Rocks near the Earth’s surface are directly exposed to elements in the environment such as air and water, and also to conditions such as temperature change as well as to living organism. And this exposure to the environment can actually cause even huge rocks to break into smaller pieces. This process is called weathering. Let’s talk about a couple ways weathering occurs.
First of all, rocks are often exposed to water. In cold wet environments rocks can break due to water freezing inside of them. How does this happen? Well, as I am sure you know, when water freezes it expands and over time this can lead to weathering.
Um, imagine a rock with a small opening or crack in it. It rains and water gets into the crack and stays there. Then, at night, the temperature drops and water inside the crack freezes. This growing, expanding ice pushes outward on either side of the crack causing it to get slightly bigger. When this happens again and again, the crack becomes larger and eventually pieces of the rock break off.
OK, weathering can also be caused by plants, by a plant growth. If a plant seed gets blown into the crack of a rock, it may take root. And its root will grow down into the rock. The plant’s roots can cause the rock to break down, uh, fracture. You may have seen this with large trees growing on top of a rock, a great example of this. Usually there’s enough dirt in a crack of a rock or on top of a rock to allow a tree to start growing there. As the tree grows over the years, the tree’s roots extend downward into the crack and crevasses of the rock in search of water and nutrients. Over time, the roots get bigger and grow deeper, widening and enlarging the cracks, causing the rock to break apart.
首先这段lecture讲了一个process called weathering，然后进行ways论述1. rocks are often exposed to water 2. It rains and water gets into the crack and stays there. Then, at night, the temperature drops and water inside the crack freezes. When this happens again and again, the crack becomes larger and eventually pieces of the rock break off 3.weathering can also be caused by plants As the tree grows over the years, the tree’s roots extend downward into the crack and crevasses of the rock in search of water and nutrients. Over time, the roots get bigger and grow deeper, widening and enlarging the cracks, causing the rock to break apart.
从备考角度来说，其实大多数学生都是知道“水滴石穿”这个学科知识。在这段话中差不多就是讲了这个水滴石穿的过程。但是要按照口语的答题内容完美的回答还是有些困难的，那么首先需要知道的是geology的一些学科词汇。比如说crack, freeze, crevasse, rock, nutrients 这些词。
Ice wedges form when water runs into cracks in the ground, the permafrost, then freezes. You ever see mud after it dries? Dried mud has cracks because when it dries it contracts, it shrinks. Well, in winter permafrost behaves similarly. It shrinks in winter because it freezes even more thoroughly then. And as it shrinks, it forms deep, deep cracks. Then in the summer, when the active layer, the top permafrost then freezes again, because that ground, the ground beneath the active layer is still below freezing. So you have wedges of ice in the permafrost.
Now, the ice wedges widen the original cracks in the permafrost because water expands when it freezes. All right, okay, then in the autumn, the active layer on top freezes again. Then in winter the permafrost starts contracting again and the cracks open up even wider. So the next summer when the active layer melts again and flows into the widened cracks and freezes, it makes the cracks even wider. So it’s sort of a cycle through which the cracks and the wedges grow wider and wider.