Lecture 20 // Marine Geology

Question 1

Earth’s Oceans

Answer 1

The Earth’s oceans cover 71% of Earth’s surface.

Shallow ocean water (< 200 m depth) is found above the continental shelf. The deep ocean covers the continental slope, rise, abyssal plain, and MORs.

Question 2

Ocean Depths

Answer 2

– Average ocean depth in the Atlantic & Pacific is ~ 4,000 m.
– The deepest point is the Mariana Trench at 11,000 m.
• The deepest point in the trench is called the Challenger Deep.

Why so deep?
Trenches surround almost the entire perimeter of the Pacific Ocean.
Subduction of oceanic plates (including the Pacific Plate) and the associated melt produced has formed the Pacific Ring of Fire.

Question 3

Active Continental Margin

Answer 3

• The west coast of North America, Central America, and South America is an active continental margin. It is a tectonically active area characterized by offshore subduction, and strike-slip faulting.
 Where subduction is present, a narrow continental shelf passes on to the continental slope (which may include an accretionary wedge), which extends to either a trench or deformation zone.

Question 4

Passive Continental Margin

Answer 4

• The east coast of North America and South America is an passive continental margin. It is a tectonically quiet area.
 A passive margin consists of a wide continental shelf, the continental slope, and a continental rise. The gently sloping rise passes seaward to the abyssal plain (not part of the continental margin).

• The continental shelf has an average slope of 0.1° and water depth from 0 at the shoreline to approx. 200 m at the shelf-slope break (shallow marine).

• The continental slope has an average slope of 3 to 5° and water depth quickly deepens (deep marine).
• The slope is riddled with submarine canyons.
The continental slope is riddled with submarine canyons.
Great turbidity currents sweep down these canyons from time to time, and deposit sediments (turbidites) as graded beds on submarine fans at the base of the slope (the continental rise).

Question 5

The True Edge of the Continent

Answer 5

• The base of the continental slope marks the true edge of the continent.
• Rifted, faulted continental crust lies below the continental shelf and slope.
• Oceanic crust lies beneath the seafloor of the continental rise and abyssal plain. This oceanic crust was originally formed at a MOR rift valley, and has since been pushed to its current
location by seafloor spreading as more and more oceanic crust was generated at the rift valley.

Question 6

The Abyssal Plain

Answer 6

• Seaward of the continental margin (shelf/slope/rise) lies
the abyssal plain.
• Deep-ocean sediment consists of material that slowly settles from the water column in deep water, called pelagic sediment. This includes abyssal clay (wind-blown dust & volcanic ash) and ooze (remains of microscopic plankton).
• Pelagic sediments blanket the underlying rough surface of the oceanic crust. The abyssal plain is mostly flat and featureless.

Question 7

Seamounts

Answer 7

• Poking up through the blanket of pelagic sediments are high
points called seamounts. These volcanic peaks formed on the
flanks of the MOR and like the oceanic lithosphere they ride on,
were pushed away with continued seafloor spreading.

Question 8

MORs

Answer 8

• As one crosses the abyssal plain, eventually the elevation of the seafloor begins to slowly, steadily rise. The oceanic crust is progressively younger and covered with less pelagic sediments as we travel up the flank of the great MOR.
• At the crest we descend down into the central rift valley.

• Recall that a MOR sits above an area of high heat flow in the asthenosphere.
• Upwards convection of this warm buoyant rock results in decompression melting.
• The magma works its way up through the fractured crust beneath the rift valley, forming new oceanic crust
Seafloor spreading

Question 9

Cross-section through

oceanic lithosphere

Answer 9

• Pelagic sediments blanket old crust that has been pushed to the side from seafloor spreading.
• Pillow basalts form as mafic lava extrudes from basalt dikes. Metallic sulphides precipitate from hydrothermal vents.
• Tension pulls the crust apart creating fractures. Magma works its way up from the vast magma chamber below and crystallizes within shallow basalt “sheeted” dikes.
• Magma at depth crystallizes into gabbro once spread away from the active spreading centre.
• Below this the magma is hotter and crystallizes into peridotite.

Question 10

Moho

Answer 10

The Moho is the boundary between the less dense gabbro of the crust, and the denser peridotite of the uppermost mantle. The Moho is a seismic discontinuity where P-wave velocity suddenly increases “jump” as they enter the denser uppermost mantle.