Origin and evolution of the ocean floor

HMS Challenger

The first major oceanographic expedition. December 1872 to May 1876. 127,500km of voyage; visited all oceans except the Arctic. Sampled ocean properties like depth using weighted lines.

Bathymetry

The measurement of ocean depths and the charting of the shape or topography of the ocean floor.

SONAR

SOund Navigation and Ranging. Basic approach in bathymetry.

Echo sounders

Work by transmitting sound waves into the water to produce an echo when it bounces off a surface.

1500 m/s

Speed of sound in water.

Sidescan SONAR

Technique that directs sound waves toward the ocean floor at an angle to map its topographic features. Provide detailed imagery but no information regarding bathymetry.

Seismic reflection profile

A cross-sectional view of the crust made by measuring the reflection of artificial seismic waves off boundaries between different layers of rock in the crust; can see beyond sediments. Produced by strong, low-frequency sounds from air guns.

Radar altimeter

Measures subtle differences in sea level by bouncing microwaves off the sea surface.

Continental margins. Deep-ocean basins. Oceanic ridges.

Provinces of the ocean floor

Passive continental margin

Margins that consist of a continental shelf, continental slope, and continental rise. They are not associated with plate boundaries and therefore experience little volcanism and few earthquakes.

Continental shelf

A gently sloping, shallow area of the ocean floor that extends outward from the edge of a continent. Inclined on 1/10th of a degree. 7.5% of the total ocean area.

Shelf valley

Large valley running from coastline. Seaward extension of land valleys. Eroded during lower sea levels.

Continental slope

A steep incline of the ocean floor leading down from the edge of the continental shelf. 5-25 degrees inclination.

Continental rise

The gently sloping section of the continental margin located between the continental slope and the abyssal plain. Sediments carried by turbidity currents. Formed from coalesced deep-sea fans.

Turbidity current

A submarine avalanche of sediment and water that speeds down a submarine slope. Flows down on submarine canyons to form deep-sea fans.

Active continental margin

Usually narrow and consisting of highly deformed sediments. Occur where oceanic lithosphere is being subducted beneath the margin of a continent. Landward wall of trench = continental slope.

Accretionary wedge

A large wedge-shaped mass of sediment that accumulates in subduction zones Here sediment is scraped from the subducting oceanic plate and accreted to the overriding crustal block.

10,984 meters

Depth of the Challenger Deep in the Marianas Trench.

Puerto Rico Trench and South Sandwich Trench

The only subduction zones present in the Atlantic Ocean.

Abyssal plain

Deep, featureless portion of the ocean. One of the most level places on Earth due to the thick accumulation of sediments transported by turbidity currents, and precipitates.

The Atlantic Ocean has the most extensive abyssal plain.

True.

Seamount

Submarine volcanoes. Form over mantle plumes while some near oceanic ridges. Sinks due to erosion as it moves away from magma source.

Guyot

A large, flat-topped seamount resulting from erosion of an island volcano when it was above sea level.

Oceanic plateau

An extensive region on the ocean floor composed of thick accumulations of pillow basalts and other mafic rocks that in some cases exceed 30 kilometers in thickness.

Atoll

A ring-like coral island or string of small islands surrounding a lagoon. Forms from a sinking, eroding volcano.

Coral

Tiny animals that appear in large numbers when linked to form colonies. External skeleton composed of CaCO3. Growth limited to no more than 45m.

Reef

A submerged ridge of rock or coral near the surface of the water. A large group of corals.

24 degrees Celsius

Prime water temperature for coral growth.

Oceanic ridges are elevated because they are hot and buoyant but sinks when they are old and cold.

True.

Slow rate (1-5 cm/year)

Characterized by prominent rift valleys, rugged topography, steep walls caused by vertical displacements along normal faults, and volcanism producing numerous cones.

Intermediate rate (5-9 cm/year)

Characterized by more subdued topography than those spreading at slow rates.

Fast rate (>9 cm/year)

Characterized by absence of rift valleys, ridge axis elevated in swells, have more gradual profiles, and less rugged portions called rise.

Swell

Elevated ridge axis present in fast-spreading spreading centers.

Rise

Less rugged portions of oceanic ridges present in fast-spreading spreading centers.

Ophiolite

An assemblage of subducting oceanic lithosphere scraped off (obducted) onto the edge of a continent.

Pelagic sediments. Pillow basalts. Sheeted basalt/diabase complex. Gabbro Peridotite

Complete sequence (from top to bottom) of an ophiolite.

Hydrothermal alteration

The process of hot water and other fluids reacting with solid rock during the formation of metasomatic rocks, such as metallic ore deposits.

Serpentine and chlorite

Secondary minerals products of hydrothermal alteration of ferromagnesian minerals.

Black smokers

Forms as hot (400 degrees Celsius) liquid mixes with cold water to precipitate Fe, Cu, Ag, and Au, and form massive sulfide deposits.

Narrow rift

Wide rift

~1000km wide. Characteristic of thin, hot, and weak lithosphere.

15 million years

Time needed for oceanic lithosphere to be denser than underlying mantle