Basalt Formation
Definition
Basalt is an extrusive igneous rock formed from solidified lava flows. In Northern Idaho, basalt formations underlie parts of the region — particularly at higher elevations and in areas outside the Rathdrum Prairie’s sedimentary aquifer zone. When a drill bit encounters basalt, penetration rates slow significantly, drill bit wear increases, and water production depends entirely on whether the drill intersects natural fractures or joints within the rock. Basalt is harder and more abrasive than sedimentary material, requiring air rotary drilling methods and specialty bits. Wells completed in basalt are typically open-hole in the bedrock section — casing is run only through the overburden above, with the borehole left open in the consolidated rock zone.
Frequently Asked Questions
Why Basalt Formation Matters
How Trident Drilling Company Uses It
Trident Drilling determines likely formation type during site evaluation by reviewing IDWR lithology logs from neighboring wells and available geological survey data. When basalt is anticipated, the company adjusts its depth range estimate, selects appropriate drill bits, and builds greater depth uncertainty into the worst-case scenario bid. Clients on basalt-zone properties are explicitly informed of this variability before any drilling begins.