Here's why:

* Density: Basalt is a dense, igneous rock with a density around 2.9 g/cm³. Water has a density of 1 g/cm³. For an object to float, it needs to be less dense than the fluid it's in. Since basalt is significantly denser than water, it will sink.

* Porosity: While some basalt can have small air pockets, these are typically not enough to make it less dense than water.

Let me know if you have any more questions!