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The Missing Piece of Wegener’s Continental Drift Puzzle
 is the study of the "beds" or "floors" of water
 Bathymetric maps illustrate the land that lies
underwater (like topographic maps).
 How do we make
these types of maps?
 Produce sound waves that travel in all directions,
bounce off the nearest object, then return to the
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:
 Mid-Ocean Ridges
 Deep Sea Trenches
 Abyssal Plains
 Long chains of mountains that rise above the seafloor
 Found at the edge of continents or in the sea near
chains of active volcanoes
 Flat areas on the ocean floor
 Many are dotted with underwater volcanoes
Scientists came up with this idea based
on various observations that were made
about the seafloor.
 Patterns in seafloor magnetism
 Changes in sediment thickness away from mid-
ocean ridges
 Age of seafloor rock
 Patterns in heat flow
 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).
 Seafloor is youngest at the mid-ocean ridges and becomes
progressively older with distance from the ridge.
1. Hot, buoyant magma from the mantle rises up a mid-ocean ridge.
2. The magma erupts as lava and forms new seafloor. Magnetic polarity
is set when rock cools.
3. The newly-formed rock is pushed away from the ridge axis as more
lava erupts.
4. If the oceanic crust reaches a deep sea trench, it sinks into the trench
and is lost into the mantle.
 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: