Here's why:

* Shale: A sedimentary rock, usually fine-grained, made up of clay minerals, silt, and sometimes organic matter.

* Phyllite: A metamorphic rock with a characteristic sheen or gloss, often described as "silky." It forms from the metamorphism of shale.

Contact Metamorphism

Contact metamorphism occurs when hot magma or lava comes into contact with existing rocks. The heat from the igneous intrusion causes the pre-existing rock (in this case, shale) to recrystallize and change its mineral composition and texture.

The Transformation:

* Heat: The heat from the igneous intrusion causes the clay minerals in the shale to start recrystallizing.

* Pressure: Although pressure is not as significant as in regional metamorphism, contact metamorphism does involve some pressure from the weight of overlying rocks.

* New Minerals: As the clay minerals recrystallize, they form new minerals like mica. The tiny mica flakes give phyllite its characteristic sheen.

* Texture: The original layering in the shale is often preserved in phyllite, but the rock becomes more compact and less brittle.

In summary: Contact metamorphism provides the heat and pressure necessary to transform the clay-rich shale into the more metamorphic phyllite.