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The sections on magnetic striping and seafloor spreading explain the cooling-rock magnetic record and the approximately paired stripes across spreading ridges.

F. J. Vine and D. H. Matthews connected the pattern of marine magnetic stripes with spreading ocean floor and changing magnetic polarity. Their model relates a spatial magnetic sequence to the history carried away from a spreading ridge.^[ref]

With an independent polarity chronology, distances from the ridge constrain spreading rates. This makes a measurable magnetic pattern evidence for a dynamical process.

Harry Hammond Hess was a geologist whose seafloor spreading hypothesis connected production of oceanic crust at spreading ridges with its return to the mantle at subduction zones. The moving ocean floor supplied a mechanism for large-scale rearrangement of continents without requiring continents to plough through a stationary ocean floor. During naval service he used sonar surveys to investigate ocean-basin bathymetry; his 1962 paper History of Ocean Basins developed the geological hypothesis.^[ref]

The hypothesis also addressed a puzzle: ancient oceans need not contain equally ancient oceanic crust if the ocean floor is continually renewed and recycled. Increasing ocean-floor age away from spreading ridges, paired marine magnetic stripes and earthquakes along descending slabs later supplied independent tests of seafloor spreading and subduction. These observations turned an initially sparsely constrained proposal into part of plate tectonics.^[ref]

A lineated pattern associated with differently magnetized crust formed at a spreading ridge during changes of field polarity. The age-distance relation is set by spreading, while the measured anomaly is a spatially filtered field of the magnetized material.

Hawaii: short-wavelength volcanic loading and long-wavelength mantle support must be distinguished. A dense volcanic edifice replacing seawater adds mass and gives a positive free-air gravity anomaly. Its load bends the lithosphere down, producing a surrounding moat and an outer flexural arch. The associated displaced density interfaces contribute negative and positive gravity signals at neighbouring distances. Observed gravity near the Hawaiian ridge includes strong positive anomalies over the ridge, negative flanks, and a broader positive belt. At much longer wavelengths, the Hawaiian swell also correlates with a positive gravity anomaly, with a smaller gravity-to-topography ratio that requires compensation; the precise source of support is not determined by that correlation alone.^[ref]

The mechanical distinction can be expressed with a simple elastic-plate model. For load height $h$, downward deflection $w$, plate rigidity $D$, loading density contrast $\Delta {\rho }_l$ relative to water, and restoring density contrast $\Delta {\rho }_b$ at an interface,For a Fourier component of horizontal wavenumber $q$, this gives $w_q=\Delta {\rho }_{l}g{h}_q/(D{q}^4+\Delta {\rho }_{b}g)$. The load's direct gravity contribution competes with the negative contribution of a depressed interface at reference depth $t$. In a linear thin-interface approximation,At short wavelengths rigidity suppresses compensation, leaving relatively high positive gravity admittance. At long wavelengths the plate can deflect and compensate more completely, reducing admittance. Fitting the wavelength dependence constrains effective elastic thickness through $D=E_Y{T}_e^3/[12(1-{\nu }^2)]$, subject to density structure, load distribution and subsurface loading assumptions. This explains why local Airy compensation alone misses the spatially extended flexural response.

The broad swell needs a deeper explanation than the narrow volcanic pile: hot, buoyant mantle and mantle flow can support regional topography, while deep density anomalies and surface elevation jointly determine its gravity. A free-air anomaly keeps the attracting mass of the topography; a Bouguer correction removes an assumed direct topographic attraction and can reveal its compensating mass deficit. Gravity plus topography constrain support as a function of wavelength; neither alone uniquely fixes the mantle density distribution.

For the OR alternative, triple junctions are points where three plate boundaries meet. Each boundary can be a spreading ridge, subduction trench, or transform fault. The three plate velocities satisfyThis velocity triangle is necessary but not enough: a single junction velocity ${\mathbf{u}}_J$ must also keep the meeting point on all three moving boundaries. For symmetric spreading between A and B, the ridge constraint is $[{\mathbf{u}}_J-({\mathbf{v}}_A+{\mathbf{v}}_B)/2]\cdot \mathbf{n}=0$. For a transform, ${\mathbf{v}}_A\cdot \mathbf{n}={\mathbf{v}}_B\cdot \mathbf{n}$, and the junction must have that same normal component. For a trench, its normal migration must match the adopted subduction/rollback model; a trench fixed relative to the overriding plate shares that plate's normal velocity.

