The hypsographic curve shows the distribution of land of various elevations and seafloor of various depths in terms of the percent of the earth's crust that they occupy. You can see that the average depth of the oceans is about 3800m (average land elevation is about 840m) and that the deepest trenches of the oceans are deeper than Mt. Everest is high.

Techniques of determining bathymetry

Early exploration of the oceans basins was mainly through the use of weighted wires or ropes to obtain a depth measurement or sounding During W.W.I the echo sounder was developed. This allowed faster and more accurate depth readings.

These readings result in a profile (of the South Pacific) or crossing of an ocean basin which shows mid-ocean ridges. By combining many of these profiles, we produce bathymetric charts - maps of bottom topography

Seismic surveys use low frequency sound which will penetrate sub-bottom layers. Sound transmission is inversely proportional to its frequency.

Isobaths are lines connecting equal depths and can be seen in a view of a submarine canyon.

Marine provinces can be divided into the continental margin and the deep-sea basins.

Continental margin - A schematic profile of the continental margin to the deep-sea shows the following zones: (see examples of the east coast of the U.S. in either plan view or in cross section). On the west coast of the U.S. we see the Santa Barbara Basin and to our south we see the continental margin in the Gulf of Mexico. and the deep-ocean basins.

1. coastal plain - to the waters edge - includes the beaches - covered by unconsolidated sediments - wave energy moves these sediments by converging on headlands

2. inner and outer continental shelf - about 70 km wide - borders continent -topography generally resembles that of the coastal plain - gentle slope - about 0.1 degree (1.9m/km). The definition can be important legally- can be wide (up to 1300km)

Averages 70 km and 135m at the break. Sediments are transported across the shelf by currents.

3. continental slope - rather steep- 4 degrees (76m/km) follows the continental shelf break - some are cut by submarine canyons such as the Hudson Submarine Canyon or the Monterey Submarine Canyon which are strongly affected by turbidity currents caused by buildup of sediments until slumping occurs or an earthquake triggers it. Speeds up to 50kts have been measured. These canyons are spaced closer together as the slope increases. Deep-sea fans are formed where these canyons empty out onto the deep-sea floor.

4. upper and lower continental rise - usually smooth due to sediment cover - slope of 0.5 degrees (9.5m/km). The Amazon Cone is a good example of this smoothing as are the cones of the Ganges and the Indus rivers in the Indian Ocean.

Deep-ocean Basin - schematic profile of an ocean basin shows:

1. volcanoes

2. seamounts isolated submarine hills, steep sides, volcano-shaped - more then 1 km in relief

3. mid-ocean rises and ridges

4. abyssal plains

5. knolls - or abyssal hills (mostly in Pacific) 30-1,000m in relief, several nm wide, origin unknown, very common less than 1000m elevation, not completely smooth

6. trenches - mostly in the Pacific - mark the transition between the continents and the ocean basins - steep sided - as deep as 11000m. Some are "higher" than Mt. Everest.

7. island arcs - on landward side of trenches - areas of active mountain building - often with high mountains on the adjacent continent

8. fracture zones - generally east-west and extend several 1000km across the Pacific

9. guyots or flat topped seamounts - eroded during exposure at the surface - more than 1 km in relief

10. atolls

Slopes on the deep-ocean floor - generally very small except on inner trench slopes where they may reach 5 degrees, the highest on earth.

Abyssal plains have the smallest slope at 0.3 degrees.


Important Terms: hypsographic curve, echo sounding, transect profiles, mid-ocean ridges, continental margin, continental shelf, shelf break, continental slope, continental rise, deep-sea floor, abyssal plain, submarine canyon, turbidity currents, seamounts, guyots (tablemounts), fracture zones, transform faults


Top of Page