Basalt

Learn about basalt, the most common extrusive igneous rock on Earth.

Overview

Basalt is a dark-colored, fine-grained igneous rock that forms from the rapid cooling of mafic lava. It is the most common extrusive rock on Earth, making up most of the ocean floor and many continental volcanic areas. Basalt is rich in iron and magnesium, giving it its characteristic dark color.

What Is Basalt?

Definition: Basalt is a fine-grained, dark-colored igneous rock composed mainly of plagioclase feldspar and pyroxene minerals. It forms from the rapid cooling of mafic (low silica) lava at or near Earth's surface. Composition: Typically contains: - Plagioclase feldspar (calcium-rich) - Pyroxene (augite, hypersthene) - Olivine (in some varieties) - Iron and magnesium oxides Color: Dark gray to black, sometimes with greenish or brownish tints. Texture: Fine-grained (aphanitic) because it cools quickly. Individual mineral grains are usually too small to see without magnification.

Formation and Occurrence

How It Forms: Basalt forms when mafic magma erupts as lava and cools rapidly at Earth's surface. The rapid cooling prevents large crystals from forming, resulting in the fine-grained texture. Where It's Found: - Ocean Floor: Makes up most of the oceanic crust - Continental Volcanic Areas: Lava flows, shield volcanoes - Volcanic Islands: Hawaii, Iceland, and many others - Flood Basalts: Large areas covered by thick basalt flows (Columbia River Plateau, Deccan Traps) Volcanic Features: Basalt creates: - Shield volcanoes (broad, gently sloping) - Lava flows (pahoehoe and aa types) - Lava tubes - Columnar jointing (hexagonal columns)

Physical Properties

Hardness: 6 on Mohs scale (can scratch glass) Density: 2.8-3.0 g/cm³ (denser than most rocks) Fracture: Conchoidal (like glass) when fine-grained Vesicles: Many basalts contain gas bubbles (vesicles) that can be filled with minerals like zeolites, calcite, or agate Weathering: Basalt weathers to form clay minerals and iron oxides, often creating red or brown soils

Varieties and Types

Vesicular Basalt: Contains many gas bubbles (vesicles). If vesicles are filled with minerals, it's called amygdaloidal basalt. Pillow Basalt: Forms underwater, creating pillow-shaped structures. Common on ocean floor. Columnar Basalt: Forms hexagonal columns due to contraction during cooling. Famous examples at Giant's Causeway (Ireland) and Devil's Postpile (California). Scoria: Highly vesicular basalt, darker and denser than pumice. Basalt Porphyry: Contains larger crystals (phenocrysts) in a fine-grained matrix.

Rockhounding Value

Mineral Collecting: Basalt can contain interesting minerals in vesicles and fractures: - Zeolites (beautiful crystals in cavities) - Agate (filling vesicles) - Calcite - Prehnite - Epidote Where to Look: - Road cuts through basalt flows - Quarries - Areas with columnar jointing - Coastal areas with basalt cliffs What to Collect: - Amygdaloidal basalt with mineral-filled vesicles - Specimens showing columnar jointing - Basalt with interesting textures or inclusions