Rock Weathering
An understanding of how rocks can change over time is necessary to interpret images of another world. But it is easy to forget how dynamic rocks are. Over the life span of a human or even a civilization they appear constant and eternal. Over the course of millions of years mountains crumble and canyons form. So lets review just what can happen in "the life" of a rock.
This section relied heavily on "The Surface of the Earth" by Dr. Arthur Bloom. This charming 150 book is part of the Foundations of Earth Science Series from Prentice-Hall and was published in 1969. If you see it in a used book store, don't hesitate, just buy it!
You can find wikipedia's description of rocks at http://en.wikipedia.org/wiki/Rock_%28geology%29 and rock weathering at http://en.wikipedia.org/wiki/Weathering.
Lots of rocks are formed under pressure. Sedimentary rocks form under oceans. They are made up from loose debris (sand, dirt, the shell of tiny sea creatures, etc.) put under intense pressure. Metamorphic rocks were once some other kind of rock that was morphed by the great heat and pressure deep in the Earth. Many igneous rocks formed when underground molten magma cooled into a solid. (For some simple animations of these processes go to http://www.fi.edu/fellows/fellow1/oct98/create/index.html.)
When these rocks reach the surface they experience only regular atmospheric pressure. Geologists call any huge reduction in pressure "unloading". And, it can cause solid rock to fracture. What formed as a huge rock can be broken into many large pieces by the time it reaches the surface just from this unloading. Exactly what happens depends on the strength of the rock, how many natural joints or weak spots it has, how much it expands when unloaded and the amount of unloading. To quote Dr. Bloom, "most rocks are thoroughly fractured into blocks or slabs that range in length from a few inches to several feet".
On some planets, like Mars, you see any water on the surface but you can find features that looked like they were formed by water. Remember that the rocks that were covered by water underwent an unloading when the water disappeared.
After a rock has suffered through unloading and is exposed at the surface its rate of weathering accelerates. One of the more powerful forces comes from freezing water. Unconfined, water will expand by 9% when it freezes. If tightly confined freezing ice can generate a pressure of up to 2100 tons per square foot. While pressures this high are only created in a lab, freezing ice in nature does create enough pressure to crack and crush most any rock.