Dating and Time

Question 1

How old are rocks and fossils?

Answer 1

1. Relative dating: putting events in order without knowing the exact date on which they happened, put rocks in order using seven stratigraphic principles of relative dating
2. Absolute dating: calculating a numerical date, atoms are the smallest components of nature with properties of a given substance, the number of protons in an atom determines what element it is, isotopes are atoms with differing number of neutrons but the same number of protons.

Question 2

Radiometric dating

Answer 2

For rocks, we assume that the initial condition was 100% parent isotope and it has been decaying ever since the rock formed. If we can measure the ratio of parent to daughter isotope and we know the half-life, then we can calculate the age of a sample.

Question 3

What are the seven stratigraphic principles of relative dating?

Answer 3

1. Principle of superposition
2. Principle of original horizontality
3. Principle of original lateral continuity
4. Principle of cross-cutting relationships
5. Principles of inclusions
6. Principle of unconformities
7. Principle of faunal succession (evolution)

Question 4

What is the principle of superposition?

Answer 4

sediments get stacked over time. Because of gravity, the oldest sediments are usually on the bottom. Basin: low spot

Question 5

What is the principle of original horizontality?

Answer 5

sediments are deposited in horizontal layers, can be deformed into folds or broken by faults, which happen post-deposition

Question 6

What is the principle of original lateral continuity?

Answer 6

because sediments are deposited in horizontal layers, you can trace undeformed layers from one rock outcrop to another, even when some of the layer has been eroded

Question 7

What is the principle of cross-cutting relationships?

Answer 7

a geologic feature that cuts across another is younger than the feature it cuts across

Question 8

What is the principle of unconformities?

Answer 8

rocks above an unconformity (an erosional surface) are younger than the rocks below. Unconformities mean there is time missing from the rock record.

Question 9

What is the principle of faunal succession (evolution)?

Answer 9

an extinct organism will only appear in rocks that formed when it was alive, fossils of different animals will succeed each other vertically in a specific, predictable order.

Question 10

Radiometric dating of igneous rocks and pyroclastic debris (ash) measures:

Answer 10

the age of crystallization

Question 11

Radiometric dating of metamorphic rocks measures

Answer 11

the age of metamorphism

Question 12

Radiometric dating of siliciclastic sedimentary rocks measures

Answer 12

the age of formation of the minerals in the rock (which have been recycled from older rocks) NOT the age of formation of the rock.

Question 13

Using absolute dating techniques, we have calculated that Earth is about:

Answer 13

4.6 billion years old. This age is based on the idea that everything in our solar system formed at the same time.

Question 14

Earth’s 4.6 By history is summarized on the

Answer 14

Geologic Time Scale

Question 15

Geologic Time:

Answer 15

Geologic Time is split into large units of time (eons) which can be split in to progressively smaller units of time (eras, periods, and epochs). The subdivisions of time are not equal. (new periods/whatever when the planet changes somehow)

Question 16

The boundaries of many units are defined by the appearance of:

Answer 16

extinction of an animal or groups of animals.

Question 17

The farther back into time you go:

Answer 17

the more different the animals looked until there are no animals at all

Question 18

Fossils are uncommon for the first ~4 By of Earth’s history. Why?

Answer 18

• Because of subduction, rocks get recycled. Therefore, very old rocks are rare.
• The further back in time we go, the less we know about Earth because the evidence has mostly been destroyed.

Question 19

How do we date fossils?

Answer 19

Bones and shells contain carbon, some of which are radioactive and can be used for absolute dating. Other radioisotopes are not common.
Carbon 14 is only useful for dating rocks and fossils that are less than 50k years old, so most fossils cannot be directly dated.

Question 20

Principle of Inclusions

Answer 20

a rock inclusion is older than its host. (the inclusion had to exist first in order to be incorporated into another unit.)