Ch. 10: Geologic Time Flashcards
Superposition
Layers of the sandwich
Correlation
Two identical halves of the sandwich
Cross-cutting relationships
The toothpick stuck into the sandwich
Inclusion
The pieces of pepper inside the cheese
Absolute (Numerical) ages
– Actual number of years
since an event
•Quantitative method
Relative ages
– Based upon sequence of events
•Older vs. younger relationships
Divisions from largest to smallest
Eon–Era–Period–Epoch
Eras
Precambrian–Paleozoic (Pz)–Mesozoic (Mz)–Cenozoic (Cz)
Principles for Determining Relative Age
uniformitarianism: The present is the key to the past
original horizontality & continuity: Strata often form laterally extensive horizontal sheets
superposition: Younger strata are on top; older strata below
cross-cutting relationships: – Younger features truncate (cut across) older features
inclusions: A rock fragment within another, inclusion is older than the material enclosing it
- xenoliths: country rock that fell into magma
- pebble: weathered debris from preexisting rocks
Angular unconformity
Mountains form and layers fold, then erosion removes the overlying rock creates new horizontal layers over old folded layers
Nonconformity
Crystalline (metamorphic or igneous) rocks overlain by sedimentary strata
Erosion removes cover, so basement lies exposed at the Earth’s surface
Crystalline igneous/ metamorphic rocks were exposed by erosion
– Sediment was deposited on this eroded surface
Found in baked rocks
Disconformity
Parallel strata above and below the buried erosion surface
Sea level drops and flat-lying strata are eroded
o Due to erosion or an interruption in sedimentation
Absolute (numerical) age or geochronology
based on radioactive decay of unstable element
Igneous rocks are best for geochronologic work, followed by metamorphic
Sedimentary rocks cannot be directly dated
Useful isotopic systems in geology and archeology:
Radiocarbon has shortest half life
Potassium-Argon in middle
Uranium-Lead has longest
Radiometric age of zircon in granite and sandstone (what did that tell us?)
granite: birthdate
sandstone: age of igneous rock that weathered to become sandstone
