When does a sedimentary rock become a metamorphic rock?

A sedimentary rock becomes a metamorphic rock through a process called metamorphism. This process involves changes in the physical and chemical conditions of the rock such as temperature, pressure, and chemical environment. Here are the steps:

1. Burial and Increase in Pressure: Sedimentary rocks are often buried deep underground over time. As more and more layers of sediment accumulate on top, the pressure on the buried sedimentary rock increases.

2. Increase in Temperature: Along with the increase in pressure, there is also an increase in temperature as the rock gets buried deeper. This is due to the geothermal gradient - the deeper you go into the Earth, the hotter it gets.

3. Metamorphism: The combination of high temperature and pressure causes the minerals in the sedimentary rock to change chemically, reorganize and recrystallize, forming a denser, more compact rock. This process is called metamorphism.

4. Formation of Metamorphic Rock: The end result of this process is a metamorphic rock. The original sedimentary rock has been transformed, or "metamorphosed," into a new type of rock.

It's important to note that this process takes a very long time - often millions of years.