Shale is the most abundant sedimentary rock, blanketing nearly the entire Earth's surface.

Nature · Published 30 September 2026 Swipe more like this →

In short: Though it makes up only a tiny fraction of the Earth's total crust, shale covers the vast majority of the planet's surface. This fine-grained rock, formed from ancient mud, holds the secrets of Earth's history and contains nearly all the organic matter found in sedimentary layers.

Shale is a clastic sedimentary rock composed of silt- and clay-sized particles. While sedimentary rocks represent only about 5 percent of the Earth's crust by volume, they mantle 80 to 90 percent of the planet's surface area. Within this category, shale is the most prevalent, accounting for roughly 70 percent of all sedimentary rocks found in the crust.

The formation of shale begins with the slow deposition of fine particles in quiet environments like river deltas, lakes, and offshore basins. Because individual clay particles settle slowly, they often clump together through flocculation or biological pelletization before reaching the seafloor. Over time, these layers undergo diagenesis—a process of compaction and lithification that transforms soft mud into solid rock. A defining characteristic of shale is its fissility, or the tendency to split into thin, parallel layers, caused by the alignment of clay mineral flakes.

The composition of shale is typically dominated by clay minerals, which make up about 58 percent of an average sample, followed by 28 percent quartz, 6 percent feldspar, 5 percent carbonate minerals, and 2 percent iron oxides. Variations in these minerals, along with the presence of carbonaceous material, dictate the rock's color. For instance, black shales form in anoxic environments and contain significant carbon, while red or green hues indicate the oxidation state of iron. Although shale contains less than one percent organic matter by mass, it is a critical reservoir, holding approximately 95 percent of all organic matter found within sedimentary rocks.

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