Earth Science 14.2

F

eatures of the Ocean Floor

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

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 are not associated with boundaries , unlike the Pacific Ocean.

in the Atlantic Ocean plate

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.

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.

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.

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.

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. narrow feature, The continental slope is a relatively averaging only about 20 kilometers in width.

Ocean Floor features

Continental Slope:  Deep, steep sided valleys known as slope.

submarine canyons are cut into the continental  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.

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.

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.

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.

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.

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 seamounts and guyots .

, and tall volcanic peaks called

Ocean Floor features

Deep Ocean Trenches :  Deep ocean trenches parts of the ocean.

are long, narrow creases in the ocean floor that form the deepest  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.

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.

Ocean Floor

Abyssal Plains:  Abyssal plains are found in all oceans of the world.  However, the Atlantic Ocean has the most extensive abyssal plains slope.

because it has few trenches to catch sediment carried down the continental

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. in the Pacific. However, the greatest number can be found  Some can be found forming at volcanic hot spots such as the Hawaiian-Emperor Seamount chain.

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.

Ocean Floor

Mid-ocean ridges:  The mid-ocean ridge most ocean basins. is located near the center of  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.

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. transfer faults slide past each other shallow earthquakes.

These segments are offset by where plates horizontally, resulting in

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.

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.