CHAPTER 6: WEATHERING AND EROSION (a) Freeze-thaw action
Water gathers in cracks in rocks. At night (or any time the temperature drops below 0°C) it freezes and expands by about 10%. This expansion places great pressure on a rock face. When daytime comes the ice melts. Repeated freezing and thawing of the water eventually splits the rock fragments off the exposed rock surfaces and they fall as scree (talus). The presence of scree is an indicator that freeze-thaw
action has occurred. An accumulation of scree can protect the lower slopes of a hill from further weathering.
Freeze-thaw action occurs:
• Where temperatures vary above and below freezing and water is present.
• In arctic as well as temperate zones such as Ireland. • On mountain tops in equatorial areas such as Kilimanjaro.
Fig. 2 How freeze-thaw action occurs
After many freeze-thaw cycles, rock fragments fall away and are called scree/talus.
Fig. 4 Scree slope at Glendalough, County Wicklow. Note the angular rocks. Why are they so angular?
Fig. 3 Freeze-thaw action creating scree slopes in the Swiss Alps.