Earth is an astounding 4.5-4.6 billion years old! But how do scientists know that, and how can we figure out what has happened throughout Earth's extraordinarily long history? Well, since humans only have a life span of about 100 years (at best), scientists have to put their detective skills to the test. They focus their efforts on understanding Earth's changing geology through the study of rocks and fossils. Like the chapters of a book, layers of rocks (and the fossils found within them) reveal Earth's history in what is known as a Geologic Timescale. The times that events occurred and now extinct organisms lived is determined in one of two ways: relative dating and absolute dating. Let's take a closer look at these methods.
Relative Dating
Relative dating is a method used by geologists to determine the order of events in Earth's history; it can't be used to give actual dates, just relative dates. It is based on a principle known as the law of superposition, which states that when rocks form layers, the oldest layer will be on the bottom while the youngest layer is on the top. Fun fact: layers of rock are known as rock strata, and the study of these layers is known as stratigraphy. The diagram below shows an example of rock strata. You can see that the first and oldest layer is on the bottom. It was the first layer of rock to form. Then, layer 2 formed on top of layer 1. This continues until you get to the youngest and top-most layer: layer 6.
Unfortunately, because Earth's surface is constantly changing, the layers don't always look quite as orderly as the layers shown in the example above. Instead, the layers of rock might...
1. bend and fold as Earth's tectonic plates slowly push into one another. This can be seen in the folded limestone in Crete, Greece (shown below in the middle).
2. shift up or down at fault lines (cracks in Earth's surface). This can be seen in normal fault along the highway between Guatemala City and Lake Amatitlan, Guatemala (shown below on the right).
3. get disrupted by magma pushing through the layers from deep inside Earth (also known as an igneous intrusion or dike). This can be seen in the Grand Canyon (shown below on the left).
Check your Understanding: Using what you have learned about relative dating, can you figure out the correct order of the rock strata shown below? Check your answers and review relative dating here.
If you struggled to put the layers of rock in the correct order, you may want to watch the video below to review a little more and go through a guided example.
Absolute Dating
To determine the actual ages of rocks and fossils, scientists use a process known as radiometric dating. This process uses the exact composition of rocks and half-life data of elements to give exact dates. To understand this, you will need to understand a few important ideas.
#1. Radioactive decay is the process by which an element decays (changes) into another element. This happens at a constant rate known as the half-life.
#2. A half-life is how long it takes for half of the original element to decay.
#3. What element you have is determined by the number of protons the atom has, so when an element decays it loses a proton.
#4. While absolute dating techniques are more accurate than relative dating, they are still an estimate. For example, if a rock or fossil is determined through carbon dating, an accurate timeline of only 50,000 years is achievable. With other isotopes, estimates can be made as far back as 4.4 billion years, which is the oldest material found on Earth!
Both of the images below model how radioactive decay is used to determine the "absolute" age of rocks and fossils. After looking over the images, watch the "How to Date a Planet" video for additional information.
Check your Understanding: Using what you have learned about absolute dating, can you figure solve the following problem?
Uranium-235 is an isotope used to date minerals in fossils and rocks. The half-life of uranium-235 is 704 million years, at which point half of the mineral’s uranium-235 will have decayed to lead-207, as shown in the graph below.
An igneous rock sample has 25% of its original uranium-235. How long ago did the igneous rock first form?
Check your answer at the bottom of this page.
Geologic Time: The Big Picture
Using the methods described above, scientists have developed a pretty clear picture of Earth's history, both in terms of geology and evolutionary biology. The study of rocks and fossils doesn't only give us a timeline of major eras; it gives us a document of the existence, diversity, extinction, and change of life forms throughout Earth's history.
Bonus: Can you find patterns in the types of fossils that have existed throughout the various eras? Use the image below as well as the interactive Geologic Time Scale link to help you with this.
Challenges/Extensions
Choose one or more of the following activities to model what you learned about Geologic Time.
#1. Choose 5 major eras or periods in Earth's history and make a geology cake modeling the rock strata for each era/period. Label each of the layers using toothpicks and tape. See Rosanna Pansino's website for an example with recipes, or watch the video below. Note that she is making soil layers, rather than rock strata.
#2. Choose the 10 events you think are most important in Earth's history and create your own Geologic Timeline using these Geologic Timeline Events & Football Field Template documents.
#3. Model radioactive decay using this Radioactive Skittles lab. There is also a video below modeling how to do the lab. You may also use M&M's, pennies, puzzle pieces, or anything else that has two different sides.
#4. Research the evolution of a particular organism (e.g., whales) and create your own mini fossils to model real-life fossils that show the evolutionary history of the organism you chose. Alternatively, you can create fossils that show how organisms in general have changed over time, as shown in the geologic timeline image above.
Bonus: Just for Fun
Absolute Dating Answer
If 25% of the original sample is left, then 2 half lives have taken place. That is because after 1 half-life there is 50% left, and after 2 half-lives 25% is left (25% is half of 50%). Since 1 half-life is 704 million years, the rock first formed 704 million x 2, or 1408 million years ago.