Plate Tectonics: History of the Idea

Plate tectonics is a major scientific discovery of the 20th century, comparable to relativity, DNA, and quantum physics.
It revolutionized geology, explaining various Earth science observations.
Many contributors to this theory are still alive.

Early observations include:
- The fit between continents, recognized as early as 1596.
- Distribution of ancient fossil plants and animals (e.g., Mesosaurus, Glossopteris).
- Geologic evidence of ancient glaciers flowing from continental edges inward.

Before 1912, explanations for these observations were lacking.
Prevailing ideas included:
- Thermal contraction: Earth cooling and shrinking, oceans as sunken continental crust, mountains as shrinkage ridges.
- Land bridges: Flora and fauna migrated across now-flooded land bridges.
- The fit of continents was dismissed as statistically insignificant.
Alfred Wegener proposed Continental Drift (Kontinentalverschiebung) between 1912 and 1929.
His book, The Origin of Continents and Oceans (1912), outlined his theory.
Wegener was inspired by the fit of continents and fossil distribution.

Fit of the continents: Continents were once amalgamated into a supercontinent called Pangaea.
Distribution of fossils: Fossil distribution makes sense when Pangaea is reconstructed.
Distribution of climatic belts: Glacial flow originated from the center of southern landmasses (Gondwana).
Contiguous geologic units and mountain belts: Mountain ranges and rock units align when Pangaea is reassembled.

Wegener's explanation for continental movement (centrifugal force, tidal waves) was flawed.
He incorrectly suggested continents moved through oceanic crust.
His theory was rejected due to unrealistic mechanisms and the implausibility of continents plowing through denser oceanic crust.
The density of oceanic crust ( $3.0g/cm^3$ ) is higher than continental crust ( $2.7g/cm^3$ )
Critics argued that such movement would cause extreme deformation of continental edges.
Wegener died in 1930, still trying to prove his ideas.

The Earth's magnetic field resembles a giant bar magnet.
Magnetic North is misaligned with the Earth's rotational axis, causing magnetic declination, which changes over time.
This misalignment leads to 'True Polar Wander,' where magnetic poles move but stay near the geographic poles.
Magnetic inclination is the dip of a compass needle aligning with the magnetic field.
Inclination varies with latitude: parallel to the surface at the equator, perpendicular at the poles.

As rocks cool from magma or sediments lithify, they record the ambient magnetic field.
Iron-containing crystals align with the magnetic field, recording declination and inclination.
Scientists used this to reconstruct the wander path of magnetic poles.
Irving and Runcorn's 1956 paper revealed that polar wander paths from different continents did not match.
This mismatch indicates that continents moved relative to each other.
The paths record the wandering of continents, not the magnetic pole (apparent polar wander).

Early seafloor mapping involved dropping a lead weight on a rope to measure depth (bathymetry).
After the sinking of the Titanic, Reginald Fessenden used the Fessenden Oscillator to bounce acoustic signals off icebergs and the seafloor, initiating acoustic exploration.

World wars, particularly German U-boats, motivated the US Navy to study the oceans.
Research vessels collected seafloor data, especially in the North Atlantic and Northeast Pacific.
Trans-Atlantic telephone cable laying and continuous echo sounding during WWII provided extensive bathymetric data.

Marie Tharp and Bruce Heezen compiled bathymetric data for the Atlantic Ocean after WWII.
They presented data as physiographic maps to bypass classified topographic maps.
Tharp identified a narrow valley along the mid-ocean ridge, similar to the East African Rift Valley, suggesting seafloor splitting.
Heezen was initially skeptical but later supported Tharp's discovery.

Mid-ocean ridges:
- Long, linear mountain chains circling the globe (e.g., Mid-Atlantic Ridge).
- The Mid-Atlantic Ridge is the longest mountain chain on Earth.
- Rift valley in the middle of the ridge (Tharp’s discovery).
- High heat flux on ridges, decreasing with distance.
- Thin sediment cover on ridges, thickening with distance.
Oceanic trenches:
- Deepest trench: Mariana Trench (>11 km deep).
Earthquakes:
- Wadati-Benioff zones: Earthquake zones dipping into the mantle, associated with trenches.

Harry Hess proposed seafloor spreading: the seafloor spreads apart at mid-ocean ridges.
Hot material wells up, forming new oceanic crust and explaining heat flux variations and sediment thickness.
Since the Earth isn't expanding, seafloor must be disappearing elsewhere, such as at oceanic trenches.
Hess suggested convection cells in the mantle drive seafloor spreading.
Hess (1962) described his paper as