Why is basalt referred to as extrusive volcanic rock?

What is extrusive volcanic rock?

Basalt is a wonderfully hard natural stone, and one of the most abundant rocks on Earth. Even on Mars, basalt thrives – making up Olympus Mons, the highest topographic features known on the planet.

Just about anywhere there's a volcano, you'll find basalt. After all, this natural stone is geologically referred to as an extrusive or volcanic rock. Let's take a look at what that means, how it determines the qualities of basalt and various uses for this brilliant stone.

What is extrusive (volcanic) rock?

Before we discuss the extrusive quality of basalt, it's important to understand that it's an igneous rock. This means it's formed through the cooling and solidification of magma or lava.

Depending on where and how the magma cools, igneous rocks are described as either intrusive or extrusive.

An intrusive rock is formed when magma solidifies below the surface of the Earth. With the insulation of the ground, the cooling process is a lot slower, allowing for the formation of larger mineral crystals. Granite is a great example of this kind of stone, as you can usually see the variation of unique minerals clearly.

Meanwhile, extrusive rock forms much more rapidly, above the surface of the Earth (and often underwater). The swift formation of the stone means that most extrusive rocks have tiny crystals, barely distinguishable by the human eye – this texture is described as aphanitic. Take obsidian, for example, which cools so quickly that crystals do not have time to form at all and a natural glass is made instead. Because extrusive rocks generally result from volcanic activity, they are also commonly called volcanic rocks.

Quite often, the magma which forms extrusive rock will have already cooled somewhat before erupting, meaning some larger crystals have already formed. This is described as porphyritic extrusive rock.

Qualities of basalt as an extrusive rock

Basalt is the most widespread extrusive rock in the world – though much of it lies beneath the ocean. Its abundance and brilliant qualities make it a popular choice for a variety of purposes in construction, interiors and landscape design. There qualities include:

Density: The relatively low silica content of basaltic lava means it has a lower viscosity and travels very quickly. As a result, the magma does not partially cool before eruption and a dense, aphanitic texture is formed.

Porosity and strength: As a result of its density and mineral makeup, basalt is both highly non-porous and strong. On the Mohs scale of mineral hardness, basalt scores a six – meaning it's harder than platinum or iron.

Colours: Another geological category basalt belongs to is mafic stone. These are stones with a high volume of iron and magnesium, which grants them a rich dark colour – often grey, black or green. In some cases, the iron content of basalt can oxidise and turn the stone red or brown. The natural variety of colour in basalt makes it a popular choice for decorative applications.

Uses of extrusive basalt

Basalt is coveted for its immense strength. As such, it's commonly incorporated into aggregate materials for building projects and road bases (e.g., asphalt and concrete). However, basalt is a wonderfully functional and attractive stone without aggregation.

This natural stone has long been used in tiling and bricks throughout history, featuring on paved roads in Rome or ancient structures in South American ruins.

Today, basalt can be used for:

  • Floors or walls, including in commercial environments.
  • Bathrooms, showers or swimming pool surrounds.
  • Monuments, columns or garden features.
  • Kitchen or bathroom countertops.
  • Outdoor or indoor design.

Limitlessly beautiful and immensely versatile in its applications, basalt is the perfect accompaniment to any design project. For more information about basalt, reach out to BauMart Natural Stone today.