Regolith

Regolithic soil from granulite rock.

Regolith - All unconsolidated earth materials above the solid bedrock. It includes material weathered in place from all kinds of bedrock and alluvial, glacial, eolian, lacustrine, and pyroclastic deposits. Soil scientists regard as soil only that part of the regolith that is modified by organisms and soil-forming processes. Most engineers describe the whole regolith, even to a great depth, as "soil."

Evolution of terminology

The term regolith was first proposed by George Perkins Merrill in 1897 and comes from the Greek words rhegos (ῥῆγος), meaning "blanket," and lithos (λίθος), meaning "stone":

"Let the reader for a moment picture to himself the present condition of the crust, with particular reference to the land areas. Everywhere, with the exception of the comparatively limited portions laid bare by ice or stream erosion, or on the steepest mountain slopes, the underlying rocks are covered by an incoherent mass of varying thickness composed of materials essentially the same as those which make up the rocks themselves, but in greatly varying conditions of mechanical aggregation and chemical combination."

"In places this covering is made up of material originating through rock-weathering or plant growth in situ. In other instances it is of fragmental and more or less decomposed matter drifted by wind, water or ice from other sources. This entire mantle of unconsolidated material, whatever its nature or origin, it is proposed to call the regolith..."

Richard Eggleton (2001) later refines the definition to be:

"The entire unconsolidated or secondarily recemented cover that overlies more coherent bedrock, that has been formed by weathering, erosion, transport and/or deposition of the older material. The regolith thus includes fractured and weathered basement rocks, saprolites, soils, organic accumulations, volcanic material, glacial deposits, colluvium, alluvium, evaporitic sediments, aeolian deposits and ground water."

Or, more simply stated:

"Everything from fresh rock to fresh air."

Composition

Regolith is composed of a variety of materials that can be derived from both from the underlying bedrock and the chemical and biologic process occurring at or near the Earth's surface, including:

• weathered rock - physically weathered rock consisting of fragments of the original bedrock or chemically weathered rock composed of new minerals and solutions
• eruptive volcanic materials - fresh or weathered, including loose or welded ash, lava or hyaloclastite
• sediments - mainly unconsolidated, but also the products of chemical alteration such as oxidation-reduction or acid-base reactions.
• gases - O₂, N₂, CH₄, H₂O
• water - aerosols and liquid that may contain ions such as iron, calcium, sodium, chlorides and carbonates.
• biota - plant roots, invertebrates and micro-organisms

Regolith science

The study of regolith is known as "regolith science" or "regolith geoscience," and includes the field of regolith stratigraphy, and the classification of regolith facies, regolith landform units and regolith-landform regimes. Regolith science is defined as the scientific study of regolith and the landscapes in which it occurs, and the interrelationship between the lithosphere, the hydrosphere, the atmosphere and the biosphere. Regolith may be thought of as originating in and existing at the interface of these four "spheres."

A regolith unit, as defined by Eggleton, is "a subdivision of the regolith, generally mappable, and having visibly distinguishable boundaries, unless defined outside the visible spectrum using remotely sensed data. The term may be used for zones or horizons of weathering profiles such as soil, duricrust, gravel, mottled regolith, saprolite, etc., or mappable entities with boundaries associated with a change in landform."

Regolith science is an important aspect to the study of ecology and biodiversity, sustainable natural resource management, climate change, soil and civil engineering, mineral exploration and agriculture. Regolith-derived natural resources include:

• Bauxite (aluminum ore)
• Alumina (an aluminum oxide)
• Gold
• Iron ore
• Magnesite (magnesium carbonate)
• Manganese (an important element of metal alloys)
• Diamond
• Clays
• Gypsum (a hydrate of calcium sulfate)
• Opal
• Alluvial sapphires (an aluminum oxide)
• Ilmenite (a titanium-iron oxide)
• Rutile (titanium oxide)
• Zircon (a silicate of zirconium)
• Salt