The geologic journey of Tyler Arboretum, Part 2

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!

The geologic journey of Tyler Arboretum, Part 1

I’ve been blogging about my journeys around Tyler Arboretum, making sure I include pieces of the fascinating history of this property that has passed through the hands of the Minshalls, Painters, and Tylers.  But the history of the Arboretum actually begins well before 1681, when Thomas Minshall purchased the property from William Penn.  There is an entire GEOLOGIC history that extends much further back in time!  As a geologist and an educator, I can’t resist the opportunity to share some of the physical and environmental changes this location has gone through during Earth’s history.  I hope you find this as interesting as I do!

First, I have to start with how geologists view the planet today.  When you visit any classroom, past and present, the walls are decorated with various posters and maps.  The maps we grew up with are political geography maps, where each country on the map is a different color.  But as we know, rivers and mountain ranges and even the ocean do not stop at political boundaries.  Geologists have a different type of map we use to look at the planet – yes, we use colors as well, but the colors define tectonic plates (see map from the U.S. Geological Survey below).

The layers of the Earth (from USGS)

On this map, you will see the familiar outline of our modern-day continents, but the boundaries on this map are defined by areas of movement and motion taking place in a zone we call the lithosphere (remember learning about the layers of the Earth – the core, mantle, and crust?  The crust and uppermost part of the mantle make up the lithosphere).  The lithosphere is broken in to several rigid “plates” (each shown in a different color on the map above), where there is minimal tectonic activity in the center of the lithospheric plates, but much more tectonic activity (volcanoes, earthquakes, etc.) at the boundaries between plates.

From the map above, can you see why Tyler Arboretum does not have any volcanoes and does not experience earthquakes, but the west coast of the USA does?  We are in the middle of a lithospheric plate – close to the coast of our continent, yes, but the edge of our continent is not in the same location as the edge of our North American plate!  Earthquake-filled California has one of the most famous transform plate boundaries running almost parallel to the coast, the San Andreas Fault.  (If you are thinking back to that 2011 earthquake that occurred in Virginia, hold on – I’ll get to that in a future post!)

Earth’s history starts 4.6 billion years ago, with the formation of our solar system (view this short NOVA video for more on the solar system, and this video from National Geographic on the early formation of Earth).  Fortunately, the outer surface of our planet has cooled enough to where we formed the solid crust, and the trapped heat inside the Earth escapes through volcanoes – which just happen to occur at plate boundaries (with some exceptions, such as the Hawaiian islands and Yellowstone).  New lithospheric material is created at some plate boundaries through volcanic activity (the cooling of lava once it erupts) at divergent margins, while old lithospheric material is destroyed and recycled back down in to the Earth as molten material at other plate boundaries called convergent margins (this Smithsonian video does a great job showing the connections between plate boundaries, volcanoes, and even earthquakes).  This constant cycle of creating new material and destroying old material, as well as boundaries where plates just slide past one another, is what has caused the plates to move and change their shapes over time.

Wait… the plates are moving???  Yes, even today, all of the plates are moving across our planet, including our own North America!  Do you feel “the Earth move under your feet?”  Don’t feel bad if you do not – the plates move on average about a few centimeters a year, the same rate that your fingernails grow.

So now that I have set the stage with a very quick overview of the structure of our planet and the constant movement occurring, we are about ready to jump in to looking at where Tyler Arboretum has journeyed over geologic time!  We can’t go back to the very beginning, as the oldest rocks on the planet only date back to about 4 billion years old (remember, plate tectonics causes the constant creation and destruction of the lithosphere, so there is a good chance that really old rocks are going to meet up with a convergent plate boundary and be destroyed).  It is through the study of unaltered rocks (meaning, rocks that have not been super-heated or crushed by colliding plates) and the history of changes in Earth’s magnetic field that geologists and geophysicists have traced the positions and pathways of the plates – and that will kick off “Part 2” of my Geologic Journey of Tyler Arboretum series!  Stay tuned to learn how our current location was once under water and even located south of the Equator!

I will not be focusing on specific rock types or the history of Appalachian Mountain building in Part 2, although I will discuss the serpentine of Tyler’s Pink Hill in a future post.  For anyone interested in this part of Pennsylvania’s geologic history, I encourage you to read the Pennsylvania Geological Survey’s The Geological Story of Pennsylvania (written for a general audience).  And if you have any Pennsylvania geology questions, please feel free to leave them in the comments section under this post, and I will be more than happy to answer them!  (One question I’ll answer right away – no, we have never found dinosaur bones in Pennsylvania, but the world’s first mounted dinosaur skeleton was put on display at the Academy of Natural Sciences – the Hadrosaurus foulkii, discovered in Haddonfield, New Jersey!)