Foundations in Deep Time

Depending on the time of the year, this street may be bustling with people or buried in snow.This unassuming street is the perfect place to study some ancient geology! The coordinates will bring you to the sidewalk in front of a small shop on the western side of Ashmun St. This storefront has an interesting stone facade over the foundation which will be the subject of our lesson today - so let's dive into the geologic history here. As you visit this main street, remember that you are an ambassador of our hobby - please be respectful while visiting.

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. 

This is the scale of time that you're staring at here at GZ. Gaze into the stone at your feet where the sidewalk meets the building (just below the store front windows) and behold the giddiness of deep 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 on the main street here in Sault Ste Marie, MI. Here you will find a lovely building stone that reached its height of popularity in the 1930s during the art deco movement. Despite the fact that you're standing in the Upper Peninsula, the gravestone that was used here 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 the 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/brown), however depending on where exactly in the deposit the stone was quarried from will determine how much of each mineral the resulting stone 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.

1. Examine the Morton Gneiss before you. What are the two predominant colors you see?
2. Based on your answer to question one, what minerals are the primary components of this section of Morton Gneiss?
3. In your own words, describe how plate tectonics enabled the interesting formation of colors you see before you.
4. Take a photo of yourself or a personal geocaching object at GZ - 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/