Friday, January 3, 2014

Rocks drying?

Some questions to Ask-a-Geologist are so off-the-wall that they rock me back on my heels and make me really think. Here's an example:

Q: is there a type of rock that dries faster than another?
- Shea P.

A: That's an interesting - and delightfully atypical - question. There are at least two issues involved:

  1. 1. The rugosity (or ruggedness) of the rock's surface. The smoother it is (like obsidian), the less surface is exposed to water and the less available in the way of nooks and crannies to trap and hold water. A sandstone would likely keep some moisture on it's surface longer than obsidian would.
  2. The surface tension/hydration of the rock's minerals. Certain minerals like clays adsorb (some also absorb) water on their surfaces in a pretty strong manner. Bentonite, for instance, will expand on contact with water and it takes forever to dry it out. When I lived in Denver I noticed that the ground sloped up to foundations of our tiny new house. I was emphatically warned by the realtor to not disturbed that grading. If water got under the eaves of my house, he told me, it could be funneled up against the side, and I could get heaving and major cracks in the basement walls as the swelling clay crushed into the concrete.
There's more to the issue than this, of course - there are other variables that include:
  • Is the entire rock in contact with air, or is some of it buried? 
  • What is the humidity of the air? 
  • Is the air circulating?
As an example of how important these are, I was once working in the Saudi Arabian desert. After a day in 40+ C temperatures, I felt sticky with dried sweat, and was determined to bathe. We carried plenty of water with us... but we were also being engulfed in a three-day sandstorm at the time. It was dark, so I took a 5-gallon Jerry can of water, a metal chair, and a towel about 100 meters out into the desert. I tied my clothes to the metal chair to keep them from blowing away, and used a heavy metal sauce-pan to load and pour water over myself. I lathered up and then poured more water over my hair and body to get the soap off. The wind was blowing so hard that I felt stinging sand up to my chest... and realized that I needed no towel after all. The air was so dessicated and moving so strongly that I was dry almost immediately.