1. The Missing Piece of Wegener’s Continental Drift Puzzle
Seafloor Spreading
The Missing Piece of Wegener’s Continental Drift Puzzle

2. Bathymetry is the study of the "beds" or "floors" of water bodies
Bathymetric maps illustrate the land that lies underwater (like topographic maps).
How do we make these types of maps?

3. Echo Sounders
Produce sound waves that travel in all directions, bounce off the nearest object, then return to the sounder.
Echo sounders calculate water depth by measuring the time it takes for the acoustic signal to reach the bottom and the echo to return to the ship.
(Courtesy of Naval Meteorology and Oceanography Command)
This animation shows how sound waves are used to create pictures of the sea floor and ocean crust:

4. Major Features of the Ocean Floor
Mid-Ocean Ridges
Deep Sea Trenches
Abyssal Plains

5. Mid-Ocean Ridges
Long chains of mountains that rise above the seafloor

6. Deep Sea Trenches
Found at the edge of continents or in the sea near chains of active volcanoes

7. Abyssal Plains Flat areas on the ocean floor
Many are dotted with underwater volcanoes

8. What Formed These Features?
Seafloor Spreading!!!
Scientists came up with this idea based on various observations that were made about the seafloor.

9. Observations/Evidence
Patterns in seafloor magnetism
Changes in sediment thickness away from mid- ocean ridges
Age of seafloor rock
Patterns in heat flow

10. Seafloor Magnetism
Alternating stripes of normal and reversed magnetic polarity in seafloor crust spread away from mid-ocean ridges in symmetrical patterns.
Stripes end abruptly at edges of continents (and sometimes at deep sea trenches).

11. Changes in Sediment Thickness

12. Age of Seafloor Rock
Seafloor is youngest at the mid-ocean ridges and becomes progressively older with distance from the ridge.

13. Patterns in Heat Flow

14. How Seafloor Spreading Works
Animations:
Hot, buoyant magma from the mantle rises up a mid-ocean ridge.
The magma erupts as lava and forms new seafloor. Magnetic polarity is set when rock cools.
The newly-formed rock is pushed away from the ridge axis as more lava erupts.
If the oceanic crust reaches a deep sea trench, it sinks into the trench and is lost into the mantle.

15. Seafloor Spreading & Continental Drift
An explanation of Seafloor Spreading finally offered scientists an explanation for HOW Wegener’s proposed Continental Drift could have occurred.
Convection currents within the mantle take the continents on a conveyor-belt ride of oceanic crust that over millions of years takes them around the planet’s surface
The breakup of Pangaea by seafloor spreading is seen in this animation: