Here's how it works:

* Weathering is the process of breaking down rocks, soil, and minerals through contact with the Earth's atmosphere, water, and biological organisms.

* There are two main types of weathering:

* Physical weathering breaks rocks down into smaller pieces without changing their chemical composition. This can happen through:

* Frost wedging: Water seeps into cracks in rocks, freezes, expands, and puts pressure on the rock, eventually causing it to break.

* Abrasion: Rocks rub against each other due to wind, water, or ice, gradually wearing them down.

* Chemical weathering changes the chemical composition of the rock, making it weaker and easier to break down. This can happen through:

* Acid rain: Rainwater with dissolved carbon dioxide can react with certain types of rocks, dissolving them.

* Oxidation: Iron in rocks reacts with oxygen, forming rust, which weakens the rock.

In the case of a large rock breaking down into small pebbles, it's most likely due to a combination of physical and chemical weathering over a long period of time.