Separate half spreading rates from plate separation

1. Define the evidence and frame

A dated crustal marker records displacement accumulated since formation, not necessarily the instantaneous present rate. On each side of an ideal ridge, distance from the ridge divided by marker age gives a mean ridge-frame half spreading rate. Comparing markers of the same age allows the two half rates to be added.

2. Connect mechanism and model

The original packet fixes marker age at 4 Ma and allows left and right distances to differ. Units matter: 1 km divided by 1 Myr equals 1 mm/yr. Thus 80 km over 4 Myr is 20 mm/yr, not 20 km/yr. Full separation concerns the increase in distance between the two plates; it is not always twice an arbitrarily chosen flank rate.

As newly formed oceanic lithosphere moves away from a ridge it cools, thickens and generally becomes denser. Its thermal evolution helps explain ridge elevation and eventual subduction of sufficiently dense lithosphere. Oceanic crust is one compositional part of the lithosphere; the lithosphere also includes mechanically strong uppermost mantle.

Constant spreading, undeformed markers and a stable ridge geometry are explicit simplifications. Ridge jumps, asymmetric accretion or changing velocities can invalidate a naive distance–age fit. A present geodetic rate and a million-year magnetic average can disagree without either measurement being an arithmetic error.

3. Reconstruct the worked packet

Left 80 km and right 120 km at 4 Ma give half rates 20 and 30 mm/yr. The marker separation is 200 km and full mean separation rate is 50 mm/yr. Doubling the left rate would incorrectly yield 40.

4. Predict and investigate

Predict first: When is doubling one half spreading rate justified?

Compare symmetric and asymmetric distance pairs. Convert every rate to both km/Myr and mm/yr, then state the assumed ridge frame and averaging interval. Keep half rates distinct from the full separation.

5. Test a changed case

Apply the evidence to a changed case

Both 4 Ma markers lie 120 km from the ridge. Find both half rates and full rate.

Retain the evidence frame, calculations and assumptions before opening public feedback.

Compare a worked response

Each half rate is 30 mm/yr; full separation is 60 mm/yr. Doubling is valid here only because the two supplied distances and ages are equal.

6. Retain a defensible evidence notebook

Preserve first predictions, full units, geometry, source provenance, assumptions and rejected alternatives. Synthetic models are not field measurements. The explicitly labeled USGS packet is actual catalog evidence, but working through a public example is not independent practical competence or an operational hazard assessment.

Mechanism and data reference, checked 30 October 2026. Original teaching prose, diagrams and exercises; actual USGS rows retain their agency and provenance.