* Sedimentary rocks form from the accumulation and cementation of sediments (like sand, mud, or shells).

* Metamorphic rocks form when existing rocks (including sedimentary) are subjected to intense heat and pressure deep within the Earth's crust.

Melting is a key difference.

* While intense heat is involved in metamorphism, it's not enough to completely melt the rock. The minerals within the rock change their structure and arrangement, but the rock itself remains solid.

* If a rock completely melts, it's no longer metamorphic. It becomes magma (molten rock) which, when cooled and solidified, turns into igneous rock.

Here's how the process works:

1. Sedimentary rock exists at the Earth's surface.

2. Tectonic plate movement or other geological processes can bury this rock deep beneath the surface.

3. Heat and pressure increase significantly.

4. Minerals within the rock recrystallize and rearrange, forming a new metamorphic rock.

So, it's not that sedimentary rocks "melt" into metamorphic rocks. It's that they are transformed under intense heat and pressure without fully melting.