Telling the time with volcanoes

Scroll to the bottom of the page for today’s task!

It’s not all gushing lava and pyroclastic flows – sometimes the excitement of volcanoes lasts long after they erupt, in the form of radioactive rocks! Find out how radioactive decay and preserved ash can be used to date volcanic eruptions…

Dating volcanic eruptions

For geographers and geologists, one of the most obvious questions to ask when we see a volcano is when did it erupt?

There are a few ways that we can answer this question. If the eruption was in recent history, people might have recorded it, and for example, we have many different record types for the eruption in Pompeii, such as diary entries, paintings, and archaeological artefacts. This type of data is fine for eruptions which happened in the recent geologic past, but what about older volcanic eruptions?


To go back into the past, we can use radioactivity to help us to date the eruption. Some elements within the minerals of a rock are radioactive, which means that they transform into different elements through time. This process is called radioactive decay. During radioactive decay, the parent (or the original) element decays into the daughter element at a certain rate through time. You can think of this process like an hourglass, with the sand at the top of the hourglass as the parent element, which will move through the narrow part of the hourglass at a fixed rate, into the bottom (the daughter elements). You can judge how much time has passed by looking at how much sand is in the top of the hour glass compared to the bottom of the hourglass. In the same way, if we compare how much of the parent element there is in relation to daughter element, we can tell how much time has passed.

We can use this idea of radioactive decay to date volcanic eruptions. One of the most common techniques uses the element Potassium. The isotope Potassium-40 is a parent element which decays into the daughter product Argon-40. Potassium-40 has a half-life of 1.25 billion years, which means it takes 1.25 billion years for half of the original Potassium-40 to decay to Argon-39. Knowing this rate, we can tell the time since a volcanic eruption by comparing the amount of Potassium-40 and Argon-39 in a lava sample.

Using radioactive decay as a way to tell the time, or to date a geologic event, is called radiometric dating. There are many other radiometric techniques, including radiocarbon dating, which you may have have heard of. These techniques are used in the science of Geochronology, which allows us to determine the age of rocks, sediments and fossils that we find on the Earth Surface.

Eruption signatures

Scientists from many different disciplines work to understand how the climate and environment of Earth has been different in the past. There are many, many different ways that this can be done. We can look at records of past vegetation by analysing fossil pollen. We can drill into ice cores to reconstruct past changes in temperature. We can look at the chemistry of sediments to trace where that sediment has come from. This list goes on…

One key thing is to know when something has changed, as well as how it has changed. This is where volcanic eruptions can help, by a dating technique called tephrochronology. Tephra is volcanic ash, and the tephra from each volcanic eruption has a unique chemical signature. This means that any tephra we find deposited can be linked back to one specific volcanic eruption.

Tephra is small and light, and when a volcano erupts, the tephra (the ash) can be carried long distances before it eventually settles onto the earth surface. When other sediments are deposited on top of the tephra, it is preserved in the sediment record. Earth Scientists when they sample the sediments can find the tephra, and know that if they can identify it, they can gain new information on how old the sediment is. As an example, the Lacher See volcano in Germany erupted between 12,900 and 11,200 years ago, and the tephra from this eruption has been found in Austria, Belgium, Denmark, France, Germany, Italy, Netherlands, Poland, Sweden and Switzerland. Finding Lacher See tephra at different locations means that we have a time marker wherever it is found.


Tephrochronology has become a very powerful tool, both for providing dates for sediment sequences around the world (both on land and in the sea), but also for linking sediment sequences from different parts of the world together.

Your task

Can you think of reasons why we would want to date volcanic eruptions?

Can you think of some things that tephrochronology can help us with?

Using what you have learned on this page, answer these questions in 300 words or less.

Further resources

Julie Durcan

Dr Julie Durcan, Supernumerary Teaching Fellow in Physical Geography
I am responsible for teaching physical geography topics at St John’s College. These currently include geomorphology, biogeography, climatology and Quaternary Science, but are subject to change in response to the department syllabi. I also teach lectures in the School of Geography and Environment.