CPT Q015 – Is it true that the total length of trenches does not closely match the total length of ridges?

15. Question. 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 20 paper by Peter Bird, “An updated digital model of plate boundaries,” Geochem., Geophys, Geosys., 4(3) here. Below is a table that summarizes the results [ed.: 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 oc 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 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 leng of convergent and divergent boundaries to be identical, they are amazingly similar.

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