Geologic Time?Dating Flashcards
(18 cards)
Absolute Dating vs. Relative dating
Absolute: Determining event’s actual time. Relative dating: putting rocks/events in proper order.
How can we tell time using geology
By looking at the physical relationships between rock units.
Stratigraphy
The description, correlation, and classification of strata in sedimentary rocks.
Formation
A series of rock layers in a region that has similar physical properties, may contain the same fossils and is mapped as a unit.
Principle of superposition (2/5)
In an undisturbed sequence of rocks, each layer is younger than the one beneath it and older than the one above it.
Principle of original horizontality (1/5)
If rocks do not lie in horizontal beds, they have be disturbed by some kind of tectonic force. i.e. folding, tilting, faulting.
Faunal succession (3/5)
Uses fossils as time pieces. Use of index fossils to correlate rocks (formations) from different locations
Unconformities (4/5)
Markers of missing time (deposition of rocks isn’t continuous forever). 1. Disconformity 2. Angular unconformity 3. Nonconformity
Disconformity
An unconformity in which the rocks above and below the unconformity are parallel.
Angular unconformity
Caused by folding and uplifting in land followed by erosion.
Nonconformity
Sedimentary rocks in contact with crystalline igneous or metamorphic rocks
Dike cross-cuts the pre-existing rock layers. Therefore, it is younger than the rock layers it cuts across. OR, fault cross-cuts all rock layers. Therefore, the fault is younger than the rock layers it cuts across.
Cross-Cutting relations (5/5)
The ratio of parent: daughter determines the age of a rock.
Radiometric dating
The time it takes for half the parent to decay into daughter product.
Half-life of a radioactive element
0 half life= 16 parent, 0 daughter. 1 half life= 8 parent, 8 daughter. 2 half-life= 4 parent, 12 daughter. 3 half-life= 2 parent, 14 daughter. Therefore, it takes 3 half-lives to get 2 parent:14 daughter. 3 half-lives x 80,000,000 years= 240,000,000 years.
ntrusion A contains radioactive isotopes with a known half-life of 80,000,000 years. How old (in years) is intrusion A if 2 atoms of the parent isotope remains for every 14 atoms of daughter isotope?
In igneous rocks, the clock begins when the molten rock cools- gets you the age of crystallization (formation). For sedimentary rocks, radiometric dating would get you the average age of all the grains which form the rock. For metamorphic rocks, radiometric dating gives the age of metamorphic event (may be more than one) rather than the initial age of crystallization.
What kind of rocks can be dated with radioactive methods?
Why don’t we use carbon-14 in rock dating?
- Most rocks do not contain carbon. 2. The half-life is only 5730 years, so you can’t date anything older than about 70,000 years.
What can be determine using carbon-14?
- Ages of recent lava flows 2. Ages of recent ash and pumice flows. 3. Ages of recent landslides, lahars.
