Basalt

Basalt is a type of rock made mostly from oxides of silicon, iron, magnesium, potassium, aluminium, titanium, and calcium. It forms when lava cools quickly at or near the surface of a planet or moon. Basalt is usually dark grey or black because of minerals like augite, but it can also be lighter if it has more plagioclase.

Basalt is very common, making up more than 90% of all volcanic rock on Earth. It is fine-grained because it cools quickly, and it has the same chemical makeup as gabbro, which cools slowly and becomes coarse-grained. The density of basalt is about 2.9 g/cm³, a bit heavier than granite. Basalt can have small holes called vesicles from gas bubbles that didn’t escape before the lava hardened.

Petrology

Basalt is made of certain minerals, mainly calcic plagioclase feldspar and pyroxene. It can also contain olivine. Other smaller minerals found in basalt include iron oxides and iron-titanium oxides such as magnetite, ulvöspinel, and ilmenite. These minerals give basalt a strong magnetic property.

Tholeiitic basalt contains pyroxene and calcium-rich plagioclase. Alkali basalts have minerals like labradorite, andesine, and augite.

Basalt has high temperatures when it becomes liquid and when it hardens. These temperatures are higher than most other volcanic rocks.

Geochemistry

Basalt has lots of magnesium and calcium but less silica and alkali oxides compared to other rocks. It contains between 45–52 percent silica.

Morphology and textures

The shape and texture of basalt tells us how it erupted. Basalt that erupts in the air can form different types of lava flows, like scoria, ash, or cinder. When it cools quickly, it can form columns with hexagonal shapes.

Subaerial eruptions

Main article: Subaerial eruption

When basalt erupts on land, it can form three types of deposits: scoria, ash or cinder, and lava flows. The top of lava flows and cinder cones often have a frothy texture. In Hawaiʻi, two common types of basalt lava flows are ʻaʻā and pāhoehoe. Pāhoehoe lava can form lava tubes.

Submarine eruption

When basalt erupts underwater, it forms pillow shapes due to the quick cooling by water. These pillow shapes are common in underwater lava flows. The island of Surtsey in the Atlantic Ocean is an example of a basalt volcano that appeared above the sea surface in 1963.

Distribution

Earth

Basalt is the most common type of volcanic rock on Earth, making up more than 90% of all volcanic rock. It forms the outer layers of ocean floors and many islands, like Hawaiʻi. Scientists see basalt lava erupting at about 20 volcanoes each year.

Basalt is also found in huge areas called large igneous provinces. These include places like the Deccan Traps in India and the Columbia River Plateau in Washington and Oregon. It is also common around volcanic arcs with thin crust.

Other bodies in the Solar System

Basalt is not just on Earth; it is found on other worlds too. It is common on Io, a moon of Jupiter, and has formed on the Moon, Mars, Venus, and the asteroid Vesta.

The Moon

The dark areas on the Moon's surface are made of basalt lava flows. Samples of these rocks were collected by both people on the Apollo program and robots from the Luna program.

Lunar basalts have more iron than those on Earth and vary in the amount of a mineral called titanium. They also look different because they have not been changed by water or oxygen.

Venus

Landers from space missions found that the surface of Venus is mostly made of basalt. Some parts look like they have been volcanically active in the last few million years.

Mars

Data from Mars shows that basalt is a common rock on its surface.

Vesta

Images from space telescopes suggest that the asteroid Vesta has a crust made of basalt.

Io

Early studies thought the lava on Io was made of sulfur, but later measurements show it is actually basalt lava. The temperatures of these lava flows are very high, at least 1,300 K and sometimes as high as 1,600 K.

Alteration of basalt

Weathering

See also: Weathering

Basalt weathers faster than granitic rocks when exposed on Earth’s surface. This is because the minerals in basalt formed at higher temperatures and in environments with less water vapor. These minerals become less stable in the colder, wetter conditions at Earth’s surface. The fine grains and volcanic glass in basalt also speed up weathering.

In humid areas, basalt often turns brown or rust-red because of iron turning into iron oxides. Weathering changes basalt into clay rich in calcium rather than potassium. In tropical areas, this clay can further change into a special type of soil called laterite. The final result of weathering can be bauxite, which is used to make aluminium.

Weathering also releases minerals like calcium, sodium, and magnesium. These minerals help protect areas with basalt from becoming too acidic. The calcium can trap carbon dioxide from the air, forming calcium carbonate.

Metamorphism

When basalt is changed by intense heat or pressure, it turns into different kinds of metamorphic rocks. Depending on the heat and pressure, basalt can change into rocks like greenschist, amphibolite, or eclogite. These changed rocks help scientists understand the conditions that affected the area.

Metamorphosed basalt can also contain important minerals, such as gold, copper, and volcanogenic massive sulfides.

Life on basaltic rocks

The way underwater volcanic basalt wears down hints that tiny living things might help change the chemicals in seawater. Basaltic rocks contain lots of iron and manganese, which can give energy to certain bacteria. Scientists have found bacteria that can live and grow using these rocks, especially around places like the Kamaʻehuakanaloa Seamount (formerly Loihi). These tiny creatures might change both the rocks and the ocean around them, which could even help explain how life first began in places like hydrothermal vents and affect the oceanic crust and the origin of life.

Uses

Basalt is used in building things like houses and roads, and it can be made into smooth stones for pathways or turned into statues. When heated and shaped, basalt can also become a kind of wool that helps keep things warm.

Scientists have looked at using basalt to help remove a gas called carbon dioxide from the air. This gas is made when people do things like driving cars. When basalt is placed deep in the ocean, the water helps keep the gas from coming back into the air.