Margins & Basins
• Ocean depths versus elevations
above sea-level.
• Difference between a continental
margin and ocean basin.
• Passive versus active continental
margins.
• Margin Features
• Basin Features
Compare Land versus
Ocean Topography:
The ocean cover over 75% of
Earth’s surface area.
Mean depth of ocean (3,790
m) is about 4-times that for
mean land elevation (840 m).
Furthermore, the total relief
(difference between high and
low points) is slightly more
for the ocean (11 km deep
trenches) than on land (8.8
km).
Defining Continental Margins and Ocean Basins:
Margins are the submerged outer edge of the continental crust.
Consisting of the “shelf “and “slope”. The continental margin extends
to where the granitic continental crustal rock stops and the
ocean crust basalt begins. This point is associated with the “rise”.
Oceans cover
Continental Crust at
the margins.
This area is shelf
and slope and
represents 11.4% of
the Earth’s area.
Ocean basins are
about evenly divided
in surface area
between ridges and
plains.
Ocean plains
29.8%
?????
T/F Continental margins are where sea meets the
land, i.e. the shoreline.
Continents are made mostly of the rock _____.
T/F On average the oceans are deeper than land
is high.
About how much of the ocean basins are plains?
Continental shelf:
Extends from coast to ~200km on average. It’s narrow for active
margins associate with subduction. Shelf width also varies
with sea level (+5 to -120 m over last 2.8 My) and marine
processes (like scouring and sediment movement, especially
for passive margins). Average slope is 0.2º, pretty flat! Shelf
area represents 7.4% of oceans, but has some of the highest
biological productivity. Max depth is 150m at shelf break.
Continental slope lies just beyond the
shelf and has a large slope average of 4º,
but steeper slopes of active margins have
been measured as steep as 25º. Ends at the
rise.
Continental rise begins at bottom of the
continent. Basalt underlies sediments of
the rise. Slope is 0.5º.
Continental rise grades into the abyssal
plains (flat) which extend to the base of
the mid-ocean ridge system.
Continental Margins:
Active: These are margins with intense earthquake and/or volcanic activity
due to their intimate association with convergent and transform plate
boundaries (e.g. west coast of South America). Short shelf; steep slopes.
Passive: These are often distant from a divergent plate boundary (e.g. east
coast of North America). Long shelf; gradual slope.
Recall that convection currents of mantle are the primary
force that drives plate movement.
Active margin of Oregon. Note crinkled
feature created by sediments scraped off
during subduction of the Jaun de Fuca
Plate (view from north to south)
???????
Choose: slope, shelf, rise, abyssal plain.
• Which is steepest?
• Which are over basalt?
• Which stops at a ridge?
• Which typically has high biological
productivity in the surface seawater above?
Which margin type typically has a divergent plate boundary
far away and nothing convergence of faulting in between?
Submarine Canyons are important features of the margin
During last glacial period sea level was ~120m lower so rivers cut
through the upper parts of the continental shelf to deposit their
sediment load at shelf break. Canyons are maintained by intense
and frequent turbidity currents.
Evidence for the formation
of submarine canyons:
The 1929 event on the Grand Banks.
Severed telecommunication cables
one after another toward deeper
depths further offshore.
A large are of abyssal plain was
covered with the sediments from off
the Grand Banks.
Ocean Basins:
Dominated by abyssal plains and mid-ocean ridges (spreading centers).
Mid-Ocean (Oceanic) Ridges
Spreading centers are associated with intense volcanism,
creating new basaltic crust. They are of immense distance (65,000
km), elevation (2km from abyssal plain), and global area
(22%). Running perpendicular to breaks in the ridge axis are
fracture zones, the active area between ridge segments is a
transform fault.
Adjacent to the ridge axis
seawater gets sucked into
fissures in the basalt. This
seawater becomes superheated by the magma, and
now contains extremely high
amounts of dissolved
minerals. When the mineral
laden super-heated water
“vents” to the surface it can
create dramatic chimney
structures that bellow “black
smoke” (actually mineral
precipitates). These
chimneys and other hot
spring features of the ridge
axis are collectively referred
to as hydrothermal vents.
There are only a few manned
submersibles in the world that
can explore hydrothermal vent
communities (e.g. Alvin)
Mid-Atlantic Ridge
seafloor with little
sediment among rocky
outcroppings
Abysal Plains and Hills
Moving away from the ridge system the basalt cools
and thins, becoming denser. Sediments accumulate and the
topography flattens as sediments cover the rough features
observed at the ridge (see echogram of younger seafloor).
By depths of 3,500 to 5,500 m the plains are mostly flat. Abyssal
hills and valleys at the edge of the ridge and beyond are due to more
dramatic faults and volcanoes (seamounts and guyots).
Sediments kilometers deep, particularly those at the
continental rise, will form into sedimentary rock due to
intense pressure from above, a process called lithification.
Sediments at 4,010 m under Drake Passage. Rippled due to
influence of the strong Antarctic Circumpolar Current.
Active margins are associated with subduction of seafloor
under continental crust, forming a deep-sea trench.
Trenches also form from subduction of old ocean crust under
younger ocean crust, on whose margin arc islands form.
Moving away from the mid-ridge axis:
T/F ocean crust gets older.
T/F ocean crust gets cooler.
T/F ocean crust gets less dense.
T/F ocean crust gets covered with thicker
sediments that can bury seamounts.
Abyssal plains have deeper depths than ridges for two
reasons. What are they?
Thick (> km) old sediments form rock by _____________.
Deepest ocean depths are due to what tectonic process,
to form what kind of basin feature?