Here's why:
* Iron and Oxygen: Iron is a metal that readily reacts with oxygen in the presence of water or moisture. This reaction forms iron oxide, commonly known as rust.
* Color Change: Rust is reddish-brown, which explains the color change you observed.
* Soft and Crumbly: Rust is a flaky and brittle substance. It weakens the original iron structure, causing the rock to become soft and crumbly.
Here's a more detailed explanation:
1. Iron in the rock: The rock contains iron, which is a reactive metal.
2. Exposure to Oxygen and Moisture: When the rock is exposed to air and water (rain, humidity, even damp soil), the iron reacts with oxygen.
3. Formation of Iron Oxide: This reaction creates iron oxide, commonly known as rust.
4. Physical Changes: Rust is a brittle and crumbly substance. As it forms, it weakens the original iron structure, causing the rock to become soft and crumble.
Note: This process is accelerated in the presence of salt, as found in seawater or road salt, which contributes to a faster rate of rust formation.
