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veillette merged 1 commit into from
Aug 5, 2026
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Carry plate boundaries on their own motion, closing the reconstruction #5

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24 changes: 23 additions & 1 deletion CLAUDE.md
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Expand Up @@ -24,10 +24,11 @@ Forked from [SceneryStackTemplate](https://github.com/OpenPhysics/SceneryStackTe
| `src/common/MapProjection.ts` | Equirectangular lon/lat ↔ view |
| `src/common/GlobeProjection.ts` | Orthographic lon/lat ↔ view, plus the globe's camera |
| `src/common/attachGlobeRotation.ts` | Drag and arrow keys → the globe's camera |
| `src/common/PlateReconstruction.ts` | Euler-pole rotation and plate velocities |
| `src/common/PlateReconstruction.ts` | Euler-pole rotation, plate velocities, `MOTION_FRAMES` |
| `src/common/data/dataTypes.ts` | Shapes of every dataset (hand-written) |
| `src/common/data/hotspots.ts` | Hand-maintained hotspot list |
| `src/common/data/generated/` | **Generated — do not edit.** `npm run build-data` owns it |
| `src/common/data/generated/motionFrameData.ts` | Rotations belonging to boundaries rather than plates |
| `src/plate-tectonics/model/PlateTectonicsModel.ts` | All AXON state |
| `src/plate-tectonics/model/EarthquakeDepthFilter.ts` | Depth bands and the filter predicate |
| `src/plate-tectonics/view/PlateTectonicsScreenView.ts` | Layout, view switching, `pdomOrder` |
Expand Down Expand Up @@ -56,6 +57,26 @@ integrity and a few facts about the Earth (deep earthquakes cluster around the
Pacific; the Chile profile's deep events sit inland of its shallow ones). Run
`npm test` after any regeneration.

`npm run build-data` with no arguments rebuilds everything. Naming steps —
`plate-model`, `land`, `earthquakes`, `volcanoes`, `relief`, `cross-sections` —
rebuilds only those, which is how the PB2002 model can be regenerated without also
pulling a newer earthquake catalogue and a fresh DEM into an unrelated diff.
`plate-model` covers the plates, their boundaries and the motion frames together,
because those three index into each other.

### What moves when the clock runs

Only `timeMillionsOfYearsProperty` evolves, and `PlateReconstruction` turns it into a
rotation per **motion frame** — `MOTION_FRAMES` is the plates first (so a plate index
is a frame index) then the rotations derived for the boundaries. Anything inside a
plate rides that plate; a boundary rides the mean of its two plates, or the overriding
plate at a trench; a plate *outline* rides a positional blend of the boundaries near
it, which is what keeps neighbouring plates edge to edge instead of overlapping and
gapping. The rules and their justification are in
[`doc/model.md`](doc/model.md#what-carries-what) — read it before changing what any
feature rides, and note that a frame index is **not** interchangeable with a plate
index outside the first `PLATES.length` entries.

### Rendering

The map and the cross-sections are `CanvasNode`s, not trees of `Path`s, because every
Expand Down Expand Up @@ -150,6 +171,7 @@ A11y strings live under `a11y.plateTectonics` in each locale JSON, exposed via
| Path | Purpose |
|---|---|
| `tests/PlateReconstruction.test.ts` | Euler-pole rotation; plate speeds against published values |
| `tests/PlateEvolution.test.ts` | The mosaic staying closed; what each boundary rides; plate areas |
| `tests/PlateTectonicsModel.test.ts` | Layer state, depth bands, time clock, reset |
| `tests/CrossSectionGeometry.test.ts` | Two-band layout, crust switching, slab fitting, ridge cooling |
| `tests/MapProjection.test.ts` | Projection round trips, 2:1 viewport, motion-arrow bearings |
Expand Down
63 changes: 58 additions & 5 deletions doc/model.md
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Expand Up @@ -11,8 +11,8 @@ The only state that evolves is a single number, `timeMillionsOfYearsProperty`: h
far the reconstruction has been run from the present day, negative into the past.

