Oceanography Chapter 4 Tom Garrison An Invitation to Marine Science, 7th Tom Garrison Chapter 4 Continental Margins and Ocean Basins

Chapter 4 Study Plan The Ocean Floor Is Mapped by Bathymetry Ocean-Floor Topography Varies with Location Continental Margins May Be Active or Passive The Topology of Deep-Ocean Basins Differs from That of the Continental Margin The Grand Tour

Chapter 4 Main Concepts Tectonics forces shape the seabed. The ocean floor is divided into continental margins and deep-ocean basins. The continental margins are seaward extensions of the adjacent continents and are usually underlain by granite; the deep seabeds have different features and are usually underlain by basalt. Continental margins may be active (earthquakes, volcanoes) or passive, depending on the local sense of plate movement. The mid-ocean ridge system is perhaps Earth’s most prominent feature. Most of the water of the world ocean circulates through hot oceanic crust in the ridges about every 10 million years. Using remote sensing methods, oceanographers have mapped the world ocean floor in surprising detail.

The Ocean Floor Is Mapped by Bathymetry The discovery and study of ocean floor contours is called Bathymetry. 85 B.C.- Posidonius- Mediterranean (≈2 km/ 1.25 mi) Used a stone on a rope 1818- Sir James Clark Ross- South Atlantic (≈16054 ft/ 4893m) 1870s- HMS Challenger Used the same techniques but used a steam powered winch to raise and lower the line and weight. Confirmed Matthew Maury’s discovery of the Mid- Atlantic Ridge.

The Ocean Floor Is Mapped by Bathymetry Echo sounding June 1922- US Navy vessel, USS Stewart, made the first continuous profile across an ocean basin. 1925 to 1927- German ship, Meteor, made 14 profiles of the Atlantic. The winding path of the Mid- Atlantic Ridge was revealed.

Echo Sounders Bounce Sound off the Seabed Echo sounding is a method of measuring seafloor depth using powerful sound pulses. The accuracy of an echo sounder can be affected by water conditions (temperature, pressure, salinity) and bottom contours. The pulses of sound energy, or “pings,” from the sounder spread out in a narrow cone as they travel from the ship. Because the first sound of the returning echo is used to sense depth, measurements over deep depressions are often inaccurate.

Multi-beam Systems Combine Many Echo Sounders Multi-beam systems provide more accurate measurements than echo sounders. Multi-beam systems collect data from up to 121 beams to measure the contours of the ocean floor.

Satellites Can Be Used to Map Seabed Contours Satellite altimetry is an indirect way of measuring ocean depth, by measuring the sea surface height from orbit. Satellites can bounce 1,000 pulses of radar energy off the ocean surface every second. (right) Geosat, a U.S. Navy satellite operated from 1985 through 1990, provided measurements of sea surface height from orbit. Moving above the ocean surface at 7 kilometers (4 miles) a second, Geosat bounced 1,000 pulses of radar energy off the ocean every second. Height accuracy was within 0.03 meters (1 inch)! (below) With the use of satellite altimetry, sea surface levels can be measured more accurately, showing sea surface distortion. Distortion of the sea surface above a seabed feature occurs when the extra gravitational attraction of the feature “pulls” water toward it from the sides, forming a mound of water over itself.

The Topography of Ocean Floors Cross section of the Atlantic ocean basin and the continental United States, showing the range of elevations. The vertical exaggeration is 100:1. Although ocean depth is clearly greater than the average height of the continent, the general range of contours is similar.

Ocean-Floor Topography Varies with Location A graph showing the distribution of elevations and depths on Earth. This graph is not a land-to-sea profile of Earth, but rather a plot of the area of Earth’s surface above any given elevation or depth below sea level. Note that more than half of Earth’s solid surface is at least 3,000 meters (10,000 feet) below sea level. The average depth of the ocean (3,790 meters or 12,430 feet) is much greater than the average elevation of the continents (840 meters or 2,760 feet).

Ocean-Floor Topography Varies with Location What are the two classifications of ocean floor? Continental Margins The submerged outer edge of a continent Marked by thicker, less dense granitic crust Ocean Basin The deep seafloor beyond the continental margin Marked by the transition to thinner, denser basaltic crust

Ocean-Floor Topography Varies with Location Continental margins have several distinct components. (above) Cross section of a typical ocean basin The submerged outer edge of a continent is called the continental margin. The deep-sea floor beyond the continental margin is properly called the ocean basin.

Ocean-Floor Topography Varies with Location What are the two types of continental margins? Passive margins, also called Atlantic-type margins, face the edges of diverging tectonic plates. Very little volcanic or earthquake activity is associated with passive margins. Active margins, known as Pacific-type margins, are located near the edges of converging plates. Active margins are the site of volcanic and earthquake activity.

Continental Margins May Be Active of Passive Typical continental margins bordering the tectonically active (Pacific-type) and passive (Atlantic-type) edges of a moving continent. The vertical scale has been exaggerated. Passive margins = continental margins facing the edges of diverging plates Active margins = continental margins near the edges of converging plates (or near places where plates are slipping past each other)

Ocean-Floor Topography Varies with Location Features of Earth’s solid surface shown as percentages of the Planet’s total surface.

Continental Margins May Be Active or Passive Continental margins have several components Continental shelf – the shallow, submerged edge of the continent. Continental slope – the transition between the continental shelf and the deep-ocean floor. Shelf break – the abrupt transition from continental shelf to the continental slope. Continental rise – accumulated sediment found at the base of the continental slope.

