Seafloor spreading shapes our oceans. New crust forms at mid-ocean ridges, pushing older crust outward. This process creates magnetic striping patterns, providing evidence for plate movement. It's the engine behind ocean basin evolution.

Ocean floors aren't flat. They feature abyssal plains, seamounts, and deep trenches. These structures form through volcanic activity, sediment accumulation, and plate interactions. Understanding them helps us grasp Earth's dynamic nature.

Seafloor Spreading and Oceanic Crust

Process of Seafloor Spreading

• Seafloor spreading is the process by which new oceanic crust is formed through volcanic activity and gradually moves away from mid-ocean ridges
• Occurs at divergent plate boundaries where tectonic plates are moving apart (East Pacific Rise)
• Magma rises from the mantle and erupts along the mid-ocean ridge, creating new oceanic crust
• As new crust is formed, older crust is pushed away from the ridge, causing the seafloor to spread outward

Magnetic Striping and Evidence for Seafloor Spreading

• Magnetic striping is a pattern of alternating magnetic polarities in the oceanic crust that provides evidence for seafloor spreading
• Basaltic rocks in the oceanic crust contain magnetic minerals that align with Earth's magnetic field at the time of their formation
• Earth's magnetic field periodically reverses polarity, causing the newly formed oceanic crust to record the alternating magnetic pattern
• Symmetrical magnetic stripes on either side of mid-ocean ridges indicate the seafloor has been spreading outward from the ridge over time (Atlantic Ocean)

Characteristics and Formation of Oceanic Crust

• Oceanic crust is the thin, dense layer of the Earth's crust that underlies the ocean basins
• Composed primarily of basalt, a dark, fine-grained igneous rock formed from the rapid cooling of magma at or near the Earth's surface
• Oceanic crust is thinner (5-10 km) and denser than continental crust
• Forms at mid-ocean ridges through seafloor spreading and volcanic activity
• As oceanic crust moves away from the ridge, it cools, contracts, and becomes denser, causing it to sink lower into the mantle

Hydrothermal Vents and Unique Ecosystems

• Hydrothermal vents are fissures in the seafloor that release geothermally heated water rich in dissolved minerals
• Form near mid-ocean ridges where seawater seeps into the oceanic crust, becomes superheated by magma, and rises back to the surface
• Hydrothermal fluids can reach temperatures up to 400°C and are highly acidic
• Vents support unique ecosystems that rely on chemosynthetic bacteria as the primary producers (tubeworms, giant clams)
• These bacteria use the dissolved minerals and chemical energy from the hydrothermal fluids to produce organic compounds, forming the base of the food chain

Ocean Floor Features

Abyssal Plains and Seamounts

• Abyssal plains are vast, flat areas of the ocean floor that cover much of the deep ocean basins
• Formed by the accumulation of fine-grained sediments that settle out of the water column over millions of years
• Seamounts are extinct submarine volcanoes that rise abruptly from the seafloor but do not reach the ocean's surface
• Formed by volcanic activity associated with hot spots or mid-ocean ridges (Emperor Seamounts)
• Seamounts provide hard substrate for the growth of deep-sea corals and sponges, creating biodiversity hotspots

Guyots and Trenches

• Guyots are flat-topped seamounts that were once above sea level but have since subsided and become submerged
• Formed when a volcanic island is eroded by waves and currents at the surface, creating a flat top, and then gradually sinks due to the cooling and contraction of the oceanic crust
• Trenches are deep, narrow depressions in the seafloor that form at convergent plate boundaries where one tectonic plate subducts beneath another
• Deepest parts of the ocean (Mariana Trench, reaching depths of over 11,000 meters)
• Trenches are often associated with high levels of seismic activity and volcanism as the subducting plate melts and generates magma

Plate Tectonics and Ocean Basins

Wilson Cycle and Ocean Basin Evolution

• The Wilson Cycle describes the formation, evolution, and destruction of ocean basins over geologic time
• Begins with the rifting of a continent and the formation of a new ocean basin through seafloor spreading (Atlantic Ocean)
• As the ocean basin widens, it reaches a mature stage characterized by a well-developed mid-ocean ridge and abyssal plains
• Eventually, subduction initiates along one margin of the ocean basin, leading to the formation of a trench and the onset of basin closure
• The ocean basin narrows as subduction continues, ultimately leading to a continent-continent collision and the formation of a mountain range (Himalayas)

Interactions Between Oceanic Crust, Trenches, and Seafloor Spreading

• Oceanic crust is formed at mid-ocean ridges through seafloor spreading and is eventually destroyed at subduction zones
• As oceanic crust moves away from the ridge, it cools, contracts, and becomes denser, causing it to subside and form abyssal plains
• Older, denser oceanic crust will subduct beneath younger, less dense oceanic crust or continental crust at convergent plate boundaries, forming deep-sea trenches
• Subduction of oceanic crust at trenches drives plate motions and is a key component of the Wilson Cycle
• The subducting oceanic crust melts as it descends into the mantle, leading to the formation of magma that rises to feed volcanic arcs (Andes, Aleutian Islands)