How do scientists establish the age of dinosaur fossils?

When paleontologist Mary Schweitzer discovered soft tissue within a Tyrannosaurus rex fossil, it prompted a crucial question—how could this tissue survive for millions of years? The bone was 68 million years old, and the prevailing theory of fossilization states that all soft tissues, from blood to brains, decay. Only hard parts like bones and teeth turn into fossils. However, for some, this discovery sparked a different inquiry. How could scientists be certain the bones were truly 68 million years old?

The current method of determining fossil ages is mainly based on radiometric dating, also known as radioactive dating. This method depends on the characteristics of isotopes, which are chemical elements like carbon or uranium. These elements are identical except for one significant difference—the number of neutrons in their nuclei.

Atoms can have an equal number of protons and neutrons. However, if there are too many or too few neutrons, the atom becomes unstable and starts shedding particles until its nucleus reaches stability. Imagine the nucleus as a pyramid of building blocks. If you try to add extra blocks to the pyramid's sides, they might stay for a while, but eventually, they'll fall off. The same happens if you remove a block from one of the pyramid's sides, causing the rest to become unstable. In time, some blocks will fall away, leaving a smaller and more stable structure.

This process is similar to a radioactive clock that keeps ticking as unstable isotopes decay into stable ones. While you can’t predict exactly when a particular unstable atom, or parent, will decay into a stable atom, or daughter, you can predict how long it will take for a large group of atoms to undergo this transformation. The element's half-life refers to the time it takes for half the parent atoms in a sample to become daughters.

To read this radioactive clock, scientists use a tool called a mass spectrometer to measure the number of parent and daughter atoms. The ratio of parents to daughters helps the researcher determine the specimen's age. The greater the number of parent isotopes—and the fewer daughter isotopes—the younger the sample. The half-life of the isotope being measured decides its effectiveness in dating ancient samples. Once all the parent atoms have transformed into daughters, there’s no longer any way to compare the two isotopes. This means isotopes with a short half-life are ineffective for dating dinosaur bones.

The short half-life is just part of the challenge when dating dinosaur bones. Researchers also need to find a sufficient quantity of both parent and daughter atoms for accurate measurements. Keep reading to learn what’s involved in dating fossils and the role volcanic ash plays in this process.

Dating Sedimentary Rock

The most well-known form of radiometric dating is carbon-14 dating, commonly used by archaeologists to determine the age of human-made artifacts. However, carbon-14 dating is not suitable for dinosaur bones. With a half-life of just 5,730 years, carbon-14 dating is only effective for samples younger than 50,000 years. Since dinosaur bones are millions, sometimes even billions, of years old, scientists rely on isotopes with much longer half-lives. Examples of these isotopes include uranium-238, uranium-235, and potassium-40, each having a half-life of more than a million years.

Unfortunately, these isotopes aren't present in the dinosaur fossils themselves. They are typically found in igneous rock, which is formed from cooled magma. Fossils, however, develop in sedimentary rock. When a dinosaur's body is covered by sediment, the bones and sediment gradually turn to rock. However, sediment usually doesn't contain enough of these necessary isotopes. Fossils also can't form in igneous rock, as the extreme heat from magma would destroy the bones.

To determine the age of sedimentary rock layers, scientists must first identify adjacent layers that contain igneous rock, such as volcanic ash. These igneous layers act like bookends, marking the start and end of the period during which the sedimentary rock was formed. By applying radiometric dating to the igneous brackets, researchers can accurately estimate the age of the sedimentary layers in between.

By using the principles of bracketing and radiometric dating, researchers have been able to date rock layers around the globe. This research has also provided insight into the age of the Earth itself. The oldest known rocks on Earth are around billion years old, but zircon crystals have been found that date back 4.3 billion years [source: USGS]. From this data, scientists estimate that the Earth is about 4.5 billion years old. Additionally, the oldest known moon rocks are also 4.5 billion years old. Since the moon and the Earth likely formed together, this supports the current estimation of the Earth’s age.

Explore more about fossils, dinosaurs, radiometric dating, and related subjects by checking out the links provided below.