Chapter 8 - Geologic Time Flashcards
What is relative dating?
A method that places events in the sequence they occurred without providing their specific age.
What is the law of superposition?
In undisturbed layers of sedimentary rock, the oldest layer is at the bottom and the youngest is at the top.
What is the principle of original horizontality?
Layers of sediment are generally deposited in a horizontal position, and any tilting or folding occurred after their deposition.
What is the principle of cross-cutting relationships?
A geological feature (like a fault or intrusion) that cuts across another feature must be younger than the feature it cuts through.
What are inclusions in geology?
Fragments of one rock contained within another. The rock containing the inclusion must be younger than the inclusion itself.
What is an unconformity?
A break or gap in the geologic record caused by erosion or non-deposition of rock units.
What are the three types of unconformities?
Angular unconformity, disconformity, and nonconformity.
How old is the Earth based on radiometric dating?
About 4.6 billion years old.
Explain how the principles of relative dating (superposition, original horizontality, cross-cutting relationships, and inclusions) can be used together to establish a sequence of geological events.
Geologists use principles like the law of superposition to determine the relative age of rock layers, while original horizontality shows if tilting occurred post-deposition. Cross-cutting relationships indicate that faults or intrusions are younger than the layers they intersect. Inclusions reveal that the surrounding rock is younger than the fragments, helping to identify the sequence of events.
What is an unconformity, and what does its presence in the geologic record indicate about Earth’s history at that location?
Unconformities mark gaps in the geologic record due to erosion or non-deposition, indicating past environmental changes like uplift or shifts in sea level.
How do angular unconformities, disconformities, and nonconformities differ in their formation, and what do they reveal about the geologic history of an area?
Angular unconformity: forms when older tilted or folded layers are eroded then overlain by younger flat layers, indicating tectonic activity.
Disconformity: parallel layers with a gap due to erosion or non-deposition.
Nonconformity: between older metamorphic or igneous rocks and younger sedimentary layers, reflecting significant time gaps, uplift, and erosion.
How does radiometric dating allow geologists to assign absolute ages to rocks, and why is this technique essential for constructing the geologic time scale?
Radiometric dating calculates a rock’s absolute age by measuring the decay of radioactive isotopes. This method provides precise numerical ages, helping geologists link relative time scales to actual dates and build Earth’s timeline.
Who is credited with recognizing the immensity of Earth’s history in the late 1700s?
James Hutton.
What did Sir Charles Lyell demonstrate about Earth’s geologic history?
Earth had undergone many mountain-building episodes.
