Age of Earth Periods 1&3

Age of Earth Periods 1&3

How did scientists calculate the age of the Earth?

Take a look at a globe or a map of the world and you may be impressed by what you see. Towering mountains, deep oceans, vast continents, and sprawling glaciers -- they make the Earth what it is today. Even more impressive, by some people's standards, is how old Earth is. Scientists have calculated the age of our planet to be approximately 4.5 billion years. But how did scientists determine that age? The answer is complicated: It involves everything from observation, to complicated mathematics, to understanding the elements that make up our planet.

In the 1800s, as scientists sought to determine the age of the Earth, they made a few missteps. In 1862, a famous Irish physicist and mathematician, Lord Kelvin, estimated that the Earth was between 20 and 400 million years old. While that is an enormous span of time, even an age of 400 million years would make the planet quite young in relation to the rest of the universe. Lord Kelvin based his conclusion on a calculation of how long it would have taken the Earth to cool if it had begun as a molten mass. While his estimate was wrong by a significant margin, his technique of drawing conclusions based on observations and calculations was an accurate scientific method.

Scientists also tried to use relative dating techniques to determine the age of the Earth. This method compares the configuration of layers of rock or sediment in order to determine how old each layer is in relation to one another. This technique can reveal which layers are older or which events happened before others. However, it yields no actual number for those layers or events, and thus no exact age. Nonetheless, even though this technique did not give scientists the precise number they were looking for, it did suggest that the Earth was most likely billions of years old, and not just millions as was previously thought.

As advances in chemistry, geology and physics continued, scientists found a method by which the absolute age - an actual number of years - of a rock or mineral sample could be determined. This method is called radiometric dating, and it involves the decay, or breakdown, of radioactive elements. Using radiometric dating techniques, it became possible to determine the actual age of a sample.

Radiometric dating requires an understanding of isotopes, or the different forms an element can take depending on the number of neutrons residing in the nucleus of an atom. The isotopes of unstable radioactive elements - known as parent isotopes - will decay into other, more stable elements - known as daughter isotopes - in a predictable manner, and in a precise amount of time called a half-life. The half-life of an element is the amount of time required for exactly half of a quantity of that element to decay. The age of a sample can be determined based on the ratio of parent to daughter isotopes within the sample.

One problem with this approach to dating rocks and minerals on Earth is the presence of the rock cycle. During the rock cycle, rocks are constantly changing between forms, going back and forth from igneous to metamorphic to sedimentary. Old rocks may even be destroyed as they slide back into the Earth's mantle, to be replaced by newer rocks formed by solidified lava. This makes finding an exact age for the Earth difficult, because the original rocks that formed on the Earth at the earliest stages of its creation are no longer here. The oldest rocks that have been found are about 3.8 billion years old, though some tiny minerals have been dated at 4.2 billion years.

To get around the difficulty presented by the rock cycle, scientists have looked elsewhere in the solar system for even older rock samples. They have examined rocks from the moon and from meteorites, neither of which have been altered by the rock cycle. The same techniques of radiometric dating have been used on those rocks. All of the data from this planet and beyond has led to the estimated age for the Earth of 4.5 billion years.

Multiple Choice

How does radiometric dating affect the way Earth's age is determined?

• (A) Radiometric dating of Earth's rocks is not useful for determining Earth's age because the rock cycle makes it impossible.

• (B) Radiometric dating of Earth's rocks is the only thing that is needed for scientists to determine the exact age of Earth.

• (C) Radiometric dating of Earth's rocks was helpful for determining Earth's age but it was not as accurate as relative dating was.

• (D) Radiometric dating of both Earth's rocks and rocks beyond Earth were instrumental in determining the latest estimate of Earth's age.

Multiple Choice

Read the following two details from the article. In 1862, a famous Irish physicist and mathematician, Lord Kelvin, estimated that the Earth was between 20 and 400 million years old. While that is an enormous span of time, even an age of 400 million years would make the planet quite young in relation to the rest of the universe. Scientists also tried to use relative dating techniques to determine the age of the Earth. This method compares the configuration of layers of rock or sediment in order to determine how old each layer is in relation to one another. Select the option that BEST explains how these details develop a central idea of the article

• (A) They both highlight processes of dating Earth's age that are still used by scientists in the current estimate.

• (B) They both highlight processes of dating Earth's age that were not accurate but still used the scientific method.

• (C) They both describe processes of dating Earth's age that use rocks that were found outside of Earth's surface.

• (D) They both describe processes of dating Earth's age that were completely unhelpful for estimating Earth's age.

Multiple Choice

Which of the following provides the BEST analysis of the article's first paragraph?

• (A) The first paragraph effectively summarizes how scientists have attempted to approximate Earth's age.

• (B) The first paragraph effectively summarizes the first method that was used to determine an estimate of Earth's age.

• (C) The first paragraph does not effectively convey the main idea of the article and instead focuses on unnecessary information.

• (D) The first paragraph does not effectively answer the question of how old Earth is and what the article is about.

Multiple Choice

What purpose is served by including in the article examples of the methods used to determine Earth's age?

• (A) to suggest that there is still a huge scientific debate about whether the Earth really is 4.5 billion years old

• (B) to show how the methods have been refined since the 1800s to get the most accurate estimation

• (C) to explain that there are many different ways to prove that the Earth is 4.5 billion years old

• (D) to prove that the first estimates of Earth's age still hold true according to the scientific community