Plate Tectonics

Earth's outer shell is divided into plates that move relative to one another at a few centimetres per year. The motion is measured directly by satellite geodesy, and the record of past motion is preserved in the seafloor.

Plate tectonics is also a case where the scientific consensus was wrong and changed. Alfred Wegener proposed continental drift in 1912 and was rejected for half a century, largely because he had no mechanism. The evidence that settled it arrived in the 1960s.

Direct measurement#

GPS and satellite laser ranging measure present-day plate motion to millimetre precision. The rates are not inferred from geology; they are observed.

At 2.5 cm/year, the 5,000 km width of the Atlantic requires roughly 200 million years — which matches the age of the oldest Atlantic seafloor determined independently.

Seafloor magnetic striping#

The strongest single line of evidence, and the finding that converted the field.

Earth's magnetic field reverses polarity at irregular intervals. Molten rock at a spreading ridge cools through the Curie point and locks in the field direction at that moment. As spreading continues, the record moves outward.

The result is a pattern of magnetic stripes parallel to the ridge and symmetrical on both sides — the same sequence of normal and reversed bands, in the same order, at the same distances, on each flank.

Vine and Matthews (1963) predicted this before it was mapped. The symmetry cannot be produced by a process that does not involve material being created at the ridge and carried away in both directions.

The stripe sequence also matches the independently dated reversal chronology established from lava flows on land.

Seafloor age#

Ocean crust ages outward from the ridges in both directions, and the pattern is continuous.

Ocean crust is young everywhere because it is continuously destroyed at subduction zones and replaced at ridges. This explains why no ocean floor is as old as the continents — a puzzle before the theory existed.

Hotspot chains#

The Hawaiian-Emperor chain is a line of volcanoes and seamounts extending 6,000 km. Ages increase steadily along it: Kilauea is active, Kauai is about 5 million years, Midway about 28 million, and the far end of the Emperor seamounts about 81 million.

Dividing distance by age difference gives a plate velocity of roughly 8–10 cm/year, matching the GPS measurement of Pacific plate motion today.

The chain also has a bend at about 47 million years, recording a change in plate direction.

Evidence Wegener had#

Wegener's original case, now supplemented rather than replaced:

• Coastlines of South America and Africa fit, better at the continental shelf edge than at the shoreline
• Glossopteris fossils across South America, Africa, India, Australia, and Antarctica, with seeds too heavy to cross oceans
• Mesosaurus, a freshwater reptile, in Brazil and South Africa only
• Matching mountain belts across the Atlantic — the Appalachians continuing into Scotland and Scandinavia
• Glacial striations in India, Australia, South Africa, and South America, radiating from a common centre when the continents are reassembled
• Coal deposits in Antarctica, requiring tropical swamps

Common objections#

"The continents moved rapidly during the flood"

Catastrophic plate tectonics, developed by John Baumgardner, a geophysicist who wrote mantle convection code used in mainstream research. It proposes runaway subduction during the flood year, with plates moving at metres per second.

Baumgardner's modelling work is genuine and his credentials are real, which distinguishes this from most young-earth proposals.

The energy is the difficulty. Moving continents thousands of kilometres in a year converts an enormous quantity of gravitational and frictional energy to heat — enough, by the model's own accounting, to vaporise the oceans and melt the crust. Baumgardner acknowledges this and invokes divine intervention to remove the heat.

The model also has to compress the magnetic reversal record into the same year, requiring hundreds of full field reversals in months, and it does not explain why the stripe widths are proportional to the independently dated reversal durations.

"Measured rates today don't prove rates were always the same"

A legitimate point, and rates have varied. Spreading was faster during parts of the Cretaceous, and this is measurable from stripe widths.

The variation is bounded and recorded. Stripe spacing gives the rate at each time, and it varies by a factor of a few rather than by the eight orders of magnitude a flood-year model requires.

Independent checks agree with the measured rates. The hotspot chain gives the same Pacific velocity as GPS, and the width of the Atlantic divided by its age gives the observed spreading rate.

"Wegener was rejected by consensus, so consensus can be wrong about deep time too"

Accurate history, and an important one. Wegener was dismissed harshly, in part because he was a meteorologist rather than a geologist, and the rejection lasted about fifty years.

The reason for rejection was specific: he proposed continents ploughing through oceanic crust, which is physically impossible, and had no viable mechanism. The objection was scientific rather than dogmatic, and it was correct as an objection to his proposed mechanism.

The resolution is the relevant part. Once seafloor spreading supplied a mechanism and magnetic striping supplied evidence, the field changed within about a decade — and the geologists who changed their minds had held the opposite view their whole careers.

Consensus being revisable by evidence is what the example demonstrates. Flood geology has been available for as long and has not produced the equivalent of magnetic striping.

"Subduction is invented to explain missing crust"

Subduction is observed rather than postulated. Wadati-Benioff zones — bands of earthquakes descending at an angle beneath island arcs and continental margins, traceable to depths of 700 km — map the descending slab directly.

Seismic tomography images slabs in the mantle using earthquake wave velocities, in the same way a medical scan images tissue.

The associated features follow: deep ocean trenches at the surface expression, volcanic arcs at a consistent distance inland, and the deepest earthquakes on Earth.

What the evidence shows#

Plate motion is measured directly by satellite at a few centimetres per year, and three independent methods agree on the rate: GPS, hotspot chain ages, and ocean basin width divided by seafloor age.

Magnetic striping on the seafloor is symmetrical about the ridges and matches the reversal sequence dated independently on land. The pattern was predicted before it was mapped.

The catastrophic alternative requires the same motion compressed into a year, which its own author acknowledges produces heat that would destroy the surface of the planet.