s crust is divided into two main types:

• Continental crust, which is composed of granite, is relatively older and thicker than oceanic crust. The thickness of the continental crust is between 25-70 km with an average thickness around 30 km. The average density of granite is 2.75 g/cm3 (read as 2.75 grams per cubic centimeter).

• Oceanic crust, which is composed of basalt, is relatively younger and thinner than continental crust. The thickness of the oceanic crust is between 5-10 km with an average thickness of 7 km. The average density of basalt is 3.0 g/cm3 (read as 3.0 grams per cubic centimeter).

1. Which crustal type is thicker?
2. Which crustal type is denser?
   1. How did you determine which crustal type was denser?
   2. Why do you think this is the case?
3. Which crustal type is more buoyant?
   1. How did you determine this?

Measuring the density of rocks is fairly easy and can be done by first weighing the rocks and then calculating their volume. The latter is best done by a method called fluid displacement using a graduated cylinder. Water is added to the cylinder and the level is recorded, a rock is then added to the cylinder and the difference in water levels equals the volume of the rock.

Density is then calculated as the mass divided by the volume (Density = Mass/Volume).

Figure 5.13 contains the information needed to calculate density. There are four rocks which have weight (in grams) as well as the volume of water recorded by a graduated cylinder (in milliliters) before and after the rock was added.

Notes:

• Each line on the graduated cylinder represents 10 milliliter (ml).
• When measuring volume, round to the nearest 10 ml line on the graduated cylinder.
• Surface tension will often cause the water level to curve up near the edges of the graduated cylinder creating a feature called a meniscus. To accurately measure the volume, use the lowest level the water looks to occupy.

Figure 5.13: Density Experiment: Calculated volumes (in ml) and masses (in g) of rock samples A, B, C, and D.

4. The rock that most closely resembles the composition of continental crust is:

1. A

2. B

3. C

4. D

5. Based on your selection above, what is the density of this rock? Note: Density = Mass/Volume. Answer unit should be in grams/milliliter. SHOW YOUR WORK.
6. The rock that most closely resembles the composition of oceanic crust is:
   1. A
   2. B
   3. C
   4. D
7. Based on your selection above, what is the density of this rock? Note: Density = Mass/Volume. Answer unit should be in grams/milliliter. SHOW YOUR WORK.
8. Based on their densities, when oceanic and continental crust collide, the ??? crust would sink below the ??? crust.
   1. continental; oceanic
   2. oceanic; continental
9. What type of boundary is represented in the question above?
   1. Convergent, subduction
   2. Convergent, C-C
   3. Divergent
   4. Transform

Attributions

• Figure 5.13: Derivative of “Density Experiment” (CC-BY-SA 3.0; Bradley Deline via LibreText) by Chloe Branciforte​