Lab 4.1

*** This data lab is under construction and has not yet been published. We anticipate releasing this edition of the plate tectonics data lab before December 1, 2025 ***

Lab 4.1 – Plate Boundaries

Fundamental concept: Identify plate tectonic boundaries and seafloor features on maps
Estimated time to complete: 15-20 minutes
Data skills preparation: Lab 3- Visualizing the ocean with maps and charts,
Materials needed: Computer

Many of the most geologically interesting landscapes on Earth occur at the boundaries of tectonic plates, and most geological activity is concentrated along these boundaries too. In the introduction to the lab, you likely noticed that there is a plate boundary that runs along the West Coast of the United States, which is characterized by steep mountain ranges, volcanoes, earthquakes and tsunamis. You may recall from lab 2 that when a continental margin coincides with the location of a plate boundary, such as the case along the West Coast, we describe it as an active margin. Conversely, when a continental margin does not coincide with the location of a plate boundary, as is the case on the East Coast of the US, we describe this type of margin as a passive margin. Although the Appalachian Mountains are a prominent geological feature on the East Coast, they are considerably older than West Coast mountains, and have been exposed to far more weathering and erosion. As a result, they have considerably lower relief than West Coast mountains, and contribute high volumes of sand to our broad, sandy East Coast beaches.

There are three dominant types of plate boundaries that we’ll explore in this module of the lab. Convergent plate boundaries occur where two plates are moving toward one another, divergent plate boundaries occur where two plates are pulling away from one another, and transform plate boundaries occur where two plates are sliding past one another.

Figure 3.1.1 Mid-ocean ridges are locations where two tectonic plates are moving apart (Right) (Copyright: Benjamin R. Jordan, used with permission; Map imagery from the Global-Resolution Topography (GMRT) Synthesis, funded by the National Science Foundation (NSF).  Deep sea trenches form where subduction occurs (Left). (Image copyright: Benjamin R. Jordan, used with permission; Map imagery from Google Earth).

 

The widget below now contains color-coded plate boundaries so that you can explore the relationship between each plate boundary type with variation in topography, seafloor age, and with the coinciding geological features. Use the widget below to answer the following questions.

How to use the widget: 

• If you hover over the map and scroll with your mouse, the map will zoom in or out. 
• You may also use the zoom features on the left to zoom in/out of the area in a more controlled method. 
• You can move the map around by clicking and dragging it. 
• To overlay the age of the seafloor and the locations of tectonic plate boundaries, earthquakes and volcanoes, toggle these layers on by [de-]selecting the boxes beside the layer name. 
• Use the measurement tool (arrow button) to measure the distance between two locations of your choosing. Select the measurement tool, click a location on the map, and then click the second location to measure the distance. Select the “X” button to remove all measurements and to stop measuring additional distances.

**** INSERT MODIFIED WIDGET FROM LINK BELOW ****

https://datalab.marine.rutgers.edu/explorations/notebook3/lab3_tectonics1.htm

Orientation Questions

**** TEXT BELOW IS NOT FINAL; NEED VIDEO – ABBREVIATE ****

Divergent Plate Boundaries (Juan de Fuca Ridge)

Divergent plate boundaries occur where two tectonic plates are pulling apart from one another, typically between two oceanic plates or between two continental plates. When two continental plates pull apart from one another, a continental rift is produced. If rifting continues long enough, the crust can become so thin that it breaks. Volcanoes may begin to erupt new oceanic crust! In the new oceanic crust, a feature called a mid-ocean ridge (MOR) forms. Here, volcanoes erupt to form the youngest seafloor crust. As the plates continue to spread and volcanoes erupt, a mirrored pattern emerges in the age of the seafloor, with the youngest crust located near the ridge, and gradually getting older on either side. For this reason, the oldest crust is located near the continents. This observation provided some of the earliest and most impactful evidence for the modern plate tectonics theory. Watch the video below to learn more about divergent plate boundaries from rifting to mid-ocean ridge spreading!

Mid-ocean ridges are by far the most common type of divergent plate boundary. As the two plates split away from each other, the mantle rises toward the surface, melting and fueling volcanic eruptions on the seafloor, which results in the generation of new oceanic crust. As more and more magma is erupted, forming progressively younger and younger seafloor, the two plates are pushed apart. This results in a mirrored pattern in the age of oceanic crust, where the crust is youngest at the axis of the mid-ocean ridge and oldest far away from the ridge axis. You can observe this pattern easily by “zooming to” the North Atlantic or Mid-Atlantic regions. Because the axis of the mid-ocean ridge is relatively warm, buoyant, and being uplifted by the rising mantle, the split forms a ridge that gets progressively deeper as you move away from the axis of the ridge. The closest divergent plate boundary to the United States is an oceanic-oceanic style boundary, which has produced a mid-ocean ridge called the “Juan de Fuca Ridge.” Here, The Axial Seamount, one of the most active volcanoes in the United States (and the topic of lab 5) rests 1,500 m beneath the sea surface. Explore this plate boundary using the “zoom to” feature and selecting “Pacific Northwest.”

