Scattered around the globe are more than 100 small regions of isolated vocanic activity known to geologists as hot spots.
    这句话当中，主干结构是 be known ... as 吗？如果是，为什么要把more than 100 small regions of isolated vocanic activity 插在are 和 known中间？
   如果不是，这句话是个什么用法？谢谢各位达人帮忙分析一下。。。。