Physical weathering is the mechanical breakdown of a material (broadly rock in this example) that fractures the material for natural pulverisation. Physical weathering makes a good catalyst for chemical weathering as well. There are a number of ways that material can be weathered in this way.
- Ice Wedging
- Ice Lens Growth
- Expansion & Contraction
- Biological Activity
Imagine you have a rock that has a few cracks in it, but it’s still solidly your rock! But you left your rock out in the cold, cold winter. Rain dropped onto your rock and found itself deep in the cracks. As the night progressed and the temperature dropped, the ice froze. Ice takes up more volume that water, so it pushed outwards against the cracks’ walls. Although this function isn’t initially catastrophic to the rock, it further weakens the material and increases its susceptibility to other weathering events.
Ice Lens Growth
Let’s say underneath your lawn there’s ice. This ice is surrounded by low pressure water because the ice pieces and rock are repulsive of each other. This low water film attracts more water, which grow the ice shards. This outwards pushing fractures your lawn. You should probably consider moving if this process continues!
Expansion & Contraction
This is a daily process, but it’s also intensified with large fires and snow storms. This is a volume change in the material. This process is dependant on wetting , drying and temperature.
So you have a rock that’s buried deep below the regolith, bedrock, etc. On all sides of the rock, there is a strong pressure that densifies this by pushing it inwards strongly. With a long series of erosion, the regolith and bedrock have disappeared, but now that rock is beginning to surface. However, the intense central push inward is no longer being applied to the top, so it expands upwards. This, in turn, fractures rock.
Things like animal burrows and plant roots cause hillslope hollowing. This loosens the regolith mantle.
Next will be chemical weathering.