But how do they know how old things are?

Relative vs absolute dating ages

Danny Hoyland and I have been talking on the radio together for 15 years, give or take. And more than once he has mentioned that his Mum always asked how scientists know how old things like fossils are. I’ve been meaning to do this show for a very long time, and I’m hoping late is better than never.

Archaeologists and geologists like to know how old certain things like rocks, pottery and bones are. By estimating their age and origins, it helps them understand how they fit into broader contexts.

There are two broad categories of dating: relative and absolute (note: relative dating does not involve hooking up with your cousin).

Relative dating

These methods estimate whether an object is older or younger than the things found alongside it but doesn’t offer specific dates. This can be achieved through:

  • stratigraphy: the oldest dating method which studies the successive placement of layers. It is based on the concept that the lowest layer is the oldest and the topmost layer is the youngest.
  • biostratigraphy: an extended version of stratigraphy where the faunal deposits are used to establish dating. Faunal deposits include remains and fossils of dead animals.
  • cross dating: compares the age of remains or fossils found in a layer with the ones found in other layers. The comparison helps establish the relative age of these remains.
  • fluorine dating: bones from fossils absorb fluorine from the groundwater, so the amount of fluorine absorbed indicates how long the fossil has been buried in the sediments.
  • seriation: looks at changes in certain styles of artifacts present at a site. It assumes that one cultural style will slowly replace an earlier style over time

Absolute dating

These methods are more specific, and provide age ranges in years:

  • radiometric dating: depends solely on the traces of radioactive isotopes found in fossils. The rate of decay of these elements helps determine their age, and in turn the age of the rocks.
  • amino acid dating: Physical structure of living beings depends on the protein content in their bodies. The changes in this content help determine the relative age of these fossils.
  • dendrochronology: Each tree has growth rings in its trunk. This technique dates the time period during which these rings were formed.
  • thermoluminescence: determines the period during which certain object was last subjected to heat. It is based on the concept that heated objects absorb light, and emit electrons. The emissions are measured to compute the age and are particularly useful for pottery and ceramics.
  • fission-track dating: determines the age of various minerals and gasses based on the trails of damage done by the spontaneous fission
  • potassium-argon and argon-argon: measures the ratio of argon gas in igneous volcanic rock to estimate how much time has elapsed since the rock cooled and solidified.
  • archaeomagnetic dating: Magnetic particles in most materials of geological origin, such as rocks and clay, are analysed to track shifts in the earth’s magnetic fields over time.

Radiometric dating

Most people would be familiar with radiometric, or radiocarbon dating. This uses the process of radioactive decay – some elements break down over time at a constant rate and form isotopes.

For example, uranium-238 breaks down to lead-206. This decay happens at a constant rate and knowing the rates of decay means you can measure the proportions of different isotopes to measure their age.

Different elements are used for different age ranges:

  • the decay of carbon-14 to nitrogen-14 (radiocarbon dating) is used to measure the age of once-living things up to about 60,000 years old
  • the decay of potassium-40 to argon-40 is used to date rocks older than 20,000 years
  • the decay of uranium-238 to lead-206 is used for rocks older than 1 million years.

Source articles: Radiometric Age Dating – Geology (U.S. National Park Service), Relative and Absolute Dating Methods in Archaeology, Relative Vs. Absolute Dating: The Ultimate Face-off – Science Struck

I talked about this with Danny Hoyland on West Bremer Radio on 8 January 2022 (but I took a break and then got really busy, so I’m late once again). Listen each week: Saturday 7.40 am, West Bremer Radio.

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