OceanographyEARTH SCIENCE 11MSUIIT – IDS (SHS)

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Calling Earth the “water planet” is certainly appropriate, because

nearly 71 percent of its surface is covered by the global ocean.

3Concepts

What is oceanography? What is the extent and distribution of the world’s oceans? What techniques are used to map the ocean floor? How does a passive continental margin differ from an active continental

margin? What are the major features of the deep-ocean basin? How are mid-ocean ridges and deep-ocean trenches related to plate

tectonics boundaries? What are the various types of seafloor sediments? How can these sediments

be used to study worldwide climate change?

4What is oceanography?

Oceanography is an interdisciplinary science that draws on the methods and knowledge of geology, chemistry, physics, and biology to

study all aspects of the world ocean.

5Geography of the Oceans

The area of Earth is about 510 million square kilometers.

360 million square kilometers (71%) is represented by oceans and marginal seas

Continents and islands comprise the remaining 29 percent, or 150 million square kilometers.

Is the distribution of land and water similar in the Northern and Southern hemispheres?

6These views of Earth show the uneven distribution of land and water between the Northern and Southern Hemispheres.

61% water

81% water

land hemisphere

water hemisphere

7What is the extent and distribution of the world’s oceans?

Between latitudes 45 degrees north and 70 degrees north, there is actually more land than water, whereas between 40 degrees south and 65 degrees south there is almost no land to interrupt the oceanic and atmospheric circulation.

8The world ocean can be divided into four main ocean basins:

9Pacific Ocean

largest ocean and the largest single geographic feature on the planet

accounts for over half of the ocean surface area on Earth

all of the continents could fit into the space occupied by it

the world’s deepest ocean, with an average depth of 3,940 meters

10Atlantic Ocean

about half the size of the Pacific Ocean and not quite as deep

relatively narrow ocean as compared to the Pacific

bounded by almost parallel continental margins.

11Indian Ocean

• slightly smaller than the Atlantic Ocean but has about the same average depth

• it is largely a Southern Hemisphere water body

12Arctic Ocean

about 7 percent the size of the Pacific Ocean

only a little more than one-quarter as deep as the rest of the oceans

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Oceanographers also recognize an additional ocean near the continent of Antarctica in the

Southern Hemisphere. Defined by the meeting of currents near

Antarctica called the Antarctic Convergence, the Southern Ocean or Antarctic Ocean is

actually those portions of the Pacific, Atlantic, and Indian Oceans south of about 50 degrees south

latitude.

14Comparing the Oceans to the Continents

Average elevation of Continents above sea level = 840 meters

Average depth of oceans = 3729 meters If Earth’s solid mass were perfectly smooth

(level) and spherical, the oceans would cover Earth’s entire surface to a uniform depth of more than 2,000 meters !

15Concept Check

How does the area of Earth’s surface covered by the oceans compare with that of the continents?

Contrast the distribution of land and water in the Northern Hemisphere and the Southern Hemisphere.

Excluding the Southern Ocean, name the four main ocean basins. Contrast them in terms of area and depth.

How does the average depth of the oceans compare to the average elevation of the continents?

16Mapping the Seafloor

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Bathymetry is the measurement of ocean depths and the charting of the

shape or topography of the ocean floor.

18The Challenger Expedition (December 1872 to May 1876)

first comprehensive study of the global ocean

sampled water depth by laboriously lowering long weighted lines overboard

The knowledge gained by the Challenger of the ocean’s great depth and varied topography expanded with the laying of transatlantic telegraph cables

19Echo Sounders

developed early in the 20th century work by transmitting a sound wave (called a

ping) into the water in order to produce an echo when it bounces off any object

A sensitive receiver intercepts the echo reflected from the bottom, and a clock precisely measures the travel time to fractions of a second.

Depth = [(1500m/s * echo travel time)/2] Depths determined from continuous monitoring

of these echoes are plotted to create a profile of the ocean floor.

20Sidescan

(used in World War 2) developed by the US Navy to look

for explosive devices that had been deployed in shipping lanes

torpedo-shaped instruments, towed behind a ship where they send out a fan of sound extending to either side of the ship’s track

produced the first photograph-like images of the seafloor but does not provide bathymetric (water depth) data

21High-Resolution Multibeam SONAR (1990s)

uses hull-mounted sound sources that send out a fan of sound, then record reflections from the seafloor through a set of narrowly focused receivers aimed at different angles

can collect bathymetric data of such high resolution that they can distinguish depths that differ by less than a meter

22Color-enhanced perspective map of the seafloor and coastal landforms in the Los Angeles area of California.

The ocean floor portion of this map was constructed from data collected using a high-resolution mapping system. (U.S. Geological Survey)

23Viewing the Ocean Floor from Space

involves measuring the shape of the ocean surface from space

Ocean surface irregularities mimic the shape of the underlying seafloor.

Satellites equipped with radar altimeters are able to measure these subtle differences by bouncing microwaves off the sea surface.

can measure variations as small as a few centimeters

Combined with traditional sonar depth measurements, the data are used to produce detailed ocean-floor maps

24Seismic Reflection Profiles

The figure on the right shows a seismic profile of a portion of the Madeira abyssal plain in the eastern Atlantic. Although the seafloor is flat, notice the irregular ocean crust buried by a thick accumulation of sediments

To construct such a profile, strong low-frequency sounds are produced by explosions (depth charges) or air guns.

