Geology

Weathered pillow lava from the Canadian Shield
Weathered pillow lava in the Temagami Greenstone Belt of the Canadian Shield
Dated at 2.7 billion years old
A scaled version of the Earth's Geologic Clock
The Earth's Geologic Clock,
Click on the image to see a larger version

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 about to cause yet another 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 as 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).

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.

Drawing of Laurentia
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

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.

Past Supercontinents:
  • Vaalbara - ~3.5 billion years ago
  • Ur - ~3.2 to ~2.9 billion years ago
  • Kenorland/Arctica - ~2.7 to ~2.1 billion years ago
  • Columbia - ~1.8 to ~1.5 billion years ago
  • Rodinia - ~1.1 billion to ~750 million years ago
  • Pannotia - ~650 to ~560 million years ago
  • Pangaea - ~300 million years ago

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 one 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...

Map of the basement rock of North America
The basement rocks of North America
Numbers mean billions of years ago

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.

Eons, Eras, Periods & Events Millions of years ago Geology Notes Life Notes
The Cenozoic Era - The Age of Mammals
Quaternary Period Present-2 The most recent Ice Ages The explosion of Man
Neogene Period 2-23 North America crashes into South America  
Mountain building 23-26 Vulcanism and mountain building in western North America: the modern Rockies are built
Paleogene Period 23-63 Continued subduction of Farallon plate on West Coast, Basin & Range Province built Explosion of Mammals
The Mesozoic Era - The Age of Reptiles
Laramide and Sevier Orogenies 63-65 Overlapping mountain building events in the western US and Canada along the Cordilleran margin, the Ancestral Rockies and the Colorado Plateau pushed up
Cretaceous-Tertiary Extinction Event, K-T Boundary 65 Asteroid impact near the Yucatan peninsula, postulated to have combined with on-going geological and atmospheric processes to cause global climatic conditions that wiped the dinosaurs out
Cretaceous Period 63-145 Laurentia was an independent continent known as North America Age of the dinosaurs, early mammals appear
Nevadan Orogeny 140-180 The breakup of Pangaea and the collision of Laurentia with the Pacific plate cause volcanic disruption: stratovolcanoes up to 20,000' high along the subduction zone, volcanoes that erode and expose their granitic basement now as the Sierra Nevada Mountains
Jurassic Period 145-201.3 Pangaea split into minor supercontinents Laurasia and Gondwana, Laurentia was part of Laurasia Age of Reptiles; first birds and lizards
Triassic-Jurassic Extinction Event 201.3 50% of all species on Earth die off
Triassic Period 201.3-251   Age of the Archosaurs
The Paleozoic Era - The Rise of Complex Life Forms
Permian-Triassic Extinction Event 251 Mass global extinction takes place in several pulses over several million years. Heavy volcanism at the Siberian Traps brings an end to the Era
End-Capitanian Extinction Event 259.8 Major species die off associated with anoxia, acidification of the oceans and general climatic cooling. First mass extinction event closely tied to a catastrophic increase in volcanic activity: the Emeishan Traps
Permian Period 251-280 Major supercontinent Pangaea formed Age of the giant hard-shelled creatures: Insects, arthropods and marine invertebrates
Alleghenian/Ouachita Orogeny 260-325 Mountain building event in eastern North America as Africa crashes into the East Coast and South America crashes into the Gulf Coast
Ancestral Rocky Mountains ~300 Frontrangia and Uncompahgria pushed up for the first time
Carboniferous Period 280-360 Early Carboniferous: end of Kanayuk Conglomerate deposition, the supercontinent Pangaea comes together Time of great rainforests, jungles, ferns and huge dragonflies, time when most of our coal was deposited
Devonian Period 360-415 Laurentia collided against Baltica, formed minor supercontinent Euramerica, deposition of Kanayuk Conglomerate in Alaska begins The Age of Fish
Silurian Period 415-440 Recovery from Snowball Earth  
Ordovician-Silurian Extinction Event 440 60% of marine invertebrates and 25% of genus families die off
Ordovician Period 440-485.4 Snowball Earth began to build, Laurentia was shrinking and Baltica was growing Ended with massive species die off as glaciers reached into the tropics
Cambrian-Ordovician Extinction Event 488 A pulse that wiped out many brachiopod, conodont and trilobyte species
Dresbachian Extinction Event 502 About 40% of marine genera died off
End Botomian Extinction Event 517 About 40% of marine genera died off
Cambrian Period 485.4-541 Laurentia was an independent continent after the break up of Pannotia The first great explosion of ambient life; the oceans are taken over by arthropods
The Precambrian World - 88% of the Earth's present age
Proterozoic Eon 541-2,500 The changeover to an oxygen atmosphere, the first orogenies as aspects of collisions among the first continents and supercontinents Eukaryotic bacteria evolve from the prokaryotic bacteria of the Archean eon and mass photosynthesis sets the stage for several massive glaciation events. The eon ends with appearance of trilobytes and reef-building marine organisms
Archean Eon 2,500-4,000 As the planet continues to cool and harden, more cratons form. The Oxygen Catastrophe begins Single-cell organisms evolve in the oceans and much of the land surface is covered with oxygen-producing cyanobacteria
Hadean Eon 4,000-4,600 Begins with formation of the planet and proceeds into the Late Heavy Bombardment period. First pieces of the Laurentia craton form deep undersea as the Slave, Superior and Wyoming cratons are created through long-term magma flows. The very earliest days of the Earth: primordial life begins to form during the Late Heavy Bombardment period and most likely survives in close proximity to geothermal vents on the floors of acidic oceans
Chart showing levels of biodiversity among the different geologic ages
Biodiversity in the Phanerozoic Ages
How today's continents fitted into the supercontinent of Pangaea
A map of today's continents at the time of Pangaea, c. 240 million years ago