Back to: Geography of Ethiopia and The Horn – Freshman Courses
2.3. The Geological Time Scale and Age Dating Techniques
- Understand the hierarchical structure of the geological time scale.
- Distinguish between relative and absolute (radiometric) dating techniques.
- Apply radiometric decay principles to determine rock ages.
- Relate geological time divisions to major biological and tectonic events.
Earth is approximately 4.5 billion years old, and life first appeared around 3.5 billion years ago. To organize this immense span, geologists developed the Geological Time Scale—a system that divides Earth’s history into hierarchical units based on major geological and biological events.
Structure of the Geological Time Scale
The time scale uses four main hierarchical units:
- Eon: Largest division (e.g., Phanerozoic, Precambrian).
- Era: Subdivision of an eon (e.g., Paleozoic, Mesozoic, Cenozoic).
- Period: Subdivision of an era (e.g., Jurassic, Cretaceous).
- Epoch: Smallest standard unit (e.g., Holocene, Pleistocene).
These units are **not of equal duration**. Boundaries are defined by **global events** such as mass extinctions, climate shifts, or the appearance of new life forms. For example:
- Paleozoic Era (“ancient life”): Age of invertebrates and early fish.
- Mesozoic Era (“middle life”): Age of reptiles and dinosaurs.
- Cenozoic Era (“recent life”): Age of mammals and humans.
| Era | Period | Began (Million Years Ago) | Ended (Million Years Ago) | Major Events |
|---|---|---|---|---|
| Cenozoic | Quaternary | 1.6 | Present | Major glaciers in North America and Europe |
| Tertiary | 70 | 1.6 | Rocky Mountains formed; continents took modern shape | |
| Mesozoic | Cretaceous | 146 | 70 | Dinosaurs extinct; Western Interior Seaway |
| Jurassic | 208 | 146 | Pangaea begins to break apart | |
| Triassic | 225 | 208 | First mammals and dinosaurs | |
| Permian | 290 | 225 | Greatest mass extinction in Earth’s history | |
| Pennsylvanian | 322 | 290 | First reptiles | |
| Paleozoic | Mississippian | 362 | 322 | Coal-forming forests |
| Devonian | 408 | 362 | First land animals and forests | |
| Silurian | 439 | 408 | Life invades land | |
| Ordovician | 510 | 439 | First fish appeared | |
| Cambrian | 600 | 510 | Explosion of marine invertebrates | |
| Proterozoic | 2,500 | 600 | Marine fossil invertebrates | |
| Archean | 4,500 | 2,500 | Earliest fossils (3,500 Ma); earliest rocks (4,000 Ma) |
Age Dating Techniques
Determining rock ages relies on two complementary methods: relative dating and absolute (radiometric) dating.
1. Relative Dating
Two key principles:
- Law of Superposition: In undisturbed sedimentary layers, the oldest layer is at the bottom.
- Index Fossils: Fossils of organisms that existed briefly but widely—used to correlate rock layers across regions.
Relative dating answers: “Which layer is older?” but not “How old is it?”
2. Absolute (Radiometric) Dating
Radioactive isotopes decay at a constant rate called the half-life—the time for half the parent atoms to decay into daughter products.
The decay formula is:
\[ N = N_0 \left( \frac{1}{2} \right)^{t / T_{1/2}} \]
Where:
- \(N\) = remaining parent isotope
- \(N_0\) = initial quantity
- \(t\) = elapsed time
- \(T_{1/2}\) = half-life
Common Radiometric Methods in Ethiopian Geology:
a. Carbon-14 Dating
- Used for **organic materials ≤ 50,000 years old**.
- Half-life of \(^{14}\text{C}\): 5,730 years.
- Not useful for most Ethiopian rocks (too old).
b. Potassium-Argon (K-Ar) Dating
- Used for **volcanic rocks > 100,000 years old**.
- \(^{40}\text{K}\) → \(^{40}\text{Ar}\) with half-life of 1.25 billion years.
- Critical in Ethiopia: Dates Cenozoic flood basalts (Trap Series) and rift volcanism (e.g., Erta Ale).
- Caveat: Argon escapes if rock is reheated >125°C, resetting the “clock.”
đź“ť Review Quiz
Q1. Which geological time unit is the largest?
A) Era
B) Period
C) Epoch
D) Eon
Explanation: As stated in the text, the geological time scale has four main units: “Eon: The largest division of time…” Eras (e.g., Cenozoic) are subdivisions of eons.
Q2. What principle states that in undisturbed sedimentary layers, the oldest layer is at the bottom?
A) Index fossil correlation
B) Law of Superposition
C) Half-life decay
D) Principle of Uniformitarianism
Explanation: Section 2.3 explicitly defines: “Law of Superposition: In undisturbed sedimentary rock layers, the oldest layer lies at the bottom…”
Q3. The half-life of Potassium-40 is 1.25 billion years. If a volcanic rock contains 25% of its original \(^{40}\text{K}\), how old is it?
A) 0.625 billion years
B) 1.25 billion years
C) 2.5 billion years
D) 3.75 billion years
Explanation: After one half-life (1.25 Ga), 50% remains. After two half-lives (2.5 Ga), 25% remains. Using the formula \( N = N_0 (1/2)^{t/T_{1/2}} \), when \( N/N_0 = 0.25 \), \( t = 2 \times T_{1/2} = 2.5 \) billion years.
Q4. Why is Carbon-14 dating NOT suitable for dating the Trap Series lava flows in Ethiopia?
A) The lava flows contain no carbon
B) Carbon-14 has a half-life too short for million-year-old rocks
C) Potassium is more abundant in basalts
D) All of the above
Explanation: The text notes Carbon-14 is used for materials “up to ~50,000 years old.” Ethiopian Trap Series lavas are 15–30 million years old—far beyond Carbon-14’s range. While (A) and (C) are true, (B) is the primary scientific reason.