Ocean basins and seafloor topography shape the underwater landscape of our planet. From continental margins to deep ocean features, these elements play a crucial role in understanding Earth's geological processes and marine ecosystems.
Plate tectonics drive the formation and evolution of ocean basins, creating mid-ocean ridges, trenches, and abyssal plains. Seafloor sediments, ranging from land-derived particles to biological remains, provide insights into ocean chemistry, climate history, and marine life.
Ocean Basin Topography
Continental Margin Features
- The continental shelf is a shallow, flat extension of the continent that ends at the shelf break formed by sediment deposition and sea level changes
- The continental slope is a steep drop-off from the shelf break to the deep ocean basin, often incised by submarine canyons shaped by turbidity currents and mass wasting
- The continental rise is a gently sloping accumulation of sediment at the base of the continental slope, formed by sediment deposition from turbidity currents and contour currents
Deep Ocean Basin Features
- The abyssal plain is a flat, deep ocean floor formed by the accumulation of fine-grained sediments covering most of the ocean basin and interrupted by other features like seamounts and mid-ocean ridges
- Mid-ocean ridges are underwater mountain ranges formed by seafloor spreading at divergent plate boundaries with a central rift valley and are the site of volcanic activity and hydrothermal vents
- Seamounts are isolated, extinct underwater volcanoes that rise abruptly from the seafloor formed by intraplate volcanism or hotspots
- Trenches are deep, narrow depressions in the seafloor formed at convergent plate boundaries where oceanic lithosphere is subducted beneath another plate and are the deepest parts of the ocean
Plate Tectonics and Ocean Basins
Plate Tectonic Theory
- Plate tectonics is the theory that the Earth's lithosphere is divided into large, rigid plates that move and interact with each other, driven by convection currents in the mantle
- The three main types of plate boundaries are divergent (plates move apart), convergent (plates collide or subduct), and transform (plates slide past each other)
- Transform boundaries, such as fracture zones, offset mid-ocean ridges and accommodate lateral motion between plates characterized by shallow, strike-slip earthquakes
- Seafloor spreading occurs at divergent boundaries, where new oceanic crust is formed by magma upwelling and solidifying, creating mid-ocean ridges and expanding ocean basins
- At convergent boundaries, old, dense oceanic crust is subducted beneath another plate, forming deep ocean trenches and causing volcanism and earthquakes, consuming oceanic crust and recycling it into the mantle
- The age of the oceanic crust increases with distance from the mid-ocean ridge, as older crust is pushed away from the spreading center with the oldest oceanic crust found near trenches, where it is subducted
- Plate tectonics explains the distribution and formation of major ocean basin features, such as mid-ocean ridges, trenches, and abyssal plains, as well as the occurrence of earthquakes and volcanism on the seafloor
Seafloor Sediments
Sediment Types and Origins
- Seafloor sediments are classified based on their origin: lithogenous (derived from land), biogenous (derived from biological activity), hydrogenous (precipitated from seawater), and cosmogenous (from space)
- Lithogenous sediments, such as clay, silt, and sand, are derived from the weathering and erosion of continental rocks transported to the ocean by rivers, glaciers, and wind, and are most abundant near continental margins
- Biogenous sediments are composed of the skeletal remains of marine organisms, such as plankton and mollusks with two main types: calcareous ooze (made of calcium carbonate) and siliceous ooze (made of silica)
- Hydrogenous sediments, such as manganese nodules and phosphorites, are formed by the direct precipitation of minerals from seawater and are most common in areas with slow sedimentation rates, such as the abyssal plains
Sediment Distribution and Influencing Factors
- Calcareous ooze is found in shallower, warmer waters above the calcite compensation depth (CCD), where calcium carbonate remains stable
- Siliceous ooze is found in deeper, colder waters and is most abundant near polar regions and upwelling zones with high biological productivity
- Cosmogenous sediments, such as micrometeorites and cosmic dust, originate from space and are a minor component of seafloor sediments
- The distribution of seafloor sediments is influenced by factors such as distance from land, water depth, ocean currents, biological productivity, and seafloor topography
- Sediment thickness generally decreases with increasing distance from continental margins and is greatest in areas of high sediment input, such as deltas and submarine fans
Interpreting Bathymetric Data
Bathymetric Maps and Profiles
- Bathymetric maps represent the topography of the ocean floor using contour lines (isobaths) that connect points of equal depth created using data from echo sounding, satellite altimetry, and other techniques
- Bathymetric profiles are cross-sectional views of the ocean floor that show the variation in depth along a specific transect useful for visualizing the shape and gradient of seafloor features
- The spacing of contour lines on a bathymetric map indicates the steepness of the seafloor gradient with closely spaced contours representing steep slopes and widely spaced contours indicating gentle slopes or flat areas
Identifying Ocean Floor Features
- Mid-ocean ridges appear as elongated, elevated features with a central rift valley on bathymetric maps and in profiles, they show a symmetric, steep-sided shape with a central depression
- Trenches are represented by closely spaced, elongated contours on bathymetric maps, indicating very deep, narrow depressions and in profiles, they appear as steep-sided, V-shaped features
- Abyssal plains are characterized by widely spaced or absent contour lines on bathymetric maps, reflecting their flat, level surface and in profiles, they appear as horizontal or gently sloping lines
- Seamounts and guyots (flat-topped seamounts) are indicated by circular or elliptical closed contours on bathymetric maps and in profiles, they show a conical or flat-topped shape rising abruptly from the seafloor
- Submarine canyons appear as sinuous, V-shaped indentations on continental slopes in bathymetric maps and in profiles, they show steep, incised channels that cut across the slope