Basalt is a common volcanic rock formed when molten rock cools quickly at or near the Earth’s surface. This dark-colored igneous rock makes up over 90% of all volcanic rock on our planet. It is a prevalent component of oceanic crust and can also be found on other planetary bodies like the Moon and Mars.
Defining Rock Hardness
Geological rock hardness refers to a material’s resistance to scratching or abrasion. This property differs from toughness, which measures resistance to fracturing, or compressive strength, which indicates resistance to crushing. The Mohs Hardness Scale is the most common method for evaluating this characteristic.
Developed in 1812 by Friedrich Mohs, this qualitative scale ranges from 1 to 10. It ranks minerals based on their ability to scratch one another, with a higher number indicating a harder material. For example, talc is rated as 1, while diamond is a 10.
Basalt’s Measured Hardness
Basalt typically exhibits a hardness ranging from 5 to 6 on the Mohs scale. This means basalt can scratch common glass, which has a Mohs hardness of approximately 5.5. Conversely, a steel file, with a Mohs hardness of about 6.5, can generally scratch basalt. Beyond scratch resistance, basalt also possesses significant compressive strength, its ability to withstand crushing forces. This strength typically ranges from 100 to 300 megapascals (MPa), with some types reaching up to 400 MPa.
What Makes Basalt Hard
Basalt’s hardness stems from its mineral composition and formation process. It is primarily composed of hard minerals like plagioclase feldspar, pyroxene, and often olivine. These mafic minerals contribute to the rock’s overall strength. Basalt forms from the rapid cooling of lava, resulting in a fine-grained, or aphanitic, texture. This rapid cooling prevents large mineral crystals from growing, leading to small, tightly interlocked grains whose compact arrangement and lack of significant pore space enhance basalt’s resistance to abrasion and crushing.
Everyday Uses of Hard Basalt
The hardness and durability of basalt make it a valuable material across various industries. Its resistance to wear and crushing leads to widespread use as construction aggregate, including road bases, concrete mixtures, and asphalt pavements. Basalt is also cut and shaped for dimension stone applications. It is used for paving stones, building facades, and wall cladding due to its resilience. Its robust nature makes it suitable for railway ballast, providing stable support and drainage for train tracks, and is also used for countertops and in hydraulic works like dams.