Sea Floor Spreading

and

Plate Tectonic theory

SEA-FLOOR SPREADING

PLATETECTONICS

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Convectional Current Theory - Tectonics

Arthur Holmes in 1930s discussed the possibility of convectioncurrents in the mantle.

These currents are generated due to radioactive elements causing thermal differences in mantle.

Mapping of the Ocean Floor Detailed research during World Wars revealed that the ocean floor

is not just a vast plain but it is full of relief with mountain ranges,deep trenches etc..

The mid-oceanic ridges were found to be most active in terms ofvolcanic eruptions.

The dating of the rocks from the oceanic crust revealed the factthat the latter is much younger than the continental areas (Rocks onocean floor are much younger than those on the continents).

Rocks on either side of the crest of oceanic ridges and having equidistant locations from the crest were found to have

remarkable similarities both in terms of their constituents and theirage.

Convectional Current Theory According to this theory, the intense heat generated by radioactive

substances in the mantle (100-2900 km below the earth surface) seeks a pathto escape, and gives rise to the formation of convention currents in themantle.

Wherever rising limbs of these currents meet, oceanic ridges are formed onthe sea floor and wherever the failing limbs meet, trenches are formed.

The Process of Sea FloorSpreading The mid-ocean ridge is the region where new oceanic crust is created. The oceanic

crust is composed of rocks that move away from the ridge as new crust is beingformed.

The formation of the new crust is due to the rising of the molten material (magma)from the mantle by convection current. When the molten magma reaches the oceaniccrust, it cools and pushes away the existing rocks from the ridge equally in bothdirections.

A younger oceanic crust is then formed, causing the spread of the ocean floor.

The new rock is dense but not as dense as the old rock that moves away from the ridge. As the rock moves, further, it becomes colder and denser until it reachesanocean trench or continues spreading.

It is believed that the successive movement of the rocks from the ridge progressivelyincreases the ocean depth and have greaterdepths in the ocean trenches.

Evidence of Sea FloorSpreading

Harry Hess’s hypothesis about seafloor spreading had collected several pieces of evidence to support the theory.

This evidence was from the investigations of the molten material, seafloor drilling, and the magnetic stripes. This evidence however was also used to support the Theory of Continentaldrift.

Molten material He described that the molten magma from the mantle arose due to the convection

currents in the interior of theearth.

The convection current was due to the radioactive energy from the earth’s core thatmakes the materials in the lower mantle to become warm, less dense and rise.

The flow of the materials goes through the upper mantle and leaks through the platesof the crust.

This makes the temperature near the mid- oceanic ridge becomes warm and the othersurface to become cold because as the molten magma continues to push upward, itmoves the rocks away from the ridge.

Seafloor drill The seafloor drilling system led to the evidence that supports the seafloor-spreading

hypothesis.

The samples obtained from the seafloor drill reveals that the rocks away from the mid-oceanic ridge were relatively older than the rocks near to it.

The old rocks were also denser and thicker compared to the thinner and less denserocks in the mid-oceanic ridge.

This means that the magma that leaks from the ridge pushes the old rocks away and asthey increasingly become distant, they more likely become older, denser, and thicker. Onthe other hand, the newest, thinnest crust is located near the center of the mid-oceanridge, the actual site of seafloorspreading.

Radiometric age dating and fossilages By the use of radiometric age dating and studying fossil ages, it was also found out the

rocks of the sea floor age is younger than the continental rocks.

It is believed that continental rocks formed 3 billion years ago, however the sediments samples from the ocean floor are found to be not exceeding 200 millionyears old. It is a clearevidence that the formation of rocks in the sea floor is due toreabsorption of materials.

Magnetic stripes In the 20th century, the magnetic survey was conducted in the Mid-ocean

ridge in order to investigate evidence of sea-floor spreading.

By using the magnetometer, the magnetic polarity will be shown through atimescale that contains the normal and a reverse polarity.

It was concluded that the sea floor is composed of different rocks accordingto ages and that they are positioned equally in opposite directions.

This records that there is a constant movement and spreading of rocks on theocean floor.

Thevolcanic rockswhich make up the sea floor have magnetism because, as theycool, magnetic minerals within the rock align to the Earth's magnetic field.

Scientists noticed that some rocks werealigned to the “positive” or north end of the Earth and some were aligned to the “negative” or south end. Also, the ages of the rocksat the ridge were much younger than those found at the subduction zones.

.

Mid-Ocean Ridge (Atlantic Ocean)There have been 170 reversal in the last 76 million years. The earth’s present orientation has existed for the past 60,000 years.

+- + - + - + - + - + -

If we collect rocks

from different latitudes,

their magnetic

direction and

inclination will reflect

the earth’s magnetic

field where they were

formed.

How could this be?Theseoceanic ridges were actually boundaries with tectonic plates pulling apart.•This movement of the plates allowed the magma to rise up and harden into new rock.•As the new rock was formed near the ridge, older rock, which formed millions of yearsago when the magnetic field was reversed, got pushed farther away, resulting in thismagneticstriping.•Rising magma assumes the polarity of Earth’s geomagnetic field before it solidifiesinto oceaniccrust.At spreading centres, this crust is separated into parallel bands of rock bysuccessive waves of emergent magma.When Earth’s geomagnetic field undergoes a reversal, the change in polarity is recordedin the magma, which contributes to the alternating pattern of magnetic striping ontheseafloor

Plate Tectonics

Objective :- Plate tectonic theory is a comprehensivetheory whichoffers explanations for various relief features and tectonic

events.

