Earth and Space Science / Unit 3: Plate Tectonics
3.4 Seafloor Spreading
Notes
In the 1950s-60s, mapping of the ocean floor supported drift.
- A mid-ocean ridge is an underwater mountain chain where new crust forms as magma rises and cools.
- Seafloor spreading: new crust forms at the ridge and moves away on both sides.
- Magnetic stripes: rocks record Earth's magnetic field when they cool. Alternating normal and reversed stripes are symmetrical on both sides of a ridge.
- Rock is youngest at the ridge and gets older farther away. Oldest ocean crust is only about 180 million years old.
- Example: ridge spreading at 2 cm/year moves 2 cm × 1,000,000 = 20 km in 1 million years.
Practice
Where is the youngest ocean crust found?
New crust forms at the ridge.
The seafloor spreads at 4 cm per year. How far does it move in 5 million years?
$4 \text{ cm/yr} \times 5{,}000{,}000 = 20{,}000{,}000$ cm $= 200$ km.
Symmetrical magnetic stripes on both sides of a ridge show that:
The pattern records magnetic reversals as crust spreads.
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