Introduction To Relative And Absolute Dating — Transcript

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1. 0:00Rocks don't come with birth certificates, yet  geologists talk about rocks forming thousands
2. 0:07millions or even billions of years ago. Do you ever  wonder how we know the age of a rock?
3. 0:15Welcome to the Geoscience Australia's education centre located on Ngunnawal and Ngambri land.
4. 0:21My name is Emily and today we're going to explore the two main ways that scientists determine the age of a rock:
5. 0:28Relative dating and absolute dating.
6. 0:32Relative dating is used to reconstruct the order of past events without knowing the exact age of a rock.
7. 0:39To find out whether a rock is younger or older than another, scientists will observe patterns and relationships between rock layers.
8. 0:49Sediments like sand and silt get transported by rivers and  streams and when they reach a calm environment,
9. 0:57like an ocean, they get deposited at the bottom in layers.
10. 1:01This container in front of me represents the ocean floor.
11. 1:06When I add some sand to this container we can see that the sand falls through the water
12. 1:12and is deposited on the bottom in a flat layer on top of the pre-existing older layers.
13. 1:21Over millions of years these loose sediments can turn into a sedimentary rock like this.
14. 1:28We know from looking at this rock and the environment it may  have formed in, that these layers must have been originally deposited like this,
15. 1:37horizontally, and that the layer on top must be younger than all of the layers below.
16. 1:44This demonstrates two laws used in relative dating: The law of original horizontality and the law of superposition.
17. 1:55If these rock layers were to contain fossils, it can provide scientists with even more information about the relative age of the rock.
18. 2:04When fossils are abundant, widely distributed and only appear for a short and distinct amount of time,
19. 2:12they are called Index Fossils. For example this trilobite Redlichia Takooensis is an index fossil.
20. 2:21It only appears in rocks between 514 and 509 million years  old and serves as a marker for rocks of the same
21. 2:32age all around the world. This one is found on  Kangaroo Island in South Australia and can also
22. 2:39be found in the Guizhou province in South China.  But how do scientists put an exact age on a rock?
23. 2:48Well they use absolute dating? In the late 1890s  scientists discovered radioactivity where some
24. 2:56atoms in nature spontaneously change or decay to  more stable atoms over time. When molten rock cools,
25. 3:04tiny minerals form and during this process some of  these radioactive atoms become trapped inside of
26. 3:12the minerals. Over time these atoms slowly decay  into more stable atoms.
27. 3:18Scientists know how long this process takes for each radioactive atom and refer to this as its half-life.
28. 3:27The half-life is the amount of time it takes for half of the original  radioactive atom to change into the decay product.
29. 3:36If we know the proportion of the original  radioactive atom compared to its decay product,
30. 3:42we can use a radioactive decay curve.  In this example we know we have 50%,
31. 3:49so we can figure out how long this process has been  happening for and therefore the age of the mineral.
32. 3:57A special instrument called a mass spectrometer is used to make these measurements.
33. 4:03Now not all radioactive atoms decay at the same rate.
34. 4:09This is the geological time scale of the Earth,
35. 4:11and it can be used to help us visualize the half- lives of two different radioactive atoms:
36. 4:18Carbon 14 and uranium 238. You might have heard of carbon dating which looks at the decay  of carbon 14.
37. 4:27Carbon 14 has a half-life of around 5,700 years which means it can be used to date
38. 4:34material up to about 50,000 years old, so carbon  dating is commonly used to date things within
39. 4:41human history, like bones and other archaeological  objects. But what about anything older than this?
40. 4:51Uranium 238 has a half-life of 4.5 billion years.  Looking at the geological time scale this means
41. 5:00it can be used to date material across  the entire history of the Earth.
42. 5:05In fact, uranium 238 was one of the radioactive  atoms that scientists used to measure
43. 5:12the world's oldest zircon mineral grain from  the Jack Hills conglomerate in Western Australia.
44. 5:18These grains are 4.4 billion years old. That's  really, really old. Almost as old as the Earth itself.
45. 5:31Today we've learned about relative dating  which all of us can do by making observations,
46. 5:36and absolute dating, which scientists use to measure  the exact age of a rock or mineral.
47. 5:42Relative dating and absolute dating are both essential tools that we use to reconstruct Earth's complex history.