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Authors: Ming Tang, Kang Chen, Roberta L. Rudnick.

Science 351, 372–375. Uses Ni/Co and Cr/Zn ratios to infer a change in upper continental crust composition during the Archean; the inferred timing of global plate tectonics is an interpretation.

Bradford J. Foley coauthored models coupling planetary cooling, melt production and the carbon cycle on stagnant-lid planets. The model balances volcanic carbon dioxide supply against weathering and tests conditions compatible with a temperate surface.^[ref]

The result makes plate tectonics a physical variable to test in a habitability model, rather than an automatic prerequisite. Initial heat and carbon inventories remain important assumptions.

A long-term carbon cycle coupling silicate weathering, carbonate formation and interior recycling with volcanic carbon dioxide supply. Its temperature- and water-dependent weathering can provide a climate feedback. Active plate tectonics can support recycling, but the effectiveness of the feedback also depends on rock exposure, water and reaction rates.

Models coupled thermal evolution and carbon cycling on stagnant-lid planets, demonstrating conditional prospects for long-lived habitability without plate tectonics. Astrobiology 18, 873–896. Author preprint: arxiv.org/abs/1712.03614

Change in relative continental positions through geological time, as explained within plate tectonics. Ancient connections must be reconstructed rather than read from modern maps.

Official United States Geological Survey account of the oceanographic, magnetic and seismic evidence behind plate tectonics. The page is a chapter of This Dynamic Earth, not a newly discovered examination solution.

Earth is a rocky planet whose surface couples a gaseous atmosphere, liquid ocean, solid crust and regions of ice. Its mean radius is about $6371\mathrm{km}$ and its mean mass density about $5.51\times 10^3\mathrm{kg}{\mathrm{m}}^{-3}$. These bulk quantities constrain, but do not uniquely specify, its differentiated interior.^[ref]

Seismic waves probe material beneath the surface, plate tectonics describes the moving outer shell, and the geodynamo explains the origin of the large-scale planetary magnetic field. At the surface, radiative transfer and ocean circulation redistribute solar energy. Earth therefore provides a nearby setting in which measurements can test coupled planetary processes that are much harder to separate in an unresolved exoplanet.

A transient release of stored mechanical energy in the solid planet, often associated with fault motion. Plate tectonics is a major terrestrial driver, but earthquakes can also occur without an active global plate system.

Geology reconstructs the composition, structure and history of the solid Earth and other rocky bodies. Its observations include rock textures, fossils, layer relationships, deformation and numerical ages. A cross-cutting intrusion, for example, must be younger than the rock it cuts, even before either has a numerical date.

Physical models connect those relationships to processes such as plate tectonics, sedimentation, metamorphism and erosion. Distinguishing a directly observed structure from the process proposed to explain it is essential: similar present-day rocks can have different formation and alteration histories.

Reusable sources for geology, planetary environments and plate tectonics.

Geophysics applies physics and mathematics to the structure, composition, and evolution of planets, including Earth. It connects measurements with mechanisms: plate tectonics explains moving crust, the geodynamo explains a planetary magnetic field, and albedo connects reflected sunlight to a planetary surface energy balance. Even a small model can reveal a useful organizing principle, such as the competition between ice-ocean albedo feedback and stabilizing thermal radiation.

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]

John Tuzo Wilson introduced transform faults as a distinct type of boundary needed to connect seafloor spreading and other relative plate motions. Where two spreading-ridge segments are offset, active sliding occurs on the connecting part of the fracture; its continuation beyond a ridge can separate crust moving together and need not remain an active plate boundary. The geometry predicts a slip direction that cannot be inferred merely by treating the offset ridge as a formerly continuous line. His 1965 proposal made a structural pattern into a test of plate tectonics.^[ref]

A geophysicist associated with transform faults and the Wilson cycle.

The mechanically strong outer shell of a rocky planet, including crust and the colder upper mantle. It can form mobile plates in plate tectonics or a relatively stagnant lid. Its temperature, water content and damage affect its ability to yield.

Ming Tang coauthored work using the chemical record of ancient rocks to investigate the evolution of continental crust. The study infers changing bulk composition from elemental ratios that remain informative after weathering and alteration.^[ref]

This turns preserved chemical evidence into a test of early crustal growth. It also requires distinguishing a change in inferred composition from a uniquely established global onset date for plate tectonics.

A regime in which surface lithosphere participates substantially in overturn and horizontal motion. Additional organization into long-lived plates and narrow boundaries is needed for a close analogue of plate tectonics.

The circumstellar habitable zone is the range of orbital distances where a terrestrial planet with a specified atmospheric inventory can retain liquid water at its surface. It is a conditional climate criterion, not a guarantee of life or a requirement for every possible subsurface habitat.

Four influential factors are:

For a Sun-like present-day star and an Earth-like planet, a useful conservative range is approximately $0.95$–$1.7\mathrm{AU}$. The inner limit depends on the adopted moist or runaway greenhouse condition, and the outer limit on the maximum greenhouse outer habitable-zone limit. More restrictive water-loss choices put the inner edge near $0.99\mathrm{AU}$. Empirical optimistic limits based on past Venus and Mars are about $0.75$–$1.8\mathrm{AU}$. These are model conventions rather than exact universal boundaries.

Plate tectonics is the movement and recycling of a planet's lithosphere as discrete plates over a deformable mantle. Mantle convection, the negative buoyancy of cool subducting slabs (slab pull), and gravitational sliding from elevated spreading ridges (ridge push) supply driving stresses; deformation and friction resist motion. The mantle mostly deforms by slow solid-state creep, rather than being a global liquid layer.

Its effects include subduction and recycling of crust, creation of new crust, mountain building, earthquakes and volcanism, transport of internal heat, and recycling of water and carbon. The carbonate-silicate cycle can couple volcanic CO2 supply to weathering and long-term climate.

Three major controls on a super-Earth's tectonic mode are:

A larger mass alone does not establish active plate tectonics. The competition between driving stress, yielding and sustained slab buoyancy must be evaluated for the planet's composition and thermal history.

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.