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The decisive change was from moving continents within a supposedly permanent ocean floor to moving coherent tectonic plates that include both continental and oceanic lithosphere. Several independent observations made that change testable.

Continental connections supplied the initial problem. In the early twentieth century, Alfred Wegener assembled matching continental margins, correlatable rock belts and mountain chains, shared fossil assemblages, and paleoclimate indicators into the case for continental drift. Reassembling Gondwana, for example, makes geographically separated glacial deposits and striation directions compatible with one ice-sheet system. Fits at the continental shelf edge are physically more meaningful than exact fits to modern coastlines, which have been modified by erosion and sea-level change. The evidence suggested relative movement, but did not justify the proposed forces: continents cannot readily plough independently through strong oceanic rock. A convincing mechanism and independent motion measurements were missing.

Oceanography changed the boundary conditions. Echo-sounding surveys revealed a connected spreading ridge system, axial valleys, subduction trenches and long fracture zones. Seismic refraction surveys established systematic differences between continental and oceanic crust. Thin oceanic sediment cover, high ridge heat flux and the lack of very old exposed oceanic basement made permanent, ancient ocean basins increasingly implausible. Harry Hammond Hess proposed creation of oceanic lithosphere at ridges and recycling at trenches: seafloor spreading without requiring an expanding Earth. Slow mantle convection supplied a physically plausible connection between surface transport and internal heat loss. The mechanism need not have been completely quantified before its kinematic predictions could be tested.^[ref]

Magnetic records supplied a clock and a speedometer. Dated continental lavas established a sequence of geomagnetic reversals. Marine magnetometers then found elongated, approximately paired marine magnetic stripes on either side of ridges. The 1963 interpretation by F. J. Vine and D. H. Matthews combined repeated reversals with seafloor spreading: cooling basalt locks in remanent magnetization, and continuous motion converts a time sequence into a spatial sequence. If a polarity interval lasts $\Delta t$ and occupies width $w$ on one ridge flank, its half-spreading rate is $w/\Delta t$. Matching several reversals tests the predicted sequence, rather than merely fitting a single stripe.^[ref] Different apparent polar wander paths for different continents also became mutually consistent after continental reconstruction, favoring relative continental motion over independent magnetic-pole histories.

Earthquakes identified creation, translation and consumption. Improved global seismology located shallow earthquakes along ridges and transforms, and dipping Wadati-Benioff zones beneath trenches. Their progressive focal depths identified cold, coherent slabs entering the mantle. Focal mechanisms showed extension at spreading boundaries, tangential slip at transforms and convergence at trenches. In 1965, John Tuzo Wilson's transform fault interpretation made a particularly discriminating prediction: active slip occurs between offset ridge segments, with a sense that can be opposite to that inferred by treating the entire lineament as an ordinary offset fault. The outer fracture zones are inherited traces, not continuing active plate boundaries. Agreement of focal mechanisms with the transform prediction linked magnetic and seismic evidence.

The late-1960s synthesis made these observations one quantitative theory. Relative motion of rigid tectonic plates on a sphere can be described by an Euler angular velocity vector, with $\mathbf{v}=\mathbf{\omega }\times \mathbf{r}$. Spreading rates, transform azimuths and slip directions must fit the same rotation, and relative angular velocities must close around plate circuits. The resulting framework accounted for global boundary geometry and earthquake mechanisms without moving continents separately from the seafloor. Deep-ocean drilling beginning in 1968 subsequently confirmed that basement and overlying sediments generally become older away from ridges; this was a powerful confirmation of an already emerging synthesis, not a prerequisite discovered decades earlier.^[ref]

Thus plate tectonics succeeded through agreement between geological reconstruction, magnetic chronology, ocean-floor structure, seismology and rigid-plate kinematics. Modern satellite geodesy verifies motions directly, but belongs to the later confirmation of the theory, not its original 1960s evidence.