NARRATOR：

Listen to part of a lecture in a geology class.

FEMALE PROFESSOR：

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 organisms.

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 of 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'm 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 the 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.

Okay.[Pause] Weathering can also be caused

by plants, by plant growth.

If a plant seed gets blown into the crack of a rock, it may take root,

and its roots will grow down into the rock.

The plant's roots can cause the rock to break down,

fracture. You may have seen this with large trees growing on top of a rock¡ª

a great example of this.

Usually there is enough dirt in the 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 cracks and crevices 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.