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Mariah
11/26/2025
Environmental Science
Geology Notes - Lesson 9
14
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Nov 26, 2025
•
Understanding Earth's History - Geologic Time and Dating Methods
Mariah
@mariah_stangl
1 / 3
Relative vs. Absolute Time in Geology
When studying Earth's history, geologists use two fundamental approaches to understand time. Relative time focuses on the sequence of events (what happened first, second, third) without specific dates. Absolute time assigns actual numerical ages in years (like 10 million years ago).
Several key principles help determine relative age of rock layers. The Principle of Superposition states that in undisturbed sedimentary layers, bottom layers are older than top layers—just like in a sandwich where the bottom bread was placed before the fillings. The Principle of Original Horizontality tells us sedimentary layers were initially deposited horizontally before any tilting occurred.
The Cross-Cutting Relationship principle shows that any feature cutting through rock (like a fault) must be younger than the rock it cuts through. Similarly, intrusions (magma bodies that push into existing rock) are always younger than the surrounding rock they intrude into.
💡 Quick Tip: Think of cross-cutting relationships like drawing on paper—the paper existed first, then you added your drawing. In geology, the rock existed first, then the cutting feature formed.
Unconformities: Gaps in Earth's History
Unconformities are fascinating features representing gaps in the rock record where erosion or non-deposition occurred. These surfaces essentially represent missing time in Earth's history and come in three main types.
A disconformity occurs when layers above and below an erosional surface remain parallel to each other. This happens when deposition stops, some erosion occurs, and then deposition continues later without any tilting of the rocks.
An angular unconformity forms through a four-stage process: initial deposition of layers, followed by deformation (tilting or folding), then erosion of the tilted layers, and finally renewed deposition on top. The distinctive angle between lower and upper rock layers makes these unconformities easy to recognize.
Nonconformities represent contacts where sedimentary rocks overlay igneous or metamorphic rocks, showing a significant gap in geological time. Understanding these discontinuities helps geologists piece together Earth's complex history.
🔍 Insight: Unconformities are like missing pages in Earth's history book—they show where chapters of the story have been erased by natural processes!
Absolute Dating: Measuring Earth's Age
Absolute dating assigns specific numerical ages to geological events and rocks. Unlike relative dating, these methods tell us exactly how old something is. Common methods include tree rings (where one ring equals one year) and isotopic dating.
Isotopes are versions of elements with varying numbers of neutrons, and some are unstable, meaning they decay over time. Scientists use this predictable decay as a geological clock. For example, Carbon-14 decays to Nitrogen, while Uranium-235 decays to Lead-207, and Uranium-238 decays to Lead-206.
The key to isotopic dating is the half-life—the time it takes for half of the parent isotope to decay into the daughter product. After one half-life, 50% of the original isotope remains; after two half-lives, 25% remains; and so on. By measuring the ratio between parent and daughter isotopes, scientists can determine when rocks formed.
🧪 Important: Uranium isotopes are particularly valuable for dating metamorphic and igneous rocks because they mark when these rocks cooled below certain temperatures—essentially recording their "birthday" in geological terms.
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The app is very easy to use and well designed. I have found everything I was looking for so far and have been able to learn a lot from the presentations! I will definitely use the app for a class assignment! And of course it also helps a lot as an inspiration.
Stefan S
iOS user
This app is really great. There are so many study notes and help [...]. My problem subject is French, for example, and the app has so many options for help. Thanks to this app, I have improved my French. I would recommend it to anyone.
Samantha Klich
Android user
Wow, I am really amazed. I just tried the app because I've seen it advertised many times and was absolutely stunned. This app is THE HELP you want for school and above all, it offers so many things, such as workouts and fact sheets, which have been VERY helpful to me personally.
Anna
iOS user
I think it’s very much worth it and you’ll end up using it a lot once you get the hang of it and even after looking at others notes you can still ask your Artificial intelligence buddy the question and ask to simplify it if you still don’t get it!!! In the end I think it’s worth it 😊👍 ⚠️Also DID I MENTION ITS FREEE YOU DON’T HAVE TO PAY FOR ANYTHING AND STILL GET YOUR GRADES IN PERFECTLY❗️❗️⚠️
Thomas R
iOS user
Knowunity is the BEST app I’ve used in a minute. This is not an ai review or anything this is genuinely coming from a 7th grade student (I know 2011 im young) but dude this app is a 10/10 i have maintained a 3.8 gpa and have plenty of time for gaming. I love it and my mom is just happy I got good grades
Brad T
Android user
Not only did it help me find the answer but it also showed me alternative ways to solve it. I was horrible in math and science but now I have an a in both subjects. Thanks for the help🤍🤍
David K
iOS user
The app's just great! All I have to do is enter the topic in the search bar and I get the response real fast. I don't have to watch 10 YouTube videos to understand something, so I'm saving my time. Highly recommended!
