# How to identify basalt Canonical page: https://lithomap.app/resources/rock-identification/basalt Last reviewed: September 17, 2026 Page purpose: Original LithoMap rock identification reference for field observations, search systems, and language models. Classification: Extrusive igneous rock ## Summary Basalt is a dark, fine-grained volcanic rock formed from relatively iron- and magnesium-rich lava; crystals are commonly too small to distinguish by eye. ## Quick identification clues - Dark gray to black on a fresh surface - Very fine-grained or glassy groundmass - May contain round gas-bubble cavities called vesicles - Dense, massive pieces are common, but some samples are porous ## Composition and classification Basalt is a mafic volcanic rock composed chiefly of microscopic plagioclase and pyroxene, commonly with olivine and iron-titanium oxides. Weathering can turn surfaces brown, rusty, greenish, or reddish. ## Appearance Most basalt looks uniform at arm’s length because its crystals cooled too quickly to become large. Some pieces contain visible olivine or plagioclase crystals set in a fine matrix. Vesicular basalt preserves bubbles from escaping volcanic gas. Later minerals may fill those cavities, producing amygdules. ## Identification tests - Grain size: Crystals mostly microscopic. Distinguishes basalt from coarse-grained gabbro. - Magnetism: Often weakly magnetic, but variable. Supports an iron-bearing volcanic rock; magnetism alone is not diagnostic. - Streak: The rock itself usually does not make a useful mineral streak. A strong red or black powder may indicate iron-rich alteration or a mineral-rich sample. ## Common lookalikes - Andesite: Often medium gray and compositionally intermediate; field boundaries can be ambiguous without chemistry. - Slag: Industrial slag may be glassier, unusually bubbly, locally metallic, or found with furnace debris. - Meteorite: Basalt commonly has vesicles and lacks a continuous fusion crust; magnetism does not make a rock a meteorite. ## Direct side-by-side comparisons - Basalt vs. gabbro — https://lithomap.app/resources/rock-identification/compare/basalt-vs-gabbro - Andesite vs. basalt — https://lithomap.app/resources/rock-identification/compare/andesite-vs-basalt ## Most relevant related guides - Andesite — https://lithomap.app/resources/rock-identification/andesite - Gabbro — https://lithomap.app/resources/rock-identification/gabbro - Amphibolite — https://lithomap.app/resources/rock-identification/amphibolite - Breccia — https://lithomap.app/resources/rock-identification/breccia ## Formation Basalt forms when mafic lava cools at or near the surface. It builds lava flows, volcanic cones, oceanic crust, dikes, and extensive volcanic plateaus. Cooling contraction can produce columns or irregular joints. Fragmental basaltic deposits can form where lava interacts explosively with water or erupts as cinders. ## Occurrence and geologic setting Basalt is abundant in oceanic crust and volcanic provinces, including Hawaii, the Columbia River Basalt Group, and many western volcanic fields. Glaciers and rivers can transport basalt cobbles beyond their source. A dark fine-grained rock is not automatically basalt. Andesite, dark sedimentary rock, hornfels, slag, and weathered intrusive rocks can look similar. ## Uses and significance Basalt is quarried for aggregate, road stone, railroad ballast, and some architectural applications. Melted basalt can also be manufactured into fiber and specialty products. ## Field safety and limitations - Use texture, geologic setting, and a fresh surface together. - Treat cliffs, lava tubes, unstable talus, and recent volcanic terrain as serious hazards. - Never enter closed volcanic areas or collect in national parks without authorization. ## Authoritative sources - Igneous Rocks — U.S. National Park Service: https://www.nps.gov/subjects/geology/igneous.htm - How Rocks Are Classified — U.S. National Park Service: https://www.nps.gov/subjects/geology/how-rocks-are-classified.htm - Rocks and Minerals — U.S. National Park Service: https://www.nps.gov/subjects/rocks/index.htm - What is the difference between a rock and a mineral? — U.S. Geological Survey: https://www.usgs.gov/faqs/what-difference-between-a-rock-and-a-mineral ## Specimen photograph - Image: https://lithomap.app/_delivery/identification/rocks/basalt.webp - Description: Dark, fine-grained basalt hand specimen - Creator: Nessa Eull - License: CC0 1.0 — https://creativecommons.org/publicdomain/zero/1.0/ - Source record: https://commons.wikimedia.org/wiki/File:Basalt_(GeoDIL_number_-_455).jpg This reference supports preliminary field identification. It is not a laboratory determination, appraisal, hazard assessment, proof of origin, or statement of land ownership, legal access, or collecting permission.