In 50 BC, a new eruption at the Kilauea volcano in Hawaii began.
Lava flows soon raced towards the ocean, covering an area littered with many other dark black
older lava flows.
However, how do scientists know that this eruption occurred in that specific year?
After all, the native Polynesians did not colonize Hawaii until at the very earliest,
124 AD.
Similar questions have been asked about other volcanoes I have covered, and as a result
I have decided to make this video which is dedicated to determining how we date volcanoes
and other rock formations which formed long before historical records were kept.
Although some other methods can be used, the most effective way of determining a rock's
age is radioactive age dating.
Three methods are commonly used.
Uranium-lead dating is used for samples older than 1 million years in age, potassium-argon
is used for dating samples between 100,000 and 4.3 billion years in age, and carbon 14
dating is used to date rocks suspected of being between 100 and 50,000 years in age.
Going beyond these aforementioned ranges is not possible due to radioactive decay which
I will explain later.
In the case of the 50 BC Kilauea eruption, the advancing aa lava flow burned and then
buried a tree which is now preserved in a layer of rock.
Since the wood is an organic product, we can determine its age based on the ratio of the
radioactive isotope carbon 14 to the stable nitrogen 14.
All living things naturally contain a small amount of carbon 14.
The rate of radioactive decay of carbon 14, along with other radioactive isotopes is measurable
in a lab which is used to determine an isotope's so called "half-life".
Half-lives measure the amount of time it takes for half of a radioactive isotope to become
a more stable product.
In the case of carbon 14, its half-life is 5,730 years.
Another way of visualizing this is imagine that you have a pizza which arrived at your
house an undetermined number of days ago.
Yet, at the end of every day something eats half of the remaining pizza.
In this example the radioactive pizza "stabilizes" to air after it is eaten each day.
So, if you open the pizza box and see that 1/8th of the pizza is left, is must have been
there for 3 days.
The same logic applies to carbon 14.
If the ratio of carbon 14 to nitrogen 14 is 1:7 it implies that three half-lives have
passed since the organic product it was in formed, totaling 17,190 years.
In the case of the aforementioned Kilauea wood fragment, 77.84% of its radioactive isotope
remained.
Thus, it had undergone 0.36 half-lives which translates to a duration of 2,070 years.
As a reminder, for carbon 14 dating to work it must be older than 100 years and younger
than 50,000 years and involve an organic piece of material.
Simply dating a layer of rock without an organic product in it using this method will never
work.
A similar methodology is used for older samples via potassium argon dating.
In this case, minerals rich in potassium are heated in a lab and then the ratio of the
radioactive potassium 40 to radioactive argon 40 is measured to determine a sample's age.
For samples older than 1 million years in age, to correctly date them a specific crystal
called a zircon must first be found.
Zircons when they form do not contain any lead, which is important.
Thus, any lead found in a zircon must have originated from radioactive decay of unstable
elements such as uranium which were present in its crystal structure.
This method relies on two separate decay chains where uranium 238 becomes lead 206, and uranium
235 becomes lead 207.
Both of these isotopes take an unusually long period to decay, which in the case of uranium
238 has a half-life of 4.47 billion years.
Thus, if you have an equal ratio of uranium 238 to lead 206 in a zircon crystal, it is
4.47 billion years old.
If all of this sounds complex, you are not alone in this thinking.
However, I hope that my explanation gave you an understanding of how us scientists determine
the age of various rock formations.
Thanks for watching.
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