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Earth's History & Geologic Time Scale Cheat Sheet
A printable reference covering relative dating, absolute dating, half-life, fossils, unconformities, and the geologic time scale for grades 8-10.
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Earth's history and the geologic time scale explain how scientists organize 4.6 billion years of change on our planet. This cheat sheet helps students connect rock layers, fossils, radioactive dating, and major events in Earth's past. It is useful for reading geologic diagrams, interpreting timelines, and understanding evidence for long-term change. The most important ideas are relative dating, absolute dating, fossil succession, and the divisions of geologic time. Relative dating places events in order using rock-layer rules, while absolute dating estimates ages using radioactive decay. The geologic time scale divides Earth history into eons, eras, periods, epochs, and ages based on major changes in life, climate, and geology.
Key Facts
- The geologic time scale is organized from largest to smallest as eon, era, period, epoch, and age.
- The major eons in order from oldest to youngest are Hadean, Archean, Proterozoic, and Phanerozoic.
- The Phanerozoic Eon contains the Paleozoic, Mesozoic, and Cenozoic Eras.
- The law of superposition states that in undisturbed sedimentary rock layers, the oldest layer is on the bottom and the youngest layer is on the top.
- The principle of cross-cutting relationships states that a fault, intrusion, or erosion surface is younger than the rock layers it cuts across.
- The principle of fossil succession states that fossil organisms appear in a predictable order, so matching fossils can help correlate rock layers.
- For radioactive decay, number of half-lives = elapsed time / half-life.
- The fraction of parent isotope remaining after n half-lives is remaining fraction = (1/2)^n.
Vocabulary
- Relative dating
- A method of placing rocks and events in order from oldest to youngest without giving an exact age.
- Absolute dating
- A method of estimating the numerical age of a rock, fossil, or event, often using radioactive decay.
- Half-life
- The time required for half of a radioactive parent isotope in a sample to decay into daughter material.
- Index fossil
- A fossil from an organism that was widespread, abundant, and lived for a short time, making it useful for matching rock ages.
- Unconformity
- A gap in the rock record caused by erosion or a period when no sediment was deposited.
- Stratigraphy
- The study of rock layers and their order, age relationships, and history.
Common Mistakes to Avoid
- Treating relative dating as an exact age is wrong because relative dating only tells whether something is older or younger than something else.
- Using superposition without checking for disturbance is wrong because folding, faulting, or overturning can change the original order of rock layers.
- Thinking an intrusion is older than the rocks around it is wrong because the intrusion had to cut into preexisting rock, so it is younger.
- Confusing parent and daughter isotopes is wrong because the parent isotope decreases over time while the daughter product increases.
- Assuming every fossil is an index fossil is wrong because useful index fossils must be widespread, easy to identify, and limited to a short time range.
Practice Questions
- 1 A rock sample contains 25 percent of its original parent isotope. How many half-lives have passed?
- 2 Potassium-40 has a half-life of about 1.3 billion years. If a sample has gone through 2 half-lives, what is its approximate age?
- 3 In an undisturbed sequence, Layer A is at the bottom, Layer B is above it, and Layer C is on top. A fault cuts all three layers. List the events from oldest to youngest.
- 4 Why are index fossils more useful for correlating rock layers than fossils from organisms that lived for hundreds of millions of years?