River Incision on Hotspot Volcanic Islands

Hotspot volcanic islands present exceptional natural laboratories for landscape evolution because they lack significant tectonic activity and exhibit relatively uniform lithology. Furthermore, in the tropics, volcanic islands experience significant climate and precipitation gradients. Erosion on volcanic islands is characterized by landslides, large-scale collapses, and landscape dissection by river incision. Here, we focus on the Reunion (Reunion and Mauritius Islands) and Hawaii (Kaua'i Island) hotspots to assess the evolution of erosion through time by using paleotopography reconstructions, published bedrock age data, and numerical modelling of fluvial incision.

Long-term basin-averaged erosion rates of Reunion Island range from 9.9 ± 0.5 mm/yr to 5.2 x 10-3 ± 2.3 x 10-4 mm/yr and show no correlation with mean annual precipitation rates ([1]; this study). Erosion rates of western Mauritius Island range from 6.5 x 10-2 ± 7.8 x 10-4 to 5.1 x 10-3 ± 3.5 x 10-4 mm/yr. Comparing Reunion, Mauritius, and Kaua'i [2] shows that erosion rates decrease with lava age from 63 kyr to ~1000 kyr whereas they do not vary significantly with lava age from ~1000 kyr to 4300 kyr. This might be related to the adjustment time of channel slopes in response to the initial volcanic activity. It implies a potential bias in studies that assess the role of climate on river incision by comparing erosion rates to precipitation rates across islands of different ages. We suggest one should only compare basins of the same age.

In order to calibrate erosion parameters for all three islands, we apply a Bayesian approach to a stream power model and obtain the erodibility K, the drainage area exponent m, and the slope exponent n. We also calculate a normalized erodibility using m/n = 0.45 and n = 1. Preliminary results yield normalized K values ranging from 1 x 10-3 to 1 x 10-6 m1-2m yr-1 and n values ranging from 6 to 0.5. For Reunion, n and normalized K do not show a clear correlation with mean annual precipitation, but in old basins (~1000 kyr) they are correlated with mean annual cyclonic precipitation. In contrast, preliminary results from Kaua'i do exhibit generally lower values of n and normalized K and do not show a clear relationship with climate variables.

References

[1] Gayer et al. (2019)

[2] Ferrier et al. (2013)