We humans often notice the passage of time by observing our watches; the movement of the Sun, Moon, and stars across the sky; or by the records left by our ancestors in diaries or other historical records—but these are just fleeting moments in the eyes of geological time. We are used to thinking about recorded history. But recorded history is just a blink when compared to the length of time called pre-history. Recorded history only goes back a few thousand years. The Earth is far older.
It’s hard for humans to grasp just how long the Earth has been here. Using a variety of methods, geologists have been able to put together many pieces of a very complicated puzzle. After all, how do you assemble a puzzle when you’re not sure what the finished picture should look like? From studying processes that are happening today, such as geological composition, rates of deposition, weathering, climatology, biology, and Earth’s magnetic field, geologists can extend these processes back to ancient times and learn what the Earth was like billions of years ago. When combined with data points such as those found in the fossil record, these extrapolations can be constrained, and the picture starts to emerge from the puzzle.
We often take our surroundings for granted. Almost everything we see around us is the result of a chain of events, each of which had an origin. The rocks and soil, the air we breathe, and the plants and animals surrounding us have not always been here. When did each of these things first appear? It’s impossible to cover all of the geological time periods in a single blog entry, but I can point out a few of the more interesting ones to highlight some new and interesting functionality in Wolfram|Alpha.
By studying the oldest rocks on Earth and even those found on the Moon during the Apollo Moon missions, scientists have found that the Earth has been around for about 4.5 billion years. During the earliest geological period (which actually predates rocks themselves, and so cannot really be called geology), the Earth was likely a ball of molten magma. This time period, because of the imagery it creates in our imagination, is known as the Hadean Eon. This period lasted for about 800 million years. That’s over 17% of the entire time span of Earth’s existence. It wasn’t until after this long time period that the Earth grew and cooled enough for the earliest rocks to form.
The Archean Eon followed, which lasted another 1.3 billion years (another 28% of the Earth’s entire history) and was marked by the formation of the first rocks, the solidification of Earth’s crust, the first mountain ranges, and the first primitive life on Earth (in the form of bacterial mats and Prokaryotic organisms capable of living in an essentially oxygen-free atmosphere). As we move closer to modern times, events start to unfold more rapidly, and the geological record becomes more forgiving at giving up its secrets.
The Proterozoic Eon lasted 1.9 billion years (another 41% of Earth’s history), and the events include the deposition of banded iron formations in the rocks. These formations are the first evidence that oxygen was starting to have a measurable footprint in our atmosphere. Oxygen was beginning to react with the rocks and oxidize the iron in them in pulses, creating bands of iron oxides in the rocks. This “oxygen catastrophe”, about 2.4 billion years ago, would have been devastating to the primitive life on Earth that was dominant at the time. Until then, the essentially anaerobic organisms thrived in a nearly oxygen-free world, but as these anaerobic forms exhaled, they expelled oxygen. Over the long time that these creatures thrived, the oxygen eventually started to fill up the oxygen sinks in the rocks and began to overflow into the atmosphere. Banded iron formations were no longer common, and aerobic and multi-cellular life had appeared.
Due to the effects of continental drift, during these early time periods, the Earth’s continents would have been unrecognizable and fragmented compared to today. By about a billion years ago, during the Stenian Period, the models of the arrangement of the continents started to become a little more reliable. Most of the continental mass was gathered together into a supercontinent known as Rodinia. We can start to create a visual picture of what the Earth looked like.
The geological record of the Cryogenian Period, which began about 850 million years ago, reveals the possible presence of glacial deposits near the paleo-equator. During this time, Earth may have experienced the deepest ice age in its history. This glacial event is often referred to as “Snowball Earth”. Rodinia was breaking up.
Soft-bodied animals such as worms started to appear on the sea floor during the Ediacaran Period, 635 million years ago, and around 542 million years ago, life started to exhibit a more “shelly” appearance. The fossil record shows an “explosion” of life forms during the Cambrian period. Many strange creatures that would look alien to us today developed during this time period. For example, Anomalocarids, such as the one shown in the following animation, were among the top predators.
Before the Ordovician Period, the only living creatures could be found in the sea. Meanwhile, life continued to evolve into forms more like those we know today. Fish were abundant during the Devonian Period, around the same time that arthropods were making their first significant excursions onto land.
During the Permian Period, which ended about 251 million years ago, some reptiles were beginning to exhibit mammalian characteristics, and atmospheric oxygen levels were at their highest levels in Earth’s history.
The Permian Period ended with the largest mass extinction in the Earth’s history around the same time that Siberia was covered in massive basalt lava flows. During this time, the continents were once again part of a supercontinent, this time called Pangea.
The Permian extinction came at the end of the Paleozoic Era and began the Mesozoic Era. Biodiversity at the start of the Mesozoic Era was low, but it slowly recovered, and by the middle of the Triassic Period, dinosaurs had appeared.
The Paleocene Epoch followed the Late Cretaceous Epoch, and the two are separated by a well-known event. During the transition between these two periods, the dinosaurs went extinct. A large crater called Chicxulub is often interpreted as the smoking gun that led to the extinction of the dinosaurs. There is some controversy as to whether the associated asteroid impact was the cause of the extinction or if it was coincidental or part of a separate chain of events, but data shows that this crater was likely formed at about the right time to possibly have had some effect.
After the dinosaur extinction, we entered the current era known as the Cenozoic Era. It was during this time that the mammals rose to dominance, India collided with Asia and began the formation of the Himalayas, the current ice age began, human civilization appeared, and this blog entry was posted.
This is just a short summary of the events that have led to where we are today. Hopefully this new content will help to create an appreciation for all that has come before us.