AboutDr Thomas Bell Expertise I can answer questions regarding surface earth processes and the chemical transformations that sediments and rocks undergo with burial. I can also answer questions regarding deep time, the evolution of the elements, and the last 4.5 billion years of earth history. I specialize in metallic ore forming processes, the major geologic time periods when they were produced and what they tell us about the evolution of our planet. Learn more about my professional interests at Stratamodel.com.
Experience I am a professional consulting geologist with a background in the petroleum, mining, environmental, and geotechnical industries with over 25 years of experience.
Education/Credentials Ph.D., Geology, University of California at Berkeley, 1984
M.A., Geology, University of California at Berkeley, 1980
B.S., Geology, San Jose State University, 1978
Question Hello. If I dug a 6 foot deep hole in my yard how far back in time would the layers of dirt be. I guess I am saying is how long would it take for the surface of my front yard to be 6 feet deep naturally? Thanks
Answer Mark,
If you live at the mouth of the Mississippi River, you could have six feet of new 'dirt' in your yard after the next flood. If you live on bedrock in New England, it could take several million years to develop 6 feet of soil.
'Dirt' or to use a more scientific term, soil, either forms by the accumulation of sediment or by chemical weathering of the local bedrock. Sediment accumulation is episodic and is referred to as alluvial or colluvial soil.
When the wind or moving water carrying suspended mineral grains slows, these grains begin to drop out of suspension. You can see these mineral grains in transit during dust storms and flood events as dusty air or muddy water. If your yard is or was a flood plain, your front yard has six feet of alluvium that may have been deposited in just a few hours sometime in the past. Or, if you live out west, those six feet of soil could have accumulated after a volcanic event dropped ash on the spot.
Chemical weathering of the parent bedrock is a gradual process that is dependent upon the local climate and some other factors. Soil formed by this process is referred to as residual soil. Residual soils are the mineral grains that are left over after more easily dissolved grains have been removed by rainwater percolating through the soil and attacking the surface and cracks in the bedrock. If you live in a hot humid climate like that of Georgia, this process is relatively fast in the context of geologic time. If you live in Arizona, the rate of soil formation is very slow because rainfall is low and episodic. Despite the heat, the amount of time the soil or bedrock is wet is relatively low so residual desert soils can take far longer to develop. If you live in Minnesota, though it is wet, the temperature is low so the rate of residual soil formation is slow.
Both soil forming processes are slowed by removal of soil at the surface by erosion. There is a simple conceptual equation that states that the rate of soil formation is equal to soil generation minus erosion. Soils on a level surface are less vulnerable to erosion than those on slopes. Vegetated soils are less vulnerable to erosion than bare soil. Frozen soil is less vulnerable to erosion than thawed soil.
Getting a real number for the rate of soil formation is not easy but with the exception of alluvial soils rates for residual soil formation are in the thousands of years. Embedded organic matter dated by the carbon14 method may help. Radiometric dating of volcanic minerals in the soil can help as well. In a seasonal tropical environment, a rate of 1 cm/1,000 years is a rough order of magnitude estimate of soil formation. In an arctic climate, the rate of soil formation might be closer to 0.01 cm/1,000 years.
