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
Salinity
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
Bromine
Fresh water
Obtained by the desalination of seawater
High cost to produce
Small total production
The oceans’s three-layered structure
Temperature
Warmest at surface
Thermocline-rapid decrease in temperature with depth
Below thermocline-little change in temperature
Salinity
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
Layered
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
Seamount
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
Atoll
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