Topography of the Ocean Floor: Sixth Grade Science
The ocean floor possesses a dramatic topography featuring continental margins, abyssal plains, volcanic seamounts, deep subduction trenches, and towering mid-ocean ridges.
Mapping the Submerged Continents and Basins
Beneath the blue surface of the sea lies a varied and rugged topography exceeding the vertical extremes found on dry land. The ocean floor is not a flat, muddy bowl; it contains mountain chains longer than the Andes, canyons vastly deeper than the Grand Canyon, and flat plains broader than the Great Plains. Oceanographers map this submerged topography using multi-beam sonar bathymetry and satellite altimetry, dividing the ocean floor into two major provinces: continental margins and deep-ocean basins.
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| OCEAN BASIN BATHYMETRIC PROFILE |
| |
| Continent Mid-Ocean Ridge |
| [ COAST ] / \ / \ |
| \ Continental Shelf / \/ \ |
| \_______ / \ |
| \ Continental Slope / \ |
| \ / \ |
| \ Continental Rise / \ |
| \_______ / \|
| \_____________________________/ |
| ABYSSAL PLAIN (Flat mud) |
| \ Trench |
| \ / |
| \/ (11,000 m Deep) |
+-----------------------------------------------------------------------------+
The Continental Margin: The Submerged Edge of Continents
The continental margin is the transition zone connecting dry continental land to the deep ocean floor. Continental margins are classified as passive (geologically quiet, like the Atlantic coast of North America) or active (associated with plate boundaries, earthquakes, and subduction trenches, like the Pacific coast):
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| THE THREE CONTINENTAL MARGIN ZONES |
+---------------------+-------------------+-----------------------------------+
| Margin Feature | Slope & Depth | Geological Characteristics |
+---------------------+-------------------+-----------------------------------+
| Continental Shelf | Extremely gentle | Submerged continent; shallow |
| | slope (0.1°); | sunlit waters (< 130 m); home to |
| | 0 to 130 m depth | 90% of all marine commercial fish |
+---------------------+-------------------+-----------------------------------+
| Continental Slope | Steep incline | The true structural boundary of |
| | (4° to 25°); | the continent; plunges down from |
| | 130 to 3,000 m | shelf to deep ocean floor |
+---------------------+-------------------+-----------------------------------+
| Continental Rise | Gentle apron | Thick wedge of sediment deposited |
| | (0.5°); | at the slope base by underwater |
| | 3,000 to 4,000 m | turbidity currents and avalanches |
+---------------------+-------------------+-----------------------------------+
Submarine Canyons and Turbidity Currents
Carved into the steep continental slope are colossal submarine canyons, some rivaling the Grand Canyon in scale (such as the Monterey Submarine Canyon off California). These canyons are carved by turbidity currents—underwater avalanches of water-saturated sediment, mud, and sand that roar down the continental slope at speeds exceeding 80 km/h, scouring deep gorges and spreading sediment out onto the continental rise as submarine fans.
Features of the Deep-Ocean Basin
Beyond the continental margin lies the deep-ocean basin, situated at depths between 3,000 and 6,000 meters:
- Abyssal Plains: Incredibly flat, featureless regions of the deep ocean floor. They are among the flattest places on Earth, formed where thick blankets of fine pelagic sediment (clay, volcanic dust, and microscopic plankton shells) have buried the rugged volcanic basalt topography beneath a smooth mud blanket up to 1 kilometer thick.
- Deep-Ocean Trenches: Long, narrow, steep-sided V-shaped depressions that mark convergent subduction zones where one tectonic plate plunges beneath another. The Mariana Trench in the western Pacific is the deepest geological feature on Earth; its lowest point, the Challenger Deep, plunges 11,034 meters (36,201 feet) below sea level—deep enough that Mount Everest could be dropped into the trench with over 2 kilometers of water above its peak!
- Seamounts and Guyots: Submerged volcanic peaks that rise more than 1,000 meters above the seafloor. Seamounts are active or extinct volcanoes formed over hot spots or spreading centers. Guyots are extinct volcanic islands that were once above sea level; wave erosion planed their summits completely flat, after which thermal subsidence drowned them into flat-topped submarine tablemounts.
- Mid-Ocean Ridges and Hydrothermal Vents: The colossal underwater volcanic mountain ranges where seafloor spreading generates new basaltic crust. Along the central rift valley, cold seawater seeps into fractured crust, is superheated to 400 degrees Celsius by shallow magma, and spews back out through mineral chimneys called black smokers. These hydrothermal vents support chemosynthetic ecosystems fueled by sulfur bacteria rather than sunlight.
Exercises and Solutions
Exercise 1: Decoding Sonar Bathymetry Data. An oceanographic research vessel uses multi-beam sonar to map the seafloor. The sonar emits sound pulses that travel through seawater at 1,500 meters per second, bounce off the ocean floor, and return to the ship's receiver. At Station A, the two-way echo return time is 0.16 seconds. At Station B, the two-way return time is 5.2 seconds. At Station C, the two-way return time is 14.6 seconds. (a) Calculate the water depth in meters at Station A, Station B, and Station C; (b) Identify the likely ocean floor feature represented by each station (choose from: Continental Shelf, Abyssal Plain, Deep-Ocean Trench).
Solution 1: (a) Depth = 0.5 x Speed x Round-Trip Time. For Station A: Depth = 0.5 x 1,500 m/s x 0.16 s = 120 meters. For Station B: Depth = 0.5 x 1,500 m/s x 5.2 s = 3,900 meters. For Station C: Depth = 0.5 x 1,500 m/s x 14.6 s = 10,950 meters. (b) Station A (120 m depth) represents the shallow, gently sloping continental shelf. Station B (3,900 m depth) represents the vast, flat abyssal plain. Station C (10,950 m depth) represents a deep-ocean subduction trench (such as the Mariana Trench).
Exercise 2: Explaining Guyot Flat-Topped Topography. While mapping the deep Pacific Ocean floor, marine geologists discover a submerged volcanic seamount standing in 4,000 meters of water, but its summit is planed completely flat like a tabletop at a depth of 1,200 meters. Explain how a volcanic cone could develop a flat summit deep beneath the ocean surface.
Solution 2: A guyot originated as an active volcanic island that erupted above sea level millions of years ago. While the volcanic summit was exposed to the atmosphere, relentless coastal ocean waves, surf, and subaerial weathering eroded and flattened the volcanic cone, planing the summit smooth at the sea surface like a table. Over subsequent millions of years, as the tectonic plate carried the extinct volcano away from the warm mantle hot spot, the underlying oceanic lithosphere cooled, contracted, and became denser. This thermal cooling caused the lithosphere to subside (sink) deeper into the asthenosphere, drowning the flat-topped volcanic island thousands of meters below sea level to form a guyot.