Oceans and the Ocean Floor Lecture Notes

GeoClassroom Physical Geology Historical Geology Structure Lab

Physical Geology

Oceans and the ocean floor

- Oceans represent 71% of the Earth's surface

- If the Earth were smooth & level, the oceans would cover the Earth to a depth of approx 6000 ft.

- Ocean floor has been extensively mapped by echo sounding

- transmit sound waves to floor, then pick them up when the reflect off the bottom. By knowing time, and how fast sound travels in water, can get an accurate picture of the topography of the sea floor.

- Most has now been confirmed by visual observation in deep-sea submersibles.

Continental Margins (or, off the coast of SC as an example)

- include the cont. shelf, slope and rise.

- found along cont margins which are not part of a subduction zone.

Shelf:

- avg 50 miles wide and 425 ft deep

- gradient of 1/10 of one degree (10 ft/mile)

- shelves represent 7.5 % of the oceans area.

- usually contain important mineral and petroleum deposits & fish.

- evidence of sea level change:

- drilling & dredging has produced remains of animals abundant during the last ice age when sea levels were much lower.

- usually consists of sediment washed off continent by rivers/streams.

Slope:

- leads into deeper water, steeper gradient

- avg 370 ft/ mile

- marks the true edge to a continent

Rise:

- slope merges into a more gradual incline

- gradient of 20-40 ft/mile

- rise may extend for a hundred miles or more into the deeper ocean

- consists of sediment moved downslope from the shelf to the deep ocean

- sediment delivered here by turbidity currents moving down the slope, which follow submarine canyons

- these turbidity currents fan out when they reach the bottom of the canyon,the rise, to form deep-sea fans.

- fans can be several miles thick, from accumulation of sediment from turbidity currents.

- Turbidity current deposits are called turbidites.

- Run down submarine canyons on the cont slope, and cause these canyons to get deeper and deeper with each current.

- Form a graded bed

- coarser material drops first, then successively lighter material.

- First evidence of this type of currents were massive earthquakes off the NE coast in the 20's which let loose turbidity currents that ran down the slope and took out underwater transatlantic phone lines.

Ocean basin

- that area between the mid-oceanic ridge system (and mountain range) and the cont shelf

Deep-ocean trenches:

- as deep as 36K feet, the deepest part of the ocean: Marianas Trench, Philipines.

- appear where subducting ocean floor is pulled beneath a continent.

Abyssal Plains:

- flat plains in the oceans

- flattest areas on Earth

- consist of clay-sized particles transported down the cont slope by turbidity currents, then carried far out to sea by ocean currents.

- basically MUD

- more common in areas where there is no subduction zone next to the continent.

Seamounts:

- isolated volcanic peaks scattered over the sea floor.

- usually form near mid-oceanic ridges where magma is being generated

- if mount rises far enough, it becomes an island, like the Azores & Bermuda.

- those which break the ocean's surface, then erode to flat become guyots as they erode below sea level.

Mid-Oceanic Ridges:

- areas where tectonic plates are pulling apart.

- represent 20% of Earth's surface

- found in all the oceans

- forms the longest mountain chain in the world: 40K miles

- 500-4000 miles wide

- stand several thousand feet above the adjacent ocean basin, but 6K' below sea level.

- elevated because they're hot

- as rx cool, the contract and become denser, thus they sink some.

- takes around 100 mya for rx to cool completely.

Black smoker:

- where sea water circulates into fractures in the bedrock, and then gets heated by magma to 400C

- hot water leaches metals from basaltic rx (Fe, Cu, S, etc.)

- heated water rises back to the surface, then immediately drops its load of metal as it encounters cold ocean water.

- form large metal ore deposits.

- discovered organisms which grow and live w/ no sunlight whatsoever, and lived on bacteria.

- generated interest on the possibilities of life elsewhere in the universe: mars, moons of Jupiter and Saturn.

Coral Reefs and Atolls

- calcareous reef forms along beach of volcanic mtn in the ocean

- as mountain weathers, reef stays in the same position

- eventually the mtn disappears altogether, leaving just the reef: Atoll.

Sediment:

- 3 types of sediment deposited in the oceans

1) Terrigenous sediment

- mineral grains transported off continents

- slow process: can take 5K to 50K for a 1cm thick layer of mud to form on the flat Abyssal Plains

- most terrigenous sed is deposited on the cont shelves

- most of this sed is red due to oxidation from floating around in sea water

2) Biogenous sediment

- CaCO3 shells and skeletons from organisms near the surface

- accumulates in warm, shallow ocean water

- dissolves in colder, deeper ocean water

3) Hydrogenous sediment

- minerals that xlize directly out of sea water: manganese nodules