Ocean Basins

1
Ocean Basins
OEAS-306
January 20, 2009

• Outline
• Review Homework Problem.
• Quick review of last lecture.
• Remote sensing of seafloor bathymetry
• Major Features of the Seafloor
• Continental Shelf
• Continental Slope
• Abyssal Plain and Deep-Sea Fans
• Volcanic Islands, Seamounts, Atolls and Guyots
• Hydrothermal Vents

5) Summary
2
In Lecture 1 we discussed the techniques
scientists used to reveal the structure of the
Earths interior. This understanding was vital
to the theory of plate tectonics

• Key Points
• The continental plates are floating over the
  denser, plastic aestheosphere.
• Motion of the plates is driven by sea-floor
  spreading at the mid-ocean ridge.
• Oceanic crust is created at mid-ocean ridges
  (divergent boundaries) and destroyed at
  convergent boundaries.

Spreading rates in cm/year
3
Modern Methods for Measuring the Bathymetry of
the Ocean Floor
Satellite Altimetry
Echo Sounding and Swath Mapping
Satellite Altimetry Satellites can measure the
height of the ocean very accurately (cms).
Distortion of the sea surface above a seabed
feature occurs when the extra gravitational
attraction of the feature pulls water toward it
from the sides, forming a mound of water over
itself. This allows the oceans bathymetry to be
mapped from space.
Echo sounding is a method of measuring seafloor
depth using powerful sound pulses. The accuracy
of an echo sounder can be affected by water
conditions and bottom contours. The pulses of
sound energy, or pings, from the sounder spread
out in a narrow cone as they travel from the
ship. When depth is great, the sounds reflect
from a large area of seabed. Because the first
sound of the returning echo is used to sense
depth, measurements over deep depressions are
often inaccurate.
4
Example of Ocean Bathymetry derived from GEOSAT
Satellite
5
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
1001. Although ocean depth is clearly greater
than the average height of the continent, the
general range of contours is similar.
6
The mid-ocean ridges are essentially a long
continuous under water volcano. Fractures and
faults along the mid-ocean ridge lead to the
rugged bathymetry.
7

• Major features of the ocean floor
• Continental Shelf Shallow (less than 100 m)
  and relatively flat region overlying continental
  crust.
• Continental SlopeTransition region between
  continental and oceanic crust.
• Continental RiseThick prism of sediment
  deposited at base of Continental Slope.
• Abyssal PlainBroad flat plain covered with
  sediment, overlying rugged ocean floor.
• Mid-ocean RidgeLong continuous ridges rising 2-3
  km above the surrounding sea floor extending
  throughout all major ocean basins.
• Deep-sea TrenchesDeep (as much as 11km) trenches
  formed by subduction of oceanic crust.

8
Continental Margins can be Active or Passive

• 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 margins are the site of volcanic and
  earthquake activity.

9
Active vs. Passive Margins

• Long, straight, sandy beaches
• Many barrier islands
• Gentling sloping continental shelf
• Backed by low lying coastal regions
• Many large Estuaries, Sounds, Bays etc...
• Sediment trapped in estuaries and bays
• Generally lower wave energy

• Rocky coast with pocket beaches
• No barrier islands
• Steeply sloping continental shelf
• Backed by coastal mountain range
• Limited, small estuaries, sounds, and bays (with
  some exceptions)
• Sediment discharged directly into ocean
• Generally higher wave energy

US East Coast
US West Coast
10
Much of the sediment on the continental shelf of
passive margins, was deposited when sea-level was
much lower.
11
Hudson Canyon

• Canyons were formed during lower sea-level by
  rivers.
• Cut across the present day continental shelf
  emptying straight on continental slope.
• Are important pathways for delivering continental
  sediments to the deep sea.

12
1) When Sea-level was lower (18,000 years ago),
rivers enter the ocean at the current day
location of the shelf break. 2) This sediment
could then move down-slope under the influence of
gravity, to reach the Abyssal Plain. 3) Sediment
moving down-slope by gravity are known as
turbidity currents. 4) Turbidity currents can
only occur where slopes are relatively steep.
Upon reaching the flat Abyssal floor, turbidity
currents weaken and deposit their sediment.
tank experiment
13
Today, most of the sediment on passive margins
does not reach the ocean. Sediment is only
transported down-slope infrequently when there
are undersea earthquakes or large-scale sediment
failures.
14
Evidence for turbidity currents from broken
telegraph wires
15
Volcanic Islands, Atolls, Seamounts and Guyots
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
16
Creation of Volcanic Islands by Hot Spots
Hot spots are poorly understood plumes in the
mantle thought to originate deep within the
earth, usually found away from plate boundaries.
17
70 Ma
PACIFIC OCEAN
Emperor Seamounts
50 Ma
HawaiianEmperor Bend
40 Ma
30 Ma
20 Ma
10 Ma
Kauai 5 Ma
Hawaii lt1 Ma
Hawaiian Ridge
Position of Loihi
Ma million years ago
18
Evolution from Island to Atoll to Guyot

• Atolls form when fringing coral reefs develop
  around an island (only in warm water).
• Sometimes island subside, leaving behind only the
  fringing coral reef (atoll).
• If coral growth cant keep up with subsidence,
  the atoll sinks becoming a guyot.
• The Darwin Point occurs when upward reef growth
  equals downward subsidence.

19
Hydrothermal Vents were discovered in the late
1970s near mid-ocean ridge systems.
20
Hydrothermal Vent Circulation

• Minerals from the hot, acidic, metal rich water
  coming out of the vents rapidly precipitates,
  leading to metal rich sediments on the seafloor
  near hydrothermal vent regions.
• Researchers have revealed thriving biological
  communities living on and around hydrothermal
  vents. Such organisms derive their energy from
  chemicals in the hydrothermal fluid NOT from the
  sun.
• Such organisms may provide clues to how life
  began on Earth.

21
vent video
22

• Summary
• Continental margins are either active or passive
  depending on whether they are on the leading or
  trailing edge of the the continental plate.
• Active and passive margins have very different
  characteristics.
• When sea-level was lower (18,000 years ago), many
  rivers empty straight onto the continental slope.
• Sediment can move down the continental slope
  under the influence of gravity as turbidity
  currents or debris flows.
• Sediments accumulate at the base of the
  continental slope forming the continental rise.
• Hot spots caused by mantle plumes can lead to
  volcanic islands or seamounts which form along
  the direction of plate motion.
• As islands subside, they can form atolls and
  guyots.
• Hydrothermal vents occur along mid-ocean ridges
  where seawater circulates down into the crust.
• These vents support diverse ecosystems that
  derive their energy from the chemicals coming
  from the vents.