Plate Tectonics
Grade 6 · Science · Study Guide
Understand how Earth's moving plates shape the continents, mountains, and ocean floors.
Earth might look solid and still, but its outer shell is actually broken into giant puzzle pieces called tectonic plates. These plates float on the hot, partially molten rock of the mantle called the asthenosphere. The plates move extremely slowly — about 2 to 10 centimeters per year, roughly the speed your fingernails grow.
There are three types of plate boundaries, and each creates different landforms. Divergent boundaries form where plates move apart. Magma rises to fill the gap, creating new ocean floor. The Mid-Atlantic Ridge is a famous divergent boundary that splits the Atlantic Ocean floor. Convergent boundaries form where plates collide. When two continental plates collide, they crumple upward to form mountain ranges like the Himalayas. When an oceanic plate collides with a continental plate, the denser oceanic plate dives beneath in a process called subduction, often creating deep ocean trenches and volcanoes. Transform boundaries form where plates slide horizontally past each other, causing earthquakes. The San Andreas Fault in California is a classic transform boundary.
The idea that continents were once connected was proposed by Alfred Wegener in 1912 as the theory of continental drift. He noticed that South America and Africa look like puzzle pieces that fit together, and that the same fossils were found on both continents. Later evidence from ocean floor mapping confirmed that the sea floor was spreading, giving rise to the modern theory of plate tectonics.
| Boundary Type | Plate Movement | Landforms Created | Example |
|---|---|---|---|
| Divergent | Plates move apart | Mid-ocean ridges, rift valleys | Mid-Atlantic Ridge |
| Convergent (ocean-continent) | Oceanic plate subducts | Ocean trenches, volcanoes | Cascade Range |
| Convergent (continent-continent) | Plates crumple together | Mountain ranges | Himalayas |
| Transform | Plates slide sideways | Faults, earthquakes | San Andreas Fault |
Key terms
- Tectonic plates
- Giant slabs of Earth's lithosphere that move slowly on the asthenosphere.
- Divergent boundary
- A plate boundary where two plates move apart, creating new crust.
- Convergent boundary
- A plate boundary where two plates collide; can form mountains or ocean trenches.
- Transform boundary
- A plate boundary where two plates slide horizontally past each other, causing earthquakes.
- Subduction
- The process where a denser oceanic plate dives beneath another plate into the mantle.
- Continental drift
- Alfred Wegener's theory that continents were once connected and have slowly drifted apart.
- Asthenosphere
- The partially molten, plastic layer of the mantle on which tectonic plates move.
- Mid-ocean ridge
- An underwater mountain range formed at a divergent boundary where new seafloor is created.
Flashcards
- Tectonic plates
- Giant slabs of Earth's crust and upper mantle that move on the asthenosphere.
- Divergent boundary
- Where plates pull apart; creates new oceanic crust and mid-ocean ridges.
- Convergent boundary
- Where plates collide; creates mountains, trenches, or volcanoes.
- Transform boundary
- Where plates slide past each other horizontally; causes earthquakes.
- Subduction
- Denser oceanic plate sinks beneath another plate into the mantle.
- Continental drift
- Wegener's theory that continents were once joined and have since drifted apart.
Practice questions
At which type of plate boundary do plates slide horizontally past each other?
- Divergent
- Convergent
- Transform
- Subduction
Show answer
Answer: Transform
At transform boundaries, plates grind past each other sideways, releasing energy as earthquakes. The San Andreas Fault is a well-known example.
The Himalayas were formed where two continental plates collided.
Show answer
Answer: True
The Indian Plate crashed into the Eurasian Plate about 50 million years ago, pushing up the Himalayan mountains — and they're still growing.
The process where an oceanic plate dives beneath another plate is called ___.
Show answer
Answer: subduction
Subduction occurs when a denser oceanic plate slides under a lighter continental plate, diving into the hot mantle where it melts.
Who proposed the theory of continental drift?
- Isaac Newton
- Charles Darwin
- Alfred Wegener
- Albert Einstein
Show answer
Answer: Alfred Wegener
Wegener proposed continental drift in 1912, noting the jigsaw fit of continents and matching fossils across oceans.
New ocean floor is created at divergent plate boundaries.
Show answer
Answer: True
At divergent boundaries, magma rises to fill the gap between separating plates, cooling to form new oceanic crust — this is called seafloor spreading.
Fun facts
- The Himalayas — Earth's tallest mountains — are still growing about 5 mm per year as India continues to push into Asia.
- Alfred Wegener was a meteorologist, not a geologist, and was widely ridiculed for his continental drift idea during his lifetime. He was proven right decades after his death.
- There is enough heat inside Earth to melt rock — the temperature at Earth's core is about 5,400°C, almost as hot as the surface of the Sun.
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