Margins and Basins

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

1. Tectonics forces shape the seabed. 2.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. Five Main Concepts

3.Continental margins may be active (earthquakes, volcanoes) or passive, depending on the local sense of plate movement. 4.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. 5.Using remote sensing methods, oceanographers have mapped the world ocean floor in surprising detail. Five Main Concepts Continued

I. Bathymetry A.The discovery and study of ocean floor contours is called bathymetry. B.Early bathymetric studies were often performed using a weighted line to measure the depth of the ocean floor. An illustration from the Challenger Report (1880): Seamen handling the steam winch aboard HMS Challenger used to lower a weight on the end of a line to the seabed to find ocean depth.

I. Bathymetry C.Advances in Bathymetry 1.Echo sounding a.a method of measuring seafloor depth using powerful sound pulses. b.The pulses of sound energy, or “pings,” from the sounder spread out in a narrow cone as they travel from the ship.

I. Bathymetry C.Advances in Bathymetry 1.Echo sounding (cont’d.) c.When depth is great, the sounds reflect from a large area of seabed. d.Because the first sound of the returning echo is used to sense depth, measurements over deep depressions are often inaccurate.

I. Bathymetry C.Advances in Bathymetry (cont’d.) 2.Multi – beam systems collect data from up to 121 beams to measure the contours of the ocean floor.

C.Advances in Bathymetry (cont’d.) 3.Satellite altimetry measures the sea surface height from orbit. a.Satellites can bounce 1,000 pulses of radar energy off the ocean surface every second. b.With satellite altimetry, sea surface levels can be measured more accurately because the precise position of the satellite can be calculated. I. Bathymetry

C.Advances in Bathymetry (cont’d.) 3.Satellite altimetry measures the sea surface height from orbit. (cont’d.) c.Distortion of the sea surface above a seabed feature occurs when the extra gravitational attraction of the feature “pulls” water toward it, forming a mound of water over itself. I. Bathymetry

Geosat, a U.S. Navy satellite operated from 1985 through 1990, provided measurements of sea surface height from orbit. Height accuracy was within 0.03 meters (1 inch)! I. Bathymetry

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. II. Topography of Ocean Floor A.Varies with location

This graph shows the distribution of elevations and depths on Earth. It 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 (12,430 feet) is much greater than the average elevation of the continents (2,760 feet). II. Topography of Ocean Floor

B.There are two primary classifications of ocean floor 1. Continental Margins = the submerged outer edge of a continent

II. Topography of Ocean Floor B.There are two primary classifications of ocean floor (cont’d.) 2. Ocean Basin = the deep seafloor beyond the continental margin

II. Topography of Ocean Floor Features of Earth’s solid surface shown as percentages of the Planet’s total surface. Text Not in notes

III. Continental Margins A.There are two types of continental margins 1. Passive margins face the edges of diverging tectonic plates. Very little to NO volcanic or earthquake activity is associated with passive margins.

III. Continental Margins A.There are two types of continental margins (cont’d.) 2. Active margins are located near the edges of converging plates. Active margins ARE the site of volcanic and earthquake activity.

B.Continental margins have several components 1. Continental shelf – the shallow, submerged edge of the continent. 2. Continental slope – the transition between the continental shelf and the deep-ocean floor. III. Continental Margins

B.Continental margins have several components 3. Shelf break – the abrupt transition from continental shelf to the continental slope. 4. Continental rise – accumulated sediment found at the base of the continental slope. III. Continental Margins

5. 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. III. Continental Margins B.Continental margins have several components (cont’d.)

III. Continental Margins 6. Turbidity currents, occur when turbulence mixes sediments into water above a sloping bottom. 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. B.Continental margins have several components (cont’d.) 7.Most geologists believe that the canyons have been formed by abrasive turbidity currents plunging down the canyons.

The features of a passive continental margin: III. Continental Margins (a) Vertical exaggeration 50:1 (b) No vertical exaggeration Text Not in notes

Active vs. Passive

1.An oceanic ridge is a mountainous chain of young, basaltic rock at an active spreading center of an ocean. If the ocean evaporated, the ridge system would be Earth’s most remarkable and obvious feature. IV. Deep Ocean Basins A.Oceanic Ridges

IV. Deep Ocean Basins The thickness of the red lines indicate the rate of spreading for some of the most rapidly spreading sections, and the numbers give spreading rates in centimeters per year. The East Pacific Rise typically spreads about six times faster than the Mid-Atlantic Ridge.

Hand-drawn map of a portion of the Atlantic Ocean floor showing some major oceanic features: mid-ocean ridge, transform faults, fracture zones, submarine canyons, seamounts, continental rises, trenches, and abyssal plains. The map is vertically exaggerated. IV. Deep Ocean Basins

2.Transform faults and fracture zones along an oceanic ridge. Because the ocean floor cannot expand evenly on the surface of a sphere, plate divergence on the spherical Earth can only be irregular and asymmetrical, and transform faults and fracture zones result. A.Oceanic Ridges (cont’d.)

IV. Deep Ocean Basins

1. Hydrothermal vents are sites where superheated water containing dissolved minerals and gases escapes through fissures, or vents. Cool water (blue arrows, next slide) is heated as it descends toward the hot magma chamber, leaching sulfur, iron, copper, zinc, and other materials from the surrounding rocks. IV. Deep Ocean Basins B.Hydrothermal Vents

IV. Deep Ocean Basins 2.The heated water (red arrows) returning to the surface carries these elements upward, discharging them at hydrothermal springs on the seafloor. B.Hydrothermal Vents

1. flat areas of sediment-covered ocean floor found between the continental margins and oceanic ridges. 2. small, extinct volcanoes or rock intrusions are found near the ridges. IV. Deep Ocean Basins C.Abyssal Plains

1. Seamounts are volcanic projections from the ocean floor that do not rise above sea level. IV. Deep Ocean Basins D.Seamounts and Guyots

2. Flat-topped seamounts eroded by wave action are called guyots. IV. Deep Ocean Basins D.Seamounts and Guyots (cont’d.)

1.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. IV. Deep Ocean Basins E.Trenches and Island Arcs

IV. Deep Ocean Basins E.Trenches and Island Arcs (cont’d.) 2.Island Arcs are curving chains of volcanic islands and seamounts and they are almost always found paralleling the concave edge of a 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. James Cameron's Deep Sea Challenge James Cameron's Deep Sea Challenge Gallo: Underwater Rivers and Lakes Gallo: Underwater Rivers and Lakes Gallo: Ocean Ridges & Hydrothermal Vents Gallo: Ocean Ridges & Hydrothermal Vents The Mariana Trench IV. Deep Ocean Basins Text Not in notes E.Trenches and Island Arcs (cont’d.)

In this unit 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. Unit in Perspective