Basalt is a dark, heavy volcanic rock formed when lava cools and hardens. When a volcano erupts and molten rock pours out onto the surface, it can cool quickly into basalt, creating smooth, solid stone that's usually dark gray or black.

Basalt is one of the most common types of rock in Earth's crust. The ocean floor is made mostly of basalt, and many volcanic islands like Hawaii sit on thick layers of it. Ancient peoples discovered that basalt could be shaped into sharp tools and weapons. The famous Giant's Causeway in Northern Ireland features thousands of hexagonal basalt columns that formed naturally as the rock cooled and cracked into geometric patterns.

Some of the most striking basalt formations occur when lava cools in unusual ways. Basalt can form rope-like patterns, glassy surfaces, or tall columns. Because basalt weathers into mineral-rich soil, volcanic regions with basalt bedrock can have exceptionally fertile farmland. The Columbia River Gorge in the Pacific Northwest showcases dramatic basalt cliffs carved by ancient floods, revealing layers of lava flows stacked like pages in a history book.