珊瑚礁是通过珊瑚虫分泌的碳酸钙物质不断沉积而形成的。

Coral reefs are formed by the gradual ___accretions of calcium carbonate secreted by coral polyps.

这座古城墙经历了几个世纪的扩建和改造，上面有许多不同时期的建筑痕迹。

The ancient city wall has many ___accretions from different periods due to centuries of expansions and renovations.

恒星是由气体和尘埃通过引力作用逐渐聚集形成的。

Stars are formed by the gradual ___accretion of gas and dust under gravitational pull.