✍️Activity 5C: Plate Densities

Answer the following questions for Activity 5C: Plate Densities.

Background Information:

An important property of tectonic 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 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 0 g/cm³ (read as 3.0 grams per cubic centimeter).

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.