The paper by Akiho Miyashiro, published in the American Journal of Science in April 1974, explores the composition and evolution of volcanic rock series in island arcs and active continental margins. Miyashiro identifies three main volcanic rock series: calc-alkalic (CA), tholeiitic (TH), and alkalic. The CA and TH series are collectively referred to as the non-alkalic series, with the CA series showing higher rates of SiO₂ increase and lower degrees of FeO* enrichment compared to the TH series. Miyashiro defines the CA and TH series quantitatively based on the slopes of variation diagrams, with the CA series having a steeper slope and the TH series a gentler slope. The main rocks in immature island arcs are typically basalts and basaltic andesites of the TH series, while well-developed island arcs with a thick continental-type crust are characterized by andesites and dacites of the TH and CA series. The proportion of CA series rocks increases with the development of the continental-type crust beneath the volcanic arc. The average SiO₂ and K₂O contents of volcanic rocks in an arc tend to increase with the development of the continental-type crust, which is attributed to the increasing proportion of CA series rocks. The K₂O and Na₂O + K₂O contents and the (Na₂O + K₂O)/Al₂O₃ ratio of volcanic rocks tend to become higher across the island arc toward the adjacent continent, indicating a trend of increasing alkalinity. Miyashiro also discusses the tectonic classification of island arcs and active continental margins, noting that the proportion of CA series rocks is generally higher in arcs with a well-developed continental-type crust. The evolution of island arcs and continental margins is hypothesized to be influenced by the progressive depletion of the upper mantle wedge overlying the descending slab, leading to changes in the composition of the parent magmas. Overall, the paper provides a comprehensive analysis of the compositional variations and tectonic implications of volcanic rock series in island arcs and active continental margins, offering insights into the petrogenetic processes and tectonic evolution of these regions.The paper by Akiho Miyashiro, published in the American Journal of Science in April 1974, explores the composition and evolution of volcanic rock series in island arcs and active continental margins. Miyashiro identifies three main volcanic rock series: calc-alkalic (CA), tholeiitic (TH), and alkalic. The CA and TH series are collectively referred to as the non-alkalic series, with the CA series showing higher rates of SiO₂ increase and lower degrees of FeO* enrichment compared to the TH series. Miyashiro defines the CA and TH series quantitatively based on the slopes of variation diagrams, with the CA series having a steeper slope and the TH series a gentler slope. The main rocks in immature island arcs are typically basalts and basaltic andesites of the TH series, while well-developed island arcs with a thick continental-type crust are characterized by andesites and dacites of the TH and CA series. The proportion of CA series rocks increases with the development of the continental-type crust beneath the volcanic arc. The average SiO₂ and K₂O contents of volcanic rocks in an arc tend to increase with the development of the continental-type crust, which is attributed to the increasing proportion of CA series rocks. The K₂O and Na₂O + K₂O contents and the (Na₂O + K₂O)/Al₂O₃ ratio of volcanic rocks tend to become higher across the island arc toward the adjacent continent, indicating a trend of increasing alkalinity. Miyashiro also discusses the tectonic classification of island arcs and active continental margins, noting that the proportion of CA series rocks is generally higher in arcs with a well-developed continental-type crust. The evolution of island arcs and continental margins is hypothesized to be influenced by the progressive depletion of the upper mantle wedge overlying the descending slab, leading to changes in the composition of the parent magmas. Overall, the paper provides a comprehensive analysis of the compositional variations and tectonic implications of volcanic rock series in island arcs and active continental margins, offering insights into the petrogenetic processes and tectonic evolution of these regions.