= Solution
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.
\b[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.
\b[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.<Developing the theory in This Dynamic Earth>[Sections Ocean floor mapping, Seafloor spreading and recycling of oceanic crust]{ref}
\b[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.<Magnetic Anomalies Over Oceanic Ridges by F. J. Vine and D. H. Matthews>[Nature 199, pp. 947–949, magnetic striping and the spreading interpretation]{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.
\b[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.
\b[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 $\mathbf v=\boldsymbol\omega\times\mathbf r$. 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.<Developing the theory in This Dynamic Earth>[Sections Magnetic striping and polar reversals, Concentration of earthquakes, and discussion of the 1968 drilling programme]{ref}
Thus \b[<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.
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