Ontogeny of islands associated with mantle-plume hotspots and its implications for biogeographical models

Abstract

Mantle-plume hotspot islands are a common focus of biogeographical studies, and models for the growth of their biodiversity often incorporate aspects of their physical evolution. The ontogenetic pathways of such islands have generally been perceived as simple, comprising successive episodes of emergence, growth, peak size, reduction and elimination. In this paper, we improve knowledge of island development by examining key physical data from 60 islands at eight archipelagoes in equatorial to mid-latitude regions of the Atlantic, Indian and Pacific oceans. Such landmasses achieve their maximum sizes within 200–500 kyrs. However, island longevity varies by up to a factor of 5 and is strongly controlled by the speed of the associated tectonic plate as it moves over the narrow, thermally-elevated conduit where volcanism is focused. At moderate to high speeds (40–90 ​mm/year; e.g., Galápagos, Hawaii), lifetimes are no more than 4–6 Myrs. In contrast, the oldest landmasses (in the Cabo Verde, Canary, and Mascarene archipelagoes) are built upon slow-travelling plates (<20 ​mm/year) and date from the Miocene. Notably, Fuerteventura in the Canary Islands, where the rate is c. 2.5 ​mm/year, has existed since 23 ​Ma. Two processes likely sustain the sub-aerial elevation of these massifs: heat from the plume expands the underlying lithosphere thus increasing its buoyancy, which in turn inhibits cooling-contraction subsidence; protracted magmatic activity counteracts denudation. Furthermore, the Cabo Verde and the Canary archipelagoes sit within dry climatic regions, which likely reduced erosion and mass-wasting. Consequently, two ontogenetic models are presented, one for the edifices on the intermediate- and fast-moving plates, and a second for the constructions on the slow-moving plates. The development path for the former is similar to the schema that is commonly envisaged (see above) and takes place over c. 5 Myrs, whereas the one for the latter is rather different and involves quasi-continuous surface renewal plus the maintenance of elevation that lasts for c. 10–25 Myrs. The new information should permit a fuller understanding of how a hotspot island's physical development shapes its biota and inform the formulation of related theoretical models.