Materials Included In Kit

Additional Materials Required

Safety Precautions

Although this activity is considered to be nonhazardous, please follow all normal laboratory safety guidelines. Wash hands thoroughly with soap and water before leaving the laboratory.

Teacher Tips

• Enough materials are provided in this kit for 30 students working in pairs.
• Due to the differences in the makeup of the rock samples, answers may vary.
• Each group should work on a specific type of sample and then rotate.
• Additional types of rocks (or other samples) may be tested as well.
• Data collection may take up to two full periods, while answering the questions can take another period or be assigned as homework.
• The idea behind this activity is that the student should be able to take the average density of Earth (5.5 g/cm³) and determine an approximation of the average density of surface rocks in the Earth’s crust.
• Although metamorphic rocks tend to be the most dense of the three rock types, the data may not always show this result.
• There are many details the teacher can address to set up the lab to achieve the sought-after results. The best results come when low density igneous rocks are used. Due to the error quotient for the equipment, metamorphic rocks may not always calculate out to be the most dense. The following reasons may be the cause:
  • Contact metamorphosed rocks (chemical alteration vs. extreme pressures) may not be as dense as those metamorphic rocks exhibiting foliation (regional metamorphism due to extreme heat and pressure).
  • Sedimentary rocks composed of felsic minerals are less dense.
  • Highly metamorphosed metamorphic rocks are more dense (e.g., gneiss, schist).
• The Styrofoam balls will float. A needle or pencil may be used to completely submerge the ball for an accurate reading.

Sample Data

Answers to Questions

1. Use the data and briefly describe 3–4 appropriate observations or trends in the properties and density of each sample type.

   See Sample Data.

2. A sample of aluminum is collected. The mass of the aluminum sample is 2.4 g, given the density of aluminum is 2.70-g/cm³, calculate the sample’s volume. Show all work.

   V = M/D

   {12631_Answers_Equation_1}
3. What are the three rock types?

   Sedimentary, metamorphic and igneous

4. Make a comparison between the (average) densities of the metamorphic rocks and the other two rock types. Base the answer on the data in the Density of Earth Data Table.

   See student data.

5. Compare the densities of granite and basalt. Which of the two rocks would make up most of the continental crust, and which would make up most of the oceanic crust? Write in the answers.

   The less dense portions of the Earth’s crust rise higher on the asthenosphere. These portions are called continental crust. These portions of Earth’s crust are thick, and composed mostly of larger crystal grains.

   Rock most commonly found: granite Rock type: igneous

   The more dense portions of the Earth’s crust rest lower on the asthenosphere. These portions are called oceanic crust. These more dense portions of Earth’s crust sit lower in elevation.

   Rock name most commonly found: basalt Rock type: igneous

   Due to the greater density of basalt, it would make sense for it to make up most of the oceanic crust. The oceanic crust is much thinner and denser than the continental crust. The continental crust is made up mostly of granite. The continental crust is therefore less dense and thicker.

6. Calculate the average density of the sedimentary, igneous, and metamorphic rocks using the data obtained for all the sample types in the data table. Show all work. This density measurement represents the average density of Earth’s lithosphere.

   See Sample Data.

7. The average density of the Earth is 5.5 g/cm³. The average density of the Earth’s lithosphere is much lower than the average density of the Earth overall. What does this tell us about the density of the internal structures of the Earth? They should be (much higher/a little bit higher/a little bit lower/much lower) than that of the lithosphere.
8. Compare the average densities of each sample type from the Density of Earth Data Table to the density of water (use the graph). How can the graph be used to determine which materials would float on water and which objects would sink?

   The density of water is 1.0 grams per milliliter. Styrofoam has a density less than the density of water and will therefore float. All other materials have densities greater than the density of water and will sink.

   On the graph the students will draw, those materials with a density greater than 1.0 g/mL will show up on the graph “above” the line drawn for the density of water. Those materials with densities less than 1.0 g/mL will show up as lines drawn “below” the line drawn for the density of water.

