Answer: scientists created the geologic time scale by studying rock layers and fossils found in different locations around the world. Here are the steps they took to make the time scale:
Explanation:
1. Rock Layers: Scientists observed that Earth's rocks are arranged in layers, with the oldest rocks at the bottom and the youngest rocks at the top. They used this principle of superposition to establish the relative age of the rocks.
2. Fossils: Fossils are the remains or traces of ancient organisms. Scientists noticed that certain fossils were only found in specific rock layers. By studying the distribution of fossils, they could determine the relative age of the rock layers.
3. Index Fossils: Some fossils were particularly useful in establishing the age of rock layers. These are called index fossils. Index fossils were widespread geographically but limited to a particular time period. By identifying and dating index fossils, scientists could correlate rock layers from different locations and establish a more accurate time scale.
4. Radioactive Dating: To assign absolute ages to rock layers and fossils, scientists used a method called radioactive dating. This technique measures the decay of radioactive isotopes in rocks and fossils to determine their age. By comparing the ratio of parent isotopes to daughter isotopes, scientists can calculate how long it took for the decay to occur.
5. Stratigraphy: Another important method used in constructing the geologic time scale is stratigraphy. This involves studying the physical characteristics and composition of rock layers. Scientists look for distinctive layers that can be correlated across different regions.
By combining these methods and analyzing data from various locations, scientists were able to create the geologic time scale. This scale divides Earth's history into different time intervals, such as eons, eras, periods, and epochs. It provides a framework for understanding the sequence of events and the relative and absolute ages of rocks and fossils. The geologic time scale continues to be refined as new discoveries are made and more accurate dating techniques are developed.