* Sedimentary rocks are formed from sediments: These sediments, like sand grains or clay particles, have spaces between them. This creates the initial high porosity in sedimentary rocks.

* Metamorphism involves heat and pressure: During metamorphism, the heat and pressure cause the mineral grains within the rock to recrystallize and interlock tightly. This process reduces the spaces between the grains, leading to a decrease in porosity.

* Recrystallization and deformation: The minerals in metamorphic rocks often grow larger and more tightly packed, further reducing the spaces for fluids to flow through.

However, there are some exceptions:

* Some metamorphic rocks, like schist, can have moderate porosity due to the presence of layered structures.

* Fracturing in metamorphic rocks can increase porosity. If the rock is subjected to tectonic forces that cause it to fracture, this can create pathways for fluids.

In general, sedimentary rocks are considered to be more porous than metamorphic rocks. This is because they are formed from loose sediments, which naturally leave spaces between them.