Welcome to Module 10: Uniformitarianism and the Age of the Earth
In Module 9, we learned how to read the rock record and write the history of the Earth, learning what happened and putting those events in order. These techniques, and the history they tell, were worked out by pioneering geologists mostly in the 1700s and 1800s. Those pioneers knew they were studying a very long history, but they couldn’t put precise numbers on exactly how long. It took until the second half of the 1900s for scientists to develop the knowledge and the sensitive instruments needed to learn how many years ago the events happened. The answer is given in this short video comparing time to distance on a US football field, and then the rest of this Module tells you a little about how the answer was discovered, with visits to Great Basin National Park and the Grand Canyon.
Video: 100 Yards of Geologic Time (3:06 minutes)
Click Here for Transcript of 100 Yards of Geologic Time Video
It is 4.6 billion years ago. You were on the goal line Beaver Stadium and you have to drive 100 yards to today where you're taking Geosciences 10. The earth is forming 4.6 billion years ago. Giant meteorites are streaming in. And when they hit the surface, they make great explosions and collisions that heat the planet so much that they evaporate the ocean. And the last one of those that was big enough to evaporate the upper part of the ocean that was warmed by the sun and given energy by the sun is about 3.8 billion years ago, which leaves you 83 yards to drive to get to the goal.
Beyond this, the continents are forming. They're no longer getting blasted. And so you start to see continents show up that the cores of the modern things and they're sitting out there very nicely. And they are formed so that you get an idea of what the world is going to be like by about 2.5 billion years ago, which is a mere 54 yards to get your touchdown.
There are bacteria in the ocean and the bacteria are committing acts of flatulence. They're putting oxygen up. The oxygen changes the composition of the atmosphere, it changes the oceans, and it eventually allows bigger critters to appear. And those bigger creators include shelly critters which suddenly make lots of interesting rocks, limestones. And so you start to get lots of shells showing up about 570 million years ago, which is a mere 12 yards to get the goal line. The shells are doing really well.
And then there's a really bad day. The ocean gets very warm from greenhouse and it belches out bad gases and most of the things alive die. And that happens about 225 million years ago at the end of the Paleozoic which is only five yards from the goal.
That clears up space so that you start to get dinosaurs. And as you know, dinosaurs were really big and they're sort of cute critters. And so you start getting dinosaurs in the Mesozoic. And here is a dinosaur if you would like one. and the dinosaurs are having a fine time and they're smiling a lot.
But there's another meteorite coming. And so the big meteorite comes screaming in and it kills the dinosaurs, and that changes the world a lot. And that happens about 65 million years ago which is only one and a half yards from the goal.
That makes room for mammals to show up, and so you start to see mammals such as this elephant that you're about to see here. This elephant happens to be running away from you. And the elephant has some big ears and a really curly tail.
And that comes up to recorded history. And recorded history, 6,000 years ago, just a little over the thickness of a sheet of paper.
And finally, to the culmination of creation to you, who are born about 1/200th of the thickness of a sheet of paper from the goal line today.
Imagine that the 100 yards of Penn State's Beaver Stadium, or any other football field, are like a timeline of all of Earth's history, and you're the star of the team, driving for glory. The planet formed on your goal line, half of the Earth's history had passed as your team marched across the 50-yard line, and now the coach personally sent you, the acme of creation, to carry the ball across the opposition's goal line of today for the winning score. If you have been carrying the ball for the whole 20 years of your life, how far did you run? (If you're not 20 years old, pretend.)
Congratulations—tomorrow's newspaper will report that you gained just a shade under 0.0002 inch, or a bit less than 1/200 of the thickness of a sheet of paper. The defense was vanquished by your onslaught, and instant replay officials were not needed to see that you broke the plane of the goal.
Written history goes back slightly less than 6000 years or so, barely the thickness of a sheet of paper on the 100 yards of Earth's "dark backward and abysm of time," as Shakespeare called it. Geologists often feel sorry for people who have restricted themselves to writings and skipped the rocks—those people may have seen the instant replay of the touchdown, but they missed the thrill of the game. So come along and see what happened before you carried the ball for those last two ten-thousandths of an inch!
Learning Objectives
- Understand that geologists learn ages of events in many ways, including counting annual layers in deposits, calculating backward from rates at which observed processes occur, and using many different radioactive-decay techniques.
- Explain how the results of these dating techniques agree with written histories as far back as writing goes, and agree with each other in demonstrating a vastly longer geologic history.
- Remember that science does not claim to be the ultimate Truth, but recognize that the science underlying age dating is very strong and that within science there is no “other side” that conflicts with the results here.
What to do for Module 10?
You will have one week to complete Module 10. See the course calendar for specific due dates.
- Take the RockOn #10 Quiz
- Take the StudentsSpeak #10 Survey
- Submit Exercise #5
- Begin working on Exercise #6
Questions?
If you have any questions, send an email via Canvas, to ALL the Teachers and TAs. To do this, add each teacher individually in the “To” line of your email. By adding all the teachers, the TAs will be included. Failure to email ALL the teachers may result in a delayed or missed response. For detailed directions on how to do this, see How to send an email in GEOSC 10 in the Important Information module.
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