Question

FORMATION OF FIRST CRUST: WHAT HAPPEND DURING THE EVENT WHICH TECTONIC PLATES WERE INVOLVED HOW IT CHANGED EARTHS SURFACE

Answer 1

Short answer
- The very first crust formed as the early molten Earth cooled and its magma ocean began to solidify (about 4.5–4.4 billion years ago). That initial crust was dense, mafic to ultramafic rock (not the silica-rich continental crust we live on).
- True continental-style crust (granitic, buoyant) grew later in the Hadean–Archean (roughly 4.4–3.0+ Ga) by repeated melting and reworking of early basaltic crust. Whether modern-style plate tectonics operated when the first continental crust formed is still debated; many models invoke proto-plates, local subduction-like behavior, plume-driven melting, or episodic “stagnant-lid” overturns rather than continuous global plate tectonics.
- The formation and stabilization of that first long-lived crust changed Earth’s surface by creating solid landmasses, concentrating incompatible elements, enabling weathering and a hydrosphere/atmosphere interaction, and setting the stage for later plate motions and life.

What happened, in more detail
1) Magma-ocean solidification and formation of a primary crust (≈4.5–4.4 Ga)
- After accretion and the giant Moon-forming impact, Earth was largely molten. As the planet radiated heat, the surface cooled and a global or regional “magma ocean” began to crystallize.
- Early crystallization produced dense mafic and ultramafic rocks (peridotitic/komatiitic compositions). These early crustal rocks were thin, patchy, and easily recycled by impacts and mantle convection.
- Evidence: detrital zircons from Jack Hills (Western Australia) dated to ~4.4 Ga show signatures consistent with reworked crust and liquid water at the surface, implying early crust formation and cooling.

2) Growth of more evolved (granitic) continental crust (Hadean → Archean, ≈4.4–3.0+ Ga)
- Continued heat loss, hydrothermal alteration, and recycling produced partial melts of basaltic crust that are richer in silica (tonalite–trondhjemite–granodiorite, or TTG) — the building blocks of the first continents.
- Mechanisms proposed for producing TTG and continental crust include:
- Subduction-like processes (early plate convergence and sinking of oceanic lithosphere), producing hydrated basalt melting.
- Delamination or “drip” of dense lower crust into the mantle, leading to partial melting of that material.
- Mantle plume–driven melting and reworking at plume–lithosphere boundaries.
- Episodic overturns of a stagnant lithospheric lid that created pulses of melting and crustal differentiation.
- Archean rock records (e.g., the Acasta Gneiss ~4.03 Ga, Isua supracrustal rocks ~3.8 Ga) document significant crustal differentiation and long-lived crustal blocks by ~4.0–3.8 Ga.

Which “plates” were involved?
- There were no continents and rigid, long-lived plates like those of today immediately after formation. Early Earth probably had:
- A transient, patchy primary crust that was frequently destroyed by impacts and convection.
- Later, “proto-plates” or blocks of lithosphere that could behave independently and sometimes converge, underthrust, or be internally reworked.
- The timing of initiation of modern-style plate tectonics is controversial. Estimates range from very early (Hadean/early Archean, >3.5–4.0 Ga) to much later (Proterozoic, <2.5 Ga). Geological evidence (ophiolites, paired metamorphic belts, high-pressure metamorphism) used to infer subduction is clearer after ~3.0 Ga but ambiguous earlier.
- So: the first crust formed largely before any globally established plate-tectonic system; later crustal growth likely involved a mix of local proto-plate motions, plume activity, and lithospheric recycling.

How the Earth’s surface changed
- Stabilization of solid crust: Euras-like rigid lithospheric blocks began to persist, allowing relief (continents, proto-continents) to exist for millions to billions of years rather than being continually molten or recycled.
- Emergence of shallow seas and weathering environments: More stable crust allowed water to pool in basins, enabling chemical weathering, sedimentary rock formation, and the cycling of elements between crust, mantle, oceans, and atmosphere.
- Chemical differentiation: Elements concentrated into continental crust (Si, Al, K, incompatible elements), while mantle retained Mg, Fe. This partitioning affected ocean and atmosphere chemistry and nutrient availability.
- Surface topography and crustal recycling: As crust thickened and some parts became buoyant, relief and continental shelves formed. Where proto-plates or subduction occurred, orogens and volcanic arcs began to form, reshaping the surface.
- Habitability and niches for life: The cooler, stable crust plus liquid water and hydrothermal systems created environments conducive to the emergence and persistence of early life.

Evidence and uncertainties
- Key evidence: ancient zircons, the oldest gneisses (Acasta), Isua supracrustal belt, geochemical signatures of TTG rocks, and metamorphic terrains showing early crustal reworking.
- Uncertainties: exact timing of the first long-lived continental crust, the dominant processes that formed it, and the timing/mechanism for the onset of continuous plate tectonics. Multiple processes probably acted in combination and varied through time and place.

Short concluding summary
The “first crust” originated as the Earth cooled from its magma-ocean state, producing an initial thin mafic crust and later more silica-rich continental crust through partial melting and reworking. Those processes happened before a clear, global plate-tectonic system existed; early crustal growth likely involved proto-plates, plumes, and episodic recycling. The result was a progressively stabilized surface, chemically distinct continents, new topography and environments, and the conditions necessary for long-term surface evolution and life.