I’m working on a review paper on evolution of volcanic ocean islands coming out of the Chapman Conference on the Galapagos I participated in last summer. Rather handily, the conference organizers have put together a nice website with all of the talks, posters, and field trip guides. If you are interested in any of the earth science aspects of volcanic ocean islands, ridge-plume interactions, etc., check out the website. Later this year there will also be an AGU monograph arising from the Conference. I’ll post details when it is published.
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The following talk will be presented in the 2011 AGU fall meeting session on “EP41F. Posteruptive Processes Operating on Volcanic Landscapes I” on Thursday, December 8th from 9:15 to 9:30 am.
Controls on the hydrologic evolution of Quaternary volcanic landscapes
Anne J. Jefferson and Noemi d’Ozouville
1. Geography and Earth Sciences, University of North Carolina at Charlotte, Charlotte, NC, United States.
2. UMR 7619 Sisyphe CNRS & UPMC, Universite Paris 6, Paris, France.
Conceptual models that explain the evolution of young volcanic landscapes require the prominent inclusion of processes which affect partitioning of water between surface and subsurface flows. Recently emplaced lava flows have no surface drainage, with infiltration to groundwater as the dominant hydrologic process. Older volcanic landscapes are often dominated by extensive drainage networks, fed by permanent or intermittent streams, which have deeply dissected the constructional topography. Drainage density, topography, and stream and groundwater discharge provide readily quantifiable measures of hydrologic and landscape evolution on volcanic chronosequences. We will use examples from the High Cascades, Galapagos, and elsewhere to illustrate the trajectories and timescales of hydrologic evolution.
We suggest that the surface-subsurface water partitioning is a function of volcanic architecture, climate-driven processes, and water-rock interactions. We will show that in mafic volcanic areas, climate-driven processes (such as weathering and dust deposition) control landscape evolution, while explosive eruptive products may be important for local hydrology. In the High Cascades, where precipitation exceeds 2 m/yr, landscape dissection has obliterated constructional morphology within 1 million years, while in the more arid Galapagos, million year old landscapes are largely undissected. Conversely, localized groundwater perching on pyroclastic layers or paleosols has been characterized in the Galapagos, but not in the Cascades, where pyroclastic activity is more limited in extent. In areas where explosive activity, including phreatomagmatism, dominates volcanism, the evolution of hydrology and topography occurs much more rapidly than in landscapes created by effusion. Hydrothermal circulation and water-rock interactions may play an important role in reducing deep permeability and altering subsurface flowpaths in some volcanic landscapes. Observed chronosequences can be complicated by juxtaposition of different age deposits, post-emplacement faulting, uplift or subsidence, and climate change, so detailed understanding of the landscape’s geologic history is a prerequisite for appropriate interpretation of hydrologic evolution in volcanic landscapes.
In July 2011, Anne was a plenary speaker at the Chapman Conference on The Galápagos as a Laboratory for the Earth Sciences in Puerto Ayora, Galapágos. Anne was tasked with reviewing the state-of-knowledge of volcanic island hydrology and identifying pressing questions for future research in this 40 minute talk. The following is the abstract which she submitted when she began the task.
Top down or bottom up? Volcanic history, climate, and the hydrologic evolution of volcanic landscapes
Volcanic landscapes are well suited for observing changes in hydrologic processes over time, because they can be absolutely dated and island chains segregate surfaces of differing age. The hydrology of mafic volcanic landscapes evolves from recently emplaced lava flows with no surface drainage, toward extensive stream networks and deeply dissected topography. Groundwater, a significant component of the hydrologic system in young landscapes, may become less abundant over time. Drainage density, topography, and stream and groundwater discharge provide readily quantifiable measures of hydrologic and landscape evolution on volcanic chronosequences. In the Oregon Cascades, for example, the surface drainage network is created and becomes deeply incised over the same million-year timescale at which springs disappear from the landscape. But chronosequence studies are of limited value if they are not closely tied to the processes setting the initial conditions and driving hydrologic evolution over time.
Landscape dissection occurs primarily by erosion from overland flow, which is absent or limited in young, mafic landscapes. Thus, volcano hydrology requires conceptual models that explain landscape evolution in terms of processes which affect partitioning of water between surface and subsurface flows. Multiple conceptual models have been proposed to explain hydrologic partitioning and evolution of volcanic landscapes, invoking both bottom up (e.g., hydrothermal alteration) and top down processes (e.g., soil development). I suggest that hydrologic characteristics of volcanic islands and arcs are a function of two factors: volcanic history and climate. We have only begun to characterize the relative importance of these two drivers in setting the hydrologic characteristics of volcanic landscapes of varying age and geologic and climatic settings.
Detailed studies of individual volcanoes have identified dikes and sills as barriers to groundwater and lava flow contacts as preferential zones of groundwater movement. Erosion between eruptive episodes and deposits from multiple eruptive centers can complicate spatial patterns of groundwater flow, and hydrothermal alteration can reduce permeability, decreasing deep groundwater circulation over time. Size and abundance of tephra may be a major geologic determinant of groundwater/surface water partitioning, while flank collapse can introduce knickpoints that drive landscape dissection. The combination of these volcanic controls will set initial conditions for the hydrology and drive bottom up evolutionary processes.
Climatic forcing drives many top down processes, but understanding the relative effectiveness of these processes in propelling hydrologic evolution requires broader cross-site comparisons. The extent of weathering may be a major control on whether water infiltrates vertically or moves laterally, and we know weathering rates increase until precipitation exceeds evapotranspiration. Weathering by plant roots initially increases porosity, but accumulation of weathered materials, such as clays in soils, can reduce near-surface permeability and promote overland flow. Similarly, eolian or glacial inputs may create low permeability covers on volcanic landscapes.
In this inaugural Scenic Saturday post, I offer up very happy volcano/landscape nerd enjoying the stunning geologic scenery on Isabella, Galápagos Islands, July 2011. I was there as a participant in the Chapman Conference on the Galápagos as a Laboratory for the Earth Sciences. I may manage to blog in more detail about the islands and the conference, but for now enjoy the scenery, just as I did on my first few days in the archipelago.
In the image above, I’ve got my back to the village of Villamil, on the southern flank of Sierra Negra volcano, and I’m actually sitting on some of the oldest exposed lavas from that volcano. You’re looking at the crust of a pahoehoe flow that is probably about 5000 to 9000 years old. A short distance up the beach, I could peek under the skin of the lava and walk a few meters into a lava tube. The floor of this lava tube was below sea level and covered by sea water, so this was really a chance to experience the water table in a very macro-pore.
The lava along this stretch of seafront is largely covered by sand that is clearly not basaltic. Instead it is made of little bits of broken shells and sea urchins from the incredibly rich marine ecosystem that surrounds the islands.
Elsewhere, the biogenic beach was covered by rather more living parts of the marine ecosystem. This sea lion put on a quite a performance for some appreciative visitors to a mangrove lagoon (and freshwater spring).