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The Origin of Ocean Basins

Ocean basin is defined as that part of the sea floor deeper than 2000 m (6000 ft).

Plate Tectonic Theory:

states that the Earth's

outermost layer (i.e.,

the lithosphere) is

fragmented

comprised of a dozen or

more large and small

plates;

plates move relative to

one another and 'glide'

atop the hot, mobile

mantle;

relatively new scientific

concept, accepted only

about ~35 years ago

• Historical Recognition of Plate Dynamics:

• many believed continents were fragmented pieces of preexisting larger landmasses

• 1596 – Dutch map maker Abraham Ortelius

– suggested that the Americas were "torn away" from Europe and Africa

• 1858 – Italian geographer Antonio Pellegrini

– constructed plate reconstruction maps

– attempted to demonstrate the fit of the American and African continents

1912 – meteorologist Alfred

Wegener introduced theory of

continental drift

noted the remarkable fit of the

South American and African

continents

identified geologic 'occurrences'

on matching coastlines of South

America, Africa

continuance of geologic

structures (mountain belts,

mineral deposits)

common plant and animal fossils

Wegener contended that the

modern landmasses had formed a

'supercontinent'

• Continental Drift Evidence:

• Wegener noted tropical plant fossils (in the form of coal deposits) in Antarctica

• suggested that Antarctica had been situated closer to equator (temperate climate)

• Wegener identified ancient (fossil) coral reefs at high latitudes

• noted that modern coral reefs do not occur where temperatures fall below 18°C

• also noted the occurrence of glacial deposits, striations in present-day arid regions

•What was the fatal flaw in Wegener's theory?

•Wegener's theory failed to explain the nature of the forces propelling the plates!

•Wegener incorrectly suggested that continents simply plowed through the ocean floor!

•Development of the 'Modern' Theory of Plate Tectonics:

•early 1950s – new evidence emerged to revive Wegener's theory

– identification of the global mid-ocean ridge system

–demonstration of the youth of the ocean floor

–magnetic 'stripes' found on the seafloor

–pattern of earthquake hypocenters in well-defined belts

•emergence of the seafloor-spreading hypothesis by Dietz, Hess, and Vine

• suggested that mantle convection drives plate motion

•demonstrated that oceanic crust formed at mid-ocean ridges and spread laterally

Sea floor spreading demonstrates that the sea floor moves apart at the oceanic ridges and new oceanic crust is added to the edges.• 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.

• Whereas oceanic ridges indicate tension, continental mountains indicate compressional forces are squeezing the land together.

3-2 Sea-Floor Spreading

•Magnetometers

detect and

measure Earth’s

magnetic field.

Source of the Earth's

magnetic field is the

liquid core

magnetic field

structure similar to

that of a bar magnet

(i.e., two

distinct poles)

magnetic field

periodically reverses

• Moving across the ocean floor perpendicularly to the oceanic ridges, magneometers alternately record stronger (positive) and weaker (negative) magnetic fields (called magnetic anomalies) in response to the influence of the sea floor rocks.

• Magnetic anomalies and the rocks causing them form parallel bands arranged 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.


Because of their paleomagnetism, rocks of the sea floor influence the magnetic field recorded by magnetometers.

• Rocks on the sea floor with normal polarity paleomagnetism locally reinforce Earth’s magnetic field making it stronger and producing a positive anomaly.

• Rocks on the sea floor with reverse paleomagnetism locally weaken Earth’s magnetic field, producing a negative anomaly.

• Rocks forming at the ridge crest record the magnetism existing at the time they solidify.


• Sea floor increases in age away from the ridge and is more deeply buried by sediment because sediments have had a longer time to collect.

• 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.


Because Earth’s size is constant, expansion of the crust in one area requires destruction of the crust elsewhere.

• Currently, the Pacific Ocean basin is shrinking as other ocean basins expand.

• Destruction of sea floor occurs in subduction zones.

• Seismicity is the frequency, magnitude and distribution of earthquakes. Earthquakes are concentrated along oceanic ridges, transform faults, trenches and island arcs.

• Tectonism refers to the deformation of Earth’s crust.

3-3 Global Plate Tectonics

Seven Major

Plates

Seven Minor

Plates

African Plate

Antarctic Plate

Eurasian Plate

Indian-Australian Plate

North American Plate

Pacific Plate

South American Plate

Arabian Plate

Caribbean Plate

Cocos Plate

Juan de Fuca Plate

Nazca Plate

Philippines Plate

Scotia Plate

• Mantle plumes originate deep within the asthenosphere as molten rock which rises and melts through the lithospheric plate forming a large volcanic mass at a “hot spot”.


