Sea Floor Spreading Theory | Geography for UPSC 2024 (Pre & Mains) PDF Download

 Page 1


 
SEA FLOOR SPREADINGH THEORY 
Particularly, the mapping of the ocean floor and palaeomagnetic studies of 
rocks from oceanic regions revealed the following facts : 
(i) It was realised that all along the midoceanic ridges, volcanic eruptions 
are common and they bring huge amounts of lava to the surface in this 
area. 
(ii)   The rocks equidistant on either sides of the crest of mid-oceanic ridges 
show remarkable similarities in terms of period of formation, chemical 
compositions and magnetic properties. Rocks closer to the mid-oceanic 
ridges are normal polarity and are the youngest. The age of the rocks 
increases as one moves away from the crest. 
(iii)   The ocean crust rocks are much younger than the continental rocks. The 
age of rocks in the oceanic crust is nowhere more than 200 million years 
old. Some of the continental rock formations are as old as 3,200 million 
years. 
(iv) The sediments on the ocean floor are unexpectedly very thin. Scientists 
were expecting, if the ocean floors were as old as the continent, to have a 
complete sequence of sediments for a period of much longer duration. 
However, nowhere was the sediment column found to be older than 200 
million years.  
 
(v) The deep trenches have deep-seated earthquake occurrences while in the 
midoceanic ridge areas, the quake foci have shallow depths. 
THEORY: 
These facts and a detailed analysis of magnetic properties of the rocks on 
either sides of the mid-oceanic ridge led Hess (1961) to propose his 
hypothesis, known as the “sea floor spreading”. Hess argued that constant 
eruptions at the crest of oceanic ridges cause the rupture of the oceanic crust 
and the new lava wedges into it, pushing the oceanic crust on either side. The 
ocean floor, thus spreads. The younger age of the oceanic crust as well as the 
fact that the spreading of one ocean does not cause the shrinking of the other, 
made Hess think about the consumption of the oceanic crust. He further 
Page 2


 
SEA FLOOR SPREADINGH THEORY 
Particularly, the mapping of the ocean floor and palaeomagnetic studies of 
rocks from oceanic regions revealed the following facts : 
(i) It was realised that all along the midoceanic ridges, volcanic eruptions 
are common and they bring huge amounts of lava to the surface in this 
area. 
(ii)   The rocks equidistant on either sides of the crest of mid-oceanic ridges 
show remarkable similarities in terms of period of formation, chemical 
compositions and magnetic properties. Rocks closer to the mid-oceanic 
ridges are normal polarity and are the youngest. The age of the rocks 
increases as one moves away from the crest. 
(iii)   The ocean crust rocks are much younger than the continental rocks. The 
age of rocks in the oceanic crust is nowhere more than 200 million years 
old. Some of the continental rock formations are as old as 3,200 million 
years. 
(iv) The sediments on the ocean floor are unexpectedly very thin. Scientists 
were expecting, if the ocean floors were as old as the continent, to have a 
complete sequence of sediments for a period of much longer duration. 
However, nowhere was the sediment column found to be older than 200 
million years.  
 
(v) The deep trenches have deep-seated earthquake occurrences while in the 
midoceanic ridge areas, the quake foci have shallow depths. 
THEORY: 
These facts and a detailed analysis of magnetic properties of the rocks on 
either sides of the mid-oceanic ridge led Hess (1961) to propose his 
hypothesis, known as the “sea floor spreading”. Hess argued that constant 
eruptions at the crest of oceanic ridges cause the rupture of the oceanic crust 
and the new lava wedges into it, pushing the oceanic crust on either side. The 
ocean floor, thus spreads. The younger age of the oceanic crust as well as the 
fact that the spreading of one ocean does not cause the shrinking of the other, 
made Hess think about the consumption of the oceanic crust. He further 
 
maintained that the ocean floor that gets pushed due to volcanic eruptions at 
the crest, sinks down at the oceanic trenches and gets consumed. 
NOTE: 
PALEOMAGNETISM: 
Paleomagnetism is the study of magnetic rocks and sediments to record the 
history of the magnetic field. Some rocks and materials contain minerals that 
respond to the magnetic field. So, when rocks form, the minerals align with the 
magnetic field preserving its position. It’s called rock magnetism when rocks 
record the position of the magnetic field. The magnetic signature of the rocks 
allows paleomagnetists to date the rocks and map the position of the field at 
the time of their formation. 
 
