Types of Plate Boundaries

Why This Matters

Plate boundaries are where Earth's geology comes alive—they're the zones where tectonic plates interact, and understanding these interactions is fundamental to explaining everything from earthquake hazards to mountain building to volcanic activity. You're being tested on your ability to connect plate motion direction to geological outcomes: why does one boundary create mountains while another creates ocean floor? Why do some produce violent earthquakes while others generate volcanic eruptions?

The key insight is that plate boundary type determines geological consequences. Each boundary represents a different relationship between plates—collision, separation, or lateral sliding—and each relationship produces predictable features. Don't just memorize that the Himalayas are at a convergent boundary; know why continental collision builds mountains instead of volcanoes, and how that differs from oceanic-continental convergence. That conceptual understanding is what earns you points on FRQs.

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Boundaries Where Plates Separate

When plates move apart, mantle material rises to fill the gap, creating new crust through volcanic activity and generating tensional stress that produces shallow earthquakes.

Divergent Boundaries

• New oceanic crust forms as magma rises from the mantle to fill the gap between separating plates—this is where Earth literally grows
• Mid-ocean ridges mark underwater divergent boundaries, featuring volcanic activity and hydrothermal vents that support unique ecosystems
• Rift valleys form when divergence occurs on land, stretching and thinning continental crust (the East African Rift is actively splitting the African continent)

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Boundaries Where Plates Collide

When plates move toward each other, the outcome depends entirely on what type of crust is involved—oceanic crust is denser and sinks, while continental crust is buoyant and resists subduction.

Subduction Zones

• Oceanic plate descends beneath another plate because oceanic crust is denser than continental crust—this density difference drives the entire process
• Deep ocean trenches mark where the descending plate bends downward (the Mariana Trench reaches nearly 11,000 meters depth)
• Volcanic arcs form as the subducting plate releases water into the mantle, lowering the melting point and generating magma that rises to the surface

Oceanic-Continental Convergence

• Volcanic mountain ranges form on the continental side as magma from the subducting oceanic plate rises through the overriding crust
• Intense seismic activity occurs at multiple depths—shallow earthquakes near the trench, deeper earthquakes along the descending slab
• The Andes Mountains exemplify this boundary type, with the Nazca Plate subducting beneath the South American Plate

Continental Collision Boundaries

• Mountain ranges form without volcanism because neither continental plate is dense enough to subduct—instead, crust crumples and thickens
• Intense metamorphism occurs as rocks experience extreme pressure and deformation during collision
• The Himalayas continue rising today as the Indian Plate pushes into the Eurasian Plate at approximately 5 cm per year

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Boundaries Where Plates Slide Past

When plates move laterally past each other, no crust is created or destroyed—but the friction between plates stores enormous elastic energy that releases as earthquakes.

Transform Boundaries

• Horizontal plate motion along strike-slip faults means crust is neither created nor destroyed—these are conservative boundaries
• Powerful shallow earthquakes result from sudden release of built-up stress when locked fault segments slip (elastic rebound theory)
• The San Andreas Fault is Earth's most famous transform boundary, where the Pacific Plate slides northwest past the North American Plate

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Quick Reference Table

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Self-Check Questions

1. Which two boundary types both involve plates moving toward each other, but produce fundamentally different geological features? What determines the difference?

2. A region experiences frequent shallow earthquakes but has no volcanic activity and no mountain building. What type of plate boundary is most likely responsible?

3. Compare and contrast the Himalayas and the Andes Mountains—both are major mountain ranges at convergent boundaries, so why does one have active volcanoes while the other does not?

4. If an FRQ asks you to explain how new oceanic crust forms, which boundary type and specific example should you reference? What process creates the new rock?

5. Why do subduction zones produce earthquakes at a range of depths (shallow to deep) while transform boundaries produce only shallow earthquakes?