How is the age of soil figured?

Question: How is the age of soil figured?

Answer: Soil scientists agree that soils form over time as the climate (temperature, rain, wind, etc.), topography (shape of the landscape), parent material (weathered rocks or stuff deposited by wind, water, ice, or gravity), and living organisms (plants and animals that live in the soil) interact.

juniper-in-soil

To determine the age of a soil, scientists look for clues. For example, a soil can’t be younger than the oldest tree growing in it. Photo of tree growing in Arches National Park, Utah, USA, by Kim Seng.

So, when soil scientists want to know how old a soil is, they look for clues.

Though the climate affects how fast weathering and erosion occur, it usually does not give many clues about how long a soil has been in a given spot. The landscape, parent materials, and trees provide better clues.

A soil can’t be younger than the oldest trees growing in it. It can’t be older than the materials in which it forms or the landscape on which it is found. Soil scientists work with geologists to determine how old the landscape is, and how long the parent materials have been there. Since most deposition of parent materials occurred before written history began, geologists make educated guesses, estimating the age of the landscape (and materials in it) relative to periods of known glaciation, volcanic activity, floods, and similar events.

Soil scientists also know that soils tend to form more rapidly on certain positions on the landscape than on others. More soil formation occurs on flat landscapes in upland positions, for example, than on slopes. Erosion on the slopes limits the rate of soil formation. Soil formation in lowland positions may be slowed by deposition of new materials on the surface by floods or gravity.

–Answered by Clay Robinson, a.k.a., Dr. Dirt

Have a question for Soils Matter? Post it here or email us at soils-matter@soils.org

 

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One response to “How is the age of soil figured?

  1. I do not entirely agree with the answer. Considering soil consists of more living organisms than minerals, the age and the soil itself would be ever changing. Also soils can become rocks ( clay to shale) making the unconsolidated material older.

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