Example - Mountain building, folding and faulting, continental drift,

vulcanicity, seismic event (Earthquakes) etc.

Divergent

Convergent

Transform

Using hands to show relative motion

Divergent Using hands to show relative motion

Convergent

Transform

Using hands to show relative motion

Divergent

ConvergentTransform

Destructive plate boundaries also known as 'Consuming plate boundaries' or 'Convergent plate boundaries' are those where two plates collide against each other and the leading edge of one plate having relatively lighter material overrides the other plate and the overridden plate boundary of relatively denser material is sub ducted or thrust into the upper mantle and thus a part of the crust is lost in the mantle.

This mechanism results in constant loss of crustal materials.

When one continental and the other oceanic plate collide due to their convergence along subduction or Benioff zone the oceanic plate boundary being heavier due to comparatively denser materials is subducted below the continental plate boundary

The sediments deposited on the continental margins are squeezed and folded due tocompressive forces caused by the subduction of oceanic plate.

The Rockies and the Andes mountains were formed due to subduction of the Pacific ocean plate under the America continental plate

The collision of two oceanic plates and subduction of the boundary of therelatively denser materials results in the formation of island arcs andfestoons.

example - Island arcs and festoons formed by Japanese island.

When two convergent plates composed of continental crusts collide againsteach other the continental plates having relatively denser materials issubducted under the other continental plate having comparatively lightermaterials than the former.

The resultant lateral compression squeezes and folds the sediments deposited on either side of continental plate margins and the sediments of the geosynclines lying between two convergent continental plates and thus forms gigantic folded mountains.

Example - The Alps and the Himalayas.

The rigid lithospheric slab or rigid and solid land masses having a thickness of about 100km composed of earth's curst and some portion of upper mantle are technically called 'Plate'.

The term 'plate' was first used by Canadian geologist J. T. Wilson in 1965.

The whole mechanism of the evolution, nature and motion and resultant reactions of plates is called 'plate tectonic'

Constructive plate boundaries also called as 'divergent plate boundaries'represent zones of divergence along the mid-oceanic ridges and arecharacterized by continuous addition (accretion) of materials as there isconstant upwelling of molten materials (basaltic lavas) from below the mid -oceanic ridges.

These basaltic lavas are cooled and solidified and are added to the trailingmarginsof the divergent plates and thus new oceanic crust is continuously formed.

Perhaps the best known example of divergent boundary is the mid Atlanticridge.

Conservative plate boundaries also known as 'shear plate boundaries'are those where two plate slip past each other without any collisionalong the transform fault and thus crust is neither created nordestroyed.

According to plate tectonic theory mountains are formed due to collision of twoconvergent plate.

Mountains are always formed along the destructive plate boundaries.

It is obvious that the process of the mountain building is associated with destructive plate boundaries of two convergent plate

The plate tectonic theory envisages the formation of mountains due to compression ofsediments caused by the collision of two convergent plate boundaries.

Two plates moving together under the impact of thermal convective currents collide against each other and the plate boundary having relatively denser materials is sub ducted under the other plate boundary of relatively lighter materials.

This seduction zone is also called Benioff zone.

The subduction of plate boundary cause lateral compressive force which ultimately squeezes and folds the sediments and materials of the margins of the plates and thus mountains are formed

Explain the formation of thousands of islands in Indonesian and Philippinesarchipelagos [20 marks - Mains 2014]

Archipelago: an extensive group of islands. Island arc: narrow chain of islands which are volcanic in origin. Island arc is usually

curved. The convex side will have a trench if it’s an oceanic arc. Japan, Philippines,Hawaii etc. are oceanic arcs. Cascade range, Western Chile range etc. are examples ofcontinental arcs.

Indonesian archipelago and Philippine archipelago are located along the platemargins.

Both the archipelagoes were formed due to ocean – oceanconvergence.

Indonesian archipelago was formed due to convergence between Sunda oceanic plate(part of Eurasian plate) and Indo – Australian plate whereas Philippine archipelagowas formed due to convergence between Sunda oceanic plate and Philippine Seaplate. [if you can’t remember names, you should avoid these kind of points]

In ocean – ocean convergence, two oceanic plates converge or collide. The denser platesubducts into the asthenosphere below the convergence zone and forms a trench atthe surface. This region below the convergence zone is called the zone of subduction.

In the zone of subduction, due to high temperature and pressure, the rocks undergometamorphosis and the sedimentsin the oceanic plate melt to form magma.

The magma being lighter moves upwards. It is at high pressure due to the buoyantforce offered by the surrounding denser medium. At the surface magma escapes inthe form of volcanic eruptions. The magma solidifies creating a volcanic layer.Subsequent volcanism builds layer over layer and a volcanic mountain if formed.Such mountains are formed all along the converging edge above the less denser plate.

Over time the mountains merge and oceanic crust gets transformed into continentalcrust.

And this is how Indonesian archipelago and Philippine archipelago are formed.