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Earth’s outer shell is broken into large, rigid tectonic plates that move slowly over the softer asthenosphere below. Where plates meet, their motion builds mountains, opens oceans, triggers earthquakes, and feeds volcanoes. Plate boundaries matter because they explain the patterns of major landforms and many natural hazards on Earth. Even a few centimeters of motion per year can reshape continents over millions of years.
There are three main types of plate boundaries: divergent, convergent, and transform. At divergent boundaries, plates move apart and magma rises to create new crust, often forming mid-ocean ridges or rift valleys. At convergent boundaries, plates collide, causing subduction, mountain building, volcanoes, and deep ocean trenches. At transform boundaries, plates slide past one another, storing stress that can be released suddenly as earthquakes.
Key Facts
- Plate speed is usually about 1 to 10 cm per year.
- Divergent boundary: plates move apart and new crust forms as magma cools.
- Convergent boundary: plates move toward each other, causing subduction or mountain building.
- Transform boundary: plates slide sideways past each other, often producing earthquakes.
- Average speed formula: speed = distance ÷ time.
- Oceanic crust is denser than continental crust, so oceanic crust usually subducts during collision.
Vocabulary
- Lithosphere
- The rigid outer layer of Earth made of the crust and the uppermost mantle.
- Asthenosphere
- The softer, slowly flowing layer of the upper mantle beneath the lithosphere.
- Subduction
- The process in which one tectonic plate sinks beneath another into the mantle.
- Divergent boundary
- A plate boundary where two plates move away from each other and new crust is created.
- Transform boundary
- A plate boundary where two plates slide horizontally past each other.
Common Mistakes to Avoid
- Thinking continents drift by themselves is wrong because continents are part of larger tectonic plates that include ocean floor.
- Assuming all plate boundaries make volcanoes is wrong because transform boundaries mainly produce earthquakes and usually do not create magma.
- Confusing convergent and divergent boundaries is wrong because convergent plates move together while divergent plates move apart.
- Believing plate motion is too slow to matter is wrong because small yearly movements add up to hundreds or thousands of kilometers over geologic time.
Practice Questions
- 1 A tectonic plate moves 4 cm per year. How far will it move in 2 million years? Give your answer in kilometers.
- 2 Two plates move away from a mid-ocean ridge at 3 cm per year each. What is the total rate at which the ocean basin widens in cm per year, and how far does it widen in 1 million years?
- 3 A coastline has frequent shallow earthquakes but no major volcanoes, and rock layers on opposite sides of a fault appear offset sideways. Which type of plate boundary is most likely present, and what evidence supports your answer?