islands

Islands and reefs

Volcanic islands are ideal natural experiments in landscape evolution. They are made of relatively homogeneous basaltic rock, each island experiences a uniform history of uplift and subsidence, and the initial slopes of the volcano can often be reconstructed and their ages measured; yet the many volcanic islands around the world have a wide range of ages and experience varied plate tectonic and climatic contexts.

Kimberly Huppert studied these many experiments to learn about connections between Earth’s surface, climate, and interior. Working with Taylor Perron and Prof. Leigh Royden, she showed how loading of Earth’s lithosphere as new volcanic islands grow can explain the complicated history of uplift and subsidence recorded by ancient shorelines in the Hawaiian Islands. Looking farther afield geographically and farther ahead in the lifetimes of islands, she proposed a model that elegantly explains how tectonic plate velocities and hotspot swell geometries control the ages at which volcanic islands subside to form coral atolls and eventually sink below the sea surface forever.

• Huppert, K.L., L.H. Royden and J.T. Perron (2015), Dominant influence of volcanic loading on vertical motions of the Hawaiian Islands, EPSL, 418, 149–171, http://doi.org/10.1016/j.epsl.2015.02.027.
• Huppert, K.L., J.T. Perron and L.H. Royden (2020). Hotspot swells and the lifespan of volcanic ocean islands. Science Advances, 6, eaaw6906, http://doi.org/10.1126/sciadv.aaw6906.
• Jefferson, A.J., K.L. Ferrier, J.T. Perron, and R. Ramalho (2014), Controls on the Hydrological and Topographic Evolution of Shield Volcanoes and Volcanic Ocean Islands, in The Galápagos: A Natural Laboratory for the Earth Sciences (eds K. S. Harpp, E. Mittelstaedt, N. d’Ozouville and D. W. Graham), John Wiley & Sons, Inc, Hoboken, New Jersey, 185–213, http://doi.org/10.1002/9781118852538.ch10.

The effects of climate variability are not limited to landscapes above sea level. Coral reefs form dramatic landscapes, particularly around ocean islands. Charles Darwin proposed a general model in which reefs progress through a well-defined sequence as an island subsides, beginning as a narrow fringing reef near the coast, then forming a barrier reef surrounding a lagoon, and finally developing into an atoll. But Darwin didn’t consider the glacial sea level cycles that have been occurring for the past few million years. How does sea level shape coral reef landscapes? Michael Toomey, in collaboration with Andrew Ashton (WHOI), has shown that reefs around many islands (such as the Hawaiian Islands) do not follow Darwin’s proposed sequence, and display a wider variety of forms. We developed a simple model for reef growth that illustrates how the rates of reef accretion and island vertical motion control a reef’s profile, and we found that glacial sea level variations are essential for reproducing the observed variety of reef forms. Pleistocene coral reefs appear to bear a strong imprint of glaciations.

• Toomey, M., A.D. Ashton and J.T. Perron (2013), Profiles of ocean island coral reefs controlled by sea-level history and carbonate accumulation rates. Geology, 41, 731–734, http://doi.org/10.1130/G34109.1.

Ocean islands are also natural experiments in wave climate, with dominant wind regimes delivering different amounts of wave energy to different sides of an island. Kim Huppert, in another collaboration with Andrew Ashton, used this strategy to show that bedrock coastal erosion rates scale with wave power in the Hawaiian Islands. Jimmy Bramante, also working with Perron and Ashton, devised an experimental oscillatory flow tunnel and performed a series of experiments showing that bedrock abrasion by wave-driven flows is fastest at intermediate sediment loads due to competing “tools” and “cover” effects, just like in bedrock rivers.

• Huppert, K.L., J.T. Perron and A.D. Ashton (2020). The influence of wave power on bedrock seacliff erosion in the Hawaiian Islands. Geology, 48, 499–503, http://doi.org/10.1130/G47113.1.
• Bramante, J.F., J.T. Perron, A.D. Ashton and J.P. Donnelly (2020). Experimental quantification of bedrock abrasion under oscillatory flow. Geology, 48, 541–545, http://doi.org/10.1130/G47089.1.