Geologic time, Lectures 2 & 3

Question 1

Pinatubo, Phillippines

when, what, causes

Answer 1

• June 1991,
• one of the largest eruptions of the 20th century
• Rich in SO2
• Aerosol dispersed globally in both hemispheres
• cooling of 0.2 degrees C for 1-2 years

Question 2

Charleston earthquake 1886

where, deaths, spatial scale, timescale

Answer 2

South Carolina, USA
60 deaths
spatial scale: 100-1000km2
timescale: seconds/minutes

Question 3

How has the relative geologic timescale been constructed?

4 principles

Answer 3

1. principle of original horizontality
2. principle of superposition
3. principle of cross-current relationships
4. principle of faunal success

Question 4

Principle of original horizontality

Answer 4

sediments are deposited as horizontal beds

Question 5

Principle of superposition

Answer 5

Each layer of sedimentary rock in a tectonically undisturbed sequence is younger than the one beneath it.

Question 6

Principle of cross-current relationships

Answer 6

An igneous intrusion or fault must be younger than the rocks it intrudes or cuts.

Question 7

Principle of faunal succession

Answer 7

The sedimentary strata in an outcrop contain fossil in a definite sequence.

Question 8

Fossils

Answer 8

Remains of ancient organisms

Question 9

Palaeontology

Answer 9

the study of ancient life and Earth’s history from the fossil record

Question 10

Body fossils

4

Answer 10

shells, teeth, bomes, impressions of plants

Question 11

Trace fossils

4

Answer 11

records of biological activity:
burrows
footprints
feeding trails
coprolites

Question 12

Another name for trace fossils

Answer 12

ichnofossils

Question 13

Coprolites

Answer 13

Pieces of fossilised dung

Question 14

Who came up with the principle of faunal succesion

Answer 14

William Smith (1769 - 1839)
‘Father of English Geology’
surveyor at coalfield and coal canal in Somerset
published first geologic map in 1815

Question 15

Absolute ages (of rocks and sediments)

Answer 15

dates in years

Question 16

When was radioactivity discovered?

Answer 16

1985

Question 17

Isotopic dating

Answer 17

the use of naturally occurring radioactive elements to determine rock ages

Question 18

Who introduced isotopic dating and when?

Answer 18

1905, Ernest Rutherford

Question 19

All matter is made up of…

Answer 19

elements which are composed of atoms

Question 20

Every atom has a…

Answer 20

nucleus containing most of the mass of the atom, surrunded by a negative cloud of electrons

Question 21

Isotopes

Answer 21

Atoms of the same element with different numbers of neutrons but the same number of protons.

Question 22

Difference between isotopes of the same element?

Answer 22

atomic mass

Question 23

Similarity between isotopes of the same element?

Answer 23

chemical properties

Question 24

Are isotopes unstable or stable?

Answer 24

some are stable but some are unstable and radioactively decay

Question 25

What is used for radioactive/isotopic dating?

Answer 25

radioactive decay

Question 26

What happens to atoms of radioactive elements?

Answer 26

they spontaneouly disintergrate (radioactively decay) to form atoms of a different element

Question 27

Radioactive parent elements…

Answer 27

decompose to form radioactive daughter elements

Question 28

Half-life

Answer 28

the time for one half of the original number of radioactive atoms to decay

Question 29

Why is radioactive decay used as a clock?

Answer 29

• decay rates/half lives of radioactive atoms known and do not vary with changes in T or P
• can measure the ratio of parent to daughter atoms in a rock samples

Question 30

Use of a mass spectrometer when carrying out isotopic dating in a lab

Answer 30

1. count the daughter element atoms
2. know the rate of decay
3. calculate the time when there were no daughters (only parents)
   = time of rock formation

Question 31

The isotopic age of a rock/sediment/fossil?

Answer 31

time since the isotopic clock was ‘reset’ when the isotopes were locked into the minerals, eg: crystallisation from melt or during metamorphism

Question 32

Radioactive isotopes are used to date

3

Answer 32

1. time of death of organisms (fossils in sedimentary rocks)
2. time of cooling and crystallisation of magma (igneous rocks)
3. time elapsed since metamorphism (metamorphic rocks)

Question 33

Age of the Earth

and how was this calculated?

Answer 33

4.6 billion years

relative and absolute dating techniques

Question 34

Relative vs absolute dating

Answer 34

relative = no specific dates, to determine if one object is older than another
absolute = origin dates, time ranges, age range in years