Chalk, which is primarily composed of calcium carbonate (CaCO3), doesn't directly turn into a metamorphic rock after heat and pressure. Instead, it re-crystallizes and forms a new mineral called marble.

Here's why:

* Chalk is a sedimentary rock formed from the accumulation of microscopic marine organisms' shells.

* Metamorphism involves the transformation of existing rocks due to intense heat and pressure, which can change their mineralogy and texture.

* Marble is a metamorphic rock formed from the recrystallization of limestone or dolostone (another type of sedimentary rock), which is essentially the same material as chalk.

So, when chalk undergoes metamorphism, the calcium carbonate crystals within it re-arrange and grow larger, forming the characteristic coarse-grained texture of marble.