Post-Drift Developments
Convection-Current Theory · Mid-Ocean Ridges & Seafloor Spreading · Paleomagnetism (28 Micro-Realms)
From Continental Drift to Seafloor Spreading (1930 – 1963)
Wegener gathered his evidence entirely from continental landmasses. After World War II, ocean-floor sonar mapping, Arthur Holmes' Convection Current Theory, and Paleomagnetic surveys of mid-oceanic ridges culminated in Harry Hess's Seafloor Spreading Hypothesis (1961)—proving that the ocean floor itself is a dynamic conveyor belt.
I. Convection-Current Theory (Holmes 1930s) & Post-WWII Sonar
Shift to the Oceans
- Drift proof was continental
- Post-WWII ocean floor focus
- Unlocked true tectonic engine
Arthur Holmes (1930s)
- Convection Current Theory
- Currents operate in Mantle
- Solved Wegener's force problem
Radioactive Thermal Engine
- Radioactive elements in mantle
- Create thermal differences
- Drive whole-mantle circulation
Rising Divergent Limbs
- Hot mantle plumes ascend
- Diverge beneath lithosphere
- Rupture crust into ridges
Sinking Convergent Limbs
- Cooled currents turn downward
- Pull crust into trenches
- Completes convective cell
Sonar Bathymetry Mapping
- Post-WWII sonic echo-sounding
- Marie Tharp & Bruce Heezen
- Mapped global ocean relief
Rugged Ocean Relief
- Not a flat featureless plain
- Submerged mountain ranges
- Deep trenches at margins
II. Mapping of the Ocean Floor & Mid-Oceanic Ridges (NCERT)
3 Ocean Floor Divisions
- 1. Continental margins
- 2. Deep-sea abyssal plains
- 3. Mid-oceanic ridges
Continental Margins
- Shelf, slope, rise & trenches
- Transition shore to abyss
- Deep-sea trenches key feature
Abyssal Plains
- Between margins & ridges
- Continental sediments settle
- Buries irregular volcanic floor
Mid-Oceanic Ridge (MOR)
- Interconnected mountain chain
- Longest chain on Earth (~70,000 km)
- Submerged beneath ocean waters
Central Rift System
- Rift valley along ridge crest
- Flanked by plateau & mountains
- Zone of intense volcanism
Focal Depth Contrast
- MOR quakes: Shallow foci
- Alpine-Himalayan & Pacific Rim:
- Deep-seated earthquake foci
Thin Ocean Sediments
- Unexpectedly thin sediment blanket
- None older than 200 Myr
- Bare rock at ridge crest
III. Paleomagnetism & Symmetrical Seafloor Zebra Stripes
What is Paleomagnetism?
- Fossil magnetism in rocks
- Magnetite aligns with field
- Locked below Curie point
Polar Wandering Curves
- Keith Runcorn (1950s)
- Separate pole paths for EU/NA
- Converge when Atlantic closes
Geomagnetic Reversals
- Periodic N–S magnetic flips
- Normal vs Reversed polarity
- Dated via potassium-argon
Magnetometer Anomalies
- Basalt has high magnetism
- Alternating +/- intensity bands
- Parallel to mid-ocean ridge
Vine–Matthews–Morley (1963)
- Twin magnetic zebra stripes
- Mirror-image across ridge crest
- Proved seafloor tape-recorder
Equidistant Rock Symmetry
- Equidistant rocks on either side
- Have identical age & composition
- And identical magnetic polarity!
Spreading Rate Calculation
- Stripe width $\div$ time = speed
- Mid-Atlantic: Slow (~2.5 cm/yr)
- East Pacific Rise: Fast (~15 cm/yr)
IV. Seafloor Spreading Hypothesis (Harry Hess 1961 — NCERT)
Harry Hess (1961)
HESS (1961)
- Proposed "Seafloor Spreading"
- With Robert Dietz (1961–62)
- Unified convection & MORs
Crest Volcanic Rupture
- Constant eruptions at MOR crest
- Ruptures oceanic crust
- New lava wedges into rift
Lateral Crustal Pushing
- New lava pushes floor aside
- Moves laterally both ways
- Ocean floor spreads apart
200-Ma vs 3,200-Ma Age
- Ocean crust <200 Myr old
- Continent rocks up to 3,200 Myr
- Youngest rocks at ridge crest
Non-Expanding Earth
- Spreading of one ocean
- Does not shrink another
- Requires floor destruction!
Trench Consumption
- Oceanic floor sinks at trenches
- Gets consumed into mantle
- Balances MOR crust creation
Birth of Plate Tectonics
- McKenzie, Parker & Morgan (1967)
- Continents ride spreading floor
- Leads into Realm 26
Core Distinctions
1) Volcanic eruptions are common all along the Mid-Oceanic Ridges.
2) Rocks equidistant on either side of the ridge crest show remarkable similarities in period of formation, chemical composition, and magnetic properties.
3) Rocks closer to the mid-oceanic ridges have normal polarity and are the youngest; age increases away from the crest.
4) Oceanic crust rocks are much younger (<200 Myr) than continental rocks (up to 3,200 Myr).
5) Sediments on the ocean floor are unexpectedly very thin (<200 Myr).
In Mid-Oceanic Ridge areas, earthquake foci are at shallow depths (divergent tensional cracking), whereas along the Alpine-Himalayan belt and Pacific Rim, earthquakes are deep-seated (subducting slabs).
Post-Drift Evolution
From Wegener (1912) to Hess (1961)