Geologic Time

Question 1

Relative dating

Answer 1

Putting everything in order without knowing the exact date on which they happened

Question 2

Principle of Superposition

Answer 2

Sediments get stacked over time. Because of gravity, the oldest sediments are usually on the bottom

Question 3

Principle of Original Horizontality

Answer 3

Sediments that have been deposited in horizontal layers can be deformed into folds or broken faults

Question 4

Principle of Original Lateral Continuity

Answer 4

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 5

Principle of Cross-Cutting Relationships

Answer 5

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

Question 6

Principle of Inclusions

Answer 6

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

Question 7

Principle of Unconformities

Answer 7

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

Question 8

Principle of Faunal Succession (Evolution)

Answer 8

• Animals evolve and go extinct, never to return again
• 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 9

Absolute dating

Answer 9

Calculating a numerical date

Question 10

Atom

Answer 10

Smallest component of nature with the properties of a given substance

Question 11

Isotopes

Answer 11

Atoms with differing numbers of neurons (different mass numbers) but same number of protons

Question 12

Difference between stable isotopes and radioactive isotopes

Answer 12

Stable isotopes do not spontaneously break down, radioactive isotopes do. Radioactive isotopes (parent isotopes) are not stable and break down into daughter isotopes over time.

Question 13

What is the rate of decay and how is it measured?

Answer 13

The rate of decay is how quickly a radioactive isotope breaks down, it is measured using half-life.

Question 14

Half-time

Answer 14

The amount of time it takes for half the sample to break down

Question 15

Radiometric dating

Answer 15

Used to calculate the age of a sample (you need to know the half-life & measure the ratio of parent to daughter isotope)

Question 16

When can you use radiometric dating?

Answer 16

When a radioactive isotope is present in the rock, and if there are measurable quantities of parent and daughter

Question 17

Things to remember when using radiometric dating

Answer 17

• Choose an isotope that doesn’t decay too quickly or slowly relative to the expected age of the rock
• If it decays too quickly, there will be no parent left
• If it decays too slowly, not enough daughter will have formed to measure reliably

Question 18

Formula for half-life

Answer 18

N(t)=N0(1/2)^t / t1/2

Question 19

What does radiometric dating measure in igneous rocks and pyroclastic debris (ash)?

Answer 19

The age of crystallization

Question 20

What does radiometric dating measure in metamorphic rocks?

Answer 20

The age of metamorphism

Question 21

What does radiometric dating measure in siliciclastic (felsic) sedimentary rock?

Answer 21

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 22

How old is the earth?

Answer 22

About 4.6 billion years old (measured by absolute dating, based on the idea that everything in our solar system formed at the same time)

Question 23

Geologic Time Scale

Answer 23

• Earth’s 4.6 By history summary chart
• Geologic time is split into large units of time (enot) which is split into progressively smaller units (eras, periods, and epochs)
• The subdivisions of time are uneven
• The boundaries of many units are defined by the appearance or extinction of an animal or groups of animals

Question 24

Can you find fossils from the first 4 billion years of the earth?

Answer 24

No

Question 25

Why is it hard to find very old rocks?

Answer 25

Because of subduction, rocks get recycled, therefore, it is hard to find very old rocks

Question 26

How do we date fossils?

Answer 26

• Bones and shells contain carbon, some of which is 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 50 000 years old, so most rocks cannot be directly dated.