Tag: (TTG) crust

A new study by Simon J. Lock (University of Bristol), published in JGR: Solid Earth, proposes that Earth’s rapid early spin after the Moon- forming impact, and its subsequent slowdown as the Moon tidally receded, strongly influenced the thickness and deformation of the planet’s first crust, potentially driving early tectonics, volcanism, and the production of more evolved (felsic) rocks.

Around 4.5 billion years ago, a Mars- sized body collided with the proto- Earth. This giant impact formed the Moon and left Earth spinning extremely rapidly, with a day lasting roughly 2- 5 hours (depending on the angular- momentum scenario).

The fast rotation made the planet highly oblate, flattened at the poles and bulging at the equator. In extreme high- angular- momentum cases, the equatorial radius could have been up to twice the polar radius.⁠

The first long- lived crust crystallized from a magma ocean while Earth was still spinning this fast. Centrifugal force partially offset gravity near the equator, so primary crust would have formed thicker there than at the poles (by factors of up to ~10 in some models).⁠

As tidal interactions transferred angular momentum, the Moon migrated outward and Earth’s spin slowed. The planet became progressively more spherical.

This shape change deformed the newly formed crust: extension (stretching) at the poles and convergence (squeezing and compression) around the equator.

In high- spin scenarios the equatorial compression rates could briefly rival those of the modern Himalayas and persist for millions to tens of millions of years.

These motions could have:

• Cracked the dense initial crust, allowing surface water and atmospheric CO₂ to interact with the interior.
• Driven burial of hydrated and weathered material, secondary melting, extensive volcanism, and degassing.
• Produced more buoyant, granitic and felsic rocks early on (helping explain evidence for such rocks within the first ~200 million years).
• Possibly contributed to early mantle heterogeneities via delamination or phase changes as internal pressures rose.⁠

The work highlights a previously under- appreciated driver of early tectonic activity that does not require modern- style plate tectonics or subduction.

It may help explain why Earth developed a dynamic, recycling crust (and ultimately a more habitable surface environment) while Venus did not.

Direct geological tests are difficult because almost none of the Hadean crust survives, but the model is consistent with several existing constraints and opens new avenues for modeling early Earth- Moon dynamics, atmosphere evolution, and core- mantle interactions.

The Phys.org piece by Sam Jarman is a popular summary of this research (also covered by Science and available in preprint form on arXiv).

Lock’s research describes a form of early tectonic activity driven by Earth’s changing shape, not by modern plate tectonics.

It offers a mechanism that could have operated in the first tens of millions of years after the Moon- forming impact, well before (or independently of) the onset of sustained plate tectonics.

Key distinctions and relationships

• Not plate tectonics: Modern plate tectonics involves rigid lithospheric plates moving relative to one another, with divergent boundaries (mid- ocean ridges), convergent boundaries (subduction zones), and transform faults. Lock’s process is a global, latitude- dependent deformation caused by the planet itself becoming less oblate (more spherical) as its spin slowed due to tidal interaction with the receding Moon. There is no requirement for discrete plates, seafloor spreading, or subduction in the modern sense.⁠
• Similar effects, different driver: The shape change produced polar extension (stretching, potentially analogous to rifting or spreading) and equatorial convergence and compression (squeezing, potentially analogous to collisional or compressional tectonics). In high- angular- momentum scenarios, equatorial convergence rates could briefly match or exceed those of the modern Himalayas and persist for tens of millions of years around the entire equator. Even in the canonical (lower angular- momentum) scenario, early rates could exceed those of average or slow modern subduction zones and mid- ocean ridges for several million years.⁠
• Possible precursor or facilitator:
  • Equatorial compression could bury hydrated surface material, enabling secondary melting and production of felsic (granite- like) magmas, something often attributed to subduction today. This provides a way to generate evolved crust early without needing modern- style subduction.
  • Polar extension could promote volcanism and degassing.
  • The resulting deformation, topography, and crustal recycling may have helped create conditions (including early continental nuclei or mantle heterogeneities via delamination) that later favored the development of plate tectonics.
  • Coverage and commentary note that this offers “another way to recycle material without needing subduction” and may help explain why Earth became geologically active and habitable while Venus (lacking a large moon and similar spin- down history) did not.⁠
• Timing context: The intense phase of this rotationally driven tectonics is concentrated in the first ~10- 100 million years after the Moon- forming impact (depending on the angular- momentum and tidal- evolution scenario). Debates about when true plate tectonics began typically focus on much later times (e.g., ~3.5- 3.0 Ga or later for sustained, modern- style plate tectonics, though some evidence suggests earlier episodic or proto- plate activity). Lock’s mechanism therefore operates in the Hadean, potentially as an independent or transitional process alongside other early regimes such as stagnant- lid, heat-pipe, or sagduction tectonics.

