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Chapter 12

Ocean Waters and the

Ocean Floor

Global extent of the world ocean

71 % of Earth

61 % of Northern hemisphere

81 % of Southern hemisphere


Size and depth of the major oceans

Pacific by far the largest

Pacific has greatest average depth

Atlantic the shallowest


Composition of seawater

3.5 % ( by Weight ) dissolved minerals


Proportion of dissolved salts

Measured in parts per thousand (0/00)

Salinity of open ocean: 33 % to 37 %

Major constituents: Cl - and Na +

Sources of salts

Chemical weathering of rocks

Outgassing - gases from volcanic eruptions


Resources from seawater

Commercial products from seawater

Salt (sodium chloride)

Magnesium - a light-weight metal


Fresh water

Obtained by the desalination of seawater

High cost to produce

Small total production


The oceans’s three-layered structure


Warmest at surface

Thermocline-rapid decrease in temperature with depth

Below thermocline-little change in temperature


In low and middle latitudes

Higher at surface

Fresh water is evaporated

Halocline-rapid decrease in salinity with depth

Below halocline-little change in salinity with depth



Measuring depth

Originally done with weighted lines

Echo sounder

Primary instrument for measuring depth

Reflects sound from ocean floor

Continental margin

1. Continental shelf

2. Continental slope

3. Continental rise


Continental margin

Continental shelf

Flooded extension of the continent

Shoreline toward ocean basin

Gently sloping

Submerged surface

Continental slope

Seaward edge of the shelf

Steep gradient into deep water

True edge of the continent

Associated with continental slopes are

Turbidity currents

Dense, sediment-laden water

Deposits are called turbidites


Graded bedding-coarse sediments on the bottom

Submarine canyons are excavated by turbidity currents


Continental rise

Found where trenches are absent

Thick accumulation of sediment

At base of slope

Turbidity currents from deep-sea fans


Ocean basin floor

1. Deep-ocean trench

2. Abyssal plain

3. Seamount


Deep-ocean trench

Deepest part of ocean

Where plates plunge into the mantle


Abyssal plain

Most level place on Earth

Thick accumulations of sediments

Found in all oceans



Isolated volcanic peak

Some form over hot spots

Mid-ocean ridge

Site of sea floor spreading

Found in all major oceans

Rift zone

On ridge crest

Geologically active

Magma moves from asthenosphere

New ocean crust forms here


Coral reefs and atolls

Coral reefs

Built up over thousands of years by

Remains and secretions of corals

Certain algae

Thrives in warm waters


Forms on the flanks of sinking volcanic island

Coral reef surrounding a lagoon

Formation explained by Darwin


Seafloor sediment

Thickness varies

Mud is the most common sediment

Types of sediments

Terrigenous sediment-weathered continental rocks

Biogenous sediment

Shells, skeletons, and plants

Calcareous and siliceous oozes

Hydrogenous sediment-chemicals precipitate directly from seawater (e.g., manganese nodules)


Climate change and seafloor sediment

Life in the sea changes with the climate

Remains of organisms in the sediments record the changes

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