2Mapping the Ocean Floor Pre-WWIRopes were lowered from boats to measure depth.1940’s/1950’sDeveloped use of sound waves to map depth of the ocean floor.Found that the ocean floor actually had a series of ridges and valleys (mid-ocean ridges).
4Seafloor Spreading Developed by Harry Hess in the 1960’s. Proposed that:Hot, less dense material beneath the ocean floor rises toward the surface at mid ocean ridgesAs the material approaches the surface it moves sideways, pulling the ocean floor apart at the center of the mid ocean ridgeMagma erupts from the resulting cracks and cools to form new ocean crustThrough time the new ocean crust moves away from the center of the mid-ocean ridge becoming cooler (and thus more dense) and sinks
5Support for Seafloor Spreading 1968 – Glomar ChallengerResearch vessel that drilled into the ocean floor to obtain rock samples.Discovered that the youngest ocean crust was closest to the center of the mid-ocean ridge.
6Support for Seafloor Spreading Magnetic ReversalsAt the time that rocks with iron-bearing minerals form, the direction of Earth’s magnetic field is recorded in the orientation of the minerals.Scientists have found that many reversals have occurred in the magnetic field and are recorded in strips of the ocean floor that run parallel to each other.