The expert spent two hours expounding his environmental proposal in detail.

expounding 是动词 expound 的现在分词形式。expound 由前缀 ex-（表示"向外"）和词根 pound（来自拉丁语 ponere，意为"放置"）组成，字面意思是"把观点摆出来"，引申为"详细解释、阐述"的意思。