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Seafloor Spreading
What evidence do scientists have to
support the fact that the Earth’s
crust is continuously moving?
Mapping the Ocean Floor
• Until the mid-1900s people thought that
the ocean floor was flat
SONAR
• A device used to map
ocean floor
•Sound waves
reflected back from
ocean floor
•Time measured and
used to calculate
distance to ocean floor
Seafloor Spreading
• Hypothesis that new ocean crust is formed
at mid-ocean ridges and destroyed at
deep sea trenches
• Occurs in a continuous cycle
• Ocean Ridges
– Underwater Mountain Chain
– Divergent Plate boundaries
• Deep Sea Trenches
– Narrow Elongated depression in seafloor with
very steep sides
– Convergent Plate boundaries
1. Magma (hotter and less dense) forced up and
fills gap in ridge
2. When hardens adds new ocean floor
3. As spreading occurs, more magma is forced
upward and the crust moves away from ridge
4. Crust is destroyed by subduction at trenches
Rock Ages
• Ridges – Younger
• Trenches – Older
• Ocean Crust – 180 Million Years Old
• Continental Crust – 3.8 Billion Years Old
Magnetism
• Paleomagnetism
– Study of Earth’s magnetic field
– Basalt is iron rich which produces magnetic
readings; acts as a compass
• Magnetic Reversal
What causes the plates to
move???
• Convection Currents
of magma in the
mantle
Trench
Ridge
Wegener’s Flaws
• Seafloor Spreading answers Wegener’s
Flaws!
– Convection Currents explain HOW the
continents “drifted”
– Continents not pushed through crust
rather they are passengers as ocean
crust moves away from ridges