Chapter 13: Origin and Evolution of the Ocean Floor

The development of bathymetric technology—from early weighted-line measurements during the HMS Challenger expedition to modern multibeam sonar and satellite altimetry—revealed the ocean floor's complex topography organized into three major provinces: continental margins, deep-ocean basins, and oceanic ridge systems. Continental margins are categorized as either passive margins, characterized by broad continental shelves and sediment-rich environments along tectonically stable coasts such as the Atlantic, or active margins found at convergent plate boundaries where subduction zones create deep trenches, volcanic arcs, and accretionary wedges typical of the Pacific Rim. The deep-ocean basins contain Earth's most extreme features, including abyssal plains—extremely flat regions formed when sediment buries underlying topography—and deep trenches marking subduction zones and generating seismic and volcanic activity. Submarine volcanism produces seamounts, volcanic islands, guyots with characteristic flat summits formed by erosion and subsidence, and massive oceanic plateaus composed of thick basaltic sequences. Coral reef systems demonstrate the relationship between reef morphology and volcanic subsidence, as proposed by Darwin's subsidence hypothesis and supported by plate tectonic theory. The oceanic ridge system, Earth's longest mountain chain extending over 70,000 kilometers, forms through seafloor spreading at divergent plate boundaries and generates new oceanic crust with distinct layers including deep-sea sediments, pillow basalts, sheeted dike complexes, and gabbroic intrusions—sequences preserved in ophiolite complexes on land. Spreading rates fundamentally influence ridge morphology and crustal properties, with slow-spreading ridges displaying rift valleys and fast-spreading ridges appearing smoother. Hydrothermal circulation at ridge crests produces black smoker vents and chemosynthetic biological communities while depositing mineral-rich compounds. The chapter concludes by describing continental rifting processes and ocean basin evolution, beginning with lithospheric doming and progressing through rift valley formation, narrow seaway development, and mature ocean basin formation, with the East African Rift and Atlantic Ocean exemplifying these stages and failed rifts illustrating incomplete rifting processes.