Ocean Floor Divided into 3 major regions Continental Margins Ocean Basins Mid-Ocean Ridges Continental Margins Include: Continental Shelves Continental Slopes Continental Rise 1
Continental Shelves Part of the continent that is underwater Extremely flat Vary in width, from almost non-existent to as much as 1500km Extend from the shoreline to the shelf edge The Shelf Edge is the boundary between the continental shelf and slope The edge has an average depth of 130m Continental Slopes Begins at the shelf edge, with a clear and abrupt boundary, marked by a massive drop off Usually less then 200 km wide and descend to a depth of 3 km The change from continental to oceanic crust happens under this area Slopes are cut by many gullies and small valleys that resulted from mud slides Submarine Canyons massive gullies that can make the Grand Canyon look small Ex: Hudson River Canyon 300km wide 2
Continental Rise Gently sloping region between the continental slope and the ocean basin Formed as a result of a massive deposition of sediments that are several kilometres thick The sediment originally came from land and was brought to this region by turbidity currents and gravity flows May be as wide as 1000 km, with a gentle slope Lower canyons, however were formed under different conditions Powerful Turbidity Currents that can run like flash floods formed as a result of massive mud slides or earthquakes These mudslides are very powerful erosion agents which can carve very large canyons, deposit large amounts of sediments at the canyon base and quite frequently broke underwater communication cables 3
Continental Margin Features associated with continental margins include; Submarine Canyon: Steep sloped valleys carved into the continental shelf that extends to ocean depths of 3000 meters. Turbidity Currents Origin of Submarine Canyons Upper part of the canyon is on the continental shelf, and the bottom part cuts deep into the continental slope Thought that the upper parts were formed during the last ice age At that time the sea level was approx. 100 m lower then it is now Large rivers cut valleys into the shelf edge Once the glaciers melted, the sea level rose and buried these valleys 4
Continental Margin Turbidity Currents: Dense sediment-rich currents created when mud and sand are lifted into suspension. These currents sweep down over the continental shelf and slope into the deep oceans. The repeated erosion of these currents are thought to be the main force in forming submarine canyons. Continental Margin Features associated with continental margins include; Turbidites: Sediment deposits on the continental rise. Form when the suspended sediment loses energy and starts to settle. Larger particles settle first, followed by smaller silt and clay. This feature of a decrease in sediment size from the bottom to the top is called Graded Bedding. Turbidity Current Graded Bedding 5
Ocean Basins Ocean basins can be described as saucerlike depressions of the seabed Features of the Basins include Abyssal Plains Abyssal Hills Seamounts Coral Atolls Guyots Abyssal Plains Plains range in depth from 3000-6000 metres Most remarkable feature is their flatness Considered to be the flattest areas on Earth Composed of sediments that can be more then 1 km thick Continental rivers deposit materials onto the edges of continental shelves and Turbidity currents carry them down the slope and spread them evenly over the Abyssal Plain Ex: Hatteras Abyssal Plain 1000km long and 150 to 300 km wide 6
Abyssal Hills Small rolling hills that occur next to continental margins and oceanic ridge systems Often found in groups Usually 1 10 km across and no higher then a few hundred metres In the North-Atlantic, they form 2 strips that are parallel to the Mid-Atlantic Ridge Seamounts, Guyots, Coral Atolls Seamounts cone-shaped mountain peaks that rise high above the deep ocean floor Typically found in clusters, most abundant in the Pacific Volcanic in origin, and related to plate boundary activity Hawaiian Islands chain of seamounts tall enough to rise above the ocean surface 7
Guyots Flat topped seamounts The peak/top of the Seamount has been removed/eroded as a result of wave action The sinking of the oceanic crust lowered the tops of the Guyots below the surface Atolls Ring shaped coral islands Forms when a coral reef forms around a volcanic island As the sea-floor (and volcanic island) sinks, so does the coral reef Because corals can only live in water less then 80 metres new corals grow on top of the old, dead corals When the reef sinks, it grows at the top trying to keep pace with the lowering seafloor Ex: Fanning Island 8
Trenches Long, narrow steep sided troughs that run parallel to either continental margins or chains of volcanic islands Almost all trenches occur around the Margin of the Pacific Ocean Trenches mark the place where one crustal plate goes beneath the other On average, trenches are 1500km long and 100 km wide Depths may be 2-4 km below the neighbouring ocean floor Bottoms are narrow, flat and filled with sediment Longest Trench Peru-Chile Trench (5900km) Deepest Trench Marianas Trench (11km below sea level) 9
Mid-Ocean Ridges Form at Divergent Plate boundaries Divergent boundaries are places where 2 lithospheric (crustal) plates move apart and new oceanic crust forms Continuous chain that crosses every ocean (approx. 80 000km long) Average depth of 2500 m and 1000km wide (fit Newfoundland in one) At the highest peaks, the ridges reach above the ocean surface to form islands The Mid-Atlantic Ridge Runs parallel to the shorelines of North and South America 10
Seamounts that rise from the ridge form the Azores Islands in the North and Ascension Islands in the South The mid-atlantic Ridge is spreading at a rate of 1 cm/year Right European Plate Left North American Plate 11
Fracture Zones Characterized by 2 factors Rugged sea floor Strike-Slip Fault Strike-Slip Fault (Transform Fault) the rocks on the opposite sides of the fault plane move horizontally past each other Fracture zones break mid-ocean ridges into sections 50-100km long Some are offset by a few kilometres and others by hundreds of kilometres The offset ridges grind together and cause earthquakes Ocean Floor Sediments 1. Oozes sediments made from microscopic shells, which are the remains of tiny floating plants and animal a) Calcareous Oozes calcium carbonate from Globigerina b) Siliceous Oozes silicon dioxide from Diatoms and radiolaria 12
2. Muds and Clays Muds are mixtures of fine particles of various sizes that have settled to the bottom of the ocean. They are often soft, plastic materials that feel greasy Clays also settle to the bottom, but because of their small size, some particles may take over 100 years to reach the ocean floor Red Clays are the most common and are found primarily in deep ocean trenches. Although they are called Red Clays, they may also be brown, yellow, gray, green and blue 13
3. Authigenic Sediments Authigenic means forms in place Form directly on the ocean floor Best known examples are Manganese Nodules lumps of material made from minerals rich in manganese and iron oxides (with small amounts of nickel, cobalt and copper) Contain twice the concentration of copper and nickel as found in deposits on land but are too difficult and expensive to mine 14