请教OG一个句子的语法

vinique

OG上的题 :
Sound can travel through water for enormous distances, its acoustic energy prevented from dissipating by boundaries in the ocean created by water layers of different temperatures and densities.

这是个正确的答案。后面这个下划线句子是个名词性从句 ，修饰的是 啥？这个用法是个啥用法？

还有一个类似的 ：
Sixty-five million years ago, according to some scientists, an asteroid bigger than Mount Everest slammed into North America, an event that caused plant and animal extinctions that mark the end of the geologic era, known as the Cretaceous Period.

这也是个正确的答案。划线的部分名词性从句修饰的是啥？

请教： 这种名词性从句是个啥用法？修饰词省掉了吗？看不懂修饰的是哪个词。

谢谢

Lmcd158

第一个是叫absolute phrase。 论坛下载ron的笔记。里面有详细介绍。