Earth Science 14.2 F eatures of the Ocean Floor

1. Earth Science 14.2 Features of the Ocean Floor

2. Ocean Floor features Oceanographers studying the topography of the ocean floor have divided it into three major zones • Continental margins • The ocean basin floor • The mid ocean ridge • The illustration at right shows the typical topography of an ocean; the continental margin, ocean basin, mid-ocean ridge and again to the next continental margin. • Scientists have discovered that each of these areas has it’s own distinct features

3. Ocean Floor features Continental Margins: • The zone of transition between a continent and the adjacent ocean basin floor is known as the continental margin. • In the Atlantic, thick layers of undisturbed sediment cover the continental margin. • This region has very little volcanic or earthquake activity. • This is because the continental margins in the Atlantic Ocean are not associated with plate boundaries, unlike the Pacific Ocean.

4. Ocean Floor features Continental Margins: • In the Pacific Ocean, where plate boundaries converge, oceanic crust is plunging beneath continental crust in a subduction zone. • This force results in a narrow continental margin that experiences both volcanic activity and earthquakes.

5. Ocean Floor features • If you could travel from one coast to another across the ocean floor, the first zone one would pass through upon leaving land would be the continental shelf. • The continental shelf is the gently sloping submerged area extending from the shoreline. • This shelf is almost nonexistant on some shorelines.

6. Ocean Floor features • However, on other shorelines in the world, the continental shelf can extend outwards as much as 1500 kilometers. • On average, the shelf is about 80 kilometers wide 130 meters deep at it’s seaward edge. • The average steepness of the drop is only about 2 meters per kilometer drop; a slope so slight that the human eye can barely perceive it.

7. Ocean Floor features • Continental shelves have political and economic significance as well. • Continental shelves contain important resources such as mineral deposits, oil and natural gas deposits, and enormous sand and gravel deposits. • The waters of the continental shelf also contain important fishing grounds, which are a significant source of food.

8. Ocean Floor features Continental Slope: • Marking the seaward edge of the continental shelf is the continental slope. • The slope is steeper than the shelf and it marks the transition from continental crust to oceanic crust. • Although the steepness of the continental slope varies from location to location; the average slope is 5 degrees. • In some places, the slope can exceed 25 degrees. The continental slope is a relatively narrow feature, averaging only about 20 kilometers in width.

9. Ocean Floor features Continental Slope: • Deep, steep sided valleys known as submarine canyons are cut into the continental slope. • These canyons may extend to the ocean basin floor. • Most information suggest that submarine canyons are formed by erosion, at least in part, by turbulent underwater currents.

10. Ocean Floor features Continental Slope: • Turbidity currents are occasional movements of dense sediment-rich water down the continental slope. • They are created when sand and mud on the continental shelf are disturbed, perhaps by an earthquake, and become suspended in the water.

11. Ocean Floor features Continental Slope: • Because the muddy water is denser than sea-water, it flows down the slope. • As it flows down, it erodes the bank away, accumulating more sediment, and eventually cuts these deep canyons into the shelf’s surface. • Erosion from these muddy torrents over time is believed to be the major force behind the carving of these large canyons.

12. Ocean Floor features Continental Slope: • Narrow continental margins, such as the one along the California coast, are marked with numerous submarine canyons created by runoff sediment from the land. • Turbidity current are know to be an important mechanism of sediment transport in the ocean. • Turbidity currents erode submarine canyons and deposit sediments on the deep-ocean floor.

13. Ocean Floor features Continental Rise: • In regions where trenches do not exist, the steep continental slope merges into a more gradual incline known as the continental rise. • Here the steepness of the slope drops to about 6 meters per kilometer. • Where the width of the continental slope averages only about 20 kilometers wide, the continental rise may be hundreds of kilometers wide.

14. Ocean Floor features Ocean Basin Floor • Between the continental margin and mid-ocean ridge, lies the oceanic basin floor. • The size of this region, nearly 30 percent of Earth’s surface, is comparably equal to the percentage of land above sea level. • This region includes deep ocean trenches, very flat areas known as abyssal plains, and tall volcanic peaks called seamounts and guyots.

15. Ocean Floor features Deep Ocean Trenches: • Deep ocean trenches are long, narrow creases in the ocean floor that form the deepest parts of the ocean. • Most trenches are located along the margins of the Pacific Ocean, and many exceed 10,000 meters (almost 30,000 feet) in depth. • A portion of one trench, the Challenger deep in the Mariana Trench has been measured at a record 11,022 meters below sea-level.

16. Ocean Floor Abyssal Plains: • Abyssal plains are deep, extremely flat features. In fact, these regions may be the most level areas on Earth. • Abyssal plains have accumulations of thick accumulations of fine sediment that have buried an otherwise rugged sea floor. • The sediments that make up the abyssal plain are carried out there by turbidity currents or as a result of suspended sediments settling.

17. Ocean Floor Abyssal Plains: • Abyssal plains are found in all oceans of the world. • However, the Atlantic Ocean has the most extensive abyssal plains because it has few trenches to catch sediment carried down the continental slope.

18. Ocean Floor Seamounts and guyots: • The submerged volcanic peaks that dot the ocean floor are called seamounts. They are volcanoes that have not reached the ocean surface. • These steep sided cone shaped peaks are found on the floors of all oceans. However, the greatest number can be found in the Pacific. • Some can be found forming at volcanic hot spots such as the Hawaiian-Emperor Seamount chain.

19. Ocean Floor Seamounts and guyots: • Once underwater volcanoes reach the surface, they form islands. Over time, running water and wave action erode these islands to near sea-level. • Over millions of years, these islands gradually sink below sea-level. • This process occurs as the moving plate carries the island away from the elevated oceanic ridge or hot spot where they originated. • These once active flat-topped but now submerged structures are called guyots.

20. Ocean Floor Mid-ocean ridges: • The mid-ocean ridge is located near the center of most ocean basins. • The mid-ocean ridge system is an interconnected system of underwater mountains that have developed on newly formed ocean crust. • This system is the longest topographic feature on Earth running 70,000 kilometers around the world’s oceans.

21. Ocean Floor Mid-ocean ridges: • The term ridge may be misleading because the mid-ocean ridge is not narrow. • It has widths from 1000 to 4000 kilometers and may occupy as much as a total of one half the total area of the ocean. • The mid ocean ridge is broken into sections. These segments are offset by transfer faults where plates slide past each other horizontally, resulting in shallow earthquakes.

22. Ocean Floor Sea-floor spreading: • A high amount of volcanic activity takes place along mid-ocean ridges. • This activity is associated with sea-floor spreading. Sea-floor spreading occurs where divergent plate boundaries are moving apart from each other. • New ocean is formed at mid-ocean ridges as magma rises between diverging plates and cools.

23. Ocean Floor Hydrothermal vents: • Hydrothermal vents form along mid-ocean ridges. • These are zones where mineral-rich water, heated by the hot, newly formed oceanic crust, escapes through cracks in the oceanic crust into the water. • As the super-heated mineral-rich water comes in contact with the surrounding cold water, minerals containing metals such as sulfur, iron, copper, and zinc precipitate out and are deposited.