Unit Overview
P.2 Energy, Forces, & Earth’s Crust

How do forces in Earth’s interior determine what will happen to the surface we see?

Unit Summary

How do forces in Earth’s interior determine what will happen to the surface we see? This unit is designed to help students build an intuitive understanding of the relationship between energy transfer and unbalanced forces as they explore science ideas related to plate tectonics, radioactivity, convection, and rock formation.

Students read about a crack that opened up suddenly in the Afar region of Ethiopia in 2005, accompanied by earthquakes and volcanos. They compare this to other earthquake events that occur in North America. This prompts them to model the events that occurred before, during, and after the crack was discovered. They figure out that changes in the structure of matter involve unbalanced forces and energy transfer, and use this idea to explain earthquakes and volcanoes at plate boundaries. They explore Earth’s interior using tomography and modeling, including radioactivity, to explain the unbalanced forces driving changes in Earth’s crust. They then investigate the interactions happening at plate boundaries and the nature of the relationship between mass and forces on the movement of tectonic plates to explain the past, present, and potential future of the Afar region. Finally, students apply these ideas in a transfer task to explain why a rift similar to the rift in the Afar region failed to create an ocean in the middle of North America 1.1 billion years ago.

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Additional Unit Information

Building Toward the Following Standards and Practices
Performance Expectations

This unit builds toward these performance expectations:

HS-ESS1-5 Evaluate evidence of the past and current movements of continental and oceanic crust and the theory of plate tectonics to explain the ages of crustal rocks.

HS-ESS2-1 Develop a model to illustrate how Earth’s internal and surface processes operate at different spatial and temporal scales to form continental and ocean-floor features.

HS-ESS2-3 Develop a model based on evidence of Earth’s interior to describe the cycling of matter by thermal convection.

HS-PS1-8 Develop models to illustrate the changes in the composition of the nucleus of the atom and the energy released during the processes of fission, fusion, and radioactive decay.

Disciplinary Core Ideas

PS2.B: Types of Interactions

  • Forces at a distance are explained by fields (gravitational, electric, and magnetic) permeating space that can transfer energy through space. Magnets or electric currents cause magnetic fields; electric charges or changing magnetic fields cause electric fields. (HS-PS2-4, HS-PS2-5) (This DCI is associated with a performance expectation that is not included in the bundle for this unit, and is covered in the first unit in this course. Applying this idea is key to building the ideas necessary to explain breaking in Earth’s crust.)
  • Attraction and repulsion between electric charges at the atomic scale explain the structure, properties, and transformations of matter, as well as the contact forces between material objects. (HS-PS2-6, secondary to HS-PS1-1, secondary to HS-PS1-3) (This DCI is associated with a performance expectation that is not included in the bundle for this unit, and is covered in the OpenSciEd HS Chemistry course. Applying this idea is key to building the ideas necessary to explain breaking in Earth’s crust.)

PS3.A: Definitions of Energy

  • At the macroscopic scale, energy manifests itself in multiple ways, such as in motion, sound, light, and thermal energy. (HS-PS3-2, HS-PS3-3) (This DCI is associated with a performance expectation that is not included in the bundle for this unit, and is covered in the first unit in this course. Applying this idea is key to building the ideas necessary to explain breaking in Earth’s crust.)
  • These relationships are better understood at the microscopic scale, at which all of the different manifestations of energy can be modeled as either motions of particles or energy stored in fields (which mediate interactions between particles). This last concept includes radiation, a phenomenon in which energy stored in fields moves across space. (HS-PS3-2) (This DCI is associated with a performance expectation that is not included in the bundle for this unit, and is covered in the first unit in this course. Applying this idea is key to building the ideas necessary to explain breaking in Earth’s crust.)

PS3.B: Conservation of Energy and Energy Transfer

  • Energy cannot be created or destroyed, but it can be transported from one place to another and transferred between systems. (HS-PS3-2) (This DCI is associated with a performance expectation that is not included in the bundle for this unit, and is covered in the first unit in this course. Applying this idea is key to building the ideas necessary to explain breaking in Earth’s crust.)