In velocity space each boundary supplies a line of possible junction velocities. A common intersection means the junction can migrate while preserving its local configuration; incompatible constraints force boundary reorganization. This is kinematic stability, not the claim that the junction is stationary. A symmetric ridge–ridge–ridge example has three arms meeting at $120^{\circ }$ and a common junction velocity; unequal spreading rates do not require those equal angles. Transform and trench junctions depend on the allowed relative motions and boundary geometry. Stable and unstable evolution were formulated through plate-velocity geometry in the original triple-junction analysis.^[ref]

These original diagrams show an illustrative flexural response and a symmetric ridge-junction construction; they are not maps or measured Hawaiian profiles.

At the north pole $\mathbf{r}=+R\hat{z}$, so the velocities reverse:The triangle still has side lengths 20, 48 and 52 mm/year. Across AB, A moves toward positive $y$ relative to B; across AC, A moves toward positive $x$ relative to C. Both boundaries therefore open and are spreading ridges, with full opening speeds 20 and 48 mm/year. Together with BC they form a ridge-ridge-transform triple junction.

In the symmetric normal-spreading model, a ridge's normal velocity is the mean normal velocity of its two plates. AB gives $U_y=(0-20)/2=-10$, and AC gives $U_x=(48+0)/2=24$. Their intersection lies on the BC transform velocity line:This northern junction is also instantaneously kinematically stable. The diagrams use the same fixed geocentric tangent axes at both poles; reversing velocities is not the same operation as reflecting a map drawn from an exterior observer's viewpoint.

Not every ridge-ridge-transform geometry works. For pure normal spreading, each ridge velocity line is the perpendicular bisector of the corresponding side of the plate velocity triangle. Their intersection is the triangle's circumcenter. A transform junction must also lie on the third side's line. A nondegenerate triangle's circumcenter lies there only at that side's midpoint, so the opposite angle is a right angle. Equivalently, $({\mathbf{v}}_B-{\mathbf{v}}_A)\cdot ({\mathbf{v}}_C-{\mathbf{v}}_A)=0$. The two ridge directions are then perpendicular, as they are here. RRF junctions are conditionally stable, with the right-angle ridge geometry in this symmetric pure-normal model; they are not all stable. Oblique or asymmetric accretion changes the velocity-line assumptions and must be specified separately.

Assume an Earth-like reversing planetary magnetic field and magnetizable crust that acquires thermoremanent magnetization on cooling. Alternating polarity intervals then produce marine magnetic stripes parallel to each spreading ridge, with matching age/polarity sequences on its two sides. AB has half-spreading rate 10 mm/year, whereas AC has half-spreading rate 24 mm/year. Thus the same polarity interval $\Delta t$ produces widths $10\Delta t$ and $24\Delta t$ mm when time is in years: the AC stripes are 2.4 times wider. Age increases away from each ridge. The transform produces no new crust; bands end or are displaced at it, rather than forming a third symmetric stripe sequence centred on BC. This is the remanent-magnetization/spreading mechanism developed from marine magnetic observations.^[ref]

A more precise local sketch can be made in the northern junction frame. Let $x,y$ be distances in kilometres along the fixed tangent axes and $t$ the crustal age in Myr; numerically, mm/year equals km/Myr. Relative to the moving junction, the velocities are ${\mathbf{w}}_A=(24,10)$, ${\mathbf{w}}_B=(24,-10)$ and ${\mathbf{w}}_C=(-24,10)$. In A the steady age field is $t_A=\min (x/24,y/10)$, since backwards advection first reaches either the AC or AB ridge. Equal-age bands therefore join as right-angle corners within A, with the corners migrating along the ray $(24,10)$. In the B wedge, $t_B=-y/10$ and the stripes meet the transform. On the C flank directly supplied from AC, $t_C=-x/24$; other parts of C carry older crust whose inherited pattern requires earlier boundary history.

The sketch displays magnetization polarity, not a computed total-field anomaly amplitude. Actual anomalies depend on the planetary field direction, source depth, magnetization strength and observation geometry. The paper supplies no field history, so a numerical reversal chronology or an absolute positive/negative anomaly sequence cannot be inferred. Without field reversals, ridge accretion alone does not imply alternating polarity stripes.

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.

Slab pull is generally the largest direct driving contribution for plates attached to substantial cold descending slabs. Cooling makes oceanic lithosphere negatively buoyant; sinking slabs transmit a pulling force to the rest of the tectonic plate. The comparison is a force balance, so the following driving and resisting processes both matter.

