Weathering Mantle

(or regolith), continental geological rock aggregates that form on the earth’s surface as a result of rock weathering. Alteration products that have remained in situ are called residual mantle, while those that have been moved short distances but have not lost their relation with the parent rock are called transported mantle. Some geologists include the products of erosion and the transportation of soils and residual mantle in the weathering mantle, calling them the accumulative mantle (proluvium, talus deposits, and the like).

By the mode of occurrence a distinction is made between areal mantle, which covers the bedrock in a sheet (from dozens of centimeters to a few dozen meters thick), and linear mantle, which extends in one direction and penetrates deep into the bedrock along cracks (tapering out at depths of several dozen meters from the earth’s surface and, more rarely, reaching depths of 100–200 to 1,500 m).

Study of the weathering mantle and the processes of its formation was begun in the mid-19th century by the Russian scientists V. V. Dokuchaev and K. D. Glinka, among others. Detailed investigations of the mantle were conducted in the 1920’s. In the first half of the 20th century the theory of the weathering mantle became an independent division of geology. The founders of this area of study were B. B. Polynoν (the present weathering mantle) and I. I. Ginzburg (the ancient weathering mantle). Significant contributions to the theory of the weathering mantle have been made abroad by the Swedish scientist W. Tarr, the American scientist W. Keller, and the German geologist H. Harraso-witz, among others.

In the process of weathering, various intermediate and final products of decomposition may be dissolved and carried away by near-surface waters. Their migration takes place in the form of suspensions and colloidal and true solutions. Although it is significant in some cases, mechanical transportation of powder-like products of the weathering mantle by water causes little change in the total chemical composition of the mantle. Colloidal and true solutions have a much more significant effect. As a result of the decomposition of the mineral mass of the bedrock and selective migration of elements, weathering mantles with different compositions and different weathering profiles and with mineral deposits characteristic of the particular mantle occur. Weathering mantles with different profiles have various mineral and chemical zones that give way to one another on the vertical, from the slightly altered bedrock to the intensively altered outcropping rocks, and that are characteristic of the particular mantle. The formation of the weathering mantle depends on climate, the composition of the bedrock, hydrogeological conditions, the relief of the terrain, tectonic structure, the duration of development, the age during which development occurred, and degree of mobility of the earth’s crust.

The thickest weathering mantles form in periods of tectonic inactivity in regions with moist, warm climates. Decomposition of a large mass of organic matter leads to the formation of CO₂ and organic acids that filter from the soil into the weathering mantle, decompose rocks at a deep level, and leach out the soluble products of weathering. Most of the mobile elements, such as Ca, Mg, Na, K, Si, and many rare metals, are removed from the mantle. The weathering mantle becomes enriched, relatively speaking, with the least mobile elements, including Fe, Al, Ti, and Zr; ferric and aluminum hydroxides, kaolinite, halloy-site, and other clay minerals also form. Ferric hydroxides give the weathering mantle a red and brown color. During a quiescent tectonic period the weathering mantle in the moist tropics reaches a thickness of dozens of meters, and in fracture zones it reaches a thickness of hundreds of meters. Depending on the mineral composition, different types of leached weathering mantles are distinguished, including kaolin and laterite mantles.

Under conditions of tectonic uplifts and broken terrain, the thickness of the weathering mantle is significantly less, even in a moist, wet climate. In a temperate moist climate, and the more so in an arid and cold climate, the processes of weathering penetrate even less deeply and the intensity of rock alteration is also minimal. In a dry climate Ca is not transported far and carbonate and even gypsum weathering mantles occur. In a cold climate and in high mountains, only a shallow detrital weathering mantle forms in some places, often coinciding with the soil.

Dependence on climate determines the latitudinal zonation of the distribution of the weathering mantle. Mantle zones are broader than geographic and soil zones. (One mantle zone is characteristic for several soil zones.) During past geological ages, in what is now the USSR, thick, acidic, leached weathering mantles formed for many millions of years in a moist, warm climate under conditions of tectonic inactivity. These “ancient weathering mantles” have been partially preserved under a layer of sedimentary deposits or crop out on the earth’s surface. In some places they have been subject to subsequent alteration—for example, gypsum formation, salinization, and gleying. The processes of formation of the ancient weathering mantle were most widespread in the Upper Triassic and Lower Jurassic, but weathering mantles of Precambrian, Paleozoic, and post-Jurassic age are also known.

In the USSR, deposits of nickel, iron, chrome, and aluminum ores, rare elements, magnesite, kaolin, and other minerals are associated with the ancient weathering mantle.