1. Superposition
The principle of superposition suggests that the layers of rock are set on top of each other in a horizontal manner. It also says that the layers that are at the bottom are the oldest, while the ones on the top are the youngest, practically meaning that the layers are arranged by age from bottom toward the top. This can be seen very well at the Grand Canyon, especially because its layers can be seen very well, and it is easily noticeable that they differ from one another and belong to different periods. This principle only applies to rock layers that have not been disturbed.
2. An atom that has more or fewer neutrons than it typically does
An isotope represents two or more forms of an identical element, having the same amount of protons, but having different number of neutrons than what the element typically has. It can also be said that an isotope represents a radioactive form of an element. There are countless number of isotopes in nature, as well as some that have been man-made. As an example we can take the carbon 12, carbon 13, and carbon 14, all of which have the same amount of protons from the same element, the carbon, but differ in the number of neutrons.
3. When the radiometric clock starts ticking in zircon minerals, there is 100% of the unstable radiometric U-235 and 0% of the stable Pb-207
We know that the Earth is very old, but in order to know how much exactly it is really hard to tell. There are numerous different methods used for dating, some of which focus on the fossils, while others focus on the rocks and minerals. The ones that focus on the fossils manage to tell us how much time has life been on our planet, but not more, so the scientists use methods to date the oldest rocks/minerals on the planet. They use the radioactive decay of uranium and lead from the zirconium in order to be able to perform proper dating, which suggest that the oldest rocks are 4.6 billion years old, though the Earth itself is probably older.
4. 1408
If U-235 isotope has a half-life of 704 million years, than that means that means that that's the amount of time for the parent material to lose half of itself. If only 25% of the parent isotope is left, and there is 75% of the daughter isotope, than we are looking for double the age of the isotope's half-life. This is because with every half-life period passed, the isotope is twice less, so with 704 million years passed, 50% of it are left, and then those 50% need 704 million years to be twice less so that only 25% of the isotope is left. This means that we just need to double the figure, thus 704 million by 2, and we have a result of 1408 million years.
5. narrow range of time in which they exist and wide geographic distribution
The index fossils are fossils that have special characteristics that make them very helpful and useful for the geologists. These fossils are used for easily determining the age of other fossils and rock layers without having to perform detailed examinations of them. The reason for this is that the index fossils are very wide spread, are found in abundance, and they only exist in relatively short period of time.
6. abundant bombardment and partially molten surface
The Hadean eon is the eon that includes the formation of the Earth. During this eon, the Earth is totally unrecognizable. Asteroids were constantly bombarding it because it still didn't had thick and nicely layer atmosphere. It was extremely hot, and the surface was initially molten and than gradually was becoming half molten. Big change during this eon was the occurrence of the first oceans on Earth, and that was a game changer for the further development on our planet.
