Past exam of the plate tectonics course of the University of Oxford / 2022 / 1

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.

Distinguish the depth-dependent mantle temperature from mantle potential temperature $T_p$: the latter is the temperature a parcel would have after reversible decompression in an adiabatic process to a chosen reference pressure. A convecting mantle is approximately adiabatic through much of its interior, with steep thermal boundary layers near the surface and the core-mantle boundary. It is neither isothermal nor wholly molten.

Early accretion, large impacts and gravitational planetary differentiation supplied heat; metal segregation released gravitational energy and extensive melting could produce a magma ocean. Rapid surface heat loss then crystallized much of the silicate mantle. Subsequent evolution reflects competition between radiogenic heating, heat entering from the core and heat carried out through the upper boundary. In a schematic mantle energy balance,Here $C_m$ is mantle heat capacity, $H_m$ mantle radioactive power and $Q_m$ heat leaving the mantle; continental crustal radioactive production is not included in $H_m$. The convective Urey ratio $H_m/Q_m$ compares internal production with mantle heat loss. A value below one does not alone prove cooling, because core heat also enters; cooling requires $H_m+Q_{\mathrm{cmb}}<Q_m$. Radioactive decay reduces the heating supply, while mantle convection, plate tectonics, melt extraction and changing viscosity control the heat loss. A hotter mantle does not automatically imply proportionally faster plate motion: melting, dehydration and boundary strength can alter that relationship.

The robust long-term picture is an energetic early Earth followed by substantial cooling and a presently mostly solid, convecting mantle. The shape of the solid-state history is less certain. Petrological reconstructions commonly place late-Archean ambient $T_p$ above modern values. One reconstruction infers about $1500$–$1600{}^{\circ }\mathrm{C}$ at $2.5$–$3$ Ga versus about $1350{}^{\circ }\mathrm{C}$ today, and permits warming before that late-Archean maximum after the initial magma-ocean cooling.^[ref] This is a model-dependent ambient-mantle history, not proof that the accreting Earth was cooler than the Archean mantle or that every mantle reservoir followed one curve. An early molten transient, subsequent solid-state heating or cooling, and later secular cooling must be distinguished.

The geological record is a collection of thermometers with different sampling biases, not a direct time series of the whole mantle.

Primary melt compositions constrain melting temperatures. Reconstruct primary magma compositions from dated, suitably unaltered basalts by correcting for fractional crystallization and checking equilibrium with olivine. Experimental phase equilibrium constraints and mineral–melt thermometers then constrain the pressure and temperature of melt generation. In laboratory calibrations, a candidate melt and mineral assemblage are equilibrated at controlled pressure and temperature, quenched, and their crystals and glass analyzed. Repetition across adjacent conditions brackets the relevant phase equilibrium boundaries. Correcting for decompression and latent heat then allows comparison as mantle potential temperature. Age-controlled, non-arc basalt suites are especially valuable for separating changes in ambient mantle from exceptional mantle plumes. The 2010 reconstruction by Claude Herzberg and colleagues combines this approach with the temporal distribution of high-Mg parental melts.^[ref]

Komatiites provide evidence for unusually hot melting, with qualifications. Many ancient high-Mg lavas, their olivine compositions and spinifex textures indicate large degrees of melting and high parental temperatures. Their abundance in Archean terrains and rarity in the recent record are consistent with a hotter past. However, a hot mantle plume, dissolved water, unusual source composition or crystal accumulation can change the interpretation. A komatiite temperature is not automatically the ambient mantle temperature, and spinifex texture alone is not a calibrated thermometer.

Melting residues and crustal products provide independent checks. Depleted peridotite mantle xenoliths, preserved mineral chemistry and compatible-element depletion constrain former melt fractions. More extensive melting can produce thicker mafic crust and more refractory residual mantle; these are predictions to compare with preserved rock packages and seismic or petrological constraints on ancient lithosphere. Metamorphic facies and pressure–temperature paths provide contextual evidence about geothermal regimes, but tectonic burial, decompression and preservation can mimic temporal changes. They do not directly measure $T_p$.

The most persuasive inference is therefore agreement between dated melt thermometry, melting residues and their predicted crustal consequences, while separating source fertility, volatile content, mantle plume anomalies, alteration and selective preservation. Those controls explain why hotter ancient melting is better established than one exact global cooling curve.