[长难句突破]

Unlike most of the world's volcanoes, they are not always found at the boundaries of the great drifting plates that make up the earth's surface; on the contrary, many of them lie deep in the interior of a plate.

主体句式：They are not always found...; many of them lie deep...

结构分析：本句由分号组成了两个部分。第一个分句中，unlike引导的壮语起到了比较的作用，主句中包含了一个that引导的定语从句、修饰plates。第二个分句中，on the contrary表示了与上文的转折关系。

As the dome grows, it develops seed fissures (cracks); in at least a few cases the continent may break entirely along some of these fissures, so that the hot spot initiates the formation of a new ocean.

主体句式：It develops seed fissures...; the continent may break so that ...

结构分析：本句也是由分号构成了两个部分。第一个分句较为简单，第二个分句则包含了一个"so that"引导的目的状语从句。

[全文译文]

Scattered around the globe are more than 100 small regions of isolated volcanic activity known to geologists as hot spots. Unlike most of the world's volcanoes, they are not always found at the boundaries of the great drifting plates that make up the earth's surface; on the contrary, many of them lie deep in the interior of a plate. Most of the hot spots move only slowly, and in some cases the movement of the plates past them has left trails of dead volcanoes. The hot spots and their volcanic trails are milestones that mark the passage of the plates.

That the plates are moving is not beyond dispute. Africa and South America, for example, are moving away from each other as new material is injected into the sea floor between them. The complementary coastlines and certain geological features that seem to span the ocean are reminders of where the two continents were once joined. The relative motion of the plates carrying these continents has been constructed in detail, but the motion of one plate with respect to another cannot readily be translated into motion with respect to the earth's interior. It is not possible to determine whether both continents are moving in opposite directions or whether one continent is stationary and the other is drifting away from it. Hot spots, anchored in the deeper layers of the earth, provide the measuring instruments needed to resolve the question. From an analysis of the hot spot population it appears that the African plate is stationary and that it has not moved during the past 30 million years.

The significance of hot spots is not confined to their role as a frame of reference. It now appears that they also have an important influence on the geophysical processes that propel the plates across the globe. When a continental plate come to rest over a hot spot, the material rising from deeper layer creates a broad dome. As the dome grows, it develops seed fissures (cracks); in at least a few cases the continent may break entirely along some of these fissures, so that the hot spot initiates the formation of a new ocean. Thus just as earlier theories have explained the mobility of the continents, so hot spots may explain their mutability (inconstancy).

地球上散落分布着100多个互不相连、面积不大的火山活动区，被地质学家称为热区。和世界上大多数火山不同的是，它们并不总是位于地球表面的巨大漂流板块之间的连接处；相反，许多热区处于板块较深的内部。大多数热区移动非常缓慢，有时，板块滑过这些热区，就会留下死火山的痕迹。热区及其火山痕迹是板块移动的标志。

板块漂移这一理论现在是无可争辩的了。以非洲和南美洲为例，由于有新的物质嵌入二者之间的海底，两个大洲之间的距离越来越远。但互相吻合的海岸线和某些似乎跨越海洋的地质特征会使人想到这两个大陆曾经是连成一片的。带着两个大陆板块漂移的相对运动已经能够被详细地阐述出来，但一个板块相对另一板块的运动还不能轻易地解释为它们相对于地球内部的运动。人们还不能确定两个大陆是在朝相反的方向运动，也不能确定是否一个大陆停止不动而另一个离它而去。位于地球更深层的热点提供了解决该问题的测量仪器。从热区的人口情况分析来看，非洲板块似乎是固定的，在过去3000万年里也没有移动过。

热区的重要意义不仅在于它们作为参照体系起到的作用。现在看来，它们还对推动板块在地球表面漂移这一地球物理过程有重要影响。当大陆板块漂移到热区上方，来自地球深层涌出的物质便形成了巨大的圆顶状隆起物。随着这个圆顶的增长，板块出现深深的裂缝。至少有几次，大陆可能会沿着其中的一些裂缝完全裂开了，因此这个热区就引发了一个新海洋的形成。这样，正像早期的理论解释了大陆的可移动性一样，热区理论或许能解释大陆板块的不稳定性。