9.1 The Topography of the Sea Floor

GEOL1340

HMS Challenger

Conducted the first dedicated oceanographic research cruise, collecting foundational data on water depths, sub-sea geology, marine biota, and ocean chemistry.

Modern Bathymetry

The measurement and mapping of underwater topography and ocean floor depths using sonar, echo sounding, deep-sea submersibles (e.g., Alvin), and satellites.

Isostatic Balance

The equilibrium between Earth's crust and mantle; dictates that thicker, less dense continental crust floats higher, while thinner, denser oceanic crust sits lower to create ocean basins.

Continental Crust

Thicker, buoyant crust composed predominantly of felsic granitic rocks and sedimentary layers.

Oceanic Crust

Thinner, denser crust composed mainly of mafic igneous rocks (basalt and gabbro) that floats lower on the mantle.

Passive Continental Margin

A interior-plate coastal margin formed by rifting and thermal subsidence with no active subduction, featuring broad shelves and quiet sedimentary accumulation (e.g., Atlantic Ocean margins).

Active Continental Margin

A coastal margin located along an active plate boundary (subduction zone), characterized by narrow shelves, steep slopes, accretionary prisms, deep trenches, and high tectonic activity.

Continental Shelf

The shallow, gently sloping region of the seafloor extending from the coast to depths of about 200 meters.

Continental Slope

The steep descent connecting the shallow continental shelf to the deep ocean floor.

Continental Rise

The gentle incline at the base of the continental slope formed by thick sediment accumulations.

Abyssal Plains

Extremely broad, flat, and deep ocean floor regions (~4,000–6,000 meters deep) underlain by oceanic crust.

Mid-Ocean Ridges

Submerged underwater mountain ranges formed at divergent plate boundaries where new oceanic crust is generated.

Deep-Sea Trenches

Narrow, deep linear troughs formed at convergent subduction zones where oceanic lithosphere bends into the mantle; represent the deepest points on Earth.

Mariana Trench

The deepest subduction trench on Earth (~11,000 meters deep), located in the southwestern Pacific near Guam.

Photic Zone (Epipelagic Zone)

The upper sunlit layer of the ocean (surface to ~200 meters) where photosynthesis occurs, supporting ~90% of all marine life.

Mesopelagic Zone

The intermediate open-ocean layer extending from 200 meters down to 1,000 meters depth.

Bathypelagic Zone

The deep-ocean layer extending from 1,000 meters down to 4,000 meters depth.

Abyssalpelagic Zone

The extremely deep open-ocean layer located deeper than 4,000 meters.

Pelagic Realm

The open-ocean water column, excluding areas near shores or the seafloor.

Thermocline

A steep vertical temperature gradient in the upper 1,000 meters of the ocean separating warm surface waters from cold deep waters.

Halocline

A sharp vertical gradient in salinity separating surface waters from uniform deep-ocean salinities.

Spring Tides

Exceptionally high tides that occur when the gravitational pulls of the Sun and Moon align during new and full moons.

Neap Tides

Moderate tides with a lower tidal range that occur when the Sun and Moon are at right angles relative to Earth.

Bay of Fundy

A bay between Nova Scotia and New Brunswick known for the world's highest tidal range (16 to 18 meters) due to its funneling coastal geometry.

Intertidal Zone

The coastal area exposed to air during low tide and submerged during high tide.

Coastal Upwelling

The wind-driven process where surface waters are pushed offshore, drawing cold, nutrient-rich deep water to the surface and driving high primary productivity.

Coastal Downwelling

The process where winds force surface waters toward the coast, causing surface water to accumulate and sink.

Thermohaline Circulation

The ocean's global density-driven circulation powered by variations in water temperature (thermo) and salinity (haline).

AMOC (Atlantic Meridional Overturning Circulation)

The North Atlantic component of global ocean circulation that carries warm surface water northward and returns cold, dense water southward along the deep seafloor.