PS4.A: Wave Properties

  • The wavelength and frequency of a wave are related to one another by the speed of travel of the wave, which depends on the type of wave and the medium through which it is passing. (HS-PS4-1) (This DCI is associated with a performance expectation that is not included in the bundle for this unit, and the parts that are crossed out here are covered in the fifth unit in this course, when students investigate electromagnetic waves.)
  • Geologists use seismic waves and their reflection at interfaces between layers to probe structures deep in the planet. (secondary to HS-ESS2-3)

PS1.C: Nuclear Processes

  • Nuclear processes, including fusion, fission, and radioactive decays of unstable nuclei, involve release or absorption of energy. The total number of neutrons plus protons does not change in any nuclear process. (HS-PS1-8)
  • Spontaneous radioactive decays follow a characteristic exponential decay law. Nuclear lifetimes allow radiometric dating to be used to determine the ages of rocks and other materials. (secondary to HS-ESS1-5, secondary to HS-ESS1-6)

ESS1.C: The History of Planet Earth

  • Continental rocks, which can be older than 4 billion years, are generally much older than the rocks of the ocean floor, which are less than 200 million years old. (HS-ESS1-5)

ESS2.A: Earth Materials and Systems

  • Earth’s systems, being dynamic and interacting, cause feedback effects that can increase or decrease the original changes. (HS-ESS2-1, HS-ESS2-2)
  • Evidence from deep probes and seismic waves, reconstructions of historical changes in Earth’s surface and its magnetic field, and an understanding of physical and chemical processes lead to a model of Earth with a hot but solid inner core, a liquid outer core, a solid mantle and crust. Motions of the mantle and its plates occur primarily through thermal convection, which involves the cycling of matter due to the outward flow of energy from Earth’s interior and gravitational movement of denser materials toward the interior. (HS-ESS2-3)

ESS2.B: Plate Tectonics and Large-Scale System Interactions

  • The radioactive decay of unstable isotopes continually generates new energy within Earth’s crust and mantle, providing the primary source of the heat that drives mantle convection. Plate tectonics can be viewed as the surface expression of mantle convection. (HS-ESS2-3)
  • Plate tectonics is the unifying theory that explains the past and current movements of the rocks at Earth’s surface and provides a framework for understanding its geologic history. (ESS2.B Grade 8 GBE) (secondary to HS-ESS1-5)
  • Plate tectonics is the unifying theory that explains the past and current movements of the rocks at Earth’s surface and provides a framework for understanding its geologic history.
  • Plate movements are responsible for most continental and ocean-floor features and for the distribution of most rocks and minerals within Earth’s crust. (HS-ESS2-1)
Science & Engineering Practices

This unit intentionally develops students’ engagement in these practice elements:

Developing and using models in 9–12 builds on K–8 experiences and progresses to using, synthesizing, and developing models to predict and show relationships among variables between systems and their components in the natural and designed worlds. The following elements of this practice are intentionally developed across this unit:

  • Evaluate merits and limitations of two different models of the same proposed tool, process, mechanism, or system in order to select or revise a model that best fits the evidence or design criteria.
  • Develop, revise, and/or use a model based on evidence to illustrate and/or predict the relationships between systems or between components of a system.
  • Develop and/or use multiple types of models to provide mechanistic accounts and/or predict phenomena, and move flexibly between model types based on merits and limitations.
  • Develop and/or use a model (including mathematical and computational) to generate data to support explanations, predict phenomena, analyze systems, and/or solve problems.

Constructing explanations and designing solutions in 9–12 builds on K–8 experiences and progresses to explanations and designs that are supported by multiple and independent student-generated sources of evidence consistent with scientific ideas, principles, and theories. The following elements of this practice are intentionally developed across this unit:

  • Make a quantitative and/or qualitative claim regarding the relationship between dependent and independent variables.
  • Construct and revise an explanation based on valid and reliable evidence obtained from a variety of sources (including students’ own investigations, models, theories, simulations, peer review) and the assumption that theories and laws that describe the natural world operate today as they did in the past and will continue to do so in the future.

Elements from the following practices are also key to the sensemaking in this unit:

  • Asking Questions
  • Obtaining, Evaluating, and Communicating Information
  • Planning and Carrying Out Investigations
  • Using Mathematics and Computational Thinking
Crosscutting Concepts

This unit intentionally develops students’ engagement in these crosscutting concept elements:

Stability and Change. For both designed and natural systems, conditions that affect stability and factors that control rates of change are critical elements to consider and understand.

  • Much of science deals with constructing explanations of how things change and how they remain stable.
  • Change and rates of change can be quantified and modeled over very short or very long periods of time. Some system changes are irreversible.

Scale, Proportion, and Quantity. In considering phenomena, it is critical to recognize what is relevant at different size, time, and energy scales, and to recognize proportional relationships between different quantities as scales change. The following elements of this crosscutting concept are intentionally developed across this unit:

  • Some systems can only be studied indirectly as they are too small, too large, too fast, or too slow to observe directly.
  • Patterns observable at one scale may not be observable or exist at other scales.

