5.4: Bathymetric Evidence for Seafloor Spreading
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How can war further science?
Oddly, the thing that was needed to further Wegener's continental drift idea was World War II. U.S. Navy ships cruised the seas. They carried instruments that helped scientists to locate the mechanism for moving continents.
During World War II, battleships and submarines carried echo sounders. Their goal was to locate enemy submarines (Figure below). Echo sounders produce sound waves that travel outward in all directions. The sound waves bounce off the nearest object and then return to the ship. Scientists know the speed of sound in seawater. They then can calculate the distance to the object that the sound wave hit. Most of these sound waves did not hit submarines. They instead were used to map the ocean floor.
A ship sends out sound waves to create a picture of the seafloor below it. The echo sounder pictured has many beams, and, as a result, it creates a three dimensional map of the seafloor beneath the ship. Early echo sounders had only a single beam and created a line of depth measurements.
Scientists expected the seafloor to be flat and featureless. So they were shocked by what they saw: tremendous topographical features like mountain ranges, rifts, and trenches. Oceanographic research vessels continue to map the seafloor as they sail across the seas today. The map below is a modern map with data from several decades.
The major features of the ocean basins and their colors on the map include:
- mid-ocean ridges: A long chain of mountains that rises up high above the deep seafloor. An example is the light blue gash in middle of Atlantic Ocean. Light blue signifies higher elevation than dark blue.
- rift zones: In the middle of the mid-ocean ridges is a rift zone. The rift cuts the ridge into pieces. It is lower in elevation than the mountains of the mid-ocean ridge.
- deep sea trenches: Trenches are found in the sea. Some are near the edges of continents. Trenches are found near chains of active volcanoes. An example is the line of the very deepest blue, off of western South America.
- abyssal plains: Flat areas that may be dotted with volcanic mountains. An example is the consistent blue off of southeastern South America.
- guyots: Flat topped mountains that appear to have been eroded. Yet these mountains are thousands of feet below sea level.
- continental margin: The transition from the land to the deep sea. The continental margin is made of continental crust. More than one-quarter of the ocean basin is continental margin.
See if you can identify each of these features on the map (Figure below).
A modern map of the southeastern Pacific and Atlantic Oceans.
Of course the first scientists to observe these features wondered how they had formed. It turns out that they were crucial for fitting together ideas about seafloor spreading. And seafloor spreading would turn out to be the mechanism for continental drift.
- Echo sounders were used to search for enemy submarines during World War II. The depths they recorded allowed scientists to piece together bathymetric maps of the seafloor. Multi-beam sounders work on research vessels today.
- These maps revealed amazing features like mid-ocean ridges, deep-sea trenches, and abyssal plains.
- The features of the seafloor helped scientists to discover the mechanism for continental drift.
- How does an echo sounder create a bathymetric map?
- What are the features of the seafloor that are elevated?
- What are the features of the seafloor that are very deep?
Use the resource below to answer the questions that follow.
- What did Henry Hess use his sonar for?
- What did Hess discover?
- What was discovered in 1953?
- What did Hess discover was occurring at the ridge?
- What did Hess call his discovery?
- What process recycles the crust?