Unit 3.5 How do the densities of Earth materials relate to the layers of the Earth?

Natalie Bursztyn, University of Montana

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Initial Publication Date: September 5, 2024

Summary

How do we measure the density of Earth materials? Students measure, calculate, and compare material densities while using models to explore fluid density variations.

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Learning Objectives

By the end of this unit, students will be able to:

  • Calculate the density and approximating the volume of various objects in order to compare and assess the accuracy of different methods of calculating density
  • Construct a scale model of our planet, examine relative thicknesses and mechanical properties of Earth's major layers in order to explain the role of density in Earth's structure.

Context for Use

This module is designed to synthesize the layered nature of our planet, from atmosphere to inner core, comparing material densities, states of matter, composition, mechanical behavior, and relative size.

These materials build a foundation for understanding the unit's motivating question, but also function well as a standalone module. The materials in this unit should take 165 min of class time. Most of the lab exercises rely on small group work and are best suited to smaller classes or a lab meet-up outside of a traditional lecture room.

In this course, plate tectonics are implicitly or explicitly themes in all three main overarching units: Unit 2.5 about the Hunga Tonga Volcano's energy, this unit on density and layering (Units 3.5 & 3.6), and Unit 4.3 on Forces. Complete all of these parts so that your students can build a deep understanding by viewing this topic through a variety of different lenses.

Description and Teaching Materials

Teaching Materials:

All Slides: U3.5 All Slides.pptx (PowerPoint 2007 (.pptx) 616kB Jul10 24)

Reflection Assignment: U3.5 Reflection.docx (Microsoft Word 2007 (.docx) 68kB Jul10 24)

Sample Student Reflections (this is Reflection 10): Reflection Examples Redacted.pdf (Acrobat (PDF) 1.8MB Jul8 24)

Measuring Densities of Solids Activity materials: Solids to measure (e.g. Wood blocks, Rocks, Steel balls), Scales, Water, Volume measurement devices. (e.g., Rulers, Graduated Cylinders and/or Beakers)

Composition and Mechanical Layers of the Earth Activity Jigsaw materials:

  • Student instructions Earth Layers Jigsaw - Student Instructions.docx (Microsoft Word 2007 (.docx) 267kB Jul10 24), Instructor Notes
  • Materials representing different rheological properties: Popsicle sticks (brittle), Slime (viscous fluid), Plastic (plastic...), Wire - paperclips? (malleable), Rubber bands (elastic), Non-Newtonian fluid (Oobleck: 1.5C cornstarch + 1C water; or silly putty)
  • Earth materials: Water, Rocks, Steel balls
  • Rulers, Slinkies, ice and water in separate containers, a single long (70+ cm) sheet of paper - like register tape or scissors and tape cut into thirds.

Pre-Class Assignment(s):

Read properties of Liquids and Solids (density section only). Be prepared to share factors that change the properties of liquids and solids and explain how they do so.

In Class, Part 1: Measuring Earth Materials' Density (75 min)

In class (15 min): Connecting atoms, minerals, rocks & density together

  • Think-pair-share cause and effect statements for factors that change the properties of liquids and solids
  • Recalling atoms, introduce what a mineral is, what a rock is, and that the Earth is made of rocks.
  • Compare the Earth, Moon, and Mars. Provide bulk planet/satellite density and surface material density data for these three space objects, which differ the most for Earth and the least for the Moon.
  • Think-pair-share: What can we infer from the discrepancy between the known surface materials and the measured bulk density? (For Earth, the surface materials have FAR too low of density to represent what the whole planet is made of. The bulk density is so much higher than the densest of surface rocks that the interior must contain materials that have far greater density. Mars' density is significantly closer to that of its surface material, but still denser. This implies that it too must have some unseen denser interior, but less dense than that of Earth. The Moon, however, has a bulk density that is almost the same as the material we see at its surface, so it is most likely almost entirely made of basalt.)
  • Briefly explain how bulk density is determined for space objects, then lead into measuring the density of solids on Earth, in a lab, which is the first activity.