PLATE TECTONICS 
 
THEORY: 
 Since the advent of the concept of sea floor spreading, the interest in the 
problem of distribution of oceans and continents was revived. It was in 1967, 
McKenzie and Parker and also Morgan, independently collected the available 
ideas and came out with another concept termed Plate Tectonics. A tectonic 
plate (also called lithospheric plate) is a massive, irregularly-shaped slab of 
solid rock, generally composed of both continental and oceanic lithosphere. 
Plates move horizontally over the asthenosphere as rigid units.  
 
The lithosphere includes the crust and top mantle with its thickness range 
varying between 5-100 km in oceanic parts and about 200 km in the 
continental areas.  
 
Page 3


 
SEA FLOOR SPREADINGH THEORY 
Particularly, the mapping of the ocean floor and palaeomagnetic studies of 
rocks from oceanic regions revealed the following facts : 
(i) It was realised that all along the midoceanic ridges, volcanic eruptions 
are common and they bring huge amounts of lava to the surface in this 
area. 
(ii)   The rocks equidistant on either sides of the crest of mid-oceanic ridges 
show remarkable similarities in terms of period of formation, chemical 
compositions and magnetic properties. Rocks closer to the mid-oceanic 
ridges are normal polarity and are the youngest. The age of the rocks 
increases as one moves away from the crest. 
(iii)   The ocean crust rocks are much younger than the continental rocks. The 
age of rocks in the oceanic crust is nowhere more than 200 million years 
old. Some of the continental rock formations are as old as 3,200 million 
years. 
(iv) The sediments on the ocean floor are unexpectedly very thin. Scientists 
were expecting, if the ocean floors were as old as the continent, to have a 
complete sequence of sediments for a period of much longer duration. 
However, nowhere was the sediment column found to be older than 200 
million years.  
 
(v) The deep trenches have deep-seated earthquake occurrences while in the 
midoceanic ridge areas, the quake foci have shallow depths. 
THEORY: 
These facts and a detailed analysis of magnetic properties of the rocks on 
either sides of the mid-oceanic ridge led Hess (1961) to propose his 
hypothesis, known as the “sea floor spreading”. Hess argued that constant 
eruptions at the crest of oceanic ridges cause the rupture of the oceanic crust 
and the new lava wedges into it, pushing the oceanic crust on either side. The 
ocean floor, thus spreads. The younger age of the oceanic crust as well as the 
fact that the spreading of one ocean does not cause the shrinking of the other, 
made Hess think about the consumption of the oceanic crust. He further 
 
maintained that the ocean floor that gets pushed due to volcanic eruptions at 
the crest, sinks down at the oceanic trenches and gets consumed. 
NOTE: 
PALEOMAGNETISM: 
Paleomagnetism is the study of magnetic rocks and sediments to record the 
history of the magnetic field. Some rocks and materials contain minerals that 
respond to the magnetic field. So, when rocks form, the minerals align with the 
magnetic field preserving its position. It’s called rock magnetism when rocks 
record the position of the magnetic field. The magnetic signature of the rocks 
allows paleomagnetists to date the rocks and map the position of the field at 
the time of their formation. 
 
PLATE TECTONICS 
 
THEORY: 
 Since the advent of the concept of sea floor spreading, the interest in the 
problem of distribution of oceans and continents was revived. It was in 1967, 
McKenzie and Parker and also Morgan, independently collected the available 
ideas and came out with another concept termed Plate Tectonics. A tectonic 
plate (also called lithospheric plate) is a massive, irregularly-shaped slab of 
solid rock, generally composed of both continental and oceanic lithosphere. 
Plates move horizontally over the asthenosphere as rigid units.  
 
The lithosphere includes the crust and top mantle with its thickness range 
varying between 5-100 km in oceanic parts and about 200 km in the 
continental areas.  
 
 
A plate may be referred to as the continental plate or oceanic plate depending 
on which of the two occupy a larger portion of the plate. Pacific plate is largely 
an oceanic plate whereas the Eurasian plate may be called a continental plate.  
 
