4th International Submarine Canyon Symposium (INCISE2018)

5-7 November 2018, Shenzhen, CHINA

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This is the detail of Post-eruptive Submarine Terrace Development by Erosion of a Surtseyan Cone at Capelinhos, Faial Island, Azores

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Zhongwei Zhao
Neil C. Mitchell, Rui Quartau, Fernando Tempera, and Lucy Bricheno
Abstarct Title
Post-eruptive Submarine Terrace Development by Erosion of a Surtseyan Cone at Capelinhos, Faial Island, Azores
Abstarct Body
Channels on steep submarine slopes can often be linked with sources of sediment, though the volumes involved can be difficult to characterize. We here describe the erosion of the Surtseyan cone, where the history of coastline retreat is well known from historical and geophysical data. The Capelinhos volcano consists of a promontory formed in 1957/58 during a Surtseyan eruption that terminated with extensive lava forming new rocky coastal cliffs. Subsequent coastal and submarine erosion has reduced the area of the promontory and created a submarine terrace. This study uses historical information, photos and marine geophysical data collected around the promontory to characterize how the submarine terrace developed following the eruption. Historical coastline positions are supplemented with coastlines interpreted from 2004 and 2014 Google Earth images in order to work out coastline retreat rate and distance for lava- and tephra-dominated cliffs. Data from swath mapping sonars are used to characterise the submarine morphology of the resulting terrace. Photographs collected by SCUBA and ROV dives on the submarine terrace reveal a rugged surface now covered with boulders. The results reveal that coastal retreat rates decreased rapidly with time after the eruption approximately in an inverse power law relationship with the retreat distance. Model calculations suggest that wave attenuation over the developing terrace can only partly explain how rapidly the retreat rate declined. The varied resistance to erosion of gradually exposed cliff base materials with the protection from eroded materials owing to rapid wave erosion, constitutes a more likely explanation. Multibeam sonar data collected below the terrace reveals several chutes of ~10 m relief and each 100-200 m across. While we cannot be sure if the chutes were carved by sedimentary flows originating from the terrace or slope failure, they head at the change of gradient marking the terrace edge and were likely pathways for much of the eroded sediment.
Session 1: Canyon processes in space and time (formation, evolution, circulation)
Presentation Type
Oral Presentation
submarine terrace, coastal erosion, wave attenuation, sediment transport, channelized movement

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