(Table 2) Analyses of prominent ash layers recovered at DSDP Site 18-178, Gulf of Alaska@en

A sequence of ash layers recovered from site 178 of the Deep Sea Drilling Project in the Gulf of Alaska was studied to determine the nature of highly explosive volcanic eruptions associated with the Aleutian Arc and Alaskan Peninsula during the last 8 m.y. The major-element chemistry of 25 distinct ash layers was determined. When the analyses are plotted on conventional major-element variation diagrams, the unusual, highly evolved, calc-alkalic characteristics of the ashes are apparent. Perhaps more significantly, there is a good correlation of certain indices of the degree of chemical evolution of each ash (SiO2 content and Larsen index) with sample age. Both parameters vary cyclically, with maximum values of both indices occurring at present, 2.5, and about 5.0 m.y. ago. The cause of the cyclic activity, as well as discontinuous volcanic activity reported for other areas by other investigators, is not precisely known. However, we suggest that variable rates of subduction provide a viable hypothesis for discontinuous volcanic activity associated with convergent plate boundaries.本研究针对阿拉斯加湾深海钻探计划（Deep Sea Drilling Project）178站位所获取的一套火山灰层序列展开分析，旨在揭示过去8百万年间与阿留申弧（Aleutian Arc）及阿拉斯加半岛相关的强爆炸性火山喷发的本质特征。研究人员对25个独立火山灰层的主量元素化学组成进行了测定。将分析结果投绘至常规主量元素变异图解后，可清晰观察到这些火山灰具有非同寻常的高度演化钙碱性（calc-alkalic）特征。更值得关注的是，各火山灰的化学演化程度相关指标（二氧化硅（SiO₂）含量与拉森指数（Larsen index））与样品年龄呈现良好的相关性。两项指标均呈周期性变化，其最大值分别出现在现今、2.5百万年前以及约5.0百万年前。目前尚不清楚该周期性火山活动的成因，也无法明确其他研究者在其他区域报道的不连续火山活动的驱动机制。不过，本研究提出，俯冲速率的变化可作为解释汇聚型板块边界相关不连续火山活动的合理假说。