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HOW OLD IS THE EARTH? The Earth is about 4,600,000,000 years old (4,600 million or 4.6 billion years old). Assuming that Earth formed at about the same time as the rest of solar system, scientists have dated meteorites and samples of Moon rocks (because they have not been re-melted and re-formed as most rocks on Earth have). Meteorites and Moon rocks are about 4.6 billion years old and so Earth is estimated to be that old too. IMAGINE: To give you some idea just how long this is, imagine that each blade of grass on a football field, including endzones, is one (1) year of Earth's age. How many football fields would you have to count before you counted 4.6 billion blades of grass? It turns out that someone has estimated how many blades of grass there are in one square foot of an average lawn—about 3,000 blades. A football field is 120 yards (360 feet) long, including endzones, and 53.5 yards (160.5 feet) wide. If you do the math, there are about 173,340,000 blades of grass on a football field. This means that if every blade of grass is one (1) year of Earth's age, you would have to count all of the blades of grass on about 26 1/2 football fields to count 4.6 billion blades of grass. GEOLOGIC TIMESCALE Paleontologists divide Earth's 4,600,000,000 year history into smaller blocks of time. Each block has a name and a beginning age and ending age. Together these blocks of time make a sort of calendar of Earth's history. We call this calendar the "Geologic Timescale." In the 1700s and 1800's, paleontologists collected fossils from rocks across Europe. They found that certain groups of rocks always had nearly the same types of fossils, and that other groups of rocks had different types of fossils. In time, paleontologists gave each group of rocks a different name, like Cambrian rocks or Silurian rocks or Cretaceous rocks, to show that different fossils were found in them. As paleontologists continued to study rocks, they found that the groups of rocks always occurred in the same order. For example, Cretaceous rocks always were found on top of Silurian rocks, and both always were found on top of Cambrian rocks. IMAGINE: Think about the dirty clothes in the hamper or in your closet. The clothes that you wore on Monday are on the bottom of the pile. The clothes that you wore on Tuesday are on top of Monday's clothes, and your Wednesday clothes are on top of everything. The oldest clothes are on the bottom and the most recent clothes are on the top. Just like your dirty clothes, rocks are found in the same order. The oldest rocks are on the bottom and most recent rocks are on the top. This "rule" is why Cretaceous rocks always were found on top of Silurian rocks, and both always were found on top of Cambrian rocks. This rule allowed the paleontologists to put the different groups of rocks they found in order from oldest to most recent. They had created the first Geologic Timescale. As more and more rocks were studied, and as more powerful ways of finding out the ages of rocks have been developed (for example radiometric dating), the Geologic Timescale has changed and become more accurate. However, finding fossils is still the first, fastest way to tell how old a rock is. |
Common Fossils of Oklahoma Geologic Time
Fossil Images What is a...? |
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