Post-WWII exploration and mapping of the ocean floor using sonar technology.
Marie Tharp and Bruce Heezen compiled sonar data to create the first map of the North Atlantic in 1957.
Discovery of a mountain range in the middle of each ocean, contributing to the development of plate tectonics.
Seafloor spreading hypothesis by Harry Hess in 1962, proposing mid-ocean ridges as divergent boundaries driven by convection in the mantle.
Locations of Volcanoes and Earthquakes (1950s)
Earthquakes and volcanoes are not randomly located; they often occur along mid-ocean ridges.
Howard Foster plotted earthquake data showing patterns correlating with ridges.
Age of the Ocean Floor (1950s-80s)
Deep Sea Drilling Project (1968-1983) studied the age of the ocean floor.
Found ocean floor is much younger than continental crust, with a pattern of age increasing away from mid-ocean ridges (MOR).
Hotspots (1960s)
Volcanism can occur away from plate boundaries, at hotspots where magma from the mantle rises to the surface.
Example: Hawaii, which forms a volcanic chain indicating plate movement across a stationary mantle plume.
Paleomagnetism (1960s)
Earth has a magnetic field with poles that reverse over time (normal and reversed polarity).
Magnetic minerals in cooling igneous rocks record the magnetic field, creating bands of normal and reversed polarity on the ocean floor.
The symmetrical pattern of magnetic stripes on either side of mid-ocean ridges provides evidence for seafloor spreading.
Paleomagnetism and Seafloor Spreading
Frederick Vine and Drummond Matthews (1968) interpreted magnetic stripes as evidence of continuous creation of new ocean crust, supporting Hess's seafloor spreading hypothesis.
The pattern of magnetic stripes is the "smoking gun" for seafloor spreading.
Comparing Theories
Continental Drift Hypothesis
Continents move independently and can plough through oceanic crust.
Lacked a science-based driving mechanism.
Evidence included coastlines, fossils, mountain ranges, rocks, and glacial evidence.
Theory of Plate Tectonics
Tectonic plates, including continents and oceanic lithosphere, move as a single slab across the asthenosphere.
Supported by a science-based driving mechanism involving convection, slab pull, and ridge push.
Evidence includes the shape and age of the ocean floor, locations of earthquakes and volcanoes, hotspots, and paleomagnetism.
Additional Notes
Paleomagnetism Practice Problems involve analyzing episodes of normal and reversed polarity and understanding the symmetrical pattern of magnetic bands relative to mid-ocean ridges.