Biology

2. Relative Dating

3. I. Relative Dating
• Relative Dating is when you give the age
of a rock or fossil compared to another
rock or fossil.
– Example: Rock A is OLDER than Rock B.
– An actual age in years is not determined.

4. II. Rules of Relative Dating
1. Law of Superposition: When sedimentary
rock layers are deposited, younger layers
are on top of older deposits.

5. Rules of Relative Dating
2. Law of Original Horizontality: Sedimentary
rock layers are deposited horizontally. If
they are tilted, folded, or broken, it
happened later.

6. Rules of Relative Dating
3. Law of Cross-Cutting Relationships: If an
igneous intrusion or a fault cuts through
existing rocks, the intrusion/fault is
YOUNGER than the rock it cuts through

7. Rules of Relative Dating
4. Law of Lateral Continuity: states that
layers of sediment initially extend laterally
in all directions; in other words, they are
laterally continuous. As a result, rocks that
are otherwise similar, but are now
separated by a valley or other erosional
feature, can be assumed to be originally
continuous.

8. 4. Law of Lateral Continuity

9. • Nicolas Steno (Danish scientist), in his
Dissertationis prodromus of 1669 is
credited with four of the defining principles
of the science of stratigraphy.

10. Index Fossils
• Some species of
organisms only lived for
a short period of time
before they became
extinct. If you use
radiometric dating to get
an age for the fossil, then
you know that the rock
the fossil is found in is
also that age.
Trilobite: Index fossil for the
Paleozoic Era
Age: 590-250 mya

11. Absolute Dating
• Determines the actual age of the fossil
• Radioactive isotopes carbon-14 (The half-
life of C-14 is only 5,700 years),
potassium-40 (1.3 billion years), and U-
235 (U-235 decays to Pb-207 with a half-
life of 704 million years.)
• Considers the half-life or the time it takes
for half of the atoms of the radioactive
element to decay

12. • The decay products of radioactive
isotopes are stable atoms.

14. • What is the limit in using carbon-14 as a
measure to determine a fossil’s age?