Lab exercise Measuring Densities of Solids (60 min)

  • Similar to the liquid density exercise, students will compare methods. This time, they will compare methods within their groups and use the whole class datasets to check their interpretations of method accuracy.
  • Working in pairs or small groups to share equipment, each group will be responsible for calculating the density for each of the solids provided using two different methods: direct volume measurement/estimation and volume by displacement.
  • Observations constitute their recorded data, in tables, and upon completing their density calculations, each group will discuss:
    • Which of these methods do you think is the most accurate? The most approximate? Why?
    • What data do you need to check your answers (hypotheses)?
  • It is expected that students will by now recognize the value of the whole class dataset and will want these data to check their hypotheses. Share out the data and use it to calculate average density by method and standard deviation. Following this is a discussion on their findings: do the data support their hypotheses?
  • The activity ends with interpretation and application questions to bring the topic of density back to the structure of the planet.
    • What can we predict from these data about the structure and composition of Earth's layers?
    • What additional information or supplemental data do you need to refine your inferences about Earth's interior?

In Class, Part 2: Visualizing Earth's Layers (90 min)

In class discussion (10 min): Earth materials' density and Earth's interior structure

  • Begin with a class discussion about the interpretation and application questions from the end of the activity.
  • Introduce important things we know about Earth's interior from other evidence that we are not able to explore in this class (magnetic field = metallic core) then lead into the next activity where students will visualize the scale of Earth's layers

Jigsaw Activity Layers of the Earth (70 min)

  • In this activity, students will produce a scaled representation of Earth's interior layers, the ocean, and atmospheric layers to be able to visualize the relative scale and the different properties of all these Earth components they have been studying in this unit.
  • We begin by splitting up into groups where each group learns about one of four specific topics related to the structure and properties of Earth's layers. Each group is assigned questions, hypotheses, predictions, and/or activities to supplement their reading. Groups then mix to reform with one student from each topic to share out, answer discussion questions and collaborate to complete a data table with information on all of Earth's layers.
  • Using these data, students will calculate, measure, draw, and annotate their scaled representations of Earth's layers. Annotations include the properties each layer has, including phase, composition, temperature, and mechanical behavior. For the latter, analog materials are provided for students to experience the concepts of "brittle", "ductile", "non-Newtonian", "elastic", etc.

In class discussion (10 min): Chemical composition, mechanical behavior, and Earth's layered interior

  • Following this second activity, introduce the two ways Earth's layered interior is usually depicted: chemical composition and mechanical behavior. Describe each.
  • Think-pair-share: Explain how Earth's interior layers are stratified in ways that are both similar and different to the layers of the atmosphere and ocean.

Teaching Notes and Tips

Completing the lab activity in the unit requires graduate cylinders that can accommodate the diameter of the rocks that you use. In a pinch, beakers can be used, but they will be so imprecise that it will be difficult to learn much from the measurements.

Reminder: water and computers are a dangerous combination. If students are using computers at their desks or lab stations then ensure that water is kept far away.

Measuring the density of solids has many similarities to measuring the density of liquids that was completed in a previous unit. We purposefully didn't provide detailed density measuring instructions - if your students are stuck, remind them to look back to what they did in that previous lab.


Assessment

  • A pre-class assignment (described above) is graded for completion only, not correctness. Administer using the same format throughout your course (through the LMS, turn in paper copies, guided discussion/participation in class, etc.). Consider setting the due date an hour or so before your class begins to give you time to summarize where your students sit with these concepts (this is a form of Just in Time Teaching).
  • The Lab is assessed as a Science Journal, as always. Science/Lab Journals General Instructions/Rubric (Microsoft Word 2007 (.docx) 2.9MB Aug30 24)
  • The reflection assignment in this unit asks students to reflect on and discuss how composition and temperature (both related to density) have resulted in the Earth's layered structure. Reflections ask students to put their learning in their own words and also to apply their knowledge in a new and novel situation. As always, reflections should be about 500 words and they should both discuss content that reflects understanding and thoughtfully reflect on the materials.
    • Read: Earth's structure
    • Can you explain, using evidence from the activities completed, how composition and temperature (both related to density) have resulted in the Earth's layered structure?

References and Resources

Readings from Vision Learning

References from the Jigsaw Activity: