Why Basaltic Lavas Are More Common on Earth Than Rhyolitic Ones

From the vastness of the ocean floors to the towering peaks of the Earth's crust, basaltic lavas have a more prominent presence compared to their rhyolitic counterparts. This article explores the key factors that contribute to the abundance of basaltic lavas on our planet, including magma composition, tectonic settings, viscosity, and the volume and frequency of volcanic eruptions.

Magma Composition and Source

The primary difference between basaltic and rhyolitic lavas lies in their composition and source. Basaltic magma, a more-commonly occurring form, originates from the deep Earth's mantle, which is rich in ultramafic rocks. This magma typically contains about 45-55% silica, making it less viscous and easier to flow compared to rhyolitic magma, which has a silica content ranging between 65-75%.

Tectonic Settings

The tectonic settings where these lava types are found also influence their prevalence. Basaltic lavas are often associated with divergent plate boundaries, such as mid-ocean ridges, and hotspot volcanism, like the Hawaiian Islands. These locations provide ideal conditions for the upwelling of basaltic magma, making eruptions more frequent and extensive.

In contrast, rhyolitic lavas are more commonly found in areas with continental crust and subduction zones. These settings are less widespread, meaning rhyolitic eruptions are less frequent on a global scale.

Viscosity and Eruption Style

The lower viscosity of basaltic magma allows it to flow more easily and frequently, leading to more expansive lava flows and shield volcanoes. This is in stark contrast to rhyolitic magma, which is much more viscous and prone to trapping gases, resulting in more explosive and less frequent eruptions.

Volume of Eruptions

Another significant factor contributing to the abundance of basaltic lavas is the volume of their eruptions. Basaltic lava flows tend to be larger, with more extensive eruptions covering vast areas. This larger scale of eruptions means that the resulting rocks are more commonly found in the geological record, making basaltic lavas more prominent.

Age of Rocks

Many of the Earth's oldest and most extensive volcanic rocks are basaltic. This contributes significantly to their overall abundance, as these rocks have been subjected to long periods of weathering and erosion, leaving behind a large volume of basaltic formations.

In conclusion, the abundance of basaltic lavas can be attributed to their source from the more accessible mantle, their prevalence in common tectonic settings, their lower viscosity enabling more frequent eruptions, and their larger eruption volumes compared to rhyolitic lavas, which are more viscous and erupt less frequently.

Understanding the reasons behind the prevalence of basaltic lavas is crucial for comprehending the geological processes occurring on our planet. These factors not only shape the Earth's surface but also play a significant role in the formation of various geological formations and the global distribution of volcanic rocks.