Pioneering plants tell us when volcanoes last erupted

Featured image: vegetated lava flows on Le Grand Brûlé, with the profile of Piton de la Fournaise behind. Image credit: Mickaël Douineau on Flickr (CC BY-NC-ND 2.0).

Paper : Dating young (<1000 yr) lava flow eruptions of Piton de la Fournaise volcano from size distribution of long-lived pioneer trees Authors: Sébastien Albert, Olivier Flores, Laurent Michon and Dominique Strasberg

A newly formed lava flow may appear to be a sterile environment: devoid of vegetation and humus. But within years, the rocky wasteland erupts into life as a host of tenacious plants take hold. The size of plants rooted on solidified lava is now being used by volcanologists working on Piton de la Fournaise, a shield volcano on the Indian Ocean island of Réunion, to date past eruptions.

On the southeastern slopes of Piton de la Fournaise, an area called Le Grand Brûlé, visitors can walk on lava fields that were incandescent rivers of basalt back in 2007. Now they are peppered with plant life. The lichen Stereocaulon vulcani was first to colonize, coating the black rocks with a white frosting. Next followed vivid green ferns and the native tree species, Agarista salicifolia, which plays a crucial role in early ecological succession. Rapid turnover: eruption, die-back and recovery, repeated over thousands of years, has shaped the island’s unique ecosystems.

Pioneer plants have already taken root on a lava flow from the 2007 eruption of Piton de la Fournaise. Image credit: MarySloA on Flickr (CC BY-NC-ND 2.0).

Piton de la Fournaise is one of the most active volcanoes on Earth. There have been more than 150 eruptions since the mid-17th century. With each eruption, vast tracts of forest were razed by fire and lava, but lush greenery soon returned. 

Although the volcano brings long-term ecological benefits and fuels the thriving geo-tourism industry, volcanologists still monitor Piton de la Fournaise closely for signs of unrest. In their quest to understand the volcano’s temperament, volcanologists have been studying the old lavas. Of key interest is the age of the lavas, and whether the frequency of eruptions has changed over time.

Lava from the 2004 eruption flowing out into the sea. Image credit: Wikipedia / Hughes Léglise-Bataille on Flickr (CC BY 2.0) 

But dating lava flows is challenging. There are few written accounts of eruptions, as Réunion Island was not settled until the mid-16th century. Charcoal for radiocarbon dating isn’t always present, and many lavas are too young for other absolute dating techniques. 

A new study, led by Sébastien Albert at the Université de La Réunion, uses ecological methods to date lava flows erupted from Piton de la Fournaise over the last 1000 years. Albert and the team used Agarista salicifolia, which quickly roots on lava fields and lives for more than 600 years as an indicator of lava age.  They recorded the height and trunk width of 711 Agarista salicifolia trees on 20 lava flows which were already dated, either via historical accounts or radiocarbon.  A pattern emerged; the older the trees, the larger their size. By fitting this simple relationship to a statistical model, Albert was able to use tree size to date 11 lavas of unknown age. 

The lava ages revealed pulses of volcanism at Piton de la Fournaise. Albert suggests that increased magma supply likely caused clusters of eruptions between 1460-1630, 1690-1840 and the current phase of unrest, which began in 1970. The team hopes their methods can be used in other areas where early pioneer species colonize volcanic slopes, including the vibrant red-flowering shrub Metrosideros in Hawaii and the feathery evergreen Casuarina, endemic to Indonesia. 

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Pioneering plants tell us when volcanoes last erupted by Erin Martin-Jones is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License.

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