4.7: Regolith

The Nature of Regolith

What is regolith? The term regolith is used for the layer or mantle of fragmental and unconsolidated rock and mineral material, whether residual or transported, that rests on bedrock. One might quibble that the definition excludes fragmental material that happens to be locally cemented by surficial processes but is otherwise closely related to nearby unconsolidated material. (You’ll be hearing more, much later in the course, about one such kind of material: caliche.) The definition is not specific about age; most regolith is young, geologically and even by human standards, but some is much older, not having been subjected to consolidation, for one reason or another, for geologically long times.

For most of us geologists, continental bedrock is our bread and butter: we inspect it minutely, hammer on it, sample it, photograph it, and think about it day and night. But in most parts of the world, finding bedrock to inspect is not easy, and in many places almost impossible: it is covered by a layer of regolith. When was the last time you walked across an outcrop of bedrock? When we are out and about in the natural world, away from the ubiquitous pavement that’s so characteristic of our civilization, we are mostly walking on regolith. If only for that reason alone, study of the Earth’s regolith should be high on the list of desiderata for Earth scientists. A good case can be made that the Earth’s mantle of regolith is for humankind by far the most important component of the solid Earth beneath our feet: it feeds us, in the form of soil. In fact, however, there is a tendency for regolith studies to be low on the priority list. (I think that there lurks in the minds of many geologists the notion that bedrock studies hold far more interest, and glory, than regolith studies.)

Regolith originates by the complex of processes collectively termed weathering, which was the subject of the preceding section. What happens to regolith once it is produced? You can detect from the foregoing definition that regolith is either residual or transported. The terms are largely self-explanatory, are they not? In areas of low relief and with a climate conducive to deep weathering, a thick mantle of regolith, in some places many tens of meters thick, lies where it has been produced. In most places, however, the regolith we see has been moved from its place of origin, nearby or far away, by various agents of transportation.

Keep in mind, however, that it’s usually not a matter of an episode of production and then a later episode of transportation: generally, both production and removal are going on at the same time. The thickness of the mantle of regolith depends in great part on the relative magnitude of the rate of production and the rate of removal. In some situations, as in regions with steep slopes, high relief, and rigorous climate, regolith is stripped away as fast as it is produced; in areas with gentle slopes and climates conducive to deep weathering, the layer of regolith is very thick.

Agents That Mobilize Regolith

Regolith resting on a sloping surface is pulled inexorably downward by the force of gravity (or, more accurately, by the downslope component of the force of gravity) (Figure 2-19). Such movement of the regolith may be imperceptibly slow or at speeds in excess of a hundred meters per second. An entire later chapter will be devoted to the subject of downslope movements of near-surface material by gravity, collectively termed mass movements (absolutely no relation to sociopolitical mass movements).

Figure 2-19. The downslope component of gravity acting on the layer of regolith. A) On a steep slope. B) On a less steep slope.

The principal agent for the transport of regolith, aside from the direct pull of gravity, is the flow of water in streams and rivers. Of course, river flow is itself a consequence of the downslope pull of gravity—but regolith transport by flowing water involves physical processes and effects that are fundamentally different from the direct pull of gravity. The mobilization and transportation of regolith by flowing water is the subject of part of the later chapter on streams and rivers. Movement of regolith by the action of land-based glaciers is of great importance in certain Earth-surface environments as well. Wind can also be a significant transporting agent, as described in the final chapter.

Kinds of Regolith

The main distinction in kinds of regolith is between residual regolith (also called sedentary regolith), on the one hand, and transported regolith, on the other hand (Table 2-1). With respect to transported regolith, several kinds are recognized, depending upon the agent of transportation. Transported regolith is collectively termed sediment. There will be much more material, later in this chapter as well as in later chapters, on sediment, and how it is eroded, transported, and deposited. Colluvial deposits (called colluvium) are those transported down steep slopes by the pull of gravity. Mainly, these are talus deposits (also called scree deposits) on steep slopes, and avalanche and landslide deposits of various kinds. Alluvial deposits (called alluvium) are those transported and deposited by rivers and streams. In addition, there are glacial deposits and eolian (wind-blown) deposits.

Table 2-1. Classification of regolith. (After Merrill, 1897.)