Energy and Matter. Tracking energy and matter flows, into, out of, and within systems helps in understanding their system’s behavior. The following elements of this crosscutting concept are intentionally developed across this unit:

  • The total amount of energy and matter in closed systems is conserved.
  • Changes of energy and matter in a system can be described in terms of energy and matter flows into, out of, and within that system.
  • Energy cannot be created or destroyed—it only moves between one place and another place, between objects and/or fields, or between systems.

Cause and Effect. Events have causes, sometimes simple, sometimes multifaceted. A major activity of science is investigating and explaining causal relationships and the mechanisms by which they are mediated. The following elements of this crosscutting concept are intentionally developed across this unit:

  • Cause-and-effect relationships can be suggested and predicted for complex natural and human-designed systems by examining what is known about smaller-scale mechanisms within the system.

Patterns. Observed patterns in nature guide organization and classification and prompt questions about relationships and underlying causes. The following elements of this crosscutting concept are intentionally developed across this unit:

  • Different patterns may be observed at each of the scales at which a system is studied and can provide evidence for causality in explanations of phenomena.
  • Classifications or explanations used at one scale may fail or need revision when information from smaller or larger scales is introduced, thus requiring improved investigations and experiments.
  • Mathematical representations are needed to identify some patterns.
  • Empirical evidence is needed to identify patterns.

The following crosscutting concepts are also key to the sensemaking in this unit:

  • Structure and Function
  • Systems and Systems Models
Connections to the Nature of Science

Which elements of NOS are developed in the unit?

  • Scientific Knowledge Is Based on Empirical Evidence. Science includes the process of coordinating patterns of evidence with current theory.
  • Scientific Investigations Use a Variety of Methods. Scientific inquiry is characterized by a common set of values that include: logical thinking, precision, open-mindedness, objectivity, skepticism, replicability of results, and honest and ethical reporting of findings.
  • Science Models, Laws, Mechanisms, and Theories Explain Natural Phenomena. Models, mechanisms, and explanations collectively serve as tools in the development of a scientific theory.
  • Science Models, Laws, Mechanisms, and Theories Explain Natural Phenomena. Scientists often use hypotheses to develop and test theories and explanations.

How are they developed?

  • In Lesson 5, students coordinate patterns of evidence with a model of Earth’s interior. They respond to a reflection question during the navigation into day 2 on how empirical evidence helped them identify patterns and anomalies in seismic velocities to support their reasoning about the plausibility of the layers model.
  • In Lesson 2, students are asked to consider whether they need to collect more trials in light of their empirical results. The class discusses data accuracy, the number of trials needed, potential causes of errors, outliers, and uncertainty in an investigation.
  • In Lesson 8, students assess the accuracy and replicability of their results as they investigate radioactive decay using a simulation. Working in groups and as a class, they discuss the importance of accuracy and replicability in scientific investigations.
    In Lesson 13, the Scale Chart is revisited to determine that the models, mechanisms identified, and explanations used in the unit up to Lesson 13 represent the Theory of Plate Tectonics. The items on the Scale Chart are grouped and labeled the Theory of Plate Tectonics.
  • In Lesson 11, students use sentence stems to develop explanatory hypotheses that describe how they predict independent variables are related to the dependent variable of the amount of friction force acting on an object.
Unit Information
What is the anchoring phenomenon and why was it chosen?

This unit is anchored by a puzzling Earth science phenomenon: the land in East Africa appears to be ripping apart. In 2005, a crack opened up very suddenly in a region called Afar in Ethiopia, accompanied by earthquakes and a volcanic eruption. This phenomenon provides the context in which to investigate the relationship between unbalanced forces and energy transfer through systems, how radioactive decay on the particle scale drives global-scale convection, and the role of plate tectonics in explaining Earth’s surface features. This unit uses fundamental physics ideas about how unbalanced forces transfer energy through systems, causing motion. The unit then uses these ideas to describe and explain fundamental Earth science ideas about how the hidden processes playing out in Earth’s interior over short and long temporal/spatial scales shape the surface patterns we see.

