The Geology of North America

The Earth's Geologic Clock,
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As the science of geology has progressed, so has our understanding of the Earth and how it came to be as it is. The amount of observational data available has exploded over the last 50 years but that has served mostly to expand on theories postulated previously. That said, the previous theories have been expanded through the study of the data and more and more intricate elements in the story have been exposed and come clear. At a time when human impact on the planet appears to be causing yet another massive global extinction event, it would be good if more people were exposed to the basics of it.

Classic creation stories aside, geologists estimate the present age of the Earth at 4.5 billion years old, give or take a few million years. During that time a lot of things have changed... and are still changing. Theories abound as to what happened (and how it happened) way back when and, as usual, the nearer to us the object of our study is, the clearer are the lessons contained. In the case of Planet Earth, some of the hard lessons began more than 4 billion years ago: geologists have discovered zircons on the surface of Australia that have been dated using radiometric processes to be about 4.4 billion years old. Other rocks from the Isua Greenstone Belt in Greenland have been dated at 3.8 billion years old. In those days the planet was just barely formed as a discernible rotating ball and the Late Heavy Bombardment was just barely underway (the Late Heavy Bombardment is defined as a large spike in the rate of asteroidal impacts with the interior planets of the Solar System, possibly caused by the movements and orbital adjustments of the larger gas giants).

Laurentia-North American Craton
The purple areas were deformed in previous continental collisions
The green area results from the on-going collision of Laurentia with the Pacific plate and with South America

Since men returned from the Moon with lunar rock souvenirs, it has been postulated that the Moon itself was originally part of the disk of the earth until that disk was impacted by another large body and the material of the Moon broken off and ejected into orbit around the Earth. Radiometric dating has suggested a date of 4.533 billion years ago for this to have happened.

An interesting note: the vast majority of water on Planet Earth was created more than a million years before the birth of our Sun. That water was deposited on the planet through the impacts of millions of ice-carrying meteorites, asteroids and comets over the last 4.5 billion years.

Plate tectonics has suggested that the original swirling ball of gases and rocks that eventually became Earth contained some materials that hardened early in the process and may still be present in the center of what are now called "cratons." Cratons form the heart of continental plates and they seem to survive as long as they retain sufficient area, mass and density to continue floating on the liquid mantle below. Most of the planet's great mining districts are centered in the heart of a craton as that is also where the geological conditions occurred for the accumulation of mineral deposits. The cratons we see today have been heavily eroded over the eons and the mineral bodies that were once deep in their hearts are now relatively exposed to the surface.

As the bodies of those cratons are still floating atop the planet's inner mantle, they appear to come together and form a supercontinent about every 450 million years or so, beginning around 3 billion years ago. Supercontinents interfere with the general flow of materials in the Earth's core and mantle and eventually, those massive bits of land are broken up by volcanic activity that usually occurs along the joints where the cratons meet. Supercontinents also interfere with global atmospheric processes and have served to promote global glaciation, the "oxygen catastrophe," global desertification and other elements we humans would consider catastrophic as it would interfere radically with the daily enjoyment of our lives... and though we've only been here but the blink of an eye we're already causing catastrophe on a global scale with many of the things we do on a daily basis to promote our enjoyment of our lives.

As the oldest material found so far on a sea floor is maybe 170 million years old and the oldest material found on the land surface dates to more than 4 billion years old, geologists have always paid more attention to the record baked into the land. That said, virtually everything ever found on the surface of the planet was at time or another underwater: even the upper levels of Mount Everest are composed of layers of sedimentary rock, some of which display fossils of ancient sea shells.

As the story of geologic history and discovery involves many places around the globe and this website is more focused on North America and the United States, our exposition here will mostly focus on those places in North America where it might be possible to see and touch the minerals and fossils that are the physical evidence on which the story is based.

Caveat: This is a brand new section of this website, required by my efforts to educate my kids in a holistic manner about the history of the Earth by hiking them through various landscapes in the western United States. Before I get too much further into it I might have to add a section to the website discussing the various indigenous people who lived in North America before the disastrous (for them) invasion by the Europeans as we live in New Mexico, surrounded by people with oral histories that reach back thousands of years. Like the rest of this website, it's all going to take a while to fill out...

So then, a chart of time that will eventually offer links to more data about each time period, and keep in mind that each of the "differentiations" in time below are also often further divided into period, events, ages... Some things I can't put on the chart, like the 200-million-year-old-and-still-going collision with the Farallon Plate. That collision began when the area that is now Utah was seafront property and what has accreted onto the land mass that is now North America since that time was scraped off the surface of the Farallon plate as it was subducted beneath the North America Plate. That ongoing collision and subduction have caused the Rocky Mountains, the Basin and Range Province, the Sevier Orogeny, the Laramide Orogeny, the Nevadan Orogeny and more.