Dating the Past
How we date the past

Radioactive Dating & Fossils

Code D7: (DVD ONLY)

This DVD contains 2 programs

Program 1

This program explains how one of the great by-products of research into radioactivity was the discovery that radioactive atoms decay, or lose electrons, at a set rate. These various clocks, once discovered, revolutionised our ability to date the past.

For the more recent past the best radioactive clock is the Carbon 14 atom. This naturally occurring isotope of the more common Carbon 12 atom is absorbed by all living things. At death the unstable Carbon 14 atom begins to decay, while the more stable Carbon 12 atom remains unchanged. As we now know that the half-life of Carbon 14 is 5730 years, we can easily measure how long ago something died.

However for dates earlier than 40,000 years, the Carbon 14 method of dating is unreliable because the remaining amount of Carbon 14 is too small to be accurately measured.

For dates older than 40,000 years another atomic clock is chosen; the Potassium 40 atom. Rather than a half-life of a mere 5700 years, the half-life of Potassium 40 is one billion, 300 million years. As we can measure the rate at which Potassium 40 atoms slowly transform into Argon 40, we can date rocks whose age stretches back to the formation of the earth.

With both Carbon 14 and Potassium-Argon dating, our own human history can be mapped, along with the much older history of our earth.

Program 2

Fossils form when water replaces the cells of dead animals or plants with minerals. These minerals then petrify into rock to form the fossils we see in museums. This process has many risks and only a small proportion of living things end up becoming a fossil.

This is especially true for fossils from the earliest and longest geological period, the Precambrian. Fossils from this remote time before 1 billion years ago are rare because the soft-cells of the plants and animals preserved poorly and many rocks that may have contained good specimens have been either destroyed or transformed by the process of Plate Tectonics.

The following period, the Paleozoic, is far better represented in the fossil record. This is helped due to invertebrate animals evolving hard outer shells at this time, along with the evolution of vertebrate animals that left more tangible remains in the fossil record. As animals left the oceans for dry land during this period, fossils are also more widespread.

Following a mass extinction at the end of the Paleozoic around 240 million years ago, the reptiles of the Mesozoic came to dominate this next geological period. As the Age of the Dinosaur this period is well known although many questions remain; in particular why, around 66 million years ago, another mass extinction occurred which wiped out the larger species of reptiles.

The present geological era is the Cenozoic and this period is the Age of the Mammals who were able to survive the mass extinctions of the Mesozoic. Of the Cenozoic’s 66 million years, humans have dominated for less than the last three million years. How long our species remains dominant is anyone’s guess, but the fossil record shows that even animals that survive for millions of years can become extinct.

For prices, see our catalogue ArrowOnly

Back to top