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.
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
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. Simple
dredge and/or scoop samples.
Advances in Bathymetry
●
Echo sounding
●
Multibeam Systems
●Satellite Altimetry
Echo Sounders Bounce Sound off
the Seabed
Multibeam Systems Combine Many
Echo Sounders
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
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
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
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
Continental margins have several distinct components.
Ocean-Floor Topography Varies
with Location
(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.
Ocean-Floor Topography Varies
with Location
Features of Earth’s
solid surface
shown as
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
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
Continental Shelves Are Seaward
Extensions of the Continents
The features of a
passive
continental
margin:
(a) Vertical
exaggeration
50:1
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
Submarine Canyons Form at the
Junction between Continental Shelf and
Continental Slope
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
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
Oceanic Ridges Circle the World
An
oceanic ridge
is a mountainous chain of young,
Oceanic Ridges Circle the World
Transform faults and fracture zones along an oceanic ridge
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
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
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.
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.
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
