Sandstone: the story on HearLore

Sandstone

Sandstone is not merely a pile of grains but a complex archive of Earth's history, composed of particles ranging from 0.0625 to 2 millimeters in diameter. These grains, primarily quartz or feldspar, are the most resistant minerals to weathering at the Earth's surface, surviving cycles of erosion that destroy softer materials. While most people see sandstone as a simple building material, it actually comprises about 20 to 25 percent of all sedimentary rocks, forming the backbone of many geological landscapes. The color of these rocks, ranging from tan and brown to vibrant reds and blacks, is dictated by impurities within the minerals, creating the iconic red rock deserts of Arches National Park and other regions of the American Southwest. This visual diversity is not accidental but a direct result of the chemical environment during the rock's formation, where red hematite forms during the early stages of burial.

From River to Rock

The journey of sandstone begins far from its final resting place, starting as bedrock subjected to physical and chemical weathering in areas of high relief such as volcanic arcs or orogenic belts. Eroded sand is transported by rivers or wind to depositional environments where tectonics creates accommodation space for sediments to accumulate. As these sediments continue to pile up, older sand is buried by younger layers, initiating a process known as diagenesis. This transformation involves compaction and lithification, where increasing pressure from overlying sediments forces grains into more compact arrangements. During the early stages of eogenesis, bioturbation and mineralogical changes occur at shallow depths, while deeper burial leads to mesogenesis, where most compaction and cementation take place. Chemical compaction via pressure solution dissolves strained mineral points, allowing grains to come into closer contact and eventually binding them together with secondary minerals like silica or calcite.

The Architecture of Grains

At the microscopic level, sandstone is defined by its framework grains, which make up the bulk of the rock and are classified into quartz, feldspar, and lithic fragments. Quartz framework grains are the dominant minerals due to their exceptional hardness and chemical stability, allowing them to survive multiple recycling events and display varying degrees of rounding. Feldspathic framework grains, including alkali feldspars and plagioclase feldspars, are commonly the second most abundant mineral, while lithic fragments are pieces of ancient source rock that have not yet weathered into individual mineral grains. Accessory minerals such as micas, olivine, and pyroxene make up a small percentage but serve as indicators of sandstone maturity through the ZTR index, which includes zircon, tourmaline, and rutile. The nature of the matrix, the very fine material within the interstitial pore space, further classifies sandstones into arenites, which are texturally clean, or wackes, which are texturally dirty with significant matrix content.

Common questions

What is the diameter range of sandstone grains?

Sandstone grains range from 0.0625 to 2 millimeters in diameter. These particles are primarily quartz or feldspar and represent the most resistant minerals to weathering at the Earth's surface.

How does sandstone form from bedrock to rock?

Sandstone forms when bedrock undergoes physical and chemical weathering to create eroded sand. This sand travels via rivers or wind to depositional environments where tectonics creates space for accumulation and diagenesis transforms the sediments into rock through compaction and lithification.

What minerals make up the framework of sandstone?

The framework grains of sandstone consist primarily of quartz, feldspar, and lithic fragments. Quartz is the dominant mineral due to its hardness and chemical stability, while accessory minerals like zircon, tourmaline, and rutile indicate the rock's maturity.

When did geologists establish the classification scheme for sandstone?

Geologists established the classification scheme for sandstone in 1964 with Dott's scheme. This system modified Gilbert's classification to incorporate dual textural and compositional maturity concepts and set the boundary between arenites and wackes at 15 percent matrix.

What is the difference between orthoquartzite and quartzite?

Orthoquartzite is a sedimentary rock that is 99 percent silica dioxide and preserves original textures, whereas quartzite is a metamorphic rock formed by heat and pressure that recrystallizes grains. Geologists recognized by 1941 that some rocks show quartzite characteristics without high-pressure metamorphism and named them orthoquartzite.

How has sandstone been used since prehistoric times?

Sandstone has been used since prehistoric times for construction, decorative art works, and tools. Examples include the Main Quadrangle of the University of Sydney, sandstone statues like Maria Immaculata by Fidelis Sporer from around 1770, and an 17,000-year-old sandstone oil lamp discovered at the caves of Lascaux, France.