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Plate Boundaries
How Plates Build Mountains Volcanoes and Earthquakes
Earth’s outer shell is broken into tectonic plates that move slowly over the softer, flowing asthenosphere beneath them. Where plates meet, their motion creates many of the planet’s most dramatic features, including mountain ranges, volcanoes, ocean trenches, and earthquake zones. Understanding plate boundaries helps explain why geologic hazards are concentrated in certain regions instead of being spread evenly across Earth. It also connects surface landforms to powerful processes deep inside the planet.
There are three main types of plate boundaries: convergent, divergent, and transform. At convergent boundaries, plates collide, causing subduction, mountain building, volcanic arcs, or deep earthquakes depending on the plate types involved. At divergent boundaries, plates move apart and magma rises to create new crust, while at transform boundaries, plates slide past each other and often produce shallow earthquakes. These interactions are driven by heat inside Earth, mantle convection, slab pull, and ridge push.
Key Facts
- Plate speed is usually measured in centimeters per year, about the same rate that fingernails grow.
- Convergent boundary: plates move toward each other and may form mountains, trenches, volcanoes, and deep earthquakes.
- Divergent boundary: plates move apart and new crust forms as magma rises and cools.
- Transform boundary: plates slide horizontally past each other and commonly produce shallow earthquakes.
- Average speed formula: speed = distance / time.
- Earthquake energy travels as seismic waves, including P waves, S waves, and surface waves.
Vocabulary
- Tectonic plate
- A large, rigid piece of Earth’s lithosphere that moves slowly over the asthenosphere.
- Lithosphere
- The stiff outer layer of Earth made of the crust and the uppermost mantle.
- Subduction
- The process in which one tectonic plate sinks beneath another into the mantle at a convergent boundary.
- Asthenosphere
- The hotter, weaker layer of the upper mantle that can flow slowly and allows tectonic plates to move.
- Seismic wave
- A wave of energy released by an earthquake that travels through Earth or along its surface.
Common Mistakes to Avoid
- Thinking plates move because continents float across the ocean, which is wrong because continents are embedded in larger lithospheric plates that include both continental and oceanic crust.
- Confusing magma and lava, which is wrong because magma is molten rock below Earth’s surface and lava is molten rock after it erupts onto the surface.
- Assuming all plate boundaries make volcanoes, which is wrong because transform boundaries usually produce earthquakes without major magma production.
- Treating earthquakes as random events everywhere, which is wrong because most earthquakes occur along plate boundaries where stress builds and is suddenly released.
Practice Questions
- 1 A tectonic plate moves 250 km in 10 million years. What is its average speed in cm per year?
- 2 Two plates move away from a mid-ocean ridge at 3 cm per year on each side. How much wider does the ocean basin become in 1 million years?
- 3 Explain why the Andes Mountains have both high peaks and many volcanoes, while the San Andreas Fault has many earthquakes but few volcanoes.