Transcript Oceanic ridges

Oceanography
An Invitation to Marine Science, 7th
Tom Garrison
Chapter 4
Continental Margins and Ocean
Basins
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.
The Ocean Floor Is Mapped by Bathymetry
• 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
– Multi-beam Systems
– Satellite Altimetry
Echo Sounders Bounce Sound
off the bottom
Echo sounding is a method of measuring seafloor
depth using powerful sound pulses.
The Topography of Ocean Floors
Cross section of the
Atlantic ocean basin
and the continental
United States,
showing the range of
elevations.
Satellites have
enabled us to rapidly
map the ocean floor
from space.
What are the two classifications of the ocean floor?
Continental Margins = the submerged outer edge
of a continent
Ocean Basin = the deep seafloor beyond the
continental margin
Continental margin types
• Passive margins, also called Atlantic-type
margins, face the edges of diverging tectonic
plates. Very little volcanic or earthquake activity,
broad have gentile inclines, and influenced more
by sea level changes.
• Active margins, known as Pacific-type margins,
are located near the edges of converging plates.
Sites of volcanic and earthquake activity, not as
broad, steeper inclines, not effected as much by
sea level changes.
Ocean-Floor Topography Varies
with Location
Features of
Earth’s solid
surface shown as
percentages of
the Planet’s total
surface.
Anatomy of a continental margin
• Continental shelf – the shallow, submerged
edge of the continent.
• Shelf break- marks the transition from the
continental shelf to the continental slope.
• Continental slope – the transition between the
continental shelf and the deep-ocean floor.
• Continental rise – accumulated sediment found
at the base of the continental slope.
Anatomy of the Continental margin
Other ocean basin features
• Submarine canyons- deep cuts into the continental
shelf, and slope.
• Abyssal plains- flat expanses of ocean basin
• Oceanic ridges- underwater mountain chains formed at
spreading zones.
• Seamounts- submerged volcanic projections
• Guyots- submerged flat top projections that formed from
eroded seamounts.
• Trenches- deep depressions formed by subducted
plates
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 deepsea floor in a fan-shaped wedge of sediment.
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.
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.
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 11km. (~ 7miles) below sea
level.
Trenches and Island Arcs continued…
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.