Basalt

Basalt is an aphanitic, or fine-grained, extrusive igneous rock. It forms when low-viscosity lava cools rapidly at or near a planet's surface. This lava is mafic, meaning it is rich in magnesium and iron. Basalt is incredibly common on Earth. In fact, more than 90% of all volcanic rock on our planet is basalt. It is also a major component of other worlds in our Solar System. For example, basalt covers about 80% of the surface of Venus. It also forms the lunar maria on the Moon and is common on Mars.

The formation of basalt begins deep within the Earth's mantle. Most basalt is created through a process called decompression melting. In the upper mantle, high pressure keeps most rock in a solid state. However, the mantle is ductile, so it can slowly deform under stress. When tectonic forces push this hot rock upward, the pressure decreases. This drop in pressure lowers the melting point of the rock. This causes the rock to partially melt into basaltic magma. This magma can then rise to the surface through various tectonic settings.

Basaltic lava has a relatively low viscosity, which means it flows easily. This is because it has a low silica content, specifically between 45% and 52%. You might compare its flow to the consistency of ketchup. This low viscosity allows lava to spread over massive areas before it solidifies. When many such flows stack up, they form thick sequences called flood basalts. These are the most voluminous volcanic formations in existence. They can cover hundreds of thousands of square kilometers.

Geologists use several methods to classify basalt. One way is by looking at the mineral content using a QAPF diagram. To be basalt, the rock must have less than 10% feldspathoid and less than 20% quartz. Additionally, plagioclase must make up at least 65% of its feldspar content. Because the grains are so small, scientists often use chemical classification instead. This is called the TAS classification. In this system, basalt must have between 45% and 52% silica and no more than 5% alkali metal oxides.

Basalt can display many different physical textures and appearances. It is often porphyritic, meaning it has large crystals called phenocrysts embedded in a fine matrix. These phenocrysts, like olivine or augite, formed before the lava erupted. You may also see vesicles, which are small holes in the rock. These form when dissolved gases bubble out of the magma as it nears the surface. If the rock is mostly made of these holes, it is called scoria.

In some cases, basalt forms striking geometric shapes. Large masses that cool slowly can develop a polygonal joint pattern. This creates tall, pillar-like columns, such as those at the Giant's Causeway.

There are several distinct types of basalt based on their chemistry. Tholeiitic basalt is rich in iron and poor in alkali metals. This type makes up most of the ocean floor and large oceanic islands. Another type is alkali basalt, which is rich in alkali metals and often found in continental rifting zones. There are also high-alumina basalts, which are common in volcanic arcs above subduction zones. Some specialized types exist, such as boninite, which is a high-magnesium form found in back-arc basins.

The study of basalt provides vital clues about the deep interior of planets. Because basaltic magmas originate in the mantle, their chemistry reflects deep processes. For instance, mid-ocean ridge basalts, or MORB, help scientists understand mantle inhomogeneity. Scientists categorize MORB into types like N-MORB, E-MORB, and D-MORB based on their element depletion. Even the magnetic signatures of cooling basalt are useful. Paleomagnetic studies use these signatures to understand the history of Earth's magnetic field.