Based on your tag word you are looking at the erosion of rubber, chalk, and wood. There are two basic kind of erosion, mechanical and chemical. Mechanical erosion is the breaking or wearing down of a substance via physical means. Chalk and wood can be broken down this way. Chalk is the easiest. It is a very brittle substance than can be worn down by just rubbing it on another surface (like a chalkboard). Wood is sturdier, but using sandpaper or a wood carving tool will erode and wear down wood.
Rubber is a bit different. It is harder to wear down mechanically without actually cutting up the material which isn't really erosion to begin with. But rubber is subject to chemical erosion. This is where the substance changes chemically at the molecular level over time to break down. Rubber molecules over the course of time will break down and become brittle. When this happens, the rubber is chemically changed and loses its elasticity. It can then crack or break which is why rubber seals need to be changed occasionally. So rubber is an example of chemical erosion.