Examples of Physical Weathering

When you pick up a rock out of a creek or stream, you are seeing an example of physical weathering, which is also referred to as mechanical weathering. Rocks often experience physical weathering as a result of exposure to swiftly moving water.

• Water flowing in a stream into a rock can eventually create a hole in the rock.
  
• When the water in a river or stream moves quickly, it can lift up rocks from the bottom of that body of water. When the rocks drop back down they bump into other rocks, and tiny pieces of the rocks can break apart.
  
• Many rock surfaces have small crevices on them. Water can freeze in these crevices when it is cold, and then melt when the weather is warmer. This repeated freezing and thawing creates ice wedges, which can cause rocks to break.
• The rushing of powerful waves towards cracks found within rocks can trap a layer of air at the bottom of the cracks. When a wave retreats, the air that was trapped can be released with a powerful force, thus weakening the rock.
  

If you have ever seen a tree growing out of a rock, you have witnessed physical weathering taking place. Being subject to environmental pressure can cause rocks to undergo physical weathering.

• Roots of trees or other plants growing into cracks in rocks may put pressure on the surrounding rock, eventually breaking rocks apart as the roots grow.
• Pressure release occurs when materials on the surface are removed from erosion or another process, and the rock underneath expands and fractures.
• Glacier movement can cause pressure release as it moves away from the surface of a rock.
• Ice wedges are a big cause of potholes in roads and streets. As ice forms in the cracks of a street, the water expands and pushes against the surrounding rock, making the cracks wider, eventually breaking apart the rock.

If you've visited a location where temperatures can vary drastically between extreme highs and lows, any cracks you saw in rocks are likely due to physical weathering. Exposure to extreme temperature fluctuations can cause rocks to weather. Rocks expand and contract as they go through heat cycles of hot and cold temperatures, which leads to the formation of cracks.

• Thermal stress weathering can occur in a desert climate that is hot during the day and cold and night. The heating and cooling processes that happen every day put stress onto rocks in the outer layer, causing the outer layers of the rock to start peeling off in thin sheets.
  
• Block disintegration occurs when rocks split along joints. This can happen as a result of repeated cycles of freezing and thawing, which can happen in climates where there is a lot of moisture and the temperature goes up and down a lot around the freezing point.
• Ice crystals can grow in the cracks of rocks. Over time this weakens the rock, and it can break apart. This frost-related shattering can result in many large loose rocks, referred to as scree, that can be found along the bottom or slopes of mountain areas.

The impacts of water, pressure and temperature fluctuations are not the only factors that can cause physical weathering in nature.

• Forest and range fires can cause rocks that are located along the surface of the ground in the affected area to undergo weathering. As a result, rocks in areas impacted by forest fires often appear quite different from those in other areas.
• Physical weathering caused by salt crystallization is common along coastlines. When saline solutions get inside of the joints or cracks in rocks, the solution can later evaporate and leave behind salt crystals. As the crystals heat up they expand, they put pressure on the rock. Sodium sulfate, calcium chloride and magnesium sulfate (Epsom salt) are salts that are quite good at disintegrating rocks.
• Salt crystallization can also occur when solutions, such as calcium chloride, cause rocks such as limestone to decompose. This creates salt solutions, which then evaporate and leave behind salt crystals. For example, calcium chloride can cause the physical weathering of rocks.

After reviewing some physical weathering examples caused by a variety of different factors, it's easy to see there are many causes of weathering. In each of the situations described, physical weathering has occurred. Now that you are familiar with physical weathering, also known as mechanical weathering, take the time to explore the difference between weathering and erosion.