Here's why:

* Dense and Crystalline Structure: Basalt is an extrusive igneous rock, meaning it cools quickly from lava flows. This rapid cooling leads to the formation of a dense, interlocking crystalline structure with relatively small pore spaces.

* Limited Porosity: The small pores in basalt are often interconnected, but their size and connectivity are limited, which restricts the flow of fluids.

* Secondary Minerals: Over time, basalt can experience weathering and alteration, leading to the formation of secondary minerals within the pores. These minerals can further reduce the permeability of the rock.

However, permeability in basalt can vary depending on several factors:

* Fracturing: Basalt can be fractured, creating pathways for fluid flow. The degree of fracturing and the size and orientation of fractures significantly influence permeability.

* Weathering: Weathering can enlarge existing pores and create new ones, increasing permeability.

* Type of Basalt: Different types of basalt have varying degrees of porosity and permeability. For example, vesicular basalt, which contains gas bubbles, can have higher permeability than dense, massive basalt.

In summary:

* Generally Low Permeability: Basalt is typically considered a low-permeability rock due to its dense, crystalline structure.

* Variable Permeability: Permeability can vary greatly depending on factors like fracturing, weathering, and the specific type of basalt.

It's important to consider these factors when evaluating the permeability of basalt in a particular application.