TPO25阅读R1有一个句子一直不理解*——*

诗音_smile

but if these volcanoes were active as recently as 100 million years ago (an estimate of the time of last eruption based on the extent of impact cratering on their slopes), some of them may still be at least intermittently active. 一直不明白：为什么一百年前活跃，现在就活跃呢？两者有什么因果联系吗？