When does rock become soil?

Rock starts becoming soil the moment it is exposed to the environment. But it’s a long transformation process from exposed rock to a mature soil. Depending on the nature of the rock and other factors in its surroundings, that time period can range between tens to tens of thousands of years!

Rocks, plants and sand
The sandstone, top layer, is weathered by climate, organisms and time. Lichen – a mixture of fungi and bacteria – help break the rock down. Eventually plants can grow in the space, adding organic matter as they grow and die. Photo: SVFisk

Soil is not simply weathered rock. Soil is a dynamic natural resource. It is comprised of minerals, water, gases, organic material, and living creatures including soil microbes and tiny animals. Calling a soil “mature” doesn’t mean that soil formation has stopped. It means the changes in the soil have become practically imperceptible as the soil comes into dynamic equilibrium with its environment. That means the rate at which soil is forming is about equal to the rate at which the soil is breaking down or naturally eroding away.

Soil scientists look at five major factors in the soil-forming process. These factors are: climate (Cl), organisms (O), relief (R) or topography and drainage of the land, the rock or other parent (P) material that will become the soil, and how much time (T) has gone by.  These can be combined into a “recipe” for soil known as ClORPT.1 Soil scientists can study each factor separately, or in combinations, to find out how a particular soil became what it is. They can even predict what kind of soil will eventually mature in that place.

Let’s look at what it takes to turn rock into a soil. Under the action of heat, cold, rain, wind, and other atmospheric factors, the rock breaks down physically into small fragments that become the parent material of the soil. The rock also chemically changes as the compounds in the rock dissolve in rain or react with air. Rock is also broken down biologically by living organisms in contact with the rock and its fragments. These processes are collectively called weathering.

Plants, soil and roots
Roots of plants help break up parent materials, and provide organic matter to maturing soils. Photo: D. Weindorf

As the soil-forming process continues over time, plants become established. Wind, birds, and other animals deposit seeds. When water comes in contact with the seeds, they may sprout. Their roots penetrate the disintegrated rock fragments, and their shoots are supported by the material. Roots take up chemicals released by the rock as it breaks down, and use some of the chemicals as nutrients. In turn, plants add organic material in the form of roots and leaves to the disintegrating rock environment. This encourages the growth of microorganisms and tiny animal populations.2 These living things degrade the fallen leaves and dead roots into organic fragments, which provide more nutrients. It also improves the capacity of the material to store moisture necessary for continual growth of plants in the forming soil.3

Weathering continues until everything that can be weathered is weathered, and the rock is no longer recognizable as rock. Disintegrated rock, however, is not yet a soil. This point must be emphasized. It is not until the dynamic equilibrium mentioned above is achieved that we truly have soil.

The rates of all these processes change with time. Rain percolating through young soil carries smaller particles and soluble compounds downward. Evaporation carries soluble compounds upward toward the surface. Sudden floods or avalanches can tear through the area, carrying weathered material away and bringing new material in its place. Human activities can change the nature of the surface.

The processes of weathering and soil formation go on, not only on the surface, but also below the surface within the soil itself. When mature, a soil can extend downward to bedrock from a few inches below the surface to up to eight feet or more, depending on environmental conditions and the original nature of the rock.

Soil is an amazing substance; human survival depends on it. The type of rock that is its parent will influence many characteristics of the soil. But climate, organisms, the shape of the land and time also influence soil formation. For more information on soils, visit www.soils.org/discover-soils.

Answered by Barbara-Ann G. Lewis, retired from Northwestern University

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  1. A more accurate form of the acronym is Cl,O,R,P,T, but it is commonly referred to as ClORPT for simplicity purposes.
  2. To learn more about large, small, and microscopic life that calls soil its home, visit these Soils Matter blogs: https://soilsmatter.wordpress.com/2015/06/30/what-types-of-animals-live-in-the-soil-why-is-soil-condition-important-to-them/ and https://soilsmatter.wordpress.com/2015/07/14/is-it-true-bacteria-live-in-the-soil-isnt-that-bad/
  3. To learn more about how soils clean and capture water, visit this SSSA YouTube video: https://www.youtube.com/watch?v=ZwQeTJEeedk

33 thoughts on “When does rock become soil?

  1. I work on the Ozark-St Francis National Forest, as the Forest Soil Scientist. The bedrock acres of the Ozark portion of the forest are located in the Boston Mountains and Ozark Plateau/Highlands. The geology is a series of shale, sandstones and limestone. There are benches with landslide deposits of various types. The soils and bedrock are deeply weathered and consist of residuum/colluvium parent materials. Using geotechnical methods to listen to the bedrock groan is an interesting idea. I would like to learn more.

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