Everything Happens for a Reason: Developing Causal Mechanistic Reasoning of Plate Tectonics
Our planet’s surface is in constant motion. Large pieces of Earth’s crust and upper mantle, known as tectonic plates, continually move toward and away from each other at a rate of millimeters to centimeters each year. Over geologic time, their relative motions determine everything from the types of boundaries they form to the distribution of rocks and landforms on Earth’s surface and the location and frequency of earthquake and volcanic eruptions (see “Monday’s Lesson“). Plate tectonic theory, the organizing paradigm that revolutionized geosciences, describes the plate and mantle system and is used to reason about how plate movements and interactions can explain where geological phenomena occur and why Earth looks the way it does. The goal of our National Science Foundation-funded Geological Models for Exploration of Dynamic Earth (GEODE) project is to help students use plate tectonics as an explanation for the landforms and geological phenomena observed on Earth’s surface.
To consider how plate movements are responsible for shaping and reshaping Earth’s surface over time, it’s best to think about plate tectonics as a system. The tectonic plate system includes both plates and the mantle, the layer of solid rock that lies between Earth’s surface and the molten core in Earth’s interior. Always in motion, the mantle acts as a major driver of the system.
Understanding this dynamic system helps us to explain everything from the mid-ocean ridges to the location of the continents and the appearance of Earth’s topographical features such as mountains and volcanoes. This systems thinking also gives us the ability to speculate about what changes might happen in the future.
Plate systems thinking
Systems thinking is the ability to think about the whole, rather than merely the parts. With the plate system, this means recognizing that what happens at one plate boundary is affected by and affects what happens along other boundaries on Earth, and that those plates are deeply coupled with the movement of rock in the mantle. Typically, we teach about plate motions along individual boundaries, focusing on convergent, divergent, and transform boundaries in isolation. However, we can better understand the distribution of features and phenomena by looking instead at the entire system.
Take, for example, the boundary found along the Mid-Atlantic Ridge in the middle of the Atlantic Ocean. At this divergent boundary, two tectonic plates are moving away from each other. As they do, magma from the mantle is added to the plates, causing them to get bigger. This phenomenon is known as seafloor spreading. The new plate material is warm and less dense than the rest of the plate. As the plate material cools, it gets denser and is pulled down and away from the ridge.
Meanwhile, convection currents move the mantle below the plates (Figure 1). The mantle is solid rock that is flowing, very slowly, like thick asphalt. It is under high pressure and is heated near the core. As the warmer mantle rock rises, it also cools, eventually pushed away by warmer rising materials and sinking back towards the core. Some of the mantle rock melts and is added to the plates along the boundary. The rest of the mantle flows below the plate, carrying the plates with it. The North American Plate is carried westward and away from the Mid-Atlantic Ridge, pushing into the Pacific Plate, thus shrinking it. Where the plates meet, additional interactions characteristic of the specific boundary types occur.
Plate tectonics system explanations
Our focus on the tectonic plate system builds on research characterizing various stages of a learning progression associated with plate tectonics. Findings from research by our partners at Pennsylvania State University suggest that the way students currently learn plate tectonics leaves them with disconnected concepts, leading to a plateau in understanding. For example, when students learn about individual plate boundaries, they have difficulty transitioning from a single boundary to the concept of a plate bounded by other plates, which all interact on all sides. This research guided the development of our interactive models, curriculum, and assessment materials.
To investigate whether or not students are developing a systems perspective of plate tectonics, we are examining student explanations for geological phenomena observed on Earth. For instance, how well can students explain the formation of the Andes Mountains or the Mid-Atlantic Ridge in terms of the underlying entities of the tectonic plate system—the plates and mantle—and the processes that occur as a result of activities the entities engage in at particular locations over long periods of geologic time?
We have developed a framework to analyze students’ written explanations, based on three key features.
- Entities are objects that comprise a system.
- Properties are well-defined characteristics of each entity.
- Activities are a series of actions and interactions produced by entities that result in changes observed over time. These actions are related to the different properties of the entities at a given location and time.
In the plate tectonics system, an expert mechanistic explanation—that is, an explanation that includes system causes and effects—should (1) identify the major entities (the plates and mantle), (2) assign and use the properties of the entities, and (3) articulate their activities (plate movements and interactions along different boundaries) in order to describe how mantle circulation, coupled with plate movement, results in phenomena observed on Earth’s surface, such as earthquakes, volcanic eruptions, and landforms.
We assess student understanding in two-part questions, with a multiple-choice component followed by an explanation. Below is an example of one multiple-choice question. Table 1 contains sample student explanations for their choices to this question, as well as our analysis of their explanations based on the entities-properties-activities framework.
Which of the following caused the separation of Africa and South America?
(1) Earth’s gravity
(2) Earth’s magnetic field
(3) Heat currents beneath the surface
(4) Earthquakes and volcanoes
(5) Wind, waves, and erosion
Student explanations help us glean a bit about what they understand regarding the underlying plate system and the causal mechanisms responsible for why Earth looks the way it does. As students become better able to describe the tectonic system, the more they are able to reason about different aspects of the dynamic Earth system they encounter in later geology units.
Why do volcanoes form where they do? How does the sea floor spread? Why do earthquakes occur at depths along convergent boundaries? How is rock formation related to tectonic environments?
Our goal is to help students develop causal mechanistic reasoning using the plate tectonics system, which can answer questions like these—and other questions they may have about our extraordinary planet.
|I think earthquakes and volcanoes cause the continents to move because in order for the earthquakes and volcanoes to occur there would have to be plates that are diverging and converging under the surface of the earth.||Though the student identifies the entities (plates) and the activities (converging and diverging), the student is clearly demonstrating reverse causality from the consequences (earthquakes and volcanoes phenomena) to plate motions, a misconception often seen in plate tectonics reasoning.|
|The core heats rock, pushes it up and causes plates to move with it. So hot rock being pushed up beneath the earth’s surface caused the continents to move with them over millions of years.||This student shows a simple mechanistic understanding, mentioning “hot rock” and “pushed up beneath the earth’s surface” as the activity moving the plate (an entity). The consequence of this activity is observed as the movement of continents over millions of years.|
|It is because the plates form those landforms to happen.||This student identifies the entities (plates), but does not include properties or activities. The consequences of the activities are described as landforms.|
|The heat currents make them [plates] move because when the crust cracks the magma comes up from the mantle pushing things out of the way and creates land which forces the plates to move.||This student includes both plates and magma as entities, and describes the activities that result both at and below the surface.|
|Because what moves the plates are movements in the mantle which are caused by heat currents because when the material near the bottom of the mantle gets heated by the core it becomes less dense and rises then the currents are separating near the crust pulling the crust apart by the heated material getting cooler, then becoming more dense causing gravity to pull down hard on it then going to the bottom near the Earth’s core becoming heated again and repeating the process.||This student includes clearly identified entities, properties of the entities, and activities responsible for the phenomenon. The answer describes how the heated mantle (entity) gets less dense (property) and rises, separating the crust (activity). It describes how materials cool (property) over time and are pulled down by gravity because they are denser (property), pulling the plate along with it (activity).|
This material is based upon work supported by the National Science Foundation under grant DRL-1621176. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.