Marques Fernando O, Ribeiro Luísa P, Hübscher Christian, Costa Ana C G, Hildenbrand Anthony
EMEPC - Task Group for the Extension of the Continental Shelf, Paço de Arcos, Portugal.
Institute of Geophysics, University of Hamburg, Hamburg, Germany.
Sci Rep. 2025 Jan 30;15(1):3835. doi: 10.1038/s41598-025-87191-5.
Mass controls two major processes in volcanic islands: large-scale collapse and vertical movements. Therefore, large islands like Hawaii are gradually subsiding and have undergone massive landsliding. What if the mass is much smaller, and there is good evidence that the vertical movement is more complex than simple loading-related subsidence? Here, we show that small volcanic islands, seemingly stable because of the small mass, can undergo sector collapses that can affect the vertical movement of the island. Santa Maria Island (Azores) is ca. 170 times smaller than Hawaii; however, it has collapsed more than once, as inferred from new onshore and marine geophysical data. From the vertical distribution of submarine and subaerial lavas, we can infer a complex subsidence and uplift history (seamount-island-seamount-island, meaning that Santa Maria has been an island twice), which is most likely the consequence of alternating volcanic construction (loading = subsidence) and large-scale destruction by sector collapse (unloading = uplift). Given the age difference between Santa Maria and the underlying oceanic crust, the island likely sits on several hundred meters of marine sediments that could behave as a weak layer (detachment) under shear. We evaluate the likely collapse mechanism by numerical modelling and conclude that small volcanic islands can collapse when both the edifice and its substrate are weak. The proximity of Santa Maria to the East Azores Fault, active during the volcanic lifetime of Santa Maria, may point to the trigger mechanism of the flank collapses. The inferred instability factor, a weak substrate, could be responsible for similar evolutions in other volcanic ocean islands sitting on sediments overlying the igneous oceanic crust.
大规模坍塌和垂直运动。因此，像夏威夷这样的大岛正在逐渐下沉，并经历了大规模的山体滑坡。如果质量小得多，而且有充分证据表明垂直运动比简单的与负载相关的下沉更为复杂，情况会怎样呢？在这里，我们表明，看似因质量小而稳定的小型火山岛，可能会发生扇形坍塌，进而影响岛屿的垂直运动。圣玛丽亚岛（亚速尔群岛）的面积约为夏威夷岛的1/170；然而，根据新的陆上和海洋地球物理数据推断，它已经坍塌过不止一次。从海底和陆上火山熔岩的垂直分布情况，我们可以推断出其复杂的下沉和隆升历史（海山-岛屿-海山-岛屿，这意味着圣玛丽亚岛曾两次成为岛屿），这很可能是火山建造（负载=下沉）与扇形坍塌造成的大规模破坏（卸载=隆升）交替出现的结果。鉴于圣玛丽亚岛与下伏洋壳之间的年龄差异，该岛可能位于几百米厚的海洋沉积物之上，这些沉积物在剪切作用下可能表现为一个软弱层（滑脱面）。我们通过数值模拟评估了可能的坍塌机制，并得出结论：当火山体及其基底都很软弱时，小型火山岛可能会发生坍塌。圣玛丽亚岛靠近东亚速尔断裂带，在圣玛丽亚岛的火山活动期该断裂带处于活跃状态，这可能指向了侧翼坍塌的触发机制。推断出的不稳定因素，即软弱的基底，可能是造成位于火成洋壳之上沉积物上的其他火山海洋岛类似演化的原因。
