Introduction: Unraveling Earth’s Secrets with Sea Floor Spreading

Post-drift studies and sea floor spreading have significantly advanced our understanding of the Earth’s dynamic geology. Building upon Alfred Wegener’s continental drift theory, these studies have revealed the intricate processes that shape our planet. The concept of sea floor spreading, driven by the movement of tectonic plates, has become a cornerstone in modern geology, elucidating the mechanisms responsible for Earth’s evolving surface.

Evidence Collection: Expanding Evidence with Sea Floor Spreading 

• While earlier studies and evidence supporting continental drift predominantly focused on data from continents, like the distribution of flora, fauna, and specific deposits (like tillite), the post–World War II era expanded this understanding by introducing data from the ocean floors.

Convectional Current Theory: Holmes’s Currents and Sea Floor Spreading Insights

• Holmes’s Contribution (1930s): Arthur Holmes introduced a pivotal theory suggesting the operation of convection currents within the Earth’s mantle.
  • These currents are believed to originate due to the thermal differences caused by radioactive elements present within the mantle.
  • Holmes proposed a system of these currents throughout the mantle, offering an alternative theory to explain the force that drives continental drift – a point of contention that led many contemporaneous scientists to discard the continental drift theory.

• Mapping of the Ocean Floor:

• • Discoveries Post-World War II: Contrary to the perception of the ocean floor being a monotonous plain, in-depth research showcased a diverse relief beneath the waters.

• • Investigations revealed submerged mountain ranges, deep trenches (primarily located near continental margins), and notably active mid-oceanic ridges known for their volcanic activities.
  • A striking observation was the age of rocks from the oceanic crust. 
  • They were considerably younger than their continental counterparts. 
  • Further, rocks flanking the crests of oceanic ridges, equidistant from the crest, shared remarkable similarities in composition and age.

Ocean Floor Configuration: Exploring Ocean Floor Configuration and Sea Floor Spreading

The ocean floor can be segmented into three based on depth and relief: 

• Continental margins, Deep-sea basins, and Mid-ocean ridges. 

Continental Margins:

• Definition: These are transition zones between continental shores and deep-sea basins.
• Components:

• • Includes continental shelf, slope, rise, and deep-oceanic trenches.                              
  • The deep-oceanic trenches are especially significant for the study of  ocean and continent distribution.         

Abyssal Plains:

• Position: Located between the continental margins and mid-oceanic ridges.
• Significance: Areas where continental sediments that move beyond the margins accumulate.

Mid-Oceanic Ridges:

• The longest submerged mountain-chain on Earth.
• They are characterised by a central rift system, a fractionated plateau, and flank zones.
• The Rift system at the crest is a hub of volcanic activity, known as mid-oceanic volcanoes.

Distribution of Earthquakes and Volcanoes: Exploring Quake and Volcano Patterns with Sea Floor Spreading

Observations:

• A visible line of seismic activity dots in the Atlantic, extending to the Indian Ocean, corresponds with the mid-oceanic ridges. 
• Another concentrated shaded belt aligns with the Alpine-Himalayan system and the Pacific Ocean rim.

Depth of Seismic Activity:

• Earthquakes in mid-oceanic ridge areas are shallow, while those along the Alpine-Himalayan belt and Pacific rim are deep-seated.    

Volcanic Activity:

• The map pattern of volcanoes matches that of seismic activity.
• The Pacific rim, dubbed the “rim of fire,” hosts numerous active volcanoes.

Concept of Sea floor Spreading: The Phenomenon of Sea Floor Spreading Explored

• • Post-drift studies brought forth significant data, largely missing during Wegener’s proposition of continental drift. 
  • Particularly, ocean floor mapping and paleomagnetic rock studies from oceanic regions brought to light crucial observations.
  • Volcanic Activity on Mid-Oceanic Ridges: Mid-oceanic ridges experience frequent volcanic eruptions, spewing vast amounts of lava on the surface.  
  • Similarities in Rocks on Either Side of Mid-Oceanic Ridges: The rocks, equidistant from the mid-oceanic ridge crest, display striking similarities in formation time, chemical compositions, and magnetic properties. 
    • Rocks nearest to the ridges possess normal polarity and are the most recent. Their age escalates with distance from the crest.
  • Age Discrepancy between Oceanic and Continental Rocks: Ocean crust rocks are substantially younger than continental rocks. 
    • The former’s age doesn’t exceed 200 million years, while some continental rocks date back as far as 3,200 million years.
  • Surprisingly Thin Ocean Floor Sediments: Contrary to expectations of a thick sediment layer if ocean floors were as ancient as continents, sediment columns on the ocean floor were found to be no older than 200 million years.
  • Earthquake Depths: Deep trenches are the epicentres for profound earthquakes, whereas the mid-oceanic ridge areas witness quakes with shallow foci.

Hess’s Sea floor Spreading Hypothesis (1961):

• Based on detailed rock magnetic studies, Hess proposed the “sea floor spreading” theory in 1961. 
• He theorised that continuous volcanic eruptions at mid-oceanic ridges cause breaks in the oceanic crust. 
• Emerging lava from these eruptions pushes the crust apart, leading to the ocean floor’s expansion. 
• Noting the youthfulness of the oceanic crust and the fact that one ocean’s widening doesn’t shrink another, Hess concluded that the spreading ocean floor eventually sinks at trenches and gets absorbed.