In short, the work expands the menu of processes that could have tectonically reworked Earth’s earliest crust. It does not claim to be the origin of plate tectonics, but it shows that significant, high- rate crustal deformation and recycling were possible very early, driven purely by the planet’s spin- down and shape change, helping set the geological stage for whatever tectonic regime followed.

Sagduction (sometimes called “drip tectonics” or vertical tectonics) is not the same as subduction.

Quick comparison

Feature	Subduction (modern plate tectonics)	Sagduction (proposed early- Earth process)
Driver	Horizontal motion of rigid lithospheric plates	Gravity (density inversion)
Motion	One plate slides sideways under another	Dense upper- crustal material sags, sinks vertically downward into weaker, lower-density crust or mantle
Geometry	Inclined Benioff zone, linear plate boundaries	Dome- and- keel, basin- and- dome structures typical of many Archean granite-greenstone terranes
Scale	Global plate system	More localized, crustal-scale
Requirements	Strong, rigid plates; cold, dense oceanic lithosphere	Hot, weak early crust; thick piles of dense volcanic rocks (e.g., komatiites or greenstones) overlying lighter felsic material

What sagduction actually is

In the hotter Hadean- Archean Earth, dense mafic and ultramafic volcanic sequences (greenstone belts) are thought to have become gravitationally unstable and slowly sank (“sagged”) into the underlying, partially molten, lower- density tonalite– trondhjemite– granodiorite (TTG) crust. This produces the classic Archean “dome- and- keel” architecture: rising felsic domes and sinking greenstone keels.

It is a form of vertical tectonics driven purely by buoyancy contrasts, not by the sideways sliding of large rigid plates.

Relevance to Lock’s mechanism

Lock’s rotationally driven process (equatorial compression and polar extension caused by Earth’s slowing spin and changing shape) is yet another independent tectonic driver that would have operated in the earliest Hadean.

It could have coexisted with, or transitioned into, other proposed early regimes such as:

• Stagnant- lid and single- lid convection
• Heat- pipe volcanism
• Sagduction and drip tectonics
• Possibly early, intermittent, or localized subduction

In short: sagduction ≠ subduction.

Sagduction is a gravity- driven vertical process often invoked for the Archean, while Lock’s mechanism is a global, latitude- dependent deformation caused by planetary spin- down. Both are alternatives (or complements) to modern- style plate tectonics on the early Earth.

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Tectonics on Early Earth Driven by Earth’s Changing Shape During Tidal Recession of the Moon

Formation of crust on a rapidly rotating early Earth is a key section in Simon J. Lock’s 2026 paper (JGR: Solid Earth; arXiv:2609.22474), which examines how Earth’s post- Moon- forming- impact spin shaped its first long- lived crust.

After the giant impact that formed the Moon (~4.5 billion years ago), Earth spun very rapidly (day length ~2- 5 hours depending on the angular-momentum model). This made the planet highly oblate (flattened at the poles, bulging at the equator). Internal pressures were substantially lower than today, and effective surface gravity varied strongly with latitude, stronger at the poles and weaker near the equator due to centrifugal force.⁠

The first long- lived crust formed as the post- impact magma ocean solidified. When the surface liquid fraction dropped below ~40%, crystals locked together, viscosity rose dramatically, and residual melt percolated upward and erupted, creating a compositionally distinct crust over a depleted mantle. Most prior models assumed present- day pressures and gravity; Lock shows that if the crust formed while Earth was still rapidly rotating (plausible given magma-ocean solidification timescales of ~10⁴- 10⁶ years, or up to ~10 Myr in some models), the geophysical setting was very different.

