oceanic structures

The Earth's crust is divided into continental crust and oceanic crust.

▼ abyssal plain : composition of oceanic crust : continental crust : continental rise : continental shelf : continental slope : creation/consumption of oceanic crust : mid-oceanic ridges : oceanic crust : oldest oceanic crust : paleomagnetic record : seamounts : submarine trenches : volcanic islands ▼

Structurally, moving outward from continental structures at sea level to those most distant from shore or at the greatest depths (image at left - elevations - click to enlarge image - mid-res, hi-res versions):

● continental shelf (continental crust) – the terrigenous-relict sediment-covered, flooded continental margin extends varying distances from shore (average 80 km), and slopes gradually from shore to the shelf break at a fairly uniform depth of 140 m. Sediments are coarser closer to shore and finer at greater distances from shore.

● continental slope (oceanic crust begins) – steeper than the sediment covered continental shelf, the continental slope extends from the shelf break to depths of 2-3 km, often cut by submarine canyons.

● continental rise – of shallower gradient than the continental slope, the rise extends up to 500 km from the slope and comprises thick sediments deposited by turbidity currents from the shelf and slope.[9]

● abyssal plain – gently sloping or flat regions that cover approximately 40% of the ocean floor, reaching depths between 2,200 and 5,500 m.

● submarine trenches – deep trenches formed by the subduction of oceanic crust beneath continental crust at convergent boundaries of tectonic plates.

● volcanic islands – basaltic shield volcanoes like the chain of Hawaiian islands above mantle plumes, oceanic rift islands (Atlantic Iceland, Jan Mayen), or volcanic arc islands at subducting margins (Mariana Islands, Aleutians, Mauritius, Tonga in the Pacific Ocean; Lesser Antilles, South Sandwich Islands in Atlantic Ocean). ◙ subduction zone magmas ◙

● seamounts – submarine mountains that typically are extinct volcanos.

● mid-oceanic ridges – spreading center mountain ranges with a total total length of about 60,000 km that ring the globe at the divergent boundaries of tectonic plates. Oceanic crust is replenished at the mid-ocean ridges and destroyed at continental margins.

Oceanic crust (right - age of oceanic crust -click to enlarge image - mid-res, hi-res) is younger than continental crust and is replenished at spreading centers. Oceanic crust is consumed when it subducts at convergent tectonic plate boundaries at continental margins. The deepest ocean lies in the Challenger Deep of the Pacific's Mariana Trench at a depth of 10,911 m below sea level [image: CD: mid-res, hi-res; MT: mid-res].

The oldest oceanic crust dates from about 180 Ma and lies adjacent to continents, while the youngest crust lies adjacent to the mid-oceanic ridge centers. As oceanic crust is followed outward from the mid-oceanic ridges, its age gradually increases (and its paleomagnetic signature records geomagnetic reversals).

Rocks of the oceanic crust are mafic basaltic rocks (sima) with a mean density of about 3.3 grams per cubic centimeter (more dense than felsic continental rocks). With an average thickness of 10 km, the oceanic crust is thinner than the continental crust [image crustal thickness].

◙ subduction zone magmas ◙

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subduction

Subduction occurs when oceanic plates collide with continental plates, and the subducting tectonic plate slides beneath the more buoyant plate, moving down into the mantle at submarine trenches.

Subduction typically proceeds at rates of mere centimeters per year, but it is associated with intense, deep-seated seismic activity and volcanic activity (volcanic island arcs). ◙ subduction zone magmas ◙

Wherever continental and oceanic plates collide, the denser oceanic plate subducts underneath the less dense continental plate, sinking toward the asthenosphere where the oceanic crust is consumed.

Any crustal rocks that are too buoyant to sink pile up as nappes or in orogenic zones.

Subduction zones are typically arcuate, with the convex arc directed toward the oceanic plate. More steeply inclined subduction plates create less pronounced arcs than do shallow subduction plates. Back-arc basins develop in the concave area between the subduction zone and the continent.

At subduction zones, the oceanic lithosphere, layers of marine sediments, and trapped water are recycled into the deep mantle. When the oceanic plate sinks to about 100 km in depth, peridotite is metamorphosed to eclogite, becoming denser and sinking farther into the mantle.

Magmatism begins at depths of 100 km or deeper in the Wadati-Benioff zone, which is typically 100-300 km inland from the subduction trench.

Island arcs contribute about 25% of the total volume of magma produced each year on Earth (~30-35 km³), which is much less than the volume produced at mid-oceanic ridges. At the spreading centers of the mid-oceanic ridge systems, convection currents within the mantle carry basaltic magmas to the surface and regenerate oceanic crust. Island arc volcanic lavas originate in a variety of magmas.

◙ subduction zone magmas ◙

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