Question on Sea-floor spreading

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Question: Explain the concept of sea-floor spreading. What is the evidence supporting sea-floor spreading? 

UPSC Syllabus: 

GS 1

Geography 

Salient features of world’s physical geography

Relevance: UPSC Previous year Mains Exam. 

2018 Define mantle plume and explain its role in plate tectonics. 

2013 What do you understand by the theory of continental drift? Discuss the prominent evidences in its support. 

ANSWER

The theory of Sea-floor spreading was proposed by Harry Hess. It is the phenomenon of formation of the new oceanic crust through volcanic activity at the mid-oceanic ridges (large submarine mountain chains) leading to the movement of oceanic crust on either side of the ridge.

Process

Sea-floor Spreading

Constant eruptions at the crests of oceanic ridges cause the rupture of the oceanic crust and the new lava wedges into it. As the lava cools, it is pushed away from the flanks of the ridges. This spreading creates a successively younger ocean floor, and the flow of material is thought to bring about the migration, or drifting apart of the continents.

Prominent evidences in support of sea-floor spreading are as follow:

Age of rocks: 

• Remarkable similarities of the rocks equidistant on either side of the crest of mid-oceanic ridges show in terms of the period of formation, chemical compositions and magnetic properties. 
• The age of the rocks increases as it moves away from the crest.

Marine sediments: 

• The oldest sediments so far recovered by coring, dredging, and deep-sea drilling dates only to the Jurassic Period (about 200 million years in age). 
• Such findings are incompatible with the doctrine of the permanency of the ocean basins that had prevailed among earth scientists for so many years.

Magnetic anomalies: 

• Studies show alternately anomalously high and low magnetic fields with increasing distance away from the axis of the mid-ocean ridge system. The anomalous features are nearly symmetrically arranged on the both sides of the axis and parallel to the axis, creating bands of parallel anomalies.

Low seismic wave velocities: 

• As per the Studies, anomalously low seismic wave velocities has been reported at the ridge crests.
• It may be due to the thermal expansion and micro fracturing associated with the upwelling magma.

Heat flow: 

• Here, the heat flow measures three to four times the normal value. 
• The anomalously high values are considered to reflect the intrusion of molten material near the crests of the ridges.

However, spreading of one ocean plate does not cause the shrinking of the other because the ocean floor that gets pushed due to volcanic eruptions at the crest, sinks down at the oceanic trenches and gets consumed. It also plays an important role in explaining continental drift in the theory of plate tectonics