Basalt

Basalt is typically dark gray to black, fine-grained, and has a dull to vitreous luster. It is hard, with a Mohs hardness of 6.0-7.0. It can be massive, vesicular, or porphyritic, with aphanitic (fine-grained) to glassy textures. When it cools slowly, it can form columnar joints, as seen in places like the Giant's Causeway.

Basalt is an extrusive igneous rock, meaning it forms from the rapid cooling of basaltic lava erupted at the Earth's surface or underwater. It is common at mid-ocean ridges, oceanic hotspots (like Hawaii), continental rifts, and flood basalt provinces. Its formation age varies widely, from very recent eruptions to billions of years old.

Basalt is widely used in construction as crushed stone for road aggregate, concrete aggregate, and railway ballast. It's also used for dimension stone, paving stones, and as a raw material for mineral wool (basalt fiber) insulation. Due to its durability, it's suitable for various outdoor applications.

Basalt is the most common rock type in the Earth's crust, forming the majority of the oceanic crust. It is also found on other terrestrial planets like Mars and the Moon. The Moon's 'maria' (dark plains) are vast basaltic lava flows. Columnar jointing, a distinctive feature of some basalt flows, forms when lava cools and contracts, creating polygonal columns.

In the field, basalt can be identified by its dark color, fine-grained texture (individual crystals often not visible without a hand lens), and relative heaviness. It may also show vesicles (gas bubbles) or columnar jointing in certain exposures. It is commonly found in areas of volcanic activity, coastlines (where oceanic crust is exposed), and flood basalt provinces worldwide. When broken, it typically produces sharp, angular fragments. The rocks in the image appear dark, angular, and fine-grained, consistent with basalt often used as riprap or fill material.