Science 7: Unit E: Planet Earth Topic 9 – Geologic Time

How Old is the Earth? The Earth is roughly 4.5 billion years old, the age of the solar system. The Earth is roughly 4.5 billion years old, the age of the solar system. The history of the Earth from start to finish is divided into separate periods called eons. These eons in turn can be divided into eras. These eras are then divided into periods. The history of the Earth from start to finish is divided into separate periods called eons. These eons in turn can be divided into eras. These eras are then divided into periods. The eras can be millions, even billions of years old and they are not all the same length of time. The eras can be millions, even billions of years old and they are not all the same length of time.

One era ends and another begins usually when there is a mass extinction of life-forms such as when the dinosaurs died out after the meteor hit. Usually when this happens, a new class of organisms (living things) become dominant on Earth. One era ends and another begins usually when there is a mass extinction of life-forms such as when the dinosaurs died out after the meteor hit. Usually when this happens, a new class of organisms (living things) become dominant on Earth. The order of eras is Precambrian, Paleozoic (ancient life), Mesozoic (middle life), and Cenozoic (recent life). The order of eras is Precambrian, Paleozoic (ancient life), Mesozoic (middle life), and Cenozoic (recent life). Because one era can be hundreds of millions years long each era is divided into periods. See p Because one era can be hundreds of millions years long each era is divided into periods. See p What are the Eras of Earth?

Where are We at Now? We are in the Cenozoic era and the Quaternary Period of that era. We are in the Cenozoic era and the Quaternary Period of that era. The Cenozoic era started after the Mass Extinction of the Dinosaurs and is characterized by the dominance of mammals and ultimately humanity. The Cenozoic era started after the Mass Extinction of the Dinosaurs and is characterized by the dominance of mammals and ultimately humanity. The Cenozoic era is roughly 65 million years old, and the Quarternary Period occupies the last 2 million years of that period. The Cenozoic era is roughly 65 million years old, and the Quarternary Period occupies the last 2 million years of that period.

How to Determine the Ages of Rocks and Fossils I. Relative Dating – is the dating of rocks by comparing them to other rocks or fossils of well known organisms. There are two methods of relative dating. I. Relative Dating – is the dating of rocks by comparing them to other rocks or fossils of well known organisms. There are two methods of relative dating. II. Absolute Dating – is the dating of rocks and fossils using the radioactive properties of certain elements (uranium and the carbon-14 isotope). Radioactivity basically means that over time, certain materials such as uranium will change into another material. By studying the amount of the original material that is left, we can calculate the age of the rock. There are two methods of absolute dating: radiometric dating and radiocarbon dating: II. Absolute Dating – is the dating of rocks and fossils using the radioactive properties of certain elements (uranium and the carbon-14 isotope). Radioactivity basically means that over time, certain materials such as uranium will change into another material. By studying the amount of the original material that is left, we can calculate the age of the rock. There are two methods of absolute dating: radiometric dating and radiocarbon dating:

Relative Dating 1. Principle of Superposition – In an undisturbed sedimentary rock formation we know that the lower down the strata/layer of rock is, the older it is. 1. Principle of Superposition – In an undisturbed sedimentary rock formation we know that the lower down the strata/layer of rock is, the older it is. 2. Index Fossils – If geologists find a fossil of a well known organism such as a dinosaur we can assume that the rock layer is as old the fossil that’s in it. For example we know that trilobites lived in the early Mesozoic Era and therefore if we find a rock containing trilobite fossils, then the rock itself is from that era as well. 2. Index Fossils – If geologists find a fossil of a well known organism such as a dinosaur we can assume that the rock layer is as old the fossil that’s in it. For example we know that trilobites lived in the early Mesozoic Era and therefore if we find a rock containing trilobite fossils, then the rock itself is from that era as well.

Absolute Dating 1. Radiometric Dating – The study of the amount of uranium (parent material) compared to amount of lead (daughter material) contained in a sample of rock. The lower the percentage of uranium, the older the rock is. The half-life of uranium is 4.5 billions old, the same age as the Earth. No rock should therefore have a percentage of uranium of less than 50% compared to lead and few if any rocks have a percentage near 50% (because rocks that old have been recycled by the Rock Cycle). 1. Radiometric Dating – The study of the amount of uranium (parent material) compared to amount of lead (daughter material) contained in a sample of rock. The lower the percentage of uranium, the older the rock is. The half-life of uranium is 4.5 billions old, the same age as the Earth. No rock should therefore have a percentage of uranium of less than 50% compared to lead and few if any rocks have a percentage near 50% (because rocks that old have been recycled by the Rock Cycle).

Absolute Dating Continued 2. Radiocarbon Dating – The study of the amount of Carbon-14 (parent material) compared to the amount of nitrogen (daughter material) contained in a fossil sample. The lower the percentage of Carbon-14, the older the fossil. This technique only works with fossils of less than 50’000 years. The half life of Carbon- 14 is 5730 a meaning that half of the carbon-14 will turn into nitrogen over that time. 2. Radiocarbon Dating – The study of the amount of Carbon-14 (parent material) compared to the amount of nitrogen (daughter material) contained in a fossil sample. The lower the percentage of Carbon-14, the older the fossil. This technique only works with fossils of less than 50’000 years. The half life of Carbon- 14 is 5730 a meaning that half of the carbon-14 will turn into nitrogen over that time.