The Prevalence of White Sand Over Black Sand: Understanding the Geology Behind It
The Prevalence of White Sand Over Black Sand: Understanding the Geology Behind It
When it comes to the most common type of sand on the world’s beaches, white sand beaches vastly outnumber their black sand counterparts. This article delves into the geology behind the distribution of white and black sand, providing a comprehensive understanding of why white sand beaches are more prevalent.
White Sand vs. Black Sand: A Comparative Overview
White sand is generally more common than black sand. White sand typically comes from the weathering of quartz and other light-colored minerals. It is often found on beaches in tropical areas such as the Caribbean and parts of Southeast Asia. In contrast, black sand is usually formed from volcanic materials, such as basalt, and is less prevalent. It can be found on beaches near volcanic islands and regions with significant volcanic activity like Hawaii and Iceland.
While black sand beaches are striking and unique, they represent a smaller percentage of global beach environments.
Why White Sand Beaches Are More Common
Simply put, white sand beaches are more common because the weathering of minerals that form white sand is generally more widespread. However, to understand this, we need to delve deeper into the geological factors at play.
The Dominance of Light-Colored Rocks
Basalt, a black volcanic rock, might be the most common rock on Earth, but most of it is located at the bottom of ocean basins. It is not easy to get most of that basalt onto a continent where it can weather down to black sand. On continents, rocks containing lighter grains are much more prevalent. When these rocks weather, they release small sediment grains that might eventually end up as part of a beach.
Mineral Weathering Rates
The weathering of minerals into sand grains is a critical factor. Different minerals weather at different rates. A key distinction can be made between two broad categories of igneous minerals: mafic and felsic.
Understanding the Bowen's Reaction Series
The mineral content of weathered sediments can be understood by examining the Bowen's Reaction Series, which describes how minerals crystallize out of igneous melts as they cool. This series uses temperature to predict which minerals form at different stages of cooling. Generally, minerals with higher iron content, known as mafic, are darker, while those with lower iron content, known as felsic, are lighter.
Fig 1. Bowen's reaction series. The color of the arrows and boxes indicates the general color of each mineral. Ultraprotic to intermediate minerals are darker while felsic ones are light.
These minerals weather more quickly and break down into sand grains more readily than their lighter counterparts. The process also involves the removal of these minerals from the rock cycle, where they undergo weathering, erosion, transport, and deposition, becoming increasingly rounded and smooth over time.
The Weathering Process and Granule Rounding
Grains of sand move through the rock cycle, being weathered, transported, deposited, and uplifted. Over time, weathering and erosion smooth the edges and make the grains more rounded. Newly formed grains are typically sharp and jagged, while older grains are well-rounded and smooth. The degree of rounding is a sign of the age of the grain: highly rounded grains are older, while subrounded grains are more recent.
Fig 2. Rounding classification of sediment grains. Well-rounded grains are older than subrounded grains.
In juvenile beaches, grains are a mix of different minerals with varying levels of roundness. As beaches mature, fewer dark grains remain, replaced by felsic minerals, such as quartz, which are much lighter. This results in mature beaches having predominantly light-colored sand, often ranging from light brown to pinkish to white.
Additional Considerations
Beaches formed from sedimentary rocks are also typically light in color. Most sediment originates from igneous rocks, and as we have established, the light-colored grains last longer. Additionally, grains like calcite, which make up limestone, are usually light-colored.
Conclusion
In summary, light sand beaches dominate because the types of minerals that form white sand are more common on continents and last longer than their darker colored counterparts.