Geologic Time?Dating

Question 1

Absolute Dating vs. Relative dating

Answer 1

Absolute: Determining event’s actual time. Relative dating: putting rocks/events in proper order.

Question 2

How can we tell time using geology

Answer 2

By looking at the physical relationships between rock units.

Question 3

Stratigraphy

Answer 3

The description, correlation, and classification of strata in sedimentary rocks.

Question 4

Formation

Answer 4

A series of rock layers in a region that has similar physical properties, may contain the same fossils and is mapped as a unit.

Question 5

Principle of superposition (2/5)

Answer 5

In an undisturbed sequence of rocks, each layer is younger than the one beneath it and older than the one above it.

Question 6

Principle of original horizontality (1/5)

Answer 6

If rocks do not lie in horizontal beds, they have be disturbed by some kind of tectonic force. i.e. folding, tilting, faulting.

Question 7

Faunal succession (3/5)

Answer 7

Uses fossils as time pieces. Use of index fossils to correlate rocks (formations) from different locations

Question 8

Unconformities (4/5)

Answer 8

Markers of missing time (deposition of rocks isn’t continuous forever). 1. Disconformity 2. Angular unconformity 3. Nonconformity

Question 9

Disconformity

Answer 9

An unconformity in which the rocks above and below the unconformity are parallel.

Question 10

Angular unconformity

Answer 10

Caused by folding and uplifting in land followed by erosion.

Question 11

Nonconformity

Answer 11

Sedimentary rocks in contact with crystalline igneous or metamorphic rocks

Question 12

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.

Answer 12

Cross-Cutting relations (5/5)

Question 13

The ratio of parent: daughter determines the age of a rock.

Answer 13

Radiometric dating

Question 14

The time it takes for half the parent to decay into daughter product.

Answer 14

Half-life of a radioactive element

Question 15

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.

Answer 15

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?

Question 16

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.

Answer 16

What kind of rocks can be dated with radioactive methods?

Question 17

Why don’t we use carbon-14 in rock dating?

Answer 17

1. 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.

Question 18

What can be determine using carbon-14?

Answer 18

1. Ages of recent lava flows 2. Ages of recent ash and pumice flows. 3. Ages of recent landslides, lahars.