The earth is an ever-changing chunk of rock to put it at the lowest level of comprehension available. This rock that we call home is brewing with billions of life forms and is constantly changing every day. This can be attributed to the various cycles that are at work on this planet of our, from the hydrological system to the rock cycle the earth is an ever changing form. Pennsylvania is just a small part of this system we call our home. Formed millions of year ago by tectonic collisions and molten rock Pennsylvania is a part of the earth full of specially minerals and geological features that stand out as being come of the best in the world. What would our state be with out its coalmines and steel mills. Which of course are only possible through the unique geographic features of our state.
The Pre-Achaean, Achaean and Proterozoic Eras.
Geologists believe that Pennsylvania was formed by parts of the Laurentian continental crust block. This crust block is made up of many micro plates, which were accreted during the period from about 2.5 billion to 1.0 billion years ago. This was previously part of an even larger crustal block called Rodinia. Geologist believes that PA is full of rocks that were once part of this crustal block. These rocks are known as the Greenville rocks. Greenville rocks are metamorphic rocks composed mainly of gneiss. Some of these rocks are visible in Southeastern PA but most are buried deep within the surface of PA.
The Cambrian and Ordovician Periods
This is really the period that life started to develop in the state of Pennsylvania. Now how did life star in this state you may ask, well here is how. At the start of the Cambrian period water from the Ocean spread inland across North America. Pennsylvania got in the way of this and was covered with a shallow layer of water which created unique environments for both life as well as the opportunity of new deposition of sediment. This water cover also had a big part in eroding away water channels as well as leaving large deposits of sediment.
During the late Ordovician era materials from the arc and the floor of the Ocean were thrust onto the North American plate. This phenomena was called Taconic orgogeny, this played an important part in the development of mountains in PA. This Taconic orogeny created the mountains that lay to the northeast of PA. The Taconic orogeny, which deposited many harmful types of sediment, also saw the end to many life forms due to this heavy disposition of harmful materials.
The Silurian Period
During this period the mountains that were formed by the Taconic orogeny were still a good source of sediments. PA was covered with clastic sediment form these mountains for the first half of this 25 million year period. Sand and gravel composed mainly of quartz were deposited by stream to the eastern parts of the state. There was also quartz sand carried further west and deposited on the beaches and shores. Even today some of this sediment can still be found along the linear ridges of the Appalachian Mountain section of the Ridge and Valley province. In the western part of the state more mud was deposited then anything and even further west was the disposition of carbonate. This was also the time when many rocks were being chemical weathered and creating large amounts of iron, which were later, mined throughout most of the 1800’s. The mountains that were created by the Taconic Orogreny were eroded during this time and stopped becoming a source of clastic sediment. In the northwestern part of PA during this time a supersaturated basin had formed with limited circulation. Once this basin evaporated the area was left full of gypsum and halite. This was also a crucial period for life. It saw the first fish appear on the earth.
The Devonian Period
This period was a relatively slow time for PA until the end of it. This period saw more deposition of carbonate for a few million more years. It also saw the formation of the Acadian Mountains. These were formed just east of PA when there was a collision between a landmass called Avalonia and Europe. This collision was called the Acadian orogeny. Once these mountains were formed they became a chief source of sediments, which were deposited in the Appalachian basin. The first of these sediments that was deposited became the black and gray shale that extends all the way from eastern PA to western Ohio. Ultimately the Acadian mountains were completely eroded and the sediment that came from them pushed forward the Appalachian basin to the Ohio border. Much of this sediment can be seen today as red clastic rock.
The Mississippi and Pennsylvania Periods: Carboniferous Time
During this time period there were really two big events that shaped PA. The first big event during this time period was the large amounts of coal found in PA were created. This coal was created when the sediment supply decreased to the swamps that had formed; the swamps slowly disappeared leaving behind a whole lot of peat. This cycle of peat formation was what is responsible for all of the coal. The second big event in this time period was that it was the last time that marine waters of the Appalachian basin covered any part of Pennsylvania. Instead swamps with huge amounts of vegetation replaced the waters. This vegetation was possible because PA was so close to the equator at this time. This was also the time period when many animals started showing up including, amphibians, reptiles and air breathing mollusks.
The Permian Period
This was a very important time period for PA. The Permian period saw a dramatic change caused by the collision of North America with Africa. This collision is known as the Aleghanian origeny. This collision created the Alleghanina Mountains, which are located on the eastern part of the state. These mountains were eroded and the sediment today can be found in the Piedmont province. By the end of this period most of the mountains were eroded away leaving PA a westward sloping alluvial plain with streams flowing across it. This period also saw a water shortage in PA being that it was in the middle of Pangea. This water shortage resulted in the drying up of the swamps and the death of much vegetation. The Alleghanina orogeny changed PA from a depositional basin to an area above the sea level that was continuously eroded placing sediment outside of the state.
The Mesozoic Era
There are three periods to the Mesozoic era, Triassic, Jurassic and the Cretaceous periods. During the Triassic period Pennsylvania was carried across the equator into the Northern Hemisphere. The climate became subtropical to tropical and the rainfall became seasonal. There were also long term wet and dry cycles during the Triassic period, which led to a large amount of erosion during this period. During the Jurassic era magma approached the surface and cooled as dark hard igneous rock. In PA the Mesozoic sediments and diabase are preserved in areas called the Gettysburg and Newark basins. Very little is know about the Jurassic and Cretaceous period in PA but what is know is that a lot of erosion occurred. This was also the period when early drainage systems were established as river flows toward the newly formed Atlantic Ocean. More peat was deposited and the dinosaurs ruled the land.
The Cenozoic Era
This was a time period of much erosion of PA. During this time probably all of PA was lowered hundreds of feet. This also meant that the differences between the highest point and the lowest point was increased dramatically. This period also saw the creation of large amounts of ice that slowly made their way into PA covering only a small part of the northern part. This ice movement allowed for significant erosion and deepening of valleys along the ice edge. The ice also scraped and lowered hills. Once the ice started to melt much sediment was left behind and can be seen today along the Appalachian Mountains. This period also saw the first sings of grasses, which allowed for the mammals to take over.
Like I had said at the beginning Pennsylvania is an ever-growing system that will probably look completely different 50 million years from now. As the plates continue to move and the continents continue to move with hem one can only guess what is in store next for our state. Hopefully with this you gain a better of idea of what was involved with the creation of PA.