1. Geologic History Relative & Absolute Dating
Chapter 32
Geologic History
Relative & Absolute Dating
BFRB Pages

2. Who’s got the TIME?
RELATIVE TIME - the order or sequence is known, but not the actual date of occurrence.
Example - the trilobite fossil is older than the dinosaur fossil
ABSOLUTE TIME – the actual age of object
Example - the dinosaur fossil is 250 million years old

3. How geologists find the RELATIVE AGES of rocks using RELATIVE TIME

4. The “DUH” Laws
Law of SUPERPOSITION – sedimentary sequence will be OLDEST on BOTTOM and YOUNGEST ON TOP (if undisturbed)
Law of Original Horizontality – rocks are usually deposited (laid down) flat and level (parallel to the Earth’s surface)
Law of Folds and Tilts - the fold or tilt is younger than the rocks that were deposited horizontally. (the rocks were there first so they are older)
Law of Cross Cutting - a fault or igneous rock intrusion is younger than the rock layers it has broken or intruded upon. (cut across)
Law of INCLUDED FRAGMENTS - pieces of rock found buried WITHIN another rock must be OLDER (formed first) ex conglomerate.

5. I. LAW OF SUPERPOSITION
YOUNGEST
OVER
OLDEST
IN OTHER WORDS, YOOOOO!

7. II - Law of Original Horizontality III – Law of Folds and Tilts
Sediments, such as those at the bottom of an ocean or lake, are usually deposited in flat layers. When geologists see sedimentary layers that aren’t horizontal, they know that the layers were deposited first, then tilted later.

8. STEP 1
STEP 2

9. Occurs when magma squeezes into or between layers of pre-existing rock
IV Law of Cross Cutting
Occurs when magma squeezes into or between layers of pre-existing rock
OR…
Layers of pre-existing rock are broken by a fault due to compressional or extensional forces

10. IV LAW OF CROSS-CUTTING BY IGNEOUS INTRUSIONS & FAULTS

13. V. LAW OF INCLUDED FRAGMENTS

15. UNCOMFORMITY A buried surface of erosion separating two rock masses
Represents a gap in geologic time (because that layer was exposed to the atmosphere for an amount of time in order for the erosion to happen, then another layer came and buried it…)

16. ….a look at the Grand Canyon and 2 types of unconformities...
2 - DISCONFORMITY
1 – ANGULAR UNCOFORMITY

17. 1 - Angular Unconformity
An unconformity in which the beds below the unconformity dip at a different angle than the beds above it.

19. 2. DISCONFORMITY
An unconformity in which the beds above the unconformity are parallel to the beds below the unconformity, though layers are “missing”.

20. “SEQUENCE” of events of the formation of an angular unconformity…
The lower sediments were deposited as horizontal layers in a body of water
These sediments were then raised above water level and tilted during a tectonic event (what type of boundary?)
Streams & other forces of erosion carved a nearly horizontal surface across the tilted beds

21. STEP 1
STEP 2

22. “SEQUENCE” of events…
The land surface subsided (or the water level raised), submerging the erosion surface
A new series of sediments deposited in horizontal layers on the erosion surface
The complicated sequence of tilted and horizontal rocks was again uplifted, exposing them to erosion and producing the outcrop we see today

23. STEP 3
STEPS 4-6

24. Practice – Describe the steps that formed this cross section.

26. PLAY THE “WHICH CAME FIRST” GAME!
 1). D or L?
 2.) M or C?
 3.) N or E?
 4.) B or A?
 5.) I or Q?
 6.) Q or J?
 7.) M or A?

28. ABSOLUTE TIME

29. Measuring Absolute Time
All methods of measuring absolute time must provide a specific date for that occurrence
Tree rings can be used to mark the passing of each season and therefore obtain a numerical age (dendrochronology)
Tree rings can be used to date events up to 3000 years ago

30. Dendrochronology
The dating of past events (climatic changes) through study of tree ring growth
Discovered by A.E. Douglass from the University of Arizona, who noted that the wide rings of certain species of trees were produced during wet years and, inversely, narrow rings during dry seasons
Each year a tree adds a layer of wood to its trunk thus creating the annual rings we see when viewing a cross section

32. RADIOACTIVE DATING Radioactive elements decay at known, steady rate
By comparing the amount of radioactive element that has decayed to the amount of decay product, an absolute age can be calculated
Compare the ratio of PARENT to DAUGTHER materials

35. Understanding RADIOACTIVITY
A radioactive element has an unstable nucleus
Over time radiation is given off from the nucleus ( Alpha, Beta, and Gamma Rays)
As the nucleus decays it becomes a different element
For example Carbon–14, a radioactive element, will change to the stable element Nitrogen-14

37. HALF-LIFE (decay rate)
A measure of the time required for half of a radioactive material (parent) to decay into a stable end product (daughter)
The amount of time required for one half life is different for each element
ESRT page 1
Example Carbon –14 requires 5700 years for half of its mass to decay into a stable product

38. VIF - THE PERIOD OF A HALF LIFE CAN NOT BE CHANGED BY CUTTING THE MATERIAL IN HALF OR BY HEATING OR BY EXTREME PRESSURES UNDER THE GROUND – that’s why radioactive dating is so accurate for dating rocks!!!

40. How does radiocarbon dating work?
All plants and animals on Earth are made principally of carbon
When an organism dies, the Carbon in begins to degrade into Nitrogen
In the 1940s, scientists succeeded in finding out that it takes 5568 years for radiocarbon to disappear, or decay, from a sample of carbon from a dead plant or animal
Radiocarbon dating is not accurately able to date anything older than ~50,000 years old because too much of the parent Carbon-14 has turned into the daughter Nitrogen 14 and the relative proportions of the 2 substances is too large to accurately measure.

41. Okay… so if we have 500g of C-14,
In 1 half-life (5,700 years) we’ll have __________
In 2 half-lives (__________years)  __________
In 3 half lives (__________years)  ____________
In 4 half-lives (__________years)  ____________
In 5 half-lives (__________years)  ____________
In 6 half-lives (__________years)  ____________