The Afar anchoring phenomenon was chosen from a group of phenomena aligned with the target performance expectations based on the results of a survey administered to almost 1000 students from across the country, and in consultation with external advisory panels that include teachers, subject matter experts, and state science administrators. The phenomena in units 1, 3, 4, and 5 were chosen because they have high relevance to students’ everyday experiences and communities. The phenomenon in this unit is playing out on a timescale at which it can be difficult to see the relevance to human lives. This more abstract and Earth-scale phenomenon was chosen with a purpose. The full physics course is designed to purposefully highlight a variety of different types of phenomena, some of which overlap: from justice-oriented (P1, P3), to everyday (P3, P5), to culturally embedded (P4), to more abstract (P2, P6). Though we design to privilege the interests of students to whom we owe an educational debt, we must not essentialize minoritized groups by assuming that a trend in the data equates to homogeneous interests and experiences. Providing a diverse suite of entry points into content and practices creates more opportunities for every student to connect with the content.

The Afar phenomenon was chosen for the following reasons:

  • Students showed high interest in explaining unique and puzzling surface features in survey data.
  • To provide a diverse suite of entry points across the course, we were seeking an event that allowed students to consider a global-scale phenomenon.
  • Teachers and administrators saw the phenomenon as interesting and on grade band.
  • Explaining the mechanisms of Earth’s interior using physics concepts grounds abstract ideas about forces.
  • Explaining the phenomenon addresses all the disciplinary core ideas in the bundle at a high school level.
  • Explaining the phenomenon requires the use of both forces and energy in order to understand sudden and long-timescale change.
How is the unit structured?

The unit is organized into two main lesson sets. Lesson Set 1 (Lessons 1-8) focuses on answering the question: How does land stretch and when/why does it break? In the first lesson set, students make observations of a series of changes in Earth’s crust over short and long timescales that reveal patterns at a local scale in a region called the Afar Triangle in Ethiopia, and at a larger scale along the eastern half of the African continent. This leads them to wonder about the nature of stretching and breaking in matter. They work with manipulatives and simulations to establish the role of unbalanced forces in natural processes, which helps them figure out why some energy transfer processes in Earth’s crust are slow and stable but others are sudden and unpredictable. Their investigations lead them to explore seismography and tomography data, which allows them to model the structure of Earth’s interior and notice dynamic patterns in the mantle. To explain these patterns, students observe a model of mantle convection and read about radioactivity, which gives them the tools to describe mantle processes from a forces, matter, and energy transfer perspective.

Lesson Set 2 (Lessons 9-13) focuses on answering the question: How do forces determine what will happen to Earth’s surface? In the second lesson set, students apply their new ideas about radioactivity to understand radiometric dating, and discover that many of the rocks in the Afar region are very young compared to the surrounding area. They explore the differences in density between oceanic and continental rocks (basalt and granite) and figure out that just as density differences can cause an unbalance of forces in the mantle, they can also cause an unbalance of forces in the crust due to an emergent relationship between mass and gravitational force, which can contribute to the movement of tectonic plates. Finally, students put the pieces together to predict what will happen to the Afar region in the geological future and apply their understanding to explain why the Midcontinent Rift failed 1.1 billion years ago.

Where does this unit fall within the OpenSciEd Scope and Sequence?

This unit is the second in the OpenSciEd High School Physics course sequence. It is designed to build on student ideas about energy transfer and matter interactions from the course’s first unit, OpenSciEd Unit P.1: How can we design more reliable systems to meet our communities’ energy needs? (Electricity Unit). In that first unit, students developed ideas around energy transfer and conservation in the context of charged particles (electrons) colliding with other electrons (electricity) to transfer energy across great distances. In this unit, Earth science phenomena that transfer energy differently across scales of time and space will motivate the need for forces to explain observations. Students establish conventions for modeling forces using free-body diagrams and think deeply about the connection between unbalanced forces, energy transfer, and motion. Although they do not quantify the relationship between forces and acceleration in this unit, they develop a robust intuitive understanding of forces, which will be fundamental to deriving and applying Newton’s second law (F=ma), which will form the basis for the way forces are applied over the next several units.

In the course’s third unit, OpenSciEd Unit P.3: What can we do to make driving safer for everyone? (Vehicle Collisions Unit), students develop a more robust understanding of forces as vectors and use conservation of momentum to make predictions about the outcomes of collisions. In the fourth unit, OpenSciEd Unit P.4: Meteors, Orbits, and Gravity (Meteors Unit), they expand their model of forces to include the force of gravity at great distances, using ideas about fields developed in the first unit to understand the relationships between gravity and energy transfer. In the fifth unit, OpenSciEd Unit P.5: How do we use radiation in our lives and is it safe for humans? (Microwave Unit), they use energy transfer, electromagnetism, wave mechanics, and forces at a distance to explain how food heats up in a microwave and how this technology might be dangerous for humans. In the final unit, OpenSciEd Unit P.6: Earth’s History and the Big Bang (Cosmology Unit), students explore cosmology and the Big Bang, applying ideas about forces and energy from all five previous units on the largest scale.