The theory of plate tectonics proposes that the earth’s lithosphere is divided 
into seven major and some minor plates. Young Fold Mountain ridges, 
trenches, and/or faults surround these major plates. 
The major plates are as follows :  
(i) Antarctica and the surrounding oceanic plate  
(ii)  North American (with western Atlantic floor separated from the 
South American plate along the Caribbean islands) plate  
(iii)  South American (with western Atlantic floor separated from the 
North American plate along the Caribbean islands) plate 
(iv)  Pacific plate  
(v) India-Australia-New Zealand plate  
(vi)  Africa with the eastern Atlantic floor plate  
(vii)  Eurasia and the adjacent oceanic plate.  
Some important minor plates are listed below:  
(i) Cocos plate : Between Central America and Pacific plate  
(ii)  Nazca plate : Between South America and Pacific plate 
(iii)  Arabian plate : Mostly the Saudi Arabian landmass  
(iv) Philippine plate : Between the Asiatic and Pacific plate 
(v)  Caroline plate : Between the Philippine and Indian plate 
(North of New Guinea)  
(vi)  Fuji plate : North-east of Australia. 
 
Page 4


 
SEA FLOOR SPREADINGH THEORY 
Particularly, the mapping of the ocean floor and palaeomagnetic studies of 
rocks from oceanic regions revealed the following facts : 
(i) It was realised that all along the midoceanic ridges, volcanic eruptions 
are common and they bring huge amounts of lava to the surface in this 
area. 
(ii)   The rocks equidistant on either sides of the crest of mid-oceanic ridges 
show remarkable similarities in terms of period of formation, chemical 
compositions and magnetic properties. Rocks closer to the mid-oceanic 
ridges are normal polarity and are the youngest. The age of the rocks 
increases as one moves away from the crest. 
(iii)   The ocean crust rocks are much younger than the continental rocks. The 
age of rocks in the oceanic crust is nowhere more than 200 million years 
old. Some of the continental rock formations are as old as 3,200 million 
years. 
(iv) The sediments on the ocean floor are unexpectedly very thin. Scientists 
were expecting, if the ocean floors were as old as the continent, to have a 
complete sequence of sediments for a period of much longer duration. 
However, nowhere was the sediment column found to be older than 200 
million years.  
 
(v) The deep trenches have deep-seated earthquake occurrences while in the 
midoceanic ridge areas, the quake foci have shallow depths. 
THEORY: 
These facts and a detailed analysis of magnetic properties of the rocks on 
either sides of the mid-oceanic ridge led Hess (1961) to propose his 
hypothesis, known as the “sea floor spreading”. Hess argued that constant 
eruptions at the crest of oceanic ridges cause the rupture of the oceanic crust 
and the new lava wedges into it, pushing the oceanic crust on either side. The 
ocean floor, thus spreads. The younger age of the oceanic crust as well as the 
fact that the spreading of one ocean does not cause the shrinking of the other, 
made Hess think about the consumption of the oceanic crust. He further 
 
maintained that the ocean floor that gets pushed due to volcanic eruptions at 
the crest, sinks down at the oceanic trenches and gets consumed. 
NOTE: 
PALEOMAGNETISM: 
Paleomagnetism is the study of magnetic rocks and sediments to record the 
history of the magnetic field. Some rocks and materials contain minerals that 
respond to the magnetic field. So, when rocks form, the minerals align with the 
magnetic field preserving its position. It’s called rock magnetism when rocks 
record the position of the magnetic field. The magnetic signature of the rocks 
allows paleomagnetists to date the rocks and map the position of the field at 
the time of their formation. 
 
PLATE TECTONICS 
 
THEORY: 
 Since the advent of the concept of sea floor spreading, the interest in the 
problem of distribution of oceans and continents was revived. It was in 1967, 
McKenzie and Parker and also Morgan, independently collected the available 
ideas and came out with another concept termed Plate Tectonics. A tectonic 
plate (also called lithospheric plate) is a massive, irregularly-shaped slab of 
solid rock, generally composed of both continental and oceanic lithosphere. 
Plates move horizontally over the asthenosphere as rigid units.  
 