These sound waves penetrate beneath the seafloor and reflect off the contacts between rock layers and fault zones.

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Oceanographers studying the topography of the ocean floor have delineated three major units: continental margins, the deep-ocean basin, and the oceanic (mid-ocean) ridge.

The map in the next slide outlines the continental margins, the deep-ocean basins, and the oceanic ridge for the North Atlantic Ocean, and the profile at the bottom of the illustration shows the varied topography.

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Provinces of the Ocean Floor

27CONTINENTAL MARGINS

Two types of continental margins have been identified: passive and active

28Passive Margins

are not associated with plate boundaries

experience very little volcanism and few earthquakes

weathered materials eroded from the adjacent landmass accumulate to form a thick, broad wedge of relatively undisturbed sediments

Passive margins are found along most of the coastal areas that surround the Atlantic Ocean, including the east coasts of North and South America, as well as the coastal areas of western Europe and Africa

Active Continental Margins occur where oceanic lithosphere is

being subducted beneath the edge of a continent

The result is a relatively narrow margin, consisting of highly deformed sediments that were scraped from the descending lithospheric slab and plastered against the margin of the overriding continent.

Active continental margins are common around the Pacific Rim, where they parallel deep oceanic trenches.

29Passive Continental MarginsFeatures: continental shelf, the continental slope, and the continental rise

Continental Shelf a gently sloping,

submerged surface extending from the shoreline toward the deep ocean basin

have economic and political significance because they contain important mineral deposits (reservoirs of oil and natural gas, sand & gravel, fishing grounds)

Continental Slope a relatively steep

zone (as compared with the shelf) that marks the boundary between continental crust and oceanic crust

ave. inclination = 5-25 degrees

ave. width = 20km

Continental Rise consists of a thick

accumulation of sediment that moved downslope from the continental shelf to the deep-ocean floor.

slope drops to about one-third degree, or about 6 meters per kilometer

may extend for hundreds of kilometers into the deep-ocean basin

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31Submarine Canyons and Turbidity Currents

Submarine canyons are deep, steep-sided valleys cut into the continental slope probably excavated by turbidity currents

Turbidity currents are downslope movements of dense, sediment-laden water, created when sand and mud on the continental shelf and slope are dislodged and thrown into suspension. Because such mud-choked water is denser than normal seawater, it hugs the seafloor as it flows downslope, eroding and accumulating more sediment.

Beds deposited by these currents are called turbidites. Each event produces a single bed characterized by a decrease in sediment size

from bottom to top, a feature known as a graded bed.

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Submarine Canyons and Turbidity Currents

33Active Continental Margins

located primarily around the Pacific Ocean in areas where oceanic lithosphere is being subducted beneath the leading edge of a continent

Along active continental margins the continental shelf is very narrow, if it exists at all, and the continental slope descends abruptly into a deep-ocean trench

sediments from the ocean floor and pieces of oceanic crust are scraped from the descending oceanic plate and plastered against the edge of the overriding continent

accumulation of deformed sediment and scraps of oceanic crust is called an accretionary wedge

Some active margins have little or no sediment accumulation, indicating that material is being carried into the mantle with the subducting plate

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Active Continental Margin

35Deep Ocean Basin

almost 30 percent of Earth’s surface Includes abyssal plains, seamounts and guyots; deep-ocean trenches, and

oceanic plateaus

36Deep-Ocean Trenches

long, relatively narrow troughs that are the deepest parts of the ocean Most trenches are located along the margins of the Pacific Ocean where many exceed

10,000 meters (33,000 feet) in depth. The Challenger Deep in the Mariana Trench—measured 11,022 meters (36,163 feet)

below sea level, the deepest known part of the world ocean. Only two trenches are located in the Atlantic: the Puerto Rico Trench and the South Sandwich

Trench. sites of plate convergence where slabs of oceanic lithosphere subduct and plunge back into

the mantle often paralleled by an arc-shaped row of active volcanoes called a volcanic island arc also paralleled by continental volcanic arcs (those that lie adjacent to continental margins) The volcanic activity associated with the trenches that surround the Pacific Ocean explains

why the region is called the Ring of Fire

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38Seamounts, Guyots, Oceanic Plateaus

Seamounts are submarine volcanoes found on the floors of all the oceans; they are most common in the Pacific.

Guyots are submerged, flat topped seamounts “The term guyot is named after Princeton University’s first

geology professor. It is pronounced “GEE-oh” with a hard g as in “give.”

Oceanic Plateaus generated from vast outpourings of fluid basaltic lavas

39Formation of a coral atoll due to the gradual sinking of oceanic crust and upward growth of the coral reef

A fringing coral reef forms around an active volcanic island.

As the volcanic island moves away from the region of hot-spot activity it sinks, and the fringing reef gradually becomes a barrier reef farther from shore.

Eventually, the volcano is completely submerged and an atoll remains.

40 Distribution of the oceanic ridge system. The map shows ridge segments that exhibit slow, intermediate, and fast spreading rates

41Types of Seafloor Sediments

Seafloor sediments can be classified according to their origin into three broad categories:

(1) terrigenous (terra = land; generare = to produce)(2) biogenous (bio = life; generare = to produce)(3) hydrogenous (hydro = water; generare = to produce)

42Resources from the Seafloor

Oil and Natural Gas Gas Hydrates

Sand and gravel evaporative salts

Manganese nodules