摘要
In subtropical volcanic environments, there are often few accessible outcrops. These are often highly weathered and of very poor quality. Soil development is rapid (1 cm/y) and small eruptions are unlikely to be preserved in the geological record. Reconstructing past eruptions and assessing hazards is a challenge. Here, we studied a poorly outcropping tuff ring (very poor, incomplete sections) with the best outcrop observed at a beach cliff (up to ca. 5 - 10 m high) at Batoke, to the south of Mt Cameroon volcano. Mt Cameroon has a few tuff rings, currently of unknown ages, near the SW coast of Cameroon. In the Batoke case, the sequence is dominated by gently dipping tuff beds varying in the proportion of lithics, juvenile clasts, and accretionary lapilli (acc-laps). Several beds are close-packed with acc-laps of up to 10 - 15 mm diameter. Part of the section is gullied by mud flow deposits. The rocks are highly weathered but differential weathering enhances relationships. Quantitative data can be extracted from a detailed study of outcrops’ external surfaces. The preserved section is close to where the deposits were initially thickest and where acc-laps were most abundant and largest. There is an empirical correlation between maximum acc-lap size in the thickest outcrop and eruption column height. This and the deposit features suggest that the Batoke eruption was pulsating but dominated by fallout, with a water and ice-rich eruption column reaching 10 - 15 km high. Recycling of water drops and ice-coated fine ash accumulated during eruption. At switch off, wholesale gravitational collapse of this material produced the mud flows, which gullied the previously-laid down deposits. Such ash fall and mud flows can represent a substantial hazard, e.g. they can gully down through towns and roads and cut evacuation routes. This study illustrates how, at subtropical tuff rings, it is possible to extract key data needed for hazard assessment from only 1 - 2 poor outcrops.
In subtropical volcanic environments, there are often few accessible outcrops. These are often highly weathered and of very poor quality. Soil development is rapid (1 cm/y) and small eruptions are unlikely to be preserved in the geological record. Reconstructing past eruptions and assessing hazards is a challenge. Here, we studied a poorly outcropping tuff ring (very poor, incomplete sections) with the best outcrop observed at a beach cliff (up to ca. 5 - 10 m high) at Batoke, to the south of Mt Cameroon volcano. Mt Cameroon has a few tuff rings, currently of unknown ages, near the SW coast of Cameroon. In the Batoke case, the sequence is dominated by gently dipping tuff beds varying in the proportion of lithics, juvenile clasts, and accretionary lapilli (acc-laps). Several beds are close-packed with acc-laps of up to 10 - 15 mm diameter. Part of the section is gullied by mud flow deposits. The rocks are highly weathered but differential weathering enhances relationships. Quantitative data can be extracted from a detailed study of outcrops’ external surfaces. The preserved section is close to where the deposits were initially thickest and where acc-laps were most abundant and largest. There is an empirical correlation between maximum acc-lap size in the thickest outcrop and eruption column height. This and the deposit features suggest that the Batoke eruption was pulsating but dominated by fallout, with a water and ice-rich eruption column reaching 10 - 15 km high. Recycling of water drops and ice-coated fine ash accumulated during eruption. At switch off, wholesale gravitational collapse of this material produced the mud flows, which gullied the previously-laid down deposits. Such ash fall and mud flows can represent a substantial hazard, e.g. they can gully down through towns and roads and cut evacuation routes. This study illustrates how, at subtropical tuff rings, it is possible to extract key data needed for hazard assessment from only 1 - 2 poor outcrops.
作者
George Teke Mafany
Edison Forbuck Njei
Gerald G. J. Ernst
Wilson Yetoh Fantong
Cheo Emmanue Suh
Samuel Ndonwi Ayonghe
Robert Stephen John Sparks
Koffi Teke Mafany
Stephen Manga Njome
George Teke Mafany;Edison Forbuck Njei;Gerald G. J. Ernst;Wilson Yetoh Fantong;Cheo Emmanue Suh;Samuel Ndonwi Ayonghe;Robert Stephen John Sparks;Koffi Teke Mafany;Stephen Manga Njome(Institute of Geological and Mining Research, Yaoundé, Cameroon;Department of Geology, University of Buea, Buea, Cameroon;Department of Geology, University of Bamenda, Bamenda, Cameroon;Department of Geology and Soil Science, University of Ghent, Ghent, Belgium;Department of Earth Sciences, University of Bristol, Bristol, UK;Department of Earth & Space/Physics, Cypress Springs High School, Houston, USA)