Effects on Crust Thickness and Properties

• Lower average gravity and shallower pressure gradients meant a larger volume of the mantle remained partially molten when the surface reached the rheological transition. This increased the total melt available to form crust.
• Melt volume and crustal thickness varied latitudinally. The depth of melting scales roughly as D∼1/g. Weaker gravity near the equator allowed greater melting depth and more melt production there.
• Simple decompression- melting models (analogous to mid-ocean-ridge processes) implycitation_id intermediate_) imply crustal thickness scales approximately as $g^{-1}$.
• Result: Primary (compositional) crust was thicker at the equator than at the poles. In the canonical (lower angular- momentum) model the equator- to-pole variation is modest (~15%). In high- angular- momentum scenarios the contrast can reach a factor of ~10, with equatorial crust potentially hundreds of km thick and polar crust an order of magnitude thinner.⁠

Melt- extraction timescales were longer near the equator (lower buoyancy drive and greater extraction depth), but overall melt extraction is still expected to have been efficient. A large fraction of the early crust may also have been “asthenospheric” (mechanically weak) because of the altered pressure/temperature structure.

This latitudinally variable primary crust set the stage for the subsequent deformation driven by Earth’s slowing spin and changing shape (as the Moon tidally receded). Equatorial compression and polar extension could then recycle hydrated material, promote secondary melting, generate more felsic rocks, and help create early mantle heterogeneities, processes that may have influenced the emergence of continental crust and surface conditions favorable for life, all within the first tens of millions of years after the Moon- forming impact.⁠

The exact magnitude depends on the poorly constrained angular momentum of the early Earth– Moon system and the relative timing of crust solidification versus tidal evolution, but the qualitative effects apply across a wide range of scenarios.

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Few fragments of Earth’s earliest crust have survived. Lock explores a previously neglected factor, rotation, after the Moon- forming impact.

Earth was rapidly rotating (day length ~2- 5 hours), and its spin slowed significantly in the first tens of millions of years as the Moon tidally receded.

Using planetary- structure and tidal- evolution calculations, the study shows:

• The initially highly oblate (flattened) Earth produced significant latitudinal variations in primary crustal thickness (thicker at the equator than at the poles, by up to a factor of ~10 in high- angular- momentum scenarios).
• As Earth’s shape became more spherical, the changing figure drove extensive tectonic activity: extension in polar regions and convergence (compression) in equatorial regions. In high- angular- momentum cases, equatorial convergence rates could briefly rival those forming the modern Himalayas.
• Depending on crustal rheology, these motions could have buried hydrated and weathered surface material, triggered extensive volcanism and degassing, caused phase- change- driven delamination (creating early mantle heterogeneities), and enabled hydrated secondary melting that produced more evolved (felsic) magmas, helping explain the early diversity of rock types within tens of millions of years of Earth’s formation.

Just after formation, Earth spun with a day shorter than ~5 hours and was strongly flattened (in extreme cases the equator was twice as far from the center as the poles). The first crust formed on this spinning, oblate planet and was therefore thicker at the equator.

As the Moon moved outward and Earth’s day lengthened, the planet rounded out, stretching the polar crust and squeezing the equatorial crust. This deformation shaped the early atmosphere and crust and helped create conditions suitable for the emergence of life.

The paper is open access. It expands on the preprint (arXiv:2609.22474) we discussed earlier and provides the formal peer- reviewed version of the research linking Earth’s early rapid spin, its changing shape, crustal thickness variations, and rotationally driven tectonics.

Journal: Journal of Geophysical Research: Solid Earth, Volume 131, Issue 11, e2025JB033676

DOI: 10.1029/2025JB033676)

First published: 02 November 2026

Authors: S. J. Lock

Abstract

Few fragments of Earth’s earliest crust have survived, and so this formative period of our planet’s history is shrouded in mystery. To leverage the limited data available, it is important to understand all the processes that could have shaped our young planet. Here, I explore a previously neglected aspect of early-Earth dynamics: rotation. Earth after the Moon-forming impact was rapidly rotating with its rotation rate slowing significantly during the first tens of millions of years after the impact as the Moon tidally receded. Using planetary structure and tidal evolution calculations, I show that the initially distorted structure of Earth would likely have led to significant latitudinal variations in primary crustal thickness. Further, the subsequent changes in Earth’s shape in response to its slowing rotation rate could have driven extensive tectonic activity and modulated the response of the crust to other forces. There was extension of the crust in polar regions and convergence in the equatorial regions at rates potentially comparable to those forming the Himalayas today in high-angular momentum scenarios. Depending on the rheology of the earliest crust, such motions could have led to burial of hydrated and weathered surface material, extensive volcanism, and degassing of Earth’s interior. Phase changes due to increases in internal pressure could have led to delamination of parts of the crust, producing early mantle heterogeneities. Hydrated secondary melting of the crust and the production of evolved magmas could explain the emergence of a diversity of rock types within tens of millions of years of Earth’s formation.