   {12631_Answers_Figure_1}

References

Flinn Scientific would like to thank Heather McArdle, Mahopac High School, Mahopac, NY, for this activity.

Introduction

The mass of the Earth is approximately 5.98 x 10²³ kg. The scale of this measurement is difficult to comprehend and impossible to measure directly. However, smaller scale measurements can be completed in the laboratory that will give insight into the density and mass of Earth.

Concepts

• Measurement of mass
• Calculating density
• Measurement of volume
• Rock types

Background

Helpful Definitions

• Density—The concentration of matter in a material. Equals the mass divided by volume of a material. D = M/V.
• Lithosphere—The rigid, outermost layer of the Earth, about 100 km thick, that includes the crust and part of the mantle.
• Asthenosphere—A structure of the Earth found beneath the lithosphere of the Earth. It consists of more dense elements in a partially liquid state. This structure has convection cells that transport heat energy from greater depths to more shallow depths. Average density is 3.3 g/cm³.
• Continental crust—The upper portion of the Earth’s lithosphere that is thicker and less dense than other regions of the lithosphere. This portion of Earth’s lithosphere tends to make up the continents. Coarse grained, felsic rocks (of granitic nature) tend to make up most of the continental crust. Average density is 2.7 g/cm³.
• Oceanic crust—The upper portion of the Earth’s lithosphere that is thinner and more dense than other regions of the lithosphere. This portion of Earth’s lithosphere tends to make up the ocean floor. Fine grained, mafic rocks (of basaltic nature) tend to make up most of the oceanic crust. Average density is 3.0 g/cm³.
• Igneous rock—Rock formed from magma or lava when it cools.
• Sedimentary rock—Rock formed when sediments become pressed or cemented together.
• Metamorphic rock—Rock formed from existing rock when the temperature or pressure changes. 

Current theories of the beginning of our solar system suggest that the early chemical composition of the solar system may still be preserved in solid remains such as comets and meteors. Comets, of course, are much more difficult to test directly than meteorites—having entered the Earth’s atmosphere, meteorites are actually found on Earth after entering Earth’s atmosphere. By studying these objects directly or indirectly and then studying the characteristics of Earth via seismic waves, scientists can calculate the density of the planet. Earth’s size and gravitational field yield clues about the “guts” of Earth—the materials and their densities that make up the structures of Earth beneath the lithosphere. The average density of the Earth has been calculated to be 5.5 g/cm³.

Experiment Overview

This activity is intended to introduce indirect evidence of the composition of Earth. Direct methods will also be used to collect composition and climate data for surface layers.

Materials

Safety Precautions

Although this activity is considered to be nonhazardous, please follow all normal laboratory safety guidelines.

Procedure

Data Collection

1. Obtain a container with three pieces of one type of sample (e.g., granite, basalt). See the Density of the Earth Data Table.
2. Using a balance, find the mass of each object in the container to the nearest tenth of a gram. Record these masses in the Density of the Earth Data Table.
3. Obtain a displacement cup. Using water displacement and a graduated cylinder, measure the volume of water each object displaced to the nearest tenth of a milliliter. (Hint: Volume can also be calculated mathematically for geometrically shaped objects). Record the volume in the data table.
4. Calculate and record the density of each object in the container in the data table.
5. Calculate and record the average density of the three objects in the data table.
6. Repeat steps 1–5 for each sample type listed in the data table.

Creating a Line Graph of Mass versus Volume

1. Label the y-axis “Mass (g)—dependent variable.”
2. Label the x-axis “Volume (mL or cm³)—independent variable.”
3. Plot a point on the graph representing the average mass (y-axis) and average volume (x-axis) in a specific color for the first sample type from the data table.
4. Draw a line from this point back to the origin, in the same color as the plotted point. The slope of this line will represent the average density of the material.
5. Repeat for the remaining objects on the data table.
6. Draw a line representing the density of water for comparison. Hint: Water has a density of 1 g/cm³.
7. Create an appropriate key for the graph.

Student Worksheet PDF

12631_Student1.pdf