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

• Subduction is the process at a trench whereby one part of the sea floor plunges below another and down into the asthenosphere.


As the Pacific Plate slides under New Zealand's North Island, it melts. Lighter rocks float up as magma, to create the mighty volcanoes of the active volcanic zone. This another Benioff zone.

• Here the plate junction is very complex since:

1) along the alpine fault (South Island) the plates are sliding past each other.

2) North, the Australian plate rides over the Pacific plate. (Kermadec trench).

3) South, the Pacific plate rides over the Australian plate. (Macquarie ridge).

4) In the volcanic triangle of the North Island, the country is tearing apart, creating the jutting East Cape in the process.The deep split in the earth's crust, which is the plate boundary, twists over at New Zealand. It slopes down to the west, as the Pacific Plate. It then flips over to slope down to the east south of the country, where the Australian Plate does the sinking

Kilauea Eruptions

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


Stages in basin history

are:

Embryonic - rift valley

forms as continent

begins to split.

Juvenile - sea floor

basalts begin forming

as continental sections

diverge.

Mature - broad ocean

basin widens, trenches

develop and

subduction begins.

Declining - subduction

eliminates much of sea

floor and oceanic

ridge.

Terminal - last of the

sea floor is eliminated

and continents collide

forming a continental

mountain chain.

• Chronology of Modern Ocean Basin Development:

• ~200 million years ago

– formation of the supercontinent Pangaea – presence of the Panthalassan 'super ocean' – Atlantic and Indian Oceans not present


– initial break up of Pangaea and formation of Laurasia and Gondwanaland – presence of an east-west trending basin (the Tethys Sea)


– separation of Eurasian and North American plates – onset of mid-Atlantic ridge development – early formation of the North Atlantic – South Atlantic still closed


– separation of the African and South American plates – early formation of the South Atlantic


– Indian plate reaches equator after separation from Australia and Antarctica – North and South Atlantic continue to widen ~40 million years ago

– onset of Indian and Asian plate collision, initial formation of the Himalayas – North and South Atlantic achieve modern appearance


– major uplift of the Himalayas and formation of the Tibetan Plateau – expansion of the Southern Ocean


– closure of mid-American seaway as North and South American plates collide – isolation of the Pacific and Atlantic Oceans

•

Ocean Floor from 2 MYBP to present

•

•

Major Ocean Basins:

• Pacific Ocean

– largest (180,000,000 km2) and deepest (averaging 3,940 m) basin – extensive marginal seas and volcanic island systems and trenches– considerable mountain building and earthquake activity along boundaries – little freshwater input

• Atlantic Ocean

– second largest basin (107,000,000 km2) – average depth 3,310 m – large freshwater input (Amazon, Congo, Mississippi, Niger, Orinoco Rivers) – small number of marginal seas (Gulf of Mexico, Caribbean, Mediterranean)

• Indian Ocean

– smallest basin (74,000,000 km2) – average depth 3,840 m – large sediment input (Indus and Ganges River deltas) – small number of marginal seas (Arabian Sea, Persian Gulf, and Red Sea)

• Arctic Ocean

– centered on the north pole – shallow and land-locked – covered by sea ice – large sediment input from active glaciers

• Southern Ocean

– continuous ring of water around the Antarctic continent (south pole) – coldest of all oceans (near freezing) – most biologically productive ocean (high nutrient concentrations) – extensive winter sea ice coverage – small number of marginal seas (Weddell and Ross Seas)

•

• Passive (Atlantic-Type) Margins:

• found on continent-bearing plates

• continental margin moving away from the mid-ocean spreading center

• not characterized by mountain building

• zone of low seismicity and no volcanism – essentially stable

• characterized by thick sediment deposits and old oceanic crust

• comprised of shelf, slope, and rise

• examples include the eastern coasts of North and South America

• Active (Pacific-Type) Margins:

• considered 'destructive'

• continental margin moving toward a subduction zone

• characterized by volcanism, many earthquakes, and active mountain building

– friction of subducting plate causes earthquake activity and heat generation

– ocean crust is heated to melting point

– molten rock (magma) rises to the surface to create island arcs and volcanoes

• dense oceanic crust is subducted beneath thicker, less dense continental crust

• Chilean (e.g., Peru, Chile) -shallow trench, accretionary prism, volcanic mountains

• Marianas (e.g., Japan, Marianas) - deep trench, volcanic island arc, back-arc basin

San Andreas fault 100

miles north of L.A

Basalt injections

in narrow

trough resulting

is separation

from Africa

New crust

forming in axial

trough via sea-

floor spreading

Hot salty brine

high in metal

concentrations

from basalt

leaching. High

enough metal

concentration to

be exploited

commercially.

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