EVOLUTION OF OCEAN FLOOR: BIRTH AND DESTRUCTION

Ocean Floor from 2 MYBP to present - current scenario….

OCEAN FLOOR

¤Ocean floor is the bottom of the ocean.

¤Ocean floors have a common structure created from tectonic movement and sediment from various sources.

¤Relatively short lived features on the planet.

¤No oceanic crust older than 180 Ma old is known from the present oceans

ContinentalShelf

Continental shelf

Continental slope

Continental slope

Seamount

Volcanic island

Continental rise

Abyssal plain

mid-ocean ridge

Rift valley

trench

EVOLUTION OF OCEAN FLOOR

• An individual ocean basin grows from an initial rift, reaches a maximum size, then shrinks and ultimately closes.

• Wilson Cycle refers to the sequence of events leading to the formation, expansion, contracting and eventual elimination of ocean basins.

The Wilson CycleRift valley forms as continent begins to split. Example: African Rift Lakes.

Sea-floor basalts begin forming and continents diverge.

Example: Red Sea.

Broad ocean basins widen, trenches develop, subduction begins. Example: Atlantic Ocean.

Subduction eliminates much of sea-floor and oceanic ridge.

Example: Pacific Ocean.

Last of sea-floor is eliminated, continents collide forming mountain chain. Example: Mediterranean Sea.

Convergence of continental plates and uplifting to form mountain range. Example: Himalayas.

Stage Examples Dominant motions Characteristic features

1.Embryonic East African rift valleys

Crustal expansion and uplift

Rift valleys

2.Young Red sea, Gulf of California

Subsidence and spreading

Narrow seas with parallel coasts and a central depression

3.Mature Atlantic ocean

Spreading Ocean basin with active mid-ocean ridge

4.Declining Pacific ocean

Spreading and shrinking

Ocean basis with active spreading axes; also numerous island arcs and adjacent trenches around margins

5.Terminal Mediterranean sea

Shrinking and uplift Young mountains

6.Relict scar/suture

Indus suture in the Himalayas

Shrinking and uplift Young mountains

Formation of incipient ocean basin

formn of ocean crust.swf

rift.swf

3-4

Juvenile Ocean Basin

DRIVING FORCES AND PROCESSES OF

EVOLUTION

• Sea floor spreading

• subduction

DRIVING FORCES AND PROCESSES OF

EVOLUTION

•Sea floor spreading– New oceanic lithosphere is created by the

upwelling and partial melting of material from the asthenosphere at the ocean ridges

•Subduction– The process of consumption of a lithospheric

plate at convergent plate margins. Here one part of the sea floor plunges below another and down into the asthenosphere.

Sea-Floor Spreading

Sea-Floor Spreading

heat

Sea-Floor Spreading

Deep Sedimentlayer

shallow Sedimentlayer

• Rift valleys along oceanic ridge crests indicate tension, are bounded by normal faults and are floored by recently-erupted basaltic lava flows.

• Axis of the oceanic ridge is offset by transform (strike-slip) faults which produce lateral displacement.

• Rates of sea-floor spreading vary from 1 to 10 cm per year for each side of the ridge and can be determined by dating the sea floor and measuring its distance from the ridge crest.

• Continents are moved by the expanding sea floor.

sea floor.swf

Magnetic anomalies form parallel bands arrange symmetrically about the axis of the oceanic ridge.

• As basaltic rocks crystallize, some minerals align themselves with Earth’s magnetic field, as it exists at that time, imparting a permanent magnetic field, called paleomagnetism, to the rock.

• Periodically Earth’s magnetic field polarity (direction) reverses poles.

Subduction is the process at a deep-sea trench whereby one part of the sea floor plunges below another and down into the asthenosphere. Destruction of sea floor occurs in subduction zones.

subduction.swf

proccess of subduction.swf

• Benioff Zone is an area of increasingly deeper seismic activity, inclined from the trench downward in the direction of the island arc.

Movement of plates is caused by thermal convection of the “plastic” rocks of the asthenosphere which drag along the overlying lithospheric plates, and gravity which pulls submerged plate downward.

TWO MODELS OF PLATE MOTION — MANTLE DRAG AND EDGE FORCE

MANTLE DRAG

•Plates are driven by the viscous drag of the asthenosphere on their bases.•Astenosphere is the top of the convection system.•Cannot account the movements of small plates.

EDGE FORCE

•Plates are driven by the forces applied to their margins/edges.•Oceanic lithosphere represents the top of the convection system.•explains the motion of small plates•Acceptable thermodynamically and much more effective to transfer heat from mantle.

CONCLUSION

¤Ocean floor is the bottom of the ocean.

¤Evolution of ocean floor is explained by wilson cycle

¤Ocean forms at the embryonic stage.¤Developes and widens in young and mature

stages.¤ spreading continues and shrinkage starts at

declining stages.¤Shrinkage continues and uplifting begins at

terminal stage and finally become a relict.

¤Sea floor spreading and subduction are the two major forces respnsible for the evolution of plates.

REFERENCES• Philip kearey, keith s klepeis and Frederick j vine ,chapters 6,7 and 9, Global Tectonics,3rd ed,wiley-blackwell-2009

• Butterworth-Heinemann, Ocean Basins : Their Structure and Evolution (2nd Edition),p 60

• Jones and Bartlett; Invitation to oceanography; origin of ocean basins

• Wikipedia and Google for images;en.wikipedia.org:sea floor.

THANKS