What modifications will I need to make if this unit is taught out of sequence?

This is the second unit of the High School Physics Course in the OpenSciEd Scope and Sequence. Given this placement, several modifications would need to be made if teaching Physics first, or teaching this unit earlier in the Physics course. These include the following adjustments:

  • If taught before OpenSciEd High School Chemistry, supplemental teaching of the particle model of matter will be required, including a conceptual understanding of the nature of electrons, nuclei, and atoms. You will also need to review the nature of thermal energy and thermal energy transfer.
  • If taught earlier in the school year, supplemental teaching around the nature of energy transfer through systems and how to represent it may be required.
  • If taught as part of an AP physics course, be prepared to provide students with additional support around equations that are not treated in depth.
What mathematics is required to fully access the unit’s learning experiences?

This unit requires knowledge of how to solve algebraic equations but is not math-intensive. Students apply the Pythagorean theorem to understand how forces function as vectors. Students need to gather and represent data in ways that can help them identify patterns in investigation results, tomography, seismic data, and maps.

The unit does not assume fluency with the mathematical practices listed below; rather, students develop these practices as part of the sensemaking.

How do I shorten or condense the unit if needed? How can I extend the unit if needed?

The following are example options to shorten or condense parts of the unit without eliminating important sensemaking:

  • Lesson 8: Shorten the investigation time within the lesson.
  • Lesson 9: Instead of conducting the Rock Sample Density Lab, give students the data from this investigation to work with instead.
  • Lessons 11-12: You could give students the data, instead of having them conduct the investigation, or give them the investigation design to collect their own data.
  • The first half of the unit attends more heavily to DCIs related to physics, whereas the second lesson set attends more heavily to the interdisciplinary connections between physics and Earth science. Though it is not recommended, there are two other options for condensing. To focus on primarily a physics perspective, you could teach Lessons 1-6, 11-12, and modify the assessment in Lesson 13. To focus primarily on an Earth science perspective, you could teach Lessons 1, 4-10, add in ideas of slab push and ridge pull to 10, and modify the assessment in Lesson 13.

To extend or enhance the unit, consider the following:

  • Lesson 1: Use the guidance given in Lesson 1 about collaborating with the English Language Arts department to create expository texts.
  • Lesson 4: There is an information tab in the simulation that describes the model rules further and a code tab that lets students inspect, modify, and recompile the code for the model, for students who are interested.
  • Lesson 10: This lesson utilizes specific features of the simulation. Consider allowing students to further explore the features of the simulator, and build in class time for them to come to consensus around their findings.
  • Lesson 12: Discussions could be led around how increasing the incline made the perpendicular component of gravity read by the digital scale decrease, and how this means that friction would decrease as the angle increased, among other discussions.
  • All lessons: Remove scaffolds provided with science and engineering practices (SEPs) as a way to give students more independent work with the elements of these practices.
Unit Acknowledgements
Unit Development Team
  • Whitney Mills, Unit Lead, BSCS Science Learning
  • Zoë Buck Bracey, Field Test Unit Lead, BSCS Science Learning
  • Diego Rojas-Perilla, Field Test Unit Lead, BSCS Science Learning
  • Michael Novak, Writer, Northwestern University
  • Rabi Whitaker, Writer, New York City Public Schools
  • Laura Zeller, Writer, BSCS Science Learning
  • Erick Arellano-Ruiz, Teacher Advisor, Denver Public Schools
  • Kathryn Fleegal, Teacher Advisor, Denver Public Schools
  • Joe Kremer, Teacher Advisor, Denver Public Schools
  • Madelyn Percy, Teacher Advisor, Denver Public Schools
  • Ann Rivet, Advisor on ESS Integration, Teachers College, Columbia University
Production Team
  • Madison Hammer, Production Manager, University of Colorado Boulder
  • Kate Herman, Copy Editor, Independent Consultant
  • Erin Howe, Project Manager, University of Colorado Boulder
Unit External Evaluation
NextGenScience’s Science Peer Review Panel

An integral component of OpenSciEd’s development process is external validation of alignment to the Next Generation Science Standards by NextGenScience’s Science Peer Review Panel using the EQuIP Rubric for Science. We are proud that this unit has earned the highest score available and has been awarded the NGSS Design Badge. You can find additional information and read this unit’s review on the nextgenscience.org website.