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首页> 外文期刊>Journal of Volcanology and Geothermal Research >Growth of an emergent tuff cone: Fragmentation and depositional processes recorded in the Capelas tuff cone, Sao Miguel, Azores
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Growth of an emergent tuff cone: Fragmentation and depositional processes recorded in the Capelas tuff cone, Sao Miguel, Azores

机译：凝灰岩锥的生长：亚速尔群岛圣米格尔角的凝灰岩锥中记录的破碎和沉积过程

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The Capelas tuff cone is an emergent Surtseyan-type tuff cone that erupted in shallow seawater off the coast of Sao Miguel, Azores. In this paper, we present a detailed stratigraphic study which is used to infer depositional processes and modes of fragmentation for the Capelas tuff cone deposits. The growth of the tuff cone can be divided into three stages based on variations in depositional processes that are probably related to differences in water/magma (W/M) ratios. The first stage corresponds well to wet Surtseyan-type activity where wet fallout is the dominant depositional process, with only minor representation of pyroclastic surge deposits. The second stage of the eruption is suggested to be the result of alternating wet and slightly drier periods of Surtseyan activity, with an overall lower W/M-ratio compared to the first stage. The drier Surtseyan periods are characterized by the presence of minor grain-flow deposits and undulating pyroclastic surge deposits that occasionally display relatively dry structures such as strongly grain-segregated layers and brittle behavior when impacted by ballistic ejecta. The first deposits of the second stage show an intense activity of pyroclastic surges but fallout, commonly modified by surges, is still the dominant depositional process during the second stage. The third stage represents a final effusive period, with the build-up of a scoria cone and ponded lava flows inside the tuff cone crater. Phreatomagmatic fragmentation, as seen by studies of the fine ash fraction (< 64 μm), is dominant in the Capelas tuff cone. However, particles with shapes and vesicularities characteristic of magmatic fragmentation are abundant in proximal deposits and present in all investigated beds (in various amounts). Emergent Surtseyan-type tuff cones are characterized by a domination of fallout deposits, both wet and dry, where dry periods are characterized by the deposition of relatively dry falling tephra transforming into grain-flow deposits. However, this study of the Capelas tuff cone shows that drier Surtseyan periods may also be represented by an increased amount of thin surge deposits that occasionally display dry features.
机译：Capelas凝灰岩锥是一种新兴的Surtseyan型凝灰岩锥，在亚速尔群岛圣米格尔海岸附近的浅海水中爆发。在本文中，我们提供了详细的地层学研究，可用于推断卡普拉斯凝灰岩锥状沉积物的沉积过程和破碎模式。根据沉积过程的变化，凝灰岩锥的生长可以分为三个阶段，这可能与水/岩浆（W / M）比的差异有关。第一阶段很好地对应于湿Surtseyan型活动，其中湿沉降是主要的沉积过程，仅有少量的火山碎屑潮沉积物。喷发的第二阶段被认为是Surtseyan活动交替的潮湿和稍干的时期的结果，与第一阶段相比，总体W / M比较低。 Surtseyan较干燥的时期的特征是存在少量的颗粒流沉积物和起伏的火山碎屑涌动沉积物，当受到弹射弹射的影响时，这些沉积物偶尔会显示相对干燥的结构，例如强烈的颗粒偏析层和脆性。第二阶段的第一批沉积物表现出强烈的火山碎屑潮活动，但通常受潮汐改变的沉降物仍然是第二阶段的主要沉积过程。第三阶段代表最后的喷发期，在煤灰锥火山口内部堆积了一个熔渣锥和堆积的熔岩流。如对细灰分（<64μm）的研究所见，在岩浆凝灰岩锥中占主导的是岩浆碎裂。然而，具有岩浆破碎特征的形状和小泡特征的颗粒在近端沉积物中含量丰富，并存在于所有研究的床层中（数量不等）。出现的Surtseyan型凝灰岩锥体的特征是，无论是干的还是湿的，沉积物都占主导地位，而干燥时期的特征是，相对干燥的下降的提夫拉沉积物转变成谷物流沉积物。但是，对Capelas凝灰岩锥的这项研究表明，较干燥的苏尔塞扬时期也可能由数量稀少的潮涌沉积物所代表，这些沉积物偶尔会显示出干燥的特征。

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