Basalt is a common extrusive igneous rock, forming from the rapid cooling of molten lava at or near the Earth’s surface. It is rich in magnesium and iron, making it a mafic rock. Basalt is the most widespread volcanic rock type on Earth, accounting for over 90% of all volcanic rocks.
The Predominant Color of Basalt
Basalt typically exhibits a dark gray to black coloration. This characteristic dark hue is primarily a direct result of its mineral composition, which includes mafic minerals rich in iron and magnesium.
The dominant minerals contributing to basalt’s dark color include pyroxene, particularly augite, along with olivine and calcic plagioclase feldspar. Iron oxides like magnetite are also present and further enhance the rock’s dark appearance. These minerals are inherently dark-colored due to their elemental makeup. Rapid cooling results in a fine-grained texture where individual mineral crystals are too small to be seen. This fine-grained nature contributes to the rock’s uniform dark color, as light is absorbed rather than reflected.
Understanding Basalt’s Color Variations
While basalt is predominantly dark, its color can vary due to several geological factors, including weathering, the presence of specific minor minerals, and alteration processes. Weathering is a significant contributor to color changes. The oxidation of iron-rich minerals within the basalt, for example, can lead to reddish or brownish hues on the rock’s surface. This occurs as iron reacts with oxygen in the air and water, forming iron oxides such as hematite or goethite, which are reddish-brown.
Chemical weathering can also break down primary minerals like pyroxene and olivine into clay minerals, altering the rock’s original color. Initially, fresh basalt may display iridescent colors like blues, purples, or greens before significant weathering begins. Over time, however, the surface can turn grayish before fully oxidizing to reddish tones.
The presence of certain minor minerals or the infilling of pores can also influence basalt’s color. Some less common types, known as leucobasalts, may appear lighter due to a higher proportion of plagioclase feldspar. Basalt often contains vesicles, which are small cavities formed by trapped gas bubbles during cooling. These vesicles can later be filled by lighter-colored secondary minerals, such as zeolites, calcite, or quartz, creating a mottled or speckled appearance that lightens the rock’s overall shade. Additionally, various alteration processes can introduce greenish colors from minerals like celadonite or brownish colors from iddingsite, which forms from altered olivine.