Sudenaz Ocak
Android user
In school I was really bad at maths but thanks to the app, I am doing better now. I am so grateful that you made the app.
Greenlight Bonnie
Android user
I found this app a couple years ago and it has only gotten better since then. I really love it because it can help with written questions and photo questions. Also, it can find study guides that other people have made as well as flashcard sets and practice tests. The free version is also amazing for students who might not be able to afford it. Would 100% recommend
Aubrey
iOS user
Best app if you're in Highschool or Junior high. I have been using this app for 2 school years and it's the best, it's good if you don't have anyone to help you with school work.😋🩷🎀
Marco B
iOS user
THE QUIZES AND FLASHCARDS ARE SO USEFUL AND I LOVE THE SCHOOLGPT. IT ALSO IS LITREALLY LIKE CHATGPT BUT SMARTER!! HELPED ME WITH MY MASCARA PROBLEMS TOO!! AS WELL AS MY REAL SUBJECTS ! DUHHH 😍😁😲🤑💗✨🎀😮
Elisha
iOS user
This app is phenomenal down to the correct info and the various topics you can study! I greatly recommend it for people who struggle with procrastination and those who need homework help. It has been perfectly accurate for world 1 history as far as I’ve seen! Geometry too!
Paul T
iOS user
The app is very easy to use and well designed. I have found everything I was looking for so far and have been able to learn a lot from the presentations! I will definitely use the app for a class assignment! And of course it also helps a lot as an inspiration.
Stefan S
iOS user
This app is really great. There are so many study notes and help [...]. My problem subject is French, for example, and the app has so many options for help. Thanks to this app, I have improved my French. I would recommend it to anyone.
Samantha Klich
Android user
Wow, I am really amazed. I just tried the app because I've seen it advertised many times and was absolutely stunned. This app is THE HELP you want for school and above all, it offers so many things, such as workouts and fact sheets, which have been VERY helpful to me personally.
Anna
iOS user
I think it’s very much worth it and you’ll end up using it a lot once you get the hang of it and even after looking at others notes you can still ask your Artificial intelligence buddy the question and ask to simplify it if you still don’t get it!!! In the end I think it’s worth it 😊👍 ⚠️Also DID I MENTION ITS FREEE YOU DON’T HAVE TO PAY FOR ANYTHING AND STILL GET YOUR GRADES IN PERFECTLY❗️❗️⚠️
Thomas R
iOS user
Knowunity is the BEST app I’ve used in a minute. This is not an ai review or anything this is genuinely coming from a 7th grade student (I know 2011 im young) but dude this app is a 10/10 i have maintained a 3.8 gpa and have plenty of time for gaming. I love it and my mom is just happy I got good grades
Brad T
Android user
Not only did it help me find the answer but it also showed me alternative ways to solve it. I was horrible in math and science but now I have an a in both subjects. Thanks for the help🤍🤍
David K
iOS user
The app's just great! All I have to do is enter the topic in the search bar and I get the response real fast. I don't have to watch 10 YouTube videos to understand something, so I'm saving my time. Highly recommended!
Sudenaz Ocak
Android user
In school I was really bad at maths but thanks to the app, I am doing better now. I am so grateful that you made the app.
Greenlight Bonnie
Android user
I found this app a couple years ago and it has only gotten better since then. I really love it because it can help with written questions and photo questions. Also, it can find study guides that other people have made as well as flashcard sets and practice tests. The free version is also amazing for students who might not be able to afford it. Would 100% recommend
Aubrey
iOS user
Best app if you're in Highschool or Junior high. I have been using this app for 2 school years and it's the best, it's good if you don't have anyone to help you with school work.😋🩷🎀
Marco B
iOS user
THE QUIZES AND FLASHCARDS ARE SO USEFUL AND I LOVE THE SCHOOLGPT. IT ALSO IS LITREALLY LIKE CHATGPT BUT SMARTER!! HELPED ME WITH MY MASCARA PROBLEMS TOO!! AS WELL AS MY REAL SUBJECTS ! DUHHH 😍😁😲🤑💗✨🎀😮
Elisha
iOS user
This app is phenomenal down to the correct info and the various topics you can study! I greatly recommend it for people who struggle with procrastination and those who need homework help. It has been perfectly accurate for world 1 history as far as I’ve seen! Geometry too!
Paul T
iOS user
Understanding Earth's History - Geologic Time and Dating Methods
Mariah
@mariah_stangl
Geology's time concepts give us a window into Earth's incredible past. Measuring and understanding geological time involves both relative and absolute dating methods. These techniques help scientists piece together Earth's 4.6-billion-year history.
