Slab Rollback & Back-Arc Basin Opening

Subducting Plate

Plate age at trench 80 Myr

Convergence rate 6.0 cm/yr

Overriding Plate & Mantle

Overriding plate motion -1.0 cm/yr

Asthenosphere viscosity 5.0 × 10²⁰ Pa·s

Regime: —

Live Readouts

Slab pull force

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Trench rollback

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Back-arc rate

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Slab dip angle

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Basin width

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Elapsed time

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How it works

An oceanic plate cools and thickens as it ages, becoming denser than the mantle beneath it. Once it sinks into the mantle at a subduction zone, that negative buoyancy pulls the whole plate downward — slab pull — and the reaction to that pull is what actually makes the trench migrate.

Thermal boundary-layer thickness:  h(t) = 2.32·√(κt)      κ ≈ 1×10⁻⁶ m²/s
Density excess:                     Δρ ≈ ρ_m·α·ΔT           (~120 kg/m³)
Slab pull per unit trench length:   F_SP = Δρ·g·h(t)·L_slab
Rollback velocity (drag-limited):   v_roll = F_SP / (η_mantle · k_drag)
Back-arc spreading rate:            v_BA = v_roll − v_overriding

Older, colder slabs pull harder. h(t) grows with √age, so a 150 Myr plate exerts far more slab pull than a young 20 Myr one — this is why old Pacific-type crust subducts more steeply and retreats faster than young crust.
Trench rollback is the trench itself migrating oceanward as the slab sinks and rolls back through the mantle, resisted by the mantle's viscosity (the drag term η·k). Low viscosity or high slab pull → fast rollback.
Back-arc basin opening happens when rollback outruns the overriding plate: v_BA > 0 stretches the crust behind the volcanic arc and opens a new, young basin floored by fresh spreading-center crust (the Mariana Trough, Lau Basin). When the overriding plate advances faster than rollback, v_BA < 0 and the margin compresses instead, building mountains and flattening the slab (the Andean margin).
The volcanic arc sits above the point where the slab reaches roughly 100–150 km depth — the pressure–temperature window where dehydration of the subducting crust drives flux melting in the mantle wedge above it.

All four sliders feed directly into the formulas above — the readouts on the left are the actual live values driving the geometry you see, not decorative numbers.

Subducting oceanic slab Overriding plate New back-arc crust Volcanic arc

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