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

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?

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: http://earthguide.ucsd.edu/eoc/teachers/t_tectonics/p_sonar.html#sthash.EpeIGLRk.dpuf

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

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

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

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

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

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

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).

Changes in Sediment Thickness

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

Patterns in Heat Flow

How Seafloor Spreading Works Animations: http://www.nature.nps.gov/GEOLOGY/usgsnps/animate/A49.gif http://www.nature.nps.gov/GEOLOGY/usgsnps/animate/A55.gif 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.

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: http://www.scotese.com/sfsanim.htm.