The lithosphere includes the crust and top mantle with its thickness range 
varying between 5-100 km in oceanic parts and about 200 km in the 
continental areas.  
 
 
A plate may be referred to as the continental plate or oceanic plate depending 
on which of the two occupy a larger portion of the plate. Pacific plate is largely 
an oceanic plate whereas the Eurasian plate may be called a continental plate.  
 
The theory of plate tectonics proposes that the earth’s lithosphere is divided 
into seven major and some minor plates. Young Fold Mountain ridges, 
trenches, and/or faults surround these major plates. 
The major plates are as follows :  
(i) Antarctica and the surrounding oceanic plate  
(ii)  North American (with western Atlantic floor separated from the 
South American plate along the Caribbean islands) plate  
(iii)  South American (with western Atlantic floor separated from the 
North American plate along the Caribbean islands) plate 
(iv)  Pacific plate  
(v) India-Australia-New Zealand plate  
(vi)  Africa with the eastern Atlantic floor plate  
(vii)  Eurasia and the adjacent oceanic plate.  
Some important minor plates are listed below:  
(i) Cocos plate : Between Central America and Pacific plate  
(ii)  Nazca plate : Between South America and Pacific plate 
(iii)  Arabian plate : Mostly the Saudi Arabian landmass  
(iv) Philippine plate : Between the Asiatic and Pacific plate 
(v)  Caroline plate : Between the Philippine and Indian plate 
(North of New Guinea)  
(vi)  Fuji plate : North-east of Australia. 
 
 
 
 
  TYPES OF PLATE BOUNDARIES: 
 
DIVERGENT BOUNDARIES  
Where new crust is generated as the plates pull away from each other. The 
sites where the plates move away from each other are called spreading sites.  
The best-known example of divergent boundaries is the Mid-Atlantic Ridge. 
At this, the American Plate(s) is/are separated from the Eurasian and African 
Plates. 
On continents, East African Rift Valley is the most important 
geomorphological feature formed due to divergence of African and Somali 
plates. 
Earthquakes (shallow focus) are common along divergent edges. 
The sites where the plates move away from each other are called spreading 
sites. 
 
Page 5


 
SEA FLOOR SPREADINGH THEORY 
Particularly, the mapping of the ocean floor and palaeomagnetic studies of 
rocks from oceanic regions revealed the following facts : 
(i) It was realised that all along the midoceanic ridges, volcanic eruptions 
are common and they bring huge amounts of lava to the surface in this 
area. 
(ii)   The rocks equidistant on either sides of the crest of mid-oceanic ridges 
show remarkable similarities in terms of period of formation, chemical 
compositions and magnetic properties. Rocks closer to the mid-oceanic 
ridges are normal polarity and are the youngest. The age of the rocks 
increases as one moves away from the crest. 
(iii)   The ocean crust rocks are much younger than the continental rocks. The 
age of rocks in the oceanic crust is nowhere more than 200 million years 
old. Some of the continental rock formations are as old as 3,200 million 
years. 
(iv) The sediments on the ocean floor are unexpectedly very thin. Scientists 
were expecting, if the ocean floors were as old as the continent, to have a 
complete sequence of sediments for a period of much longer duration. 
However, nowhere was the sediment column found to be older than 200 
million years.  
 
(v) The deep trenches have deep-seated earthquake occurrences while in the 
midoceanic ridge areas, the quake foci have shallow depths. 
THEORY: 
These facts and a detailed analysis of magnetic properties of the rocks on 
either sides of the mid-oceanic ridge led Hess (1961) to propose his 
hypothesis, known as the “sea floor spreading”. Hess argued that constant 
eruptions at the crest of oceanic ridges cause the rupture of the oceanic crust 
and the new lava wedges into it, pushing the oceanic crust on either side. The 
ocean floor, thus spreads. The younger age of the oceanic crust as well as the 
fact that the spreading of one ocean does not cause the shrinking of the other, 
made Hess think about the consumption of the oceanic crust. He further 
 
maintained that the ocean floor that gets pushed due to volcanic eruptions at 
the crest, sinks down at the oceanic trenches and gets consumed. 
NOTE: 
PALEOMAGNETISM: 
Paleomagnetism is the study of magnetic rocks and sediments to record the 
history of the magnetic field. Some rocks and materials contain minerals that 
respond to the magnetic field. So, when rocks form, the minerals align with the 
magnetic field preserving its position. It’s called rock magnetism when rocks 
record the position of the magnetic field. The magnetic signature of the rocks 
allows paleomagnetists to date the rocks and map the position of the field at 
the time of their formation. 
 
