Here's why:

* Composition: Shale is primarily composed of clay minerals, quartz, and other minerals. The density of these minerals varies, but they are generally not extremely dense.

* Porosity: Shale can be quite porous, meaning it has many small spaces within the rock. These spaces can be filled with air or water, which reduces the overall density.

* Specific Gravity: The specific gravity of shale typically ranges from 2.4 to 2.8. This is considered moderate compared to other rocks.

For comparison:

* High density rocks: Granite, basalt, and other igneous rocks have higher densities due to their denser mineral composition.

* Low density rocks: Sandstone and limestone tend to have lower densities due to their larger grain size and higher porosity.

Therefore, while shale is not the lightest rock, it is not exceptionally heavy either. Its density falls somewhere in the middle range.