A Microsoft® PowerPoint® Link Tool An Invitation to Marine Science Multimedia Manager A Microsoft® PowerPoint® Link Tool for Oceanography An Invitation to Marine Science 6th Edition by Tom Garrison http://oceanography.brookscole.com/garrison6e

Chapter 4 Continental Margins and Ocean Basins

Look For The Following Key Ideas In Chapter 4 New bathymetric devices used to study seabed features include multibeam echo sounder systems and satellites that use sensitive radar for altimetry. Their use has revolutionized our understanding of ocean floor formation and topography. Seafloor features result from a combination of tectonic activity and the processes of erosion and deposition. Near shore, the features of the ocean floor are similar to those of the adjacent continents because they share the same granitic basement. The transition to basalt marks the true edge of the continent and divides ocean floors into two major provinces. 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. Features of the continental margins include continental shelves, continental slopes, submarine canyons, and continental rises. Features of the deep-ocean basins include oceanic ridges, hydrothermal vents, abyssal plains and hills, seamounts, guyots, trenches, and island arcs.

The Ocean Floor Is Mapped by Bathymetry The discovery and study of ocean floor contours is called Bathymetry. (left) An illustration from the Challenger Report (1880). Seamen are handing the steam winch used to lower a weight on the end of a line to the seabed to find ocean depth.

Bathymetry: The Study of Ocean Floor Contours How did early scientists study the ocean floor? Early bathymetric studies were often performed using a weighted line to measure the depth of the ocean floor. Advances in Bathymetry Echo sounding Multibeam Systems Satellite Altimetry

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 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. When depth is great, the sounds reflect from a large area of seabed. Because the first sound of the returning echo is used to sense depth, measurements over deep depressions are often inaccurate.

Multibeam Systems Combine Many Echo Sounders Multibeam systems can provide more accurate measurements than echo sounders do. Multibeam systems collect data from as many as 121 beams to measure the contours of the ocean floor.

Satellites Can Be Used to Map Seabed Contours Satellite altimetry measures 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 Ocean Basin – the deep seafloor beyond the continental margin 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.

Ocean-Floor Topography Varies with Location Continental margins have several distinct components. (above) Cross section of a typical ocean basin flanked by passive continental margins. 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 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 slopes – the transition between the continental shelf and the deep-ocean floor. Shelf break – the abrupt transition from continental shelf to the continental slope. Continental rises – accumulated sediment found at the base of the continental slope.

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. Continental margins facing the edges of diverging plates are called passive margins. Continental margins near the edges of converging plates (or near places where plates are slipping past each other) are called active margins.

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 Form at the Junction between Continental Shelf and Continental Slope (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 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.

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. Flat-topped seamounts eroded by wave action are called guyots 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.

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 Summary 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?

End of Chapter 4