Tectonic Plates: The Physics of a Restless Earth

A planet with a soft inside

Earth looks solid from the outside, but its interior is layered like an onion, and one of those layers is soft enough to flow. The crust and the rigid uppermost mantle together form the lithosphere — a shell roughly 70-100 km thick, broken into about seventeen major pieces we call tectonic plates. Below it sits the asthenosphere, a partially molten zone in the upper mantle that is mechanically weak enough for the rigid plates above to slide on it. It is on this narrow, sluggish layer that the entire outer shell of the planet quietly drifts.

Deeper still, the mantle behaves like an almost unimaginably viscous fluid — roughly 10²¹ Pa·s, about a trillion trillion times more viscous than water — yet given millions of years, it convects. Hot rock near the core boundary is buoyant and rises; cooler rock near the base of the lithosphere sinks. That circulation is the engine, however indirect, behind every earthquake, mountain range and ocean basin on the map.

When does convection start? The Rayleigh number

Whether a fluid layer convects or just sits there conducting heat quietly is governed by a single dimensionless quantity, the Rayleigh number:

Ra = (ρ · g · α · ΔT · d³) / (η · κ)

ρ  density          α  thermal expansion coefficient
g  gravity           ΔT temperature difference across the layer
d  layer thickness    η  viscosity
κ  thermal diffusivity

Convection begins once Ra exceeds roughly 1,000.
For Earth's mantle, Ra ≈ 10⁷ — vigorously convecting.

The governing physics underneath that number is the Stokes equations coupled to heat transport: at mantle scales inertia is negligible, so pressure gradients and viscous forces balance gravity exactly, while temperature is carried along by the flow and slowly smoothed by conduction. Solving that system in full 3D is a job for supercomputers — which is why every real-time simulation, including the one on this page, works with a drastically simplified stand-in.

Three ways plates meet

Wherever two plates share a border, the relative motion between them falls into one of three categories, each with a distinctive geological signature.

Divergent boundaries pull apart. Magma wells up to fill the gap and solidifies into new oceanic crust — seafloor spreading. The Mid-Atlantic Ridge, 16,000 km of underwater mountain chain, is the result. Convergent boundaries collide: the denser oceanic plate is forced beneath the lighter continental one in subduction, producing deep earthquakes and explosive arc volcanism — the Pacific "Ring of Fire" traces a chain of these zones. Where two continental plates meet, neither wants to subduct, so the crust crumples upward instead — that collision built the Himalayas when India rammed into Asia around 40-50 million years ago. Transform boundaries slide past one another horizontally, locking and slipping in fits that release as earthquakes — California's San Andreas Fault is the textbook case, with the Pacific Plate creeping northwest relative to North America at about 5 cm a year.

What actually does the pulling

It is tempting to picture convection currents dragging the plates around like conveyor belts, but the dominant driver is something blunter. Slab pull — old, cold, dense oceanic lithosphere sinking under its own weight at a subduction zone — accounts for roughly 90% of the net driving force, hauling the rest of the plate behind it like a tablecloth being pulled off a table. Ridge push, the gravitational slide of elevated new crust away from a mid-ocean ridge, contributes a smaller share. Mantle drag beneath the plate can help or resist depending on the geometry of the convection cell underneath, and localized basal suction from rising mantle plumes can tear plates apart and start new rifts. None of this happens quickly — typical plate speeds are 1-10 cm per year, roughly the rate a fingernail grows, though GPS can now measure it directly: the Atlantic widens about 2.5 cm every year.

How the browser simulation fakes it convincingly

Nobody is solving the Stokes equations in a browser tab at planetary resolution, so Tectonic Plates uses three coupled shortcuts that reproduce the qualitative behaviour cheaply. First, the globe's surface is partitioned into a Voronoi diagram — each plate is the region of mesh closest to a moving seed point. Second, a coarse 2D grid underneath stores a simplified temperature and velocity field: each frame it advects temperature along the flow, diffuses it with a small Laplacian stencil, and derives a buoyancy velocity from the resulting temperature gradient. That velocity field is what nudges the Voronoi seeds around, which is what makes the plates drift.

function updateBoundaryMesh(plateA, plateB, velocity) {
  const relSpeed = velocity.dot(boundary.normal);  // + convergent, - divergent
  if (relSpeed > 0) {
    // convergent: raise terrain along the boundary, then smooth
    boundary.vertices.forEach(v => v.y = Math.min(v.y + relSpeed * dt * MOUNTAIN_SCALE, MAX_HEIGHT));
    smoothTerrain(plateA, 3); smoothTerrain(plateB, 3);
  } else {
    // divergent: insert new vertices as young, hot crust
    insertVertices(plateA, interpolateBoundary(boundary, -relSpeed * dt), HOT_CRUST_COLOR);
  }
}

The third layer handles the boundaries themselves: at every shared edge, the relative velocity between neighbouring plates decides whether the region opens, collides or shears. Divergent edges insert new triangles coloured as fresh, hot crust; convergent edges buckle the surface upward proportional to the closing speed, then apply Gaussian smoothing so mountains rise gradually instead of spiking; transform edges accumulate sliding stress as an "earthquake energy" value that releases as a particle burst once a threshold is crossed. Roughly every sixty frames the whole mesh is re-triangulated with constrained Delaunay triangulation to keep boundary geometry clean — but only the vertices near an active edge need it, since the bulk of each plate's interior stays untouched. None of this is geologically rigorous, but it captures the right causal chain: convection drives motion, motion at boundaries builds or destroys crust, and crust shape feeds back into what you see on screen.

Frequently asked questions

Try it live

Everything above runs in your browser — open Tectonic Plates and adjust mantle viscosity, core heat flux and plate count to watch a supercontinent cycle play out in fast-forward.