Relative vs. Absolute Time in Geology
When studying Earth's history, geologists use two fundamental approaches to understand time. Relative time focuses on the sequence of events (what happened first, second, third) without specific dates. Absolute time assigns actual numerical ages in years (like 10 million years ago).
Several key principles help determine relative age of rock layers. The Principle of Superposition states that in undisturbed sedimentary layers, bottom layers are older than top layers—just like in a sandwich where the bottom bread was placed before the fillings. The Principle of Original Horizontality tells us sedimentary layers were initially deposited horizontally before any tilting occurred.
The Cross-Cutting Relationship principle shows that any feature cutting through rock (like a fault) must be younger than the rock it cuts through. Similarly, intrusions (magma bodies that push into existing rock) are always younger than the surrounding rock they intrude into.
💡 Quick Tip: Think of cross-cutting relationships like drawing on paper—the paper existed first, then you added your drawing. In geology, the rock existed first, then the cutting feature formed.
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Unconformities: Gaps in Earth's History
Unconformities are fascinating features representing gaps in the rock record where erosion or non-deposition occurred. These surfaces essentially represent missing time in Earth's history and come in three main types.
A disconformity occurs when layers above and below an erosional surface remain parallel to each other. This happens when deposition stops, some erosion occurs, and then deposition continues later without any tilting of the rocks.
An angular unconformity forms through a four-stage process: initial deposition of layers, followed by deformation (tilting or folding), then erosion of the tilted layers, and finally renewed deposition on top. The distinctive angle between lower and upper rock layers makes these unconformities easy to recognize.
Nonconformities represent contacts where sedimentary rocks overlay igneous or metamorphic rocks, showing a significant gap in geological time. Understanding these discontinuities helps geologists piece together Earth's complex history.
🔍 Insight: Unconformities are like missing pages in Earth's history book—they show where chapters of the story have been erased by natural processes!
Sign up to see the contentIt's free!
Access to all documents
Improve your grades
Join milions of students
By signing up you accept Terms of Service and Privacy Policy
Absolute Dating: Measuring Earth's Age
Absolute dating assigns specific numerical ages to geological events and rocks. Unlike relative dating, these methods tell us exactly how old something is. Common methods include tree rings (where one ring equals one year) and isotopic dating.
Isotopes are versions of elements with varying numbers of neutrons, and some are unstable, meaning they decay over time. Scientists use this predictable decay as a geological clock. For example, Carbon-14 decays to Nitrogen, while Uranium-235 decays to Lead-207, and Uranium-238 decays to Lead-206.
The key to isotopic dating is the half-life—the time it takes for half of the parent isotope to decay into the daughter product. After one half-life, 50% of the original isotope remains; after two half-lives, 25% remains; and so on. By measuring the ratio between parent and daughter isotopes, scientists can determine when rocks formed.
🧪 Important: Uranium isotopes are particularly valuable for dating metamorphic and igneous rocks because they mark when these rocks cooled below certain temperatures—essentially recording their "birthday" in geological terms.
We thought you’d never ask...
What is the Knowunity AI companion?
Our AI companion is specifically built for the needs of students. Based on the millions of content pieces we have on the platform we can provide truly meaningful and relevant answers to students. But its not only about answers, the companion is even more about guiding students through their daily learning challenges, with personalised study plans, quizzes or content pieces in the chat and 100% personalisation based on the students skills and developments.
Where can I download the Knowunity app?
You can download the app in the Google Play Store and in the Apple App Store.
Is Knowunity really free of charge?
That's right! Enjoy free access to study content, connect with fellow students, and get instant help – all at your fingertips.
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The app is very easy to use and well designed. I have found everything I was looking for so far and have been able to learn a lot from the presentations! I will definitely use the app for a class assignment! And of course it also helps a lot as an inspiration.
Stefan S
iOS user
This app is really great. There are so many study notes and help [...]. My problem subject is French, for example, and the app has so many options for help. Thanks to this app, I have improved my French. I would recommend it to anyone.
Samantha Klich
Android user
Wow, I am really amazed. I just tried the app because I've seen it advertised many times and was absolutely stunned. This app is THE HELP you want for school and above all, it offers so many things, such as workouts and fact sheets, which have been VERY helpful to me personally.
Anna
iOS user
I think it’s very much worth it and you’ll end up using it a lot once you get the hang of it and even after looking at others notes you can still ask your Artificial intelligence buddy the question and ask to simplify it if you still don’t get it!!! In the end I think it’s worth it 😊👍 ⚠️Also DID I MENTION ITS FREEE YOU DON’T HAVE TO PAY FOR ANYTHING AND STILL GET YOUR GRADES IN PERFECTLY❗️❗️⚠️
Thomas R
iOS user
Knowunity is the BEST app I’ve used in a minute. This is not an ai review or anything this is genuinely coming from a 7th grade student (I know 2011 im young) but dude this app is a 10/10 i have maintained a 3.8 gpa and have plenty of time for gaming. I love it and my mom is just happy I got good grades
Brad T
Android user
Not only did it help me find the answer but it also showed me alternative ways to solve it. I was horrible in math and science but now I have an a in both subjects. Thanks for the help🤍🤍
David K
iOS user
The app's just great! All I have to do is enter the topic in the search bar and I get the response real fast. I don't have to watch 10 YouTube videos to understand something, so I'm saving my time. Highly recommended!
