Heat/Cold Cycles and Ice

• Cycles of warming and cooling can wreak havoc on many objects. Rather than thinking of what exposure will do to an exterior wooden door, consider first what it will do to an acoustic guitar. Tune the guitar first thing in the morning. If it's in direct sunlight and warms from 60 degrees to 85 degrees, it will raise the pitch of each string -- significantly. It may even ruin the guitar. Physically, that's a lot of stress to go through. Molecules become more active with heat. They become more passive as they cool. Expansion and contraction is a powerful physical strain. But no natural expansion is as powerful as ice. Water, as it's freezing, can begin crystallizing in the nooks and crevices of granite; for example; as it continues to freeze, the ice can expand with enough force to cleave solid rock.
  
  

Humid/Dry Cycles

• Cycles of humidity and dryness punish things in similar ways as heat/cold cycles. Moisture causes expansion in many materials, just as dryness causes contraction in many. While some materials may be less affected by the moisture content than heat, others may endure the heat cycles but decay rapidly with high humidity. Rock, for example, may be be affected by humidity cycles, whereas tress may be far more reactive to changes in moisture than they are to heat.
  
  

Solar Radiation

• You may not consider the effects of solar radiation on natural things. We know what the sun can do to a paint job or or a vinyl car top. They can fade, oxidize, dry out and crack. We notice, because we know what these man-made things look like when they're sheltered from the elements. Natural objects undergo the same radiation, but they endure it constantly without any kind of protection. When you see driftwood on the beach, it's a washed-out gray color. It got that way in part from solar radiation.
  
  

Chemical Weathering

• Humidity, temperature and radiation can all take their toll. So do chemicals. You can see chemical decomposition in forest floors. Plant life and trees die. Fungus and bacteria play a big role in creating a chemical change. As with most weathering, not everything responds the same. For example, Northern bogs can actually preserve organic material. Foliage falls to the ground. Snow and ice melt. The bog turns into a tea pot, and the tannins help preserve fibrous material that would be quickly broken down if it were in a warmer forest floor, exposed to oxygen. This illustrates how everything weathers in different ways from different causes.