Mechanical Weathering
Turning big pieces into little ones requires cracking the big ones. Cracks in rocks are caused or enlarged by processes including:
- Mountain-building stresses or earthquakes;
- Expansion as erosion removes the weight of overlying rock;
- Expansion and contraction during heating and cooling (especially very near the surface during forest fires; a really hot fire followed by a rainstorm can change temperature a lot in a hurry);
- Growth of things in cracks (tree roots, various minerals, and especially ice).
Probably the most important mineral that grows in cracks is ice, but others do too. For example, the mineral thenardite, Na2SO4 (no, you don’t have to memorize the mineral or the formula!) can add a lot of water to its structure (10 molecules of water for each Na2SO4, to make mirabilite, Na2SO4·10H2O, and you still don’t need to memorize the mineral or the formula), expanding in the process. Some pieces of the “dry” mineral, thenardite, may fall into a crack in a dust storm during the dry season, and then change to the much bigger mirabilite during the rainy season as the air gets humid, wedging open the crack. Too much rain may dissolve the mirabilite and move it deeper into the crack where it can lose water during the next dry season and then get wet and expand again, and again… This process is breaking many of the ancient monuments of Egypt as increased irrigation and other activities give seasonal increases in humidity in some places. (The story is even a little more complex than this, but, as shown below, the growth of minerals in cracks really does break rocks!)
Video: Rock Weathering (1:44)
Rock weathering really does happen. Things break rocks. Here is an example of tree roots breaking a human-made rock, or pavement, and we have to worry about a lot of human-made things being broken. The National Park Service preserves so many historical things that they actually have a National Center for Preservation Technology and Training to learn how to preserve our past for the future, as shown here. These pictures are just a little bit old, but they show an attempt by this National Center and Princeton University to learn how to save rocks from certain kinds of weathering. And they were worried about sodium sulfate salts that get into cracks and then can pick up water and expand, and in doing so they can break the rocks. And this would be thenardite changing to mirabilite. You don't need to know the detail, but it really does happen. And these pictures are slightly old, but what you can see here is they had developed a treatment that was used on some of these that protected them. And others, that were put in the salt and did not have the treatment, were very clearly not protected. So there are things that we humans can do to help save the things we made, but there is no question whatsoever, that nature does know how to break rocks.
Want to see more?
Enrichment: Another vintage movie for you to enjoy--and it won't be on a quiz. Here, National Park Service Ranger Jan Stock and the CAUSE students explore the freeze-thaw cycle in Bryce Canyon National Park.
Video: The Freeze-Thaw Cycle: Bryce Canyon National Park (1:17)
This white feature here is called the Wall of Windows. It's actually limestone. And it started out with a little tiny hole, like the hole on the left, and then it's eroding and eroding and eroding.
You've got the bridge on top. And then the bridge on top will fall away. And then you'll have the two hoodoos left standing. The difference between a natural bridge and an arch? Natural bridges are formed by streams and rivers running through underneath, cutting away.
Whereas this one is?
Whereas this is actually frost wedging.
OK.
Where it freezes and thaws and freezes and thaws and freezes and thaws and then it breaks away in the different layers.