Plate Tectonics
Grade 7 · Science · Study Guide
Learn how Earth's outer layer is broken into moving plates that drive earthquakes, volcanoes, and mountain building.
Earth's outermost layer, the lithosphere, isn't one solid shell — it's cracked into about 15 major pieces called tectonic plates. These plates float on the partially molten asthenosphere, a layer of the mantle that flows very slowly over millions of years. The plates move because of convection currents in the mantle: hot rock rises, spreads outward, cools, and sinks back down, dragging the plates along like conveyor belts.
Plates interact at their boundaries in three main ways. At divergent boundaries, plates pull apart. Magma rises to fill the gap, creating new ocean floor — this is how the Mid-Atlantic Ridge formed, and it grows by a couple centimeters every year. At convergent boundaries, plates push together. If an ocean plate meets a continental plate, the denser ocean plate sinks beneath it in a process called subduction, often triggering earthquakes and forming volcanic mountain ranges like the Andes. If two continental plates collide, neither sinks — they crumple upward, forming mountain ranges like the Himalayas. At transform boundaries, plates slide horizontally past each other, causing earthquakes. California's San Andreas Fault is a famous example.
The evidence for plate tectonics goes beyond just plate movements. The continents fit together like puzzle pieces — South America and Africa look like they were once joined. Identical fossils and rock formations appear on opposite sides of oceans. Magnetic striping on the ocean floor records reversals of Earth's magnetic field symmetrically on both sides of mid-ocean ridges, proving new seafloor is forming there.
The theory of plate tectonics explains why the map of Earth has looked very different throughout history. About 225 million years ago, all the continents were joined in one supercontinent called Pangaea. They've been slowly drifting apart ever since.
| Boundary Type | Plate Movement | Features Formed | Example |
|---|---|---|---|
| Divergent | Plates move apart | Mid-ocean ridges, rift valleys | Mid-Atlantic Ridge |
| Convergent (ocean-continent) | Plates collide; ocean plate subducts | Volcanoes, trenches | Andes Mountains |
| Convergent (continent-continent) | Plates collide; both crumple up | Mountain ranges | Himalayas |
| Transform | Plates slide past each other | Faults, earthquakes | San Andreas Fault |
Key terms
- Tectonic plates
- Large, rigid sections of Earth's lithosphere that move slowly over the asthenosphere.
- Divergent boundary
- A plate boundary where two plates move apart, allowing magma to rise and create new crust.
- Convergent boundary
- A plate boundary where two plates move toward each other, causing subduction or mountain building.
- Transform boundary
- A plate boundary where two plates slide horizontally past each other, causing earthquakes.
- Subduction
- The process where a denser plate sinks beneath a lighter plate at a convergent boundary.
- Pangaea
- The supercontinent that existed about 225 million years ago before breaking apart into today's continents.
Flashcards
- Lithosphere
- Earth's rigid outer layer composed of the crust and upper mantle; broken into tectonic plates.
- Asthenosphere
- The partially molten, slowly flowing layer of the mantle on which tectonic plates ride.
- Convection current
- Circular flow of material caused by heat differences; drives plate movement in the mantle.
- Mid-ocean ridge
- Underwater mountain chain formed at divergent boundaries where new ocean floor is created.
- Subduction zone
- Region where one plate sinks beneath another, often forming deep ocean trenches and volcanoes.
- Transform boundary
- Where plates grind past each other horizontally; source of major earthquakes.
Practice questions
What causes tectonic plates to move?
- Earth's rotation
- Convection currents in the mantle
- The pull of the Moon's gravity
- Solar wind
Show answer
Answer: Convection currents in the mantle
Convection currents in the mantle transfer heat from Earth's interior. The rising and sinking of hot and cool rock drags tectonic plates along.
The Himalayas were formed at a divergent plate boundary.
Show answer
Answer: False
The Himalayas formed at a convergent boundary where the Indian Plate and Eurasian Plate collided, pushing rock upward into mountains.
The supercontinent that existed about 225 million years ago is called ___.
Show answer
Answer: Pangaea
Alfred Wegener proposed the existence of Pangaea in 1912 based on the matching shapes of continents and matching fossil evidence.
At which type of boundary do plates slide horizontally past each other?
- Divergent
- Convergent
- Transform
- Subduction
Show answer
Answer: Transform
At transform boundaries, plates grind horizontally past each other. This movement releases stress as earthquakes. The San Andreas Fault is a well-known example.
Subduction occurs when a denser oceanic plate sinks beneath a lighter plate.
Show answer
Answer: True
Oceanic crust is denser (made of basalt) than continental crust (made of granite), so it sinks — or subducts — beneath the lighter plate.
Fun facts
- The Himalayas are still growing — India is slowly crashing into Asia at about 5 cm per year.
- The Pacific Ocean is actually getting smaller as plates around it slowly converge, while the Atlantic Ocean is slowly widening.
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