The Oldest Building in Chicago
As you look at this unassuming 24 hour coffee shop (Does Chicago Run on It?), you are actually standing in front of the oldest building in Chicago - in a sense. The rocks that are used to build the facade of the columns of this coffee shop are some of the oldest building stones in use anywhere. Let's dive into the geologic history here.
The earth is really old. Incredibly old. Unfathomably old. When we examine the creation of the earth we're working on a scale of 4.54 billion years. A billion is almost impossible to comprehend - if you counted to a billion and didn't stop (and could say the number 457,323,987 in about 3 seconds) it would take you nearly 30 years to count to a billion. If you wanted to "count" each year the earth has revolved around the sun it would take you more than 135 years. This is deep time, a phrase coined by Geologist John McPhee back in 1981 to describe timeframes and scales that seem to boggle the mind.
"Consider the Earth's history as the old measure of the English yard, the distance from the King's nose to the tip of his outstretched hand. One stroke of a nail file on his middle finger erases human history."
John McPhee
This is the scale of time that you're staring at here at GZ. Gaze into the paneling before you and behold the giddiness of time. This is an Earthcache - there is no container here to find! In order to claim this Earthcache you'll read through this description, make some observations, and then message the CO with answers to some questions that you'll find at the end of the write up.
Deep Time
Stretching back 4.54 billion years, the great abyss of deep time helps quantify how the world looks the way it does. Deep time is what made the formation of the Great Lakes possible. Deep Time is what allowed microscopic organisms to evolve into the myriad of complex species and ecosystems that grace, have graced, and will grace our planet. Deep time is what allows you to read this earthcache description on a device made from minerals that formed millions of years ago.
Although deep time manifests itself in many different ways, it resonates more strongly in some locations. One such place is here in downtown Chicago at the SE corner of Jackson and Dearborn St. Here you will find a lovely building stone that was popular in the art deco age that covers the columns of this coffee shop. Despite the fact that you're standing in Illinois, the building stone that was used for the facade of the building however is Morton Gneiss, which was quarried in Minnesota.
With swirls of bright and dark colors, this slab of rock has a dynanic feel. What is not immediately evident, it's the most interesting quality of this rock is its age. Morton Gneiss is 3,524,000,000 years old - that's some serious deep time! As John Playfair wrote in the 18th century "the mind seemed to grow giddy by looking so far into the abyss of time." This rock is so old that when it formed earth didn't look anything like it does now. The oldest evidence for life is also about 3.5 billion years ago, which means that the surface of th planet back then lacked any of the plants or animals or other life forms that provide the colors and textures and chaos that we know today. Instead, the surface was probably fairly muted in palette except for the contrasts provided by water and lava.
Before and After Plate Tectonics
Much of our understanding of geology rests on the concept of plate tectonics - the idea that portions of the earth's crust, floating over a denser magma mantle slowly move over time. While plate tectonics eventually plays a part in the formation of Morton Geneiss it's not there in the beginning. The earliest estimations on when plate tectonics began put the process inception at about 3 billion years ago.
Thus when a chunk of grey granite formed about 3.5 billion years ago deep beneath the surface of the earth, the plates above it weren't yet shifting. This allowed the granite to cool exceedingly slowly over time as it rose to the surface forming grains of crystalized minerals. This rock was the basis for the slabs you see before you. About a billion years later, two plates of the earth's crust collided at the future location of southwestern Minnesota subjecting the then 500 million year old granite to heat and pressure. These forces melted it once again and allowed intrusions of molten pink granite. The two granites folded and twisted; when they eventually solidified again those twists and folds remained. 800 million years later another geologic heating event added additional color and texture.
The rocks colors comes from quartz (white), potassium feldspar (pink), plagioclase feldspar (light gray) and biotite/amphibole (black), however depending on where exactly in the deposit the stone was quarried from will determine how much of each mineral the resulting facade will have.
Questions
After reading the description above, answer the following questions below and send the answers to the cache owner to log your find on this earthcache. If there are any issues with your answers I will reach out to you and ask for clarification and we can work through the answers together.
- Examine the Morton Gneiss before you. What are the two predominant colors you see?
- Based on your answer to question one, what minerals are the primary components of this section of Morton Gneiss?
- In your own words, describe how plate tectonics enabled the interesting formation of colors you see before you.
- Take a photo of yourself or a personal geocaching object on the corner of Jackson and Dearborn - make sure the Gneiss is not in your photo!
References
- https://streetsmartnaturalist.substack.com/p/the-giddiness-of-time
- https://www.mnopedia.org/thing/morton-gneiss
- https://www.earthmagazine.org/article/when-and-how-did-plate-tectonics-begin-earth/