In my first post on Tyler’s geologic journey, I discussed how the planet is viewed by geologists, how the surface of the Earth is broken into tectonic plates that are either pulling apart (divergent boundaries), coming together (convergent boundaries), or sliding past one another (transform boundaries) – see this map and view this animation for a quick review. These plates have been shifting and moving throughout all of Earth’s history, which means Tyler Arboretum has also traveled through geologic time.
Something else to keep in mind – our global sea level has risen and fallen throughout geologic time. There are various causes for why ocean level fluctuates, but the main causes of this fluctuation are the changing size of the ocean basins, and the change in the volume of glacial ice over time. The more water we have locked up in ice sheets, the less water we have in our ocean basins, and vice versa. So when you look at a map of our modern-day continent and the position of its shoreline, note that our shoreline has changed through time, depending upon how high or low the water levels are in the ocean basins.
Comparison of two sea level reconstructions during the last 500 million years. Note that sea level has changed over a scale of hundreds of meters. It is estimated that if all of the current glacial ice sheets melt, global sea level will rise an additional 80 meters.
OK, so to review… we know there are tectonic plates that are in constant motion across the surface of our planet, and our ocean levels have gone up and down. So what does this mean for the geologic journey of Tyler Arboretum? Time to investigate through paleogeographic maps (thanks to the efforts of C. R. Scotese, PALEOMAP Project (www.scotese.com))! The maps below were constructed from evidence found in the rock record. Dark blue represents deep water, light blue represents shallow water, and the lightest blue/white color represents ice sheets. Brown is for the land exposed above sea level, and the curved lines show the position of plate boundaries (those divergent/convergent/transform motions). The white outlines are just for reference – although you will see the familiar shape of North America on most of these maps, please note that North America did not exist through most of Earth’s history – certainly not as we know it today!
Late Precambrian Supercontinent and Ice House World
This is the best reconstruction that we can make from the data and technology we have available for what our planet might have looked like 650 million years ago (the age is in the upper left corner, during a time period we call the Late Proterozoic Era). Notice the Southern Hemisphere is covered with much ice, as well as portions of the Northern Hemisphere. Can you find Pennsylvania? We were part of a landmass called Laurentia in the Southern Hemisphere and rotated 90 degrees to the right. You can see the outline of the modern-day west coast under water and the east coast – well, under ice!
Cambrian: the beginning of the Paleozoic Era
At 514 million years ago, the oceans are becoming filled with hard-shelled organisms for the first time, which is not surprising given the extensive shallow seas covering the continents. Note that Laurentia has moved up to the Equator, but we know that pieces of New England and Florida were still at the South Pole.
The Late Carboniferous a Time of Great Coal Swamps
Zipping ahead to the period of time also known as the Pennsylvania Period, you can see that we are still south and not that far from the Equator. These were the perfect conditions for the famous Pennsylvania coal deposits to form in the Equatorial Zone, while extensive ice sheets were covering the Southern Hemisphere.
At the end of the Permian was Greatest Extinction of All Time
And here we are, with Pennsylvania north of the Equator, and at one of the most significant times of geologic history (certainly for the biologic record). When the Pangea formed ~245 million years ago, pulling together all of the landmasses into one giant supercontinent, it is estimated that anywhere from 95-99% of the file that existed on the planet at that time went extinct. This is the largest mass extinction event in Earth’s history (not the most popular, of course). Note that the dinosaurs still had not yet appeared on the planet!
Pangea Begins to Rift Apart
The “Jurassic” Period of geologic time really did exist – and there were dinosaurs around at the time. It also marks the time of the formation of the Northern Atlantic Ocean and the splitting of the supercontinent Pangea, when we finally become North America!
The End of the Dinosaurs
And just for those curious, here is what our planet looked like 65 million years ago when the dinosaurs went extinct, showing the site of an asteroid impact that contributed to the demise of the dinosaurs. Notice how high the water levels are and the absence of ice.
We are still drifting apart from Europe and Africa, as the Atlantic Ocean continues to widen at the divergent plate boundary that runs under the middle of the ocean basin.
So there you have it! Tyler Arboretum has been under water, under ice, south of the Equator, and neighbors with fellow continents during Pangea’s formation. Where will Tyler be in the future? There have been maps constructed for where geoscientists believe the plates are heading – look at the maps for the future world 50 million years in to the future, 150 million years, and 250 million years!
Journeys of Dr. G at Tyler Arboretum 