Each plate carries an **Euler pole** — an axis through the centre of the Earth — and
a rotation rate about it. Moving a plate by `t` million years is one rigid rotation
of every point on it:
a rotation rate about it. Moving a point on a plate by `t` million years is one
rotation:

```
θ = rate (°/Myr) × t (Myr) about the plate's pole
Expand Down Expand Up @@ -61,6 +61,47 @@ Nazca ≈ 78 mm/yr ENE, Australia ≈ 67 mm/yr NNE, North America ≈ 16 mm/yr W
`tests/PlateReconstruction.test.ts` checks them, so a sign error anywhere in that
chain fails the test suite rather than quietly drawing the Atlantic closing.

Two independent checks confirm the derivation. PB2002 also publishes a **relative**
velocity across every boundary step, a number the poles here were not built from;
recomputing it as `|ω₁ × r − ω₂ × r|` matches the published value to a median of
0.04 mm/yr over all 1 580 segments (`tests/geophysicalData.test.ts`). And integrating
`r × (ω × r)` over the whole globe — the condition that defines the no-net-rotation
frame — leaves a residual equivalent to 0.009 °/Myr, about 1 mm/yr at the equator, so
the frame really is the one it claims to be.

## What carries what

A plate's interior moves with the plate. A plate **boundary** cannot: it belongs to
two plates at once, and carrying it with either one drives it into the other. That is
where the gaps and overlaps in a naive reconstruction come from — run the clock to the
end of the slider and the two sides of a typical boundary end up some 1 600 km apart,
tearing the map open along the ridges and piling it up at the trenches.

So boundaries are given rotations of their own:

| Boundary | What it rides | Why |
|---|---|---|
| Spreading ridge | mean of the two plates' rotation vectors | where the axis sits when accretion is symmetric |
| Transform fault | mean of the two | stationary with respect to a fault the plates merely slide along |
| Subduction zone | the **overriding** plate | a trench is a feature of the plate that stays; the other is being consumed |

PB2002 names each boundary section with a separator that doubles as a cross-section
through it — `-` where neither plate descends, `\` where the left-hand plate descends
beneath the right, `/` where the right-hand one does — so `NZ\SA` is Nazca going down
under South America and `TO/PA` is the Pacific going down under Tonga. That is where
the overriding plate is read from.

Plate outlines are then carried by the boundary network rather than by the plate
inside them, each vertex taking a distance-weighted blend of the boundary motions near
it. Because the blend depends on *position alone*, two plates that share an edge carry
it identically and the mosaic stays a mosaic. What changes through time is each
plate's **area**: it grows along its spreading ridges and shrinks at its trenches,
which is sea floor being made and unmade, and is the thing worth watching.

The outlines are subdivided until this stops showing: an edge whose ends ride motions
far enough apart to stretch it by more than 200 km over the slider's range is split
and reconsidered. `tests/PlateEvolution.test.ts` holds the whole scheme in place.

## Earthquake depth bands

Hypocentres are grouped at **70 km** and **300 km**, the conventional shallow /
Expand Down Expand Up @@ -119,14 +160,26 @@ distorting the picture silently.

## What this model is not

- **Plates are rigid.** Real plates deform, especially at their edges. Run the clock
and rigid plates overlap and leave gaps; that is the deformation the model does not
do, not a drawing error.
- **Plate interiors are rigid.** Only the boundaries deform, and only in the sense
above — a plate changes area but never changes shape internally. The deforming belts
along real plate edges, which is where the Andes, the Himalaya and the Basin and
Range are, are drawn as though they were not deforming at all.
- **Velocities are today's velocities.** Extrapolating them is reasonable over a few
million years, a sketch at ±50 Myr (the ends of the slider), and wrong beyond that:
ridges and subduction zones are born and die, and plates that existed 50 Myr ago —
the Farallon plate, for one — are missing entirely because the model has no record of
them. The range is capped at ±50 Myr for that reason.
- **The microplates are the first thing to stop meaning anything.** PB2002 resolves
plates a couple of degrees across whose poles sit almost on top of them, so they
spin: ten of the fifty-two turn through more than half a revolution over 50 Myr, and
Manus through seven full turns. Nothing like that happened — such plates are
transient features that do not survive tens of millions of years — and because a
boundary is shared, a spinning microplate drags its larger neighbour's edge with it.
That is why the south-west Pacific and the Galápagos region look scribbled at the
ends of the slider while Africa, the Americas, Eurasia, Australia, Antarctica and
the Pacific stay clean. The sixteen labelled plates hold their area to within a
factor of four; the microplates do not, and no rule about how boundaries move can
rescue an Euler pole extrapolated that far.
- **Earthquakes and volcanoes are present-day observations.** They ride their plate
when the clock runs, so the picture stays coherent, but a 1994 earthquake did not
happen 20 Myr ago somewhere else.
Expand Down
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