Chapter 30: The Oceans and Ocean Basins
The shape of the ocean floor and the physical properties of seawater underlie soundings, sound propagation, currents and ice.
Basin topography
From the shore, the continental shelf slopes gently to a shelf break near 130 m, then steepens down the continental slope and merges into the continental rise and abyssal plain at 3000–6000 m. Mid-ocean ridges mark spreading centres; trenches such as the Mariana exceed 10 000 m at subduction zones. Seamounts, guyots and pinnacles are the practical hazard: many are inadequately surveyed and rise steeply from deep water with little warning on the echo sounder.
Seawater properties
Average salinity is about 35 parts per thousand, higher in evaporation-dominated subtropics and enclosed seas, lower near river outflows and melting ice. Density increases with salinity and pressure and decreases with temperature, producing a stratified ocean: a mixed surface layer, a thermocline of rapid temperature decrease, and cold deep water. Sound speed depends on temperature, salinity and pressure and creates refraction and channelling that affect echo sounding and sonar.
- Shelf break near 130 m depth
- Salinity ~35 ppt average; density driven by temperature, salinity, pressure
- Thermocline causes acoustic refraction and false bottoms
Key takeaways
- • The continental shelf ends at a break near 130 m and drops steeply.
- • Density stratification governs both currents and sound propagation.
- • Poorly surveyed seamounts are a real and continuing hazard offshore.
Knowledge check
Immediate feedback with rationales. Answer each question to see the explanation.
Chapter 30 quiz
0 of 3 answered
1.The continental shelf break typically occurs at a depth of about:
2.Average ocean salinity is about:
3.Seawater density increases with: