The biogeochemical cycle that involves the erosion of rocks and is considered a slow cycle is the phosphorus cycle.

Here's why:

* Slow Process: Unlike cycles like the carbon or nitrogen cycles, which involve rapid movement of elements through living organisms and the atmosphere, the phosphorus cycle is much slower. This is because the primary source of phosphorus is rocks.

* Erosion and Weathering: Phosphorus is released from rocks through a slow process of weathering and erosion. Rainwater and other natural forces break down rocks, gradually releasing phosphorus into the soil and water.

* Long-term Storage: A significant portion of phosphorus is stored in rocks for very long periods. This makes it a limiting nutrient in many ecosystems, meaning its availability can restrict plant growth.

Let me know if you'd like more information on the phosphorus cycle or other biogeochemical cycles!