>他还能说准确的估计是多少?让我们开始说,地壳是异构的。没有一个“地壳的组成”。如果你去一个地方富含石灰岩、然后Ca和C(不含O)是最常见的元素,如果你去一个地方丰富的花岗岩,硅和铝是最常见的。去的地方丰富的玄武岩和得到很多毫克。这意味着,准确性是无关紧要的。如果你有整个大陆的平均水平,没有一个岩石或一个区域组成。如果有,它是纯粹的巧合。>这方法存在和被用来估计什么?发现平均的方法之一是知道你每个岩石的多少(这是非常简单的,只要你拥有一切地质地图)和每个岩石的成分是什么。 However, there is a more clever way of doing it. You can sample sediments. Take major rivers that have huge areas in their drainage basin. These rivers transport the fragments of those rocks as sediments and deposit them in one place. All you have to do is to analyse those sediments. Surprisingly (or not), when you actually do this for rivers in Europe, America, Australia, etc, the composition of them turns out to be almost identical! So that's one way to derive an approximation. > up to 50km deep You can't directly probe that deep. Our deepest drill hole was just over 10 km, and that was almost impossible (and extremely expensive to do). Fortunately, we have an indirect method of sampling rocks from 50 km and even deeper. Sometimes magma erupted from depth tears away pieces of rock from the walls of the conduit. This rock is then embedded in in the volcanic rock. Here's an example: [![Xenolith][1]][1] Source: James St. John, [flickr][2] You have the black basalt and in it a rock that came from several tens of km below the surface (or more). By collecting a lot of those deep-seated rocks from volcanic rocks, we can get a good idea of what the lower crust and the upper mantle look like. [1]: http://i.stack.imgur.com/ODaiR.jpg [2]: https://www.flickr.com/photos/jsjgeology/15614514062