1. Water: This is the most common cause of erosion. Rain, rivers, and ocean waves can all wear away at the surface of the Earth.

2. Wind: Wind can pick up and carry away soil, sand, and rocks, especially in dry areas. This can lead to the formation of sand dunes and dust storms.

3. Ice: Glaciers, which are slow-moving rivers of ice, can carve out valleys and move large amounts of rock and sediment.

4. Gravity: Gravity can cause landslides and rockfalls, which can quickly erode large amounts of material.

It's important to remember that these four causes often work together, and erosion can be a complex process.