The first two gravitational contributions ultimately belong to the same coupled mantle convection system: the plates are its cold upper thermal boundary layer. They are not passive rafts on a prescribed conveyor belt. A plate with little attached descending slab need not have slab pull as its largest local driver, and a large gross slab pull can be partly consumed by bending and interface resistance. Geological observations and force comparisons nonetheless support the general dominance of slab pull for rapidly moving subducting plates.^[ref]

Plate tectonics changes the environments in which natural selection, gene flow and extinction operate. It influences both the persistence of habitable conditions and the geographical and chemical opportunities for evolution. It does not prescribe a unique sequence of biological innovations.

Separation of continents by continental rifting divides populations and reduces gene flow, promoting allopatric speciation. Continental collision or newly exposed land connections can reunite populations, permit dispersal and introduce competitors or predators. These mechanisms connect biogeography to continental configuration. Changing the amount and fragmentation of shallow continental shelf habitat can also change the opportunities for diversification of marine organisms.

Over long intervals, volcanic outgassing, silicate weathering, carbonate burial and subduction exchange carbon among the interior, atmosphere and ocean. In a suitable regime, warmer, wetter conditions increase silicate weathering, remove atmospheric carbon dioxide and weaken the greenhouse effect. This negative feedback can support long-lived habitability, though it depends on exposed fresh rock, available water and reaction rates.^[ref] Mountain building changes precipitation, erosion and delivery of nutrients to the ocean, affecting primary production and organic carbon burial.

The opening and closure of ocean gateways changes ocean circulation, transport of heat and the supply of oxygen to deep water. Together with movement of land through different latitudes and changes in sea level, this reorganizes climate and habitat. Enhanced organic carbon burial can change atmospheric oxygen over long intervals, while a poorly ventilated ocean can develop ocean anoxia. Such chemical changes alter which forms of metabolism are viable.

Hydrothermal circulation at spreading ridges and through fractured rock supplies chemical disequilibrium and mineral surfaces. For example, serpentinization can produce hydrogen usable by chemolithotrophy when suitable electron acceptors are available. This supplies a possible setting for abiogenesis and early ecosystems; it does not establish that life originated there.

There can also be abrupt disruption. Rapid emplacement of a large igneous province can change climate through gas release; sulfur-bearing aerosols can initially cool, whereas sustained carbon dioxide release can warm and contribute to ocean acidification and ocean anoxia. Such perturbations can contribute to mass extinction, followed by adaptive radiation among survivors. The Deccan Traps exemplify the scale of volcanism involved, but their association with the Cretaceous–Palaeogene boundary does not justify attributing the entire biological crisis to volcanism alone. Both gradual environmental reorganization and rare rapid perturbations belong in the explanation.

Hotter Archean mantle would generally produce thicker, more magnesium-rich oceanic crust, together with a more strongly depleted residual mantle. Greater melt production, not merely thermal expansion of an unchanged modern column, is the principal reason for the increased crustal thickness.

At a spreading ridge, upwelling peridotite undergoes adiabatic decompression melting. A higher mantle potential temperature makes the ascending mantle intersect the solidus at greater pressure and depth. It therefore melts over a longer decompression interval and generally reaches a larger integrated melt fraction. With comparable melt focusing, more melt is supplied per unit area of newly formed oceanic crust.

Modern normal oceanic crust is about 6–7 km thick and contains basaltic lavas, a sheeted dike complex, and gabbro with mafic cumulates at depth. An illustrative reconstruction for hot ambient Archean mantle gives about 25–35 km of oceanic crust, rather than a universal fixed thickness. Its primary liquids are more magnesium-rich than typical modern primary ridge liquids; their crystallization produces more olivine- and pyroxene-rich cumulates. The extracted liquid and its crystal products must be distinguished: the entire final oceanic crust need not be a homogeneous komatiite layer.^[ref]

The residual peridotite loses more melt components, becoming more refractory and commonly harzburgite-rich. Thick buoyant basaltic crust, its later eclogitization, and the depleted underlying mantle can change the mechanics of subduction. A hotter mantle does not by itself establish how fast plates moved or whether a globally connected modern plate tectonics regime operated.

Komatiites, with very magnesium-rich primary liquids and high inferred eruption temperatures, provide evidence for hot melting, but may sample especially hot mantle plumes. Estimates of ambient mantle conditions also use less exceptional non-arc basalts. Variable source composition, water content, cooling, melt extraction and later alteration all limit the precision of a single inferred Archean thickness. The robust prediction is a larger melting column and melt yield, thicker igneous oceanic crust, and a more depleted residue.

Cascadia subducts a young, warm plate and lacks the enormous plateau and crustal root of the central Andes; the central Andean Nazca Plate system also supports much deeper intraslab earthquakes. These are regional comparisons: neither western margin is uniform along its length.

