Why Does Some Sandstone Have Stripes?
Stratification & Segregation
Sandstone sample from Petra
Once you've made mashed potatoes, could you turn them back into whole potatoes? After you've scrambled eggs, could you separate the yolks and whites, restore them to their original form and put the eggs back into their shells? Maybe not, but here's a story about natural phenomena in geology called Stratification & Segregation that achieve something similar. These phenomena are responsible for the layers we see in some types of sedimentary rocks.
How is sandstone formed?
In Petra, a 2000 year old city in the Jordanian desert, many of the ancient buildings were built and carved out of the local sandstone. As in Petra, sandstone can often be found in areas where windblown sand accumulates over thousands or millions of years. Sand deposits also build up on the ocean floor, or in lakes and rivers. When the sand particles, mixed with cement-like minerals such as quartz and calcite, are compressed under great pressure over long periods of time they stick together to become stone.
accumulated sand + cementing minerals + pressure + time = sandstone
Buildings in Petra were carved out of the local sandstone.
Not all sand is the same. There are various colors and the grains may be of different sizes and shapes. This stone from Petra appears to be made of at least two kinds of sand that are different colors, separated into layers. How did the layers form? Maybe in prehistoric times one kind of sand accumulated for a few millennia and then another kind for a few millennia, and then the first kind again, and so on. This doesn't seem too likely and there is no geological evidence to support this theory. Hernán Makse and his colleagues believe they may have an explanation. Their theory is that there was originally a mixture of different kinds of sand grains that separated into layers as the sand piled up millions of years ago. In order to understand how sand may have sorted itself out millions of years ago, we can look at a similar process as it occurs today in avalanches.