Geology of the Seafloor

 Before the early 20 th century, study of the seafloor was impossible because: ◦ Humans cannot dive to the seafloor ◦ Limitations of technology prevented study of the seafloor  Most scientists assumed the seafloor was a flat, featureless plain

 Two new technologies made studying the seafloor possible in the early 20 th century; sonar and submersibles ◦ Sonar measures the speed and direction of reflected sound waves to determine the depth and slope of the seafloor ◦ Sonar made it possible to detect and map otherwise invisible features on the seafloor ◦ The German Meteor expedition mapped the South Atlantic using sonar in 1925 and became the basis for the first maps of the seafloor.  Mapping of the seafloor is called bathymetry

 A little review...  Submersibles are vehicles that can dive to great depths for extended periods of time ◦ By the 1950’s subs were used by scientists to study the seafloor. ◦ Subs can allow filming of the seafloor and the collection of samples for study on the surface Rule #1: Do NOT open hatch underwater

 The ability to map the seafloor revealed many new features: ◦ Continental shelf, slope, and rise ◦ Undersea canyons and trenches ◦ Mid-ocean ridges ◦ The abyssal plain ◦ Seamounts and guyots (GEE-ohs)

 The flooded coasts of the continents make up the continental shelf, slope, and rise  Continental Shelf ◦ Gently sloping and covered in thin layers of sediment ◦ Extends 0-50 km from the coast ◦ Water is less than 500ft deep

 Continental Slope ◦ Steeper slope than the shelf, covered in thick sediments ◦ Marks the transition from continental to oceanic crust ◦ Sediment from the shelf flows down the slope in underwater landslides (turbidity currents)  Continental Rise ◦ Sediments from the slope are deposited here

 Abyssal Plain ◦ Broad, flat plain ◦ Covered in thick layers of sediments ◦ Marked with seamounts and guyots  Seamounts and Guyots (gee-yoes) ◦ Underwater volcanoes ◦ Guyots are old, extinct volcanoes ◦ Guyots are flat-topped due to erosion from ocean currents ◦ Important habitats for marine life

 Mid-Ocean Ridges ◦ Volcanic mountain ranges that encircle the entire globe ◦ Thousands of miles long ◦ Can be over a mile high  Trenches ◦ Deep underwater chasms ◦ Deepest points in the ocean  Ridges and trenches are both formed where two tectonic plates meet

 Active margins: subject to earthquakes and volcanic activity ◦ Results in little to no continental rise, narrow continental shelves, and trenches  Passive margins: not geologically active ◦ Allow for broad continental shelves and prominent rises to develop

 Almost completely surrounded by narrow continental shelves and deep trenches

 Composes 50% of the total ocean area.  Has a mid-ocean ridge called the East Pacific Rise  Features thousands of islands, seamounts and atolls

 Broader shelf system (larger continental shelf area than the Pacific Ocean)  Fewer seamounts  Smoother abyssal plain

 1/5 of Earth’s surface area ◦ Less than half the size of the Pacific  Center of the Atlantic Ocean is the Mid- Atlantic Ridge ◦ High mountain range composed of virtually all seamounts below the surface of the water  Lacks deep sea trenches

 Massive abyssal fans  Extensive (but more shallow) abyssal plains

 Center of the Indian Ocean is the Southwest Indian Ridge  Similar to the Pacific and the Atlantic

 Hypso = height  Graphic = drawn  Shows the relationship between height of the land and depth of the ocean  The bar graph (left side) gives the percentage of Earth’s surface area at various ranges of elevation and depth  The cumulative hypsographic curve (right side) gives the percentage of the surface area from the highest peak to the deepest depth of the ocean – puts it in picture form.