新东方背诵50篇——托福听力专用(18)

的海水中显得优雅堂皇，令人浮想联翩。

但是人们亦把冰山称为恐怖的和危险的。它们的确如此--在夜间，雾天和风暴肆虐时。即便是在晴朗的天气里，与它们保持一段安全距离也是明智的。冰山的大部分体积稳藏于水下，因此其水下部分的伸展远远超过可见的顶部。冰山也可能出人意料地翻滚，剧烈地搅动周围的水体。

冰山是冰川的一部分，从冰川断裂漂流进水中，一段时间后融化。今天的冰山由多年前降落的雪花形成。它们的体内是数百年，或数千年，有时甚至是数百万年前的降雪。这些雪花落在极地或寒冷的山上，仅有少量融化或根本不融化，这样经过许多年或许多世纪后积累了巨大的深度。

由于每年的雪花积累在表面之上，蒸发和融化使得雪花慢慢失去其羽状尖端而变成微小的冰粒。当新的雪花降落到旧的表面上，也变成了冰粒。因而雪花覆盖层和冰 粒层层堆积起来直到如此之大的厚度以致较上层的重量压缩较下层。在时间和压力的作用下，许多小冰粒结合到一起变成更大的晶体，最终较底层的晶体合并成庞大 而坚固的冰块。

Plate Tectonics and Sea-floor Spreading 板块结构与海床扩展

2009-09-30 来源: 【大 中 小】 我要投稿 下载音频

The theory of plate tectonics describes the motions of the lithosphere, the comparatively rigid outer layer of the Earth that includes all the crust and part of the underlying mantle. The lithosphere(n.[地]岩石圈)is divided into a few dozen plates of various sizes and shapes, in general the plates are in motion with respect to one another. A mid-ocean ridge is a boundary between plates where new lithospheric material is injected from below. As the plates diverge from a mid-ocean ridge they slide on a more yielding layer at the base of the lithosphere.

Since the size of the Earth is essentially constant, new lithosphere can be created at the mid-ocean ridges only if an equal amount of lithospheric material is consumed elsewhere. The site of this destruction is another kind of plate boundary: a subduction zone. There one plate dives under the edge of another and is reincorporated into the mantle. Both kinds of plate boundary are associated with fault systems, earthquakes and volcanism, but the kinds of geologic activity observed at the two boundaries are quite different.

The idea of sea-floor spreading actually preceded the theory of plate