The geologic journey of Tyler Arboretum, Part 1

November 24, 2013 by Dr. G 1 Comment

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!)