First, the Juan de Fuca Plate entering Cascadia is young, typically of order 10 million years old, because its spreading ridge lies close to the margin. Its lithosphere is comparatively thin and warm. Much of the Nazca Plate entering the central Andes is older and colder. A cold plate carries low temperatures farther into the mantle, sustaining deep intraslab seismicity; the warm Cascadia slab loses its brittle seismogenic character much shallower. This concerns earthquakes within the slab, not the size of megathrust earthquakes: both margins can generate very large plate-interface events. The official warm-slab synthesis treats Cascadia as an endmember.^[ref]

Second, the central Andes have undergone major sustained crustal shortening and tectonic thickening, building a high plateau and locally roughly 60–70 km thick continental crust. Cascadia has an active volcanic arc and mountains but no equivalent continuous central-Andean plateau and crustal root. The difference reflects the accumulated history of convergence, upper-plate deformation, slab dip, trench motion and mechanical coupling, not just the instantaneous existence of subduction. The Andes include flat-slab segments with gaps in arc volcanism as well as normally dipping segments.^[ref][ref]

A young warm incoming plate and a faster, colder incoming plate can therefore generate different thermal structures, dehydration patterns and deformation histories. Plate age alone is insufficient to predict the thickness of the overriding continental crust.

A lies on the continuation of the offset outside the two ridge tips: it is the fracture zone, whose two sides are carried in the same overall plate direction. A bathymetric step can remain because the two sides have different ages and thermal histories, without continuing plate-boundary strike-slip faulting.

B is the black segment connecting the two white spreading ridges. Construct the plate motion from divergence away from those ridge axes. Between their tips, the upper side of B belongs to the plate west of the right-hand ridge and moves approximately west-northwest; the lower side belongs to the plate east of the left-hand ridge and moves approximately east-southeast. Looking across B from either side, the opposite side moves to the observer's left. B is therefore a left-lateral strike-slip faulting on a transform fault. The offset preserved outside the active segment must not be read as the sense of present slip.

The grey arrows in the source image are pointers to detachment faults, not plate-motion vectors. The original publication identifies white lines as spreading ridges, black lines as transform faults, and arrows as pointers to detachments. Its Figure 1b is the unlettered source of the exam's Figure 3.^[ref]

C lies on the white spreading ridge extending down from the left-hand ridge-transform intersection. Relative motion there is extensional, so its tectonic faults have normal slip. Dike intrusion also accommodates extension at a spreading ridge; “normal slip” is the appropriate category among the four offered, not a claim that all spreading is achieved by a single normal fault.

Cooling makes the oceanic lithosphere denser and contracts it; isostatic adjustment therefore lowers the seafloor. The cooling layer includes substantial mantle, not just the approximately fixed-thickness igneous oceanic crust.

New plate near a spreading ridge is hot and relatively buoyant. As it moves away, conductive cooling penetrates more deeply, and material formerly in the hot asthenosphere becomes part of the colder thermal boundary layer. To first order,where $\alpha$ is the volumetric thermal expansion coefficient, ${\rho }_m$ a reference mantle density, $T_m$ the hot reference temperature, and $t$ plate age. A positive depth-integrated density anomaly makes the column negatively buoyant relative to the ridge column. Isostasy is restored by downward displacement and mantle redistribution; cold rock also occupies less volume.

At a fixed sufficiently deep datum, deepening the seafloor by $\Delta d$ removes an equivalent thickness of rock from the column and replaces it with less dense seawater. In the usual constant-property approximation this giveswhere ${\rho }_w$ is seawater density and $z_c$ lies below the relevant thermal anomaly. This explains why contraction alone and water-loaded seafloor subsidence are not identical numerical quantities.

For a half-space cooling model,so integration yieldsHere $\kappa$ is thermal diffusivity and $z$ is measured downward from the surface of the thermal model. The leading oceanic thermal subsidence is proportional to $\sqrt{t}$. A finite oceanic plate cooling model limits this growth at old ages. Sediment loading, dynamic topography, anomalous crust thickness and hydrothermal heat transport can produce departures from the simple relation.^[ref]

Seafloor spreading creates oceanic crust at a spreading ridge and carries it away with the adjoining plates. With symmetric steady spreading, an age interval maps to distance by the half-spreading rate, not the full relative opening speed. A time sequence of field polarities can therefore become paired marine magnetic stripes.

An assemblage of many intrusive dikes, representing conduits feeding lavas at a spreading ridge.