Plates Shape
Continents
Discover how continental shapes and seafloor features provide powerful evidence that Earth's tectonic plates are in constant motion.
What Are Tectonic Plates?
Earth's outer shell, the lithosphere, is broken into large, rigid pieces called tectonic plates. These plates float on the hot, slowly flowing asthenosphere below them: like crackers floating on thick soup.
The Continental Puzzle
In 1912, Alfred Wegener noticed that the coastlines of South America and Africa fit together like jigsaw pieces. He proposed that all continents were once joined in a supercontinent called Pangaea.
Evidence Beyond Shape
Matching coastlines alone weren't enough to convince scientists. Additional geological evidence sealed the case for plate motion:
Seafloor Spreading: The Engine
Mid-ocean ridges are underwater mountain chains where hot mantle material rises, cools, and forms new oceanic crust. This process pushes plates apart.
The Plate Tectonics Rule
Three Types of Plate Boundaries
Continental Drift vs. Plate Tectonics
| Continental Drift | Plate Tectonics | |
|---|---|---|
| Proposed by | Alfred Wegener (1912) | Multiple scientists (1960s) |
| Main idea | Continents move through ocean crust | Entire plates (continent + ocean floor) move together |
| Mechanism | Unknown at the time | Convection currents in mantle |
| Evidence used | Coastline fit, fossils, rocks | All of Wegener's + seafloor data |
| Accepted? | Rejected for decades | Widely accepted today |
How to Evaluate Evidence of Plate Motion
Example 1: Coastline Fit
Example 2: Fossil Evidence
Example 3: Seafloor Age Pattern
Magnetic Stripes on the Seafloor
As new oceanic crust cools at a ridge, iron minerals lock in the direction of Earth's magnetic field. Since the field flips (reverses polarity) periodically, the seafloor records these reversals as alternating stripes.
Modern Evidence: GPS Tracking
Today, scientists use GPS satellites to directly measure plate movement. Results confirm that plates move at rates of \(1\) to \(15\) cm per year.
Common Mistakes to Avoid
Evidence Summary
| Evidence Type | What It Shows | Example |
|---|---|---|
| Coastline fit | Continents were once joined | South America + Africa |
| Fossil matches | Same species, different continents | Mesosaurus, Glossopteris |
| Rock/mountain matches | Continuous formations split apart | Appalachians + Scottish Highlands |
| Seafloor age pattern | New crust at ridges, old near edges | Mid-Atlantic Ridge |
| Magnetic stripes | Symmetrical polarity reversals | Mirror stripes at all ridges |
| GPS measurements | Plates are moving right now | \(2.5\) cm/yr Atlantic widening |
Question 1 of 5
The coastlines of South America and Africa appear to fit together like puzzle pieces. What is the best explanation for this observation?
Question 2 of 5
Mesosaurus was a freshwater reptile whose fossils are found in both South America and Africa. Why is this strong evidence for plate motion?
Question 3 of 5
At a mid-ocean ridge, scientists find that the oceanic crust closest to the ridge is the youngest. What does this pattern tell us?
Question 4 of 5
Scientists observe a symmetrical pattern of magnetic stripes on either side of a mid-ocean ridge. What causes this pattern?
Question 5 of 5
A student claims: "Continental drift and plate tectonics are the same theory." Which statement best corrects this misconception?