Book 24, Question 13
Why does frozen ground push rocks up to the surface?
When water in soil freezes, it expands and pushes the soil upward. This is called frost heave. When the ground thaws, the soil settles, but stones that were lifted do not drop back down exactly. Over many winters, rocks slowly work their way to the surface.
More fun facts
- Frost heave needs three things: soil that can form ice, enough water, and cold strong enough to freeze them.
- As soil freezes, water creeps from unfrozen soil up to the freezing layer and builds thin sheets of ice called ice lenses.
- Sand and gravel usually do not heave, but clay, silt, and very fine sand can, even in small amounts.
- In spring, the extra water from melting ice lenses makes the soil soggy and weak.
- Lifting during freezing and softening during thawing is how many dips and potholes form in roads.
- Builders sometimes swap frost-prone soil near foundations for coarser material so frost heaving cannot happen.
Sources: U.S. Department of Agriculture
Questions kids ask
Why are there so many potholes in spring?
Freezing lifts and cracks the road, and when the ice melts, the soggy soil underneath gets weak, so pieces break away.