Reader Question: How do geological layers work?

This post is a response to a reader question. Please submit suggestions for posts to metaist.blog@gmail.com. Not all questions will be answered.

Question
Reader Matthew Garland asks:

Where do geological layers come from? If remains of civilizations and eras are buried, where did the material for the top layers come from? There can't be more material now than five thousand or million years ago.

The answer to this reader question comes from Richard Lupia.

Summary
The formation of geological layers is a continuous process based on the settling of sediment and shifts in tectonic plates.

A geologic deli roll.
(Photo: Crinity at Flickr)

Explanation
Geological layers, also called beds or strata, which contain buried civilizations are composed for the most part of fragments of rocks that form when bigger rocks are broken down by water, wind, or even organisms like plants. Once broken down, the fragments, called sediment, are transported by flowing water or blowing wind. Eventually, water stops flowing or wind stops blowing and the sediment settles, be it on the river bed or on the ocean bottom, or on floodplains alongside rivers when they flood. The fossil record is primarily a record of what lived in the oceans and in and along rivers.

Inasmuch as humans are dependent on water for sustenance, irrigation, etc., the bric-a-brac of human civilizations is ideally located to be buried by sediment when the rivers flood, which many do seasonally. It is worth noting that floods add new sediment that maintains the fertility of the floodplains for agriculture which also keeps humans near rivers.

The Earth does not run out of sediment because mountains constantly form and grow (slowly) as the plates--the 14 or so massively large blocks Earth's outer "shell" made of continents and ocean floor--shuffle around and collide. As mountains go up, there is always more rock to break down.

And although adding layer upon layer of sediment does indeed produce an increasingly thick pile of rock, it need not get higher and higher. There are two main reasons. First, as old sediment is buried by new sediment, water and air are squeezed out from the spaces between the sediment, and the old layer becomes thinner. For example, 10 feet of mud will on average yield a layer of rock about 1 foot thick. Second, the colliding of plates I mentioned above warps the ground over a very large area often bending it down to form a basin which lowers the Earth's surface.

Finally, I briefly add that sediment, and the oceanic part of the plates, are eventually recycled by sinking back into the deeper layers of the Earth. Where plates sink are called 'subduction zones' and are often apparent on maps as oceanic trenches (e.g., Mariana Trench, the deepest place on the surface of the Earth). The sinking of plates at subduction zones initiates the formation and growth of another type of mountain: volcanos. The ash and debris associated with volcanic eruptions blanket the surface and frequently bury human structures. The burial of Pompeii and Herculaneum by ash from the eruption of Mount Vesuvius in 79 CE is one famous example.

Richard Lupia is an Associate Professor in the School of Geology and Geophysics at the University of Oklahoma.

Explanation text licensed under Creative Commons Attribution 3.0 Unported License. Additional editing by The Metaist.

Reader Post: Spicy Food

This post was submitted by reader Josh Vogel. Please submit ideas for posts to metaist.blog@gmail.com.

Commentary
Many of us enjoy spicy food. At the same time, many of us really don't like spicy food. According to an article in The New York Times, there may be more of a personality trait to this than one might have originally thought. Additionally, the fact that any human being likes spicy food might say a lot about the evolution of our species.

Not to be confused with the spice.
(Photo: Will Clayton at Flickr)

Researchers at the University of Pennsylvania say that our enjoyment (or non-enjoyment) of spicy food might have to with the unique human capability of getting pleasure out of painful experiences: "Humans and only humans get to enjoy events that are innately negative, that produce emotions or feelings that we are programmed to avoid when we come to realize that they are actually not threats," said Dr. Paul Rozin of University of Pennsylvania, who calls this trait "benign masochism". After all, we are the only animal that likes spicy food, as evidenced by the inclusion of Capsaicin (the chemical compound that makes chilies spicy) in many animal repellents.

What this might say about people who do and do not like spicy food is not covered by the article, but it doesn't take too much thinking to see if people who like spicy foods are also ones who have a high tolerance for pain.

Josh Vogel is a Candidate for the Master of Public Health at Boston University School of Public Health.

See Also

• A Perk of Our Evolution: Pleasure in Pain of Chilies at The New York Times for the article.
• Food for Thought: Paul Rozin's Research and Teaching at Penn for a lengthy interview with Paul Rozin from 1997 where he discusses many of the aspects of his research.

Commentary text licensed under Creative Commons Attribution 3.0 Unported License. Additional editing by The Metaist.