Continental Shelves Are Seaward Extensions of the Continents The features of a passive continental margin: (a) Vertical exaggeration 50:1 (b) No vertical exaggeration margin.

Continental Shelves Are Seaward Extensions of the Continents Changes in sea level over the last 250,000 years, as traced by data taken from ocean-floor cores. The rise and fall of sea level is due largely to the coming and going of ice ages – periods of increased and decreased glaciation, respectively. Because water that formed the ice-age glaciers came from the ocean, sea level dropped. Point a indicates a low stand of -125 meters (-410 feet) at the climax of the last ice age some 18,000 years ago. Point b indicates a high stand of +6 meters (+19.7 feet) during the last interglacial period about 120,000 years ago. Point c shows the present sea level. Sea level continues to rise as we emerge from the last ice age and enter an accelerating period of global warming.

Submarine Canyons Form at the Junction between Continental Shelf and Continental Slope Submarine canyons are a feature of some continental margins. They cut into the continental shelf and slope, often terminating on the deep-sea floor in a fan-shaped wedge of sediment.

Submarine Canyons (right) A turbidity current flowing down a submerged slope off the island of Jamaica. The propeller of a submarine caused the turbidity current by disturbing sediment along the slope Avalanche-like sediment movement caused when turbulence mixes sediments into water above a sloping bottom are called turbidity currents.

The Topology of Deep-Ocean Basins Differs from That of the Continental Margin What are some features of the deep-ocean floor? Oceanic Ridges Hydrothermal Vents Abyssal Plains and Abyssal Hills Seamounts and Guyots Trenches and Island Arcs

Oceanic Ridges Circle the World An oceanic ridge is a mountainous chain of young, basaltic rock at an active spreading center of an ocean.

Oceanic Ridges Circle the World Stetches across 65,000 km (40,000 miles) Can rise 2 km above the seafloor Can lead to the formation of islands Iceland The Azores Easter Island Slowly spreading centers have a steeper rise than rapidly cooling ones

Oceanic Ridges Circle the World Transform faults and fracture zones along an oceanic ridge Transform faults are fractures along which lithospheric plates slide horizontally past one another. Transform faults are the active part of fracture zones. Shallow earthquakes are common at these faults.

Hydrothermal Vents Are Hot Springs on Active Oceanic Ridges Hydrothermal vents are sites where superheated water containing dissolved minerals and gases escapes through fissures, or vents. Cool water (blue arrows) is heated as it descends toward the hot magma chamber, leaching sulfur, iron, copper, zinc, and other materials from the surrounding rocks. The heated water (red arrows) returning to the surface carries these elements upward, discharging them at hydrothermal springs on the seafloor.

Volcanic Seamounts and Guyots Project above the Seabed Seamounts are volcanic projections from the ocean floor that do not rise above sea level. Some form at hotspots, but most are believed to be submerged in active volcanoes that formed at spreading centers. ≈ 10,000 (1/2 total) are found in the Pacific. Guyots are flat-topped seamounts eroded by wave action. They once were tall enough to penetrate the sea surface, but were eroded by waves. Abyssal hills are flat areas of sediment-covered ocean floor found between the continental margins and oceanic ridges. Abyssal hills are small, extinct volcanoes or rock intrusions near the oceanic ridges. Earth’s most common landform.

Trenches and Island Arcs Form in Subduction Zones Trenches are arc-shaped depressions in the ocean floor caused by the subduction of a converging ocean plate. Most trenches are around the edges of the active Pacific. Trenches are the deepest places in Earth’s crust, 3 to 6 kilometers (1.9 to 3.7 miles) deeper than the adjacent basin floor. The ocean’s greatest depth is the Mariana Trench where the depth reaches 11,022 meters (36,163 miles) below sea level.

Trenches and Island Arcs Form in Subduction Zones The Mariana Trench (a) Comparing the Challenger Deep and Mount Everest at the same scale shows that the deepest part of the Mariana Trench is about 20% deeper than the mountain is high. (b) The Mariana Trench shown without vertical exaggeration.

Chapter 4 in Perspective In this chapter you learned how difficult it has been to discover the shape of the seabed. Even today, the surface contours of Mars are better known than those of our ocean floor. We now know that seafloor features result from a combination of tectonic activity and the processes of erosion and deposition. The ocean floor can be divided into two regions: continental margins and deep-ocean basins. The continental margin, the relatively shallow ocean floor nearest the shore, consists of the continental shelf and the continental slope. The continental margin shares the structure of the adjacent continents, but the deep ocean floor away from land has a much different origin and history. Prominent features of the deep ocean basins include rugged oceanic ridges, flat abyssal plains, occasional deep trenches, and curving chains of volcanic islands. The processes of plate tectonics, erosion, and sediment deposition have shaped the continental margins and ocean basins. In the next chapter you will learn that nearly all the ocean floor is blanketed with sediment. With the exception of the spreading centers themselves, the broad shoulders of the oceanic ridge systems are buried according to their age – the older the seabed, the greater the sediment burden. Some oceanic crust near the trailing edges of plates may be overlain by sediments more than 1,500 meters (5,000 feet) thick. Sediments have been called the “memory of the ocean.” The memory, however, is not a long one. Before continuing, can you imagine why that is so?