PLATE TECTONICS 
 
THEORY: 
 Since the advent of the concept of sea floor spreading, the interest in the 
problem of distribution of oceans and continents was revived. It was in 1967, 
McKenzie and Parker and also Morgan, independently collected the available 
ideas and came out with another concept termed Plate Tectonics. A tectonic 
plate (also called lithospheric plate) is a massive, irregularly-shaped slab of 
solid rock, generally composed of both continental and oceanic lithosphere. 
Plates move horizontally over the asthenosphere as rigid units.  
 
The lithosphere includes the crust and top mantle with its thickness range 
varying between 5-100 km in oceanic parts and about 200 km in the 
continental areas.  
 
 
A plate may be referred to as the continental plate or oceanic plate depending 
on which of the two occupy a larger portion of the plate. Pacific plate is largely 
an oceanic plate whereas the Eurasian plate may be called a continental plate.  
 
The theory of plate tectonics proposes that the earth’s lithosphere is divided 
into seven major and some minor plates. Young Fold Mountain ridges, 
trenches, and/or faults surround these major plates. 
The major plates are as follows :  
(i) Antarctica and the surrounding oceanic plate  
(ii)  North American (with western Atlantic floor separated from the 
South American plate along the Caribbean islands) plate  
(iii)  South American (with western Atlantic floor separated from the 
North American plate along the Caribbean islands) plate 
(iv)  Pacific plate  
(v) India-Australia-New Zealand plate  
(vi)  Africa with the eastern Atlantic floor plate  
(vii)  Eurasia and the adjacent oceanic plate.  
Some important minor plates are listed below:  
(i) Cocos plate : Between Central America and Pacific plate  
(ii)  Nazca plate : Between South America and Pacific plate 
(iii)  Arabian plate : Mostly the Saudi Arabian landmass  
(iv) Philippine plate : Between the Asiatic and Pacific plate 
(v)  Caroline plate : Between the Philippine and Indian plate 
(North of New Guinea)  
(vi)  Fuji plate : North-east of Australia. 
 
 
 
 
  TYPES OF PLATE BOUNDARIES: 
 
DIVERGENT BOUNDARIES  
Where new crust is generated as the plates pull away from each other. The 
sites where the plates move away from each other are called spreading sites.  
The best-known example of divergent boundaries is the Mid-Atlantic Ridge. 
At this, the American Plate(s) is/are separated from the Eurasian and African 
Plates. 
On continents, East African Rift Valley is the most important 
geomorphological feature formed due to divergence of African and Somali 
plates. 
Earthquakes (shallow focus) are common along divergent edges. 
The sites where the plates move away from each other are called spreading 
sites. 
 
 
CONVERGENT BOUNDARIES  
Where the crust is destroyed as one plate dived under another.  
The zone of collision may undergo crumpling and folding and folded 
mountains may emerge. 
This is an orogenic collision. Himalayan Boundary Fault is one such example. 
The location where sinking of a plate occurs is called a subduction zone.  
There are three ways in which convergence can occur.  
These are:  
(i) between an oceanic and continental plate;  
(ii)  between two oceanic plates; and  
(iii)  between two  continental plates. 
TRANSFORM BOUNDARIES  
Where the crust is neither produced nor destroyed as the plates slide 
horizontally past each other.  
Transform faults are the planes of separation generally perpendicular to the 
midoceanic ridges. 
 As the eruptions do not take all along the entire crest at the same time, there is 
a differential movement of a portion of the plate away from the axis of the 
earth.  
Also, the rotation of the earth has its effect on the separated blocks of the plate 
portions. 
San Andreas Fault along the western coast of USA is the best example for a 
transcurrent edge on continents. 
RATES OF PLATE MOVEMENT  
The strips of normal and reverse magnetic field that parallel the mid-oceanic 
ridges help scientists determine the rates of plate movement.