Sudenaz Ocak
Android user
In school I was really bad at maths but thanks to the app, I am doing better now. I am so grateful that you made the app.
Greenlight Bonnie
Android user
I found this app a couple years ago and it has only gotten better since then. I really love it because it can help with written questions and photo questions. Also, it can find study guides that other people have made as well as flashcard sets and practice tests. The free version is also amazing for students who might not be able to afford it. Would 100% recommend
Aubrey
iOS user
Best app if you're in Highschool or Junior high. I have been using this app for 2 school years and it's the best, it's good if you don't have anyone to help you with school work.😋🩷🎀
Marco B
iOS user
THE QUIZES AND FLASHCARDS ARE SO USEFUL AND I LOVE THE SCHOOLGPT. IT ALSO IS LITREALLY LIKE CHATGPT BUT SMARTER!! HELPED ME WITH MY MASCARA PROBLEMS TOO!! AS WELL AS MY REAL SUBJECTS ! DUHHH 😍😁😲🤑💗✨🎀😮
Elisha
iOS user
This app is phenomenal down to the correct info and the various topics you can study! I greatly recommend it for people who struggle with procrastination and those who need homework help. It has been perfectly accurate for world 1 history as far as I’ve seen! Geometry too!
Paul T
iOS user
The app is very easy to use and well designed. I have found everything I was looking for so far and have been able to learn a lot from the presentations! I will definitely use the app for a class assignment! And of course it also helps a lot as an inspiration.
Stefan S
iOS user
This app is really great. There are so many study notes and help [...]. My problem subject is French, for example, and the app has so many options for help. Thanks to this app, I have improved my French. I would recommend it to anyone.
Samantha Klich
Android user
Wow, I am really amazed. I just tried the app because I've seen it advertised many times and was absolutely stunned. This app is THE HELP you want for school and above all, it offers so many things, such as workouts and fact sheets, which have been VERY helpful to me personally.
Anna
iOS user
I think it’s very much worth it and you’ll end up using it a lot once you get the hang of it and even after looking at others notes you can still ask your Artificial intelligence buddy the question and ask to simplify it if you still don’t get it!!! In the end I think it’s worth it 😊👍 ⚠️Also DID I MENTION ITS FREEE YOU DON’T HAVE TO PAY FOR ANYTHING AND STILL GET YOUR GRADES IN PERFECTLY❗️❗️⚠️
Thomas R
iOS user
Knowunity is the BEST app I’ve used in a minute. This is not an ai review or anything this is genuinely coming from a 7th grade student (I know 2011 im young) but dude this app is a 10/10 i have maintained a 3.8 gpa and have plenty of time for gaming. I love it and my mom is just happy I got good grades
Brad T
Android user
Not only did it help me find the answer but it also showed me alternative ways to solve it. I was horrible in math and science but now I have an a in both subjects. Thanks for the help🤍🤍
David K
iOS user
The app's just great! All I have to do is enter the topic in the search bar and I get the response real fast. I don't have to watch 10 YouTube videos to understand something, so I'm saving my time. Highly recommended!
Sudenaz Ocak
Android user
In school I was really bad at maths but thanks to the app, I am doing better now. I am so grateful that you made the app.
Greenlight Bonnie
Android user
I found this app a couple years ago and it has only gotten better since then. I really love it because it can help with written questions and photo questions. Also, it can find study guides that other people have made as well as flashcard sets and practice tests. The free version is also amazing for students who might not be able to afford it. Would 100% recommend
Aubrey
iOS user
Best app if you're in Highschool or Junior high. I have been using this app for 2 school years and it's the best, it's good if you don't have anyone to help you with school work.😋🩷🎀
Marco B
iOS user
THE QUIZES AND FLASHCARDS ARE SO USEFUL AND I LOVE THE SCHOOLGPT. IT ALSO IS LITREALLY LIKE CHATGPT BUT SMARTER!! HELPED ME WITH MY MASCARA PROBLEMS TOO!! AS WELL AS MY REAL SUBJECTS ! DUHHH 😍😁😲🤑💗✨🎀😮
Elisha
iOS user
This app is phenomenal down to the correct info and the various topics you can study! I greatly recommend it for people who struggle with procrastination and those who need homework help. It has been perfectly accurate for world 1 history as far as I’ve seen! Geometry too!
Paul T
iOS user