CPT Q. 015: Is it true that the total length of trenches does not closely match the total length of ridges?
Q. 15. Why is it that the total length of trenches does not approximately match the total length of ridges, as one might expect from plate tectonics theory?
Response: This perception is simply not true. Detailed estimates of plate convergence and divergence is provided in a 2003 paper by Peter Bird, “An updated digital model of plate boundaries,”1. Below is a table that summarizes the results (see ibid. p. 46).
Table 3. Collective Properties of Plate Boundaries by Class
| Class | Total length (km) | Mean velocity (mm/yr) | Area production (m2/s) |
|---|---|---|---|
| Continental Convergent | 23,003 | 26.2 | -0.013616 (-12.6%) |
| Continental Transform | 26,132 | 24.7 | -0.000599 ( -0.5%) |
| Continental Rift | 27,472 | 17.6 | +0.011502 (+10.7%) |
| Oceanic Ridge | 67,338 | 46.6 | +0.095348 (+88.4%) |
| Oceanic Transform | 47,783 | 40.5 | +0.001022 ( +1.0%) |
| Oceanic Convergent | 17,449 | 17.6 | -0.007141 ( -6.7%) |
| Subduction Zone | 51,310 | 62.3 | -0.086516 ( -80.1%) |
| Totals | 260,487 | 39.6 | 0 |
The total length of convergent boundaries is 91,762 km, while the total length of divergent boundaries is 94,810. If we neglect the continental convergent and rift boundaries and consider only convergent and divergent boundaries in the ocean basins, the total length of subduction zones and other convergent segments is 68,759 km, while the total length of ocean ridges is 67,338 km, again very similar.
The current rate of area increase along the oceanic ridges is 0.095 m2/s, which is very close to the current rate of area loss along convergent boundaries in the oceans, 0.094 m2/s. While there is no logical or geometrical requirement for the total lengths of convergent and divergent boundaries to be identical, they are amazingly similar.
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Ibid., Geochem., Geophys, Geosys., 4(3). ↩︎