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What is the scientific reason for the perpetual need to pick rocks from a farmer's field, i.e. how do they continually rise to the surface?
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3 Answers
There are several factors influencing the surface emergence of rocks in fields. The following is not meant to be a conclusive list:
1) Frost heaving. Freezing temperatures penetrates the soil from top to bottom, stone is a better heat (or cold) conductor than the surrounding soil and therefore a pillar of soil (or really water in soil) under each rock will freeze more efficiently/faster than the surrounding soil. The freezing process actually continues by sourcing water from outside the pillar creating an ice lens. Since water expands when it freezes this frozen pillar/ice lens will lift the rock relative to the surrounding soil. Over time the rock will move towards the surface (direction of least resistance).
Factors: climate (freezing temperatures), ground-water supply, soil type, ...
2) Erosion. The process of erosion will carry smaller soil particles and leave rocks in the field.
3) Tillage. The mechanical tillage process can lift rocks to the surface. This combines with frost heaving, erosion etc.
1) Frost heaving. Freezing temperatures penetrates the soil from top to bottom, stone is a better heat (or cold) conductor than the surrounding soil and therefore a pillar of soil (or really water in soil) under each rock will freeze more efficiently/faster than the surrounding soil. The freezing process actually continues by sourcing water from outside the pillar creating an ice lens. Since water expands when it freezes this frozen pillar/ice lens will lift the rock relative to the surrounding soil. Over time the rock will move towards the surface (direction of least resistance).
Factors: climate (freezing temperatures), ground-water supply, soil type, ...
2) Erosion. The process of erosion will carry smaller soil particles and leave rocks in the field.
3) Tillage. The mechanical tillage process can lift rocks to the surface. This combines with frost heaving, erosion etc.
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