Continents can also split apart, sometimes stretching and thinning the crust so much that it will break, causing volcanic activity. If this continental rifting is successful, the stretching will continue until new oceanic crust starts to form between the two now-separated continents in the form of a mid-ocean ridge. There is no better example on Earth of continental rifting than in the East African Rift region, where three plates, the African Plate, the Arabian Plate, and the Somalian Plate are pulling apart from one another. Here, the rifting results in some of the most interesting terrestrial and submarine volcanoes on Earth! Explore this unique plate boundary using the “zoom to” feature and selecting “East African Rift.”

Convergent Plate Boundaries (Western US – Cascadia Subduction Zone)

Convergent plate boundaries occur where two tectonic plates are colliding toward each other, and can occur when two lithospheric plates collide that are oceanic and oceanic, continental and continental, or when a continental and oceanic plate collide.The features that form and phenomena that occur along convergent plate boundaries depend almost exclusively on the types of lithosphere involved. Watch the video below to learn more about the tectonic features that form at convergent plate boundaries.

Continental – Oceanic Convergent Plate Boundaries

When a continental and oceanic plate collide with one another, the oceanic plate will be recycled back into the Earth’s mantle through the process of subduction because it is more dense (3.3 g/cm3) than the continental plate (2.7 g/cm3). The oceanic crust is destroyed, providing the fuel to form volcanoes in the overriding continental plate. As the oceanic crust subducts back into the Earth’s mantle, friction between the two plates causes the oceanic crust to bow, forming deep ocean trenches along these subduction zones. The Pacific Northwest is a fantastic example of this type of plate boundary. Here, the Juan de Fuca Plate is subducting beneath the North American Plate, producing some of the most iconic volcanoes in the United States! Use the “zoom to” feature on the widget to select the “Pacific Northwest” to explore this plate boundary

Oceanic – Oceanic Convergent Plate Boundaries

When two oceanic plates collide with one another, the more dense of the two oceanic plates will be recycled back into the Earth’s mantle through the process of subduction. Typically, the older of the two plates will subduct because it is colder and more dense. The deepest ocean trenches also form at these subduction zones, and volcanoes form in the overriding oceanic plate. One of the best examples of this type of plate boundary occurs in the deepest part of the Ocean basin–the 10,984m Mariana Trench! Here, the Pacific Plate is subducting beneath the Mariana, producing the trench and the Mariana Islands. To explore this plate boundary, use the “zoom to” feature and select “Mariana Islands.”

Continental-Continental Convergent Plate Boundaries

When two continental plates collide with one another, neither of the two plates subduct, but rather, both plates uplift to form some of the tallest mountain ranges on Earth. This happens because continental crust has a much lower density than oceanic crust, which allows it to buoyantly “float.” Continental crust is also considerably thicker than that of oceanic crust, making continental lithosphere nearly impossible to subduct. Without subduction, there is no “fuel” for volcanism, and therefore volcanoes do not form at this type of plate boundary. The Himalayan Mountains provide one of the greatest examples of this type of plate boundary. Here, the Indian Plate is colliding with the Eurasian Plate, producing many of Earth’s tallest mountains,including Mount Everest, which stands 8,848 m high! Explore this plate boundary using the “zoom to” feature and selecting “Himalayas.”

Transform Plate Boundaries (Western US – San Andreas Fault)

When two plates slide past each other, neither moving away from or towards each other, this is a transform plate boundary. Perhaps the most famous, at least to North Americans, is the San Andreas Fault, where the western part of California is sliding north past the rest of North America. These plate boundaries always connect other types of plate boundaries–they “transform” movement at one boundary to movement at another. Use the “zoom to” feature and select “Western US” to explore the San Andreas Fault.

Interpretation Questions

1. Where in the Pacific ocean do you observe the oldest oceanic crust? Does it coincide with a specific plate boundary type? 
2. In the Atlantic Ocean where in the oceans do you observe the youngest oceanic crust? Does it coincide with a specific plate boundary type?

Reflection Questions

1. Mid-ocean ridges produce a mirrored pattern in the age of seafloor crust on either side of the ridge axis. You can see this beautifully in the Atlantic Ocean. Observe the Pacific Ocean. Why do you think we are not able to observe the mirrored pattern in the Pacific Ocean? 
2. Because of the configuration of the plate boundaries, some oceans are growing to a greater size, while others are shrinking. Is the Atlantic Ocean growing or shrinking? The Pacific? How do you know?
3. Like the East Coast Appalachian Mountains, the Colorado Rocky Mountains are located far from a tectonic plate boundary. Why do you think that there is a large mountain range located here?