Geologic Time Flashcards
What is geologic time?
It’s the timescale of Earth’s history—both the relative order of events and their absolute ages.
What is relative time?
It arranges events from oldest to youngest using principles like superposition and cross-cutting.
What does uniformitarianism mean?
“The present is the key to the past”—today’s processes explain past events.
What is original horizontality?
Sediments are initially deposited in horizontal layers; any tilt indicates later deformation.
What is the principle of superposition?
In an undeformed sequence, the oldest layers lie at the bottom and the youngest at the top.
What does lateral continuity imply?
Sedimentary layers extend laterally until they thin out or change into different rock types.
What do cross-cutting relationships show?
Features like faults or dikes that cut through rock layers are younger than the rocks they intersect.
What are unconformities?
They are gaps in the geologic record where erosion or non-deposition interrupts the sequence of layers.
What does an inclusion indicate?
Rock fragments enclosed in another rock are older than the host rock.
How do fossils help in relative dating?
Fossils—especially index fossils—allow correlation of rock layers across regions by indicating similar ages.
How is the geologic time scale often illustrated?
By compressing Earth’s 4.6 billion-year history into one calendar year with key events on specific dates.
What is radioactive decay?
It’s the process by which unstable parent isotopes transform into daughter isotopes at a constant rate (the half-life).
What assumptions underlie radiometric dating?
Constant decay rates, a closed system (no loss or gain of parent/daughter isotopes), and known initial daughter amounts.
What makes Carbon-14 dating unique?
It’s used for dating recent organic materials (up to ~70,000 years) and relies on known 14C/12C ratios, though its production rate can vary.
