Plate Tectonics
Grade 8 · Science · Study Guide
Discover how Earth's tectonic plates move and the geological features their interactions create.
Earth's lithosphere is divided into about 15 major tectonic plates that move slowly over the asthenosphere at a rate of a few centimeters per year. This movement is driven by convection currents in the mantle and creates most of Earth's major geological features.
Plates interact at three types of boundaries: convergent (plates move toward each other, forming mountains or trenches), divergent (plates move apart, forming rift valleys or mid-ocean ridges), and transform (plates slide past each other horizontally, causing earthquakes like along the San Andreas Fault).
Alfred Wegener proposed the theory of continental drift in 1912, noting that continents fit together like puzzle pieces. Evidence includes matching fossils across continents, similar rock formations on different continents, and paleomagnetic striping on the ocean floor — all supporting the idea of one ancient supercontinent called Pangaea.
| Boundary Type | Movement | Features Created | Example |
|---|---|---|---|
| Convergent | Plates move together | Mountains, trenches, volcanoes | Himalayas, Mariana Trench |
| Divergent | Plates move apart | Rift valleys, mid-ocean ridges | Mid-Atlantic Ridge |
| Transform | Plates slide past each other | Earthquakes, fault lines | San Andreas Fault |
| Evidence | Explanation |
|---|---|
| Puzzle-piece fit of continents | Africa and South America shorelines match closely |
| Matching fossils | Same species found on continents now separated by oceans |
| Similar rock formations | Identical rock layers across ocean-separated continents |
| Seafloor spreading | New crust forms at mid-ocean ridges as plates diverge |
Key terms
- Tectonic Plate
- A large section of Earth's lithosphere that moves slowly on the asthenosphere.
- Convergent Boundary
- Where two tectonic plates move toward each other; creates mountains or ocean trenches.
- Divergent Boundary
- Where two plates move apart; creates rift valleys or mid-ocean ridges.
- Subduction
- When a denser plate sinks beneath a less dense plate at a convergent boundary.
Practice questions
At which type of plate boundary would you expect to find mid-ocean ridges?
- Convergent
- Transform
- Divergent
- Subduction
Show answer
At divergent boundaries, plates move apart and magma rises to fill the gap, creating new seafloor and mid-ocean ridges.
The San Andreas Fault in California is an example of a ___ boundary.
- Convergent
- Divergent
- Transform
- Subduction
Show answer
The San Andreas Fault is where the Pacific and North American plates slide horizontally past each other — a transform boundary.
Alfred Wegener's theory of continental drift was immediately accepted by the scientific community.
Show answer
Wegener's theory was largely rejected in his lifetime because he couldn't explain the mechanism. Seafloor spreading evidence in the 1960s finally confirmed it.
When a denser oceanic plate sinks beneath a less dense plate at a convergent boundary, the process is called ___.
Show answer
Subduction creates deep ocean trenches and volcanic arcs on the overriding plate.
Describe two pieces of evidence that support the theory that all continents were once joined as Pangaea.
Show answer
These multiple independent lines of evidence strongly support Wegener's continental drift theory.
Fun facts
- The Himalayas, Earth's tallest mountain range, formed and continue to grow because the Indian plate is still pushing into the Eurasian plate.
- The Pacific Ring of Fire, a zone around the Pacific Ocean, has about 75% of the world's volcanoes and 90% of all earthquakes due to intense tectonic activity.
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