2.3 Geochronological Principles

Question 1

What two methods are used to calculate the age of rocks?

Answer 1

• relative dating

* absolute dating

Question 2

What is relative dating?

Answer 2

Stating whether one rock or geological event is older than another

Question 3

What is absolute dating?

Answer 3

Giving a precise age to a rock or geological event

Question 4

What are orgoenies?

Answer 4

A time of mountain building

Question 5

What must the dating of rocks and events always be related to?

Answer 5

The geological time scale

Question 6

How many major orogenies have affected the British isles over the last 600Ma?

Answer 6

3

Question 7

Which major orogenies have affected the British isles over the last 600Ma?

Answer 7

• Alpine
• Variscan
• Caledonian

Question 8

What are orogenies caused by?

Answer 8

Plate tectonics

Question 9

What are the ways of dating one rock relative to another?

Answer 9

• superposition
• fossils and correlation
• unconformities
• cross-cutting relationships
• included fragments

Question 10

What does superposition state?

Answer 10

The rocks at the bottom of a series of beds are the oldest, while those at the top are the youngest, unless they have been overturned

Question 11

What is correlation?

Answer 11

When fossils are used to find the relative ages of rocks

Question 12

What are zones?

Answer 12

What rocks are divided into, based on the fossil or fossil they contain

Question 13

What can a zone fossil occur in?

Answer 13

Any sedimentary rock type

Question 14

What are qualities that a good zone fossil needs?

Answer 14

• to be easily recognised
• to have existed for a short time as possible, that is to have evolved rapidly
• to be preserved easily
• to be geographically widespread
• to be abundant so as to be easily found

Question 15

What are the best fossils for zoning?

Answer 15

Floating forms e.g. ammonites and graptolites

Question 16

What is a diachronous rock?

Answer 16

When different zone fossils are found in the same rock, showing that the rock was formed at different times in different places

Question 17

What group are ammonoids part of?

Answer 17

A group of molluscs called the cephalopods

Question 18

How can an ammonoid’s shell form best be understood?

Answer 18

By looking at the modern cephalopod Nautilus

Question 19

Why can geologists allocate very specific portions of time to each time zone on ammonoids?

Answer 19

They evolved rapidly into many different forms

Question 20

What is the ammonoid shell divided into?

Answer 20

Many chambers, with the dividing partitions between chambers known as septa

Question 21

On an ammonoid shell, what is it called where the septa meet the shell?

Answer 21

Suture lines

Question 22

What happened to suture lines on ammonoid shells further on in time?

Answer 22

The suture lines became more complex

Question 23

What are the three main types of ammonoid?

Answer 23

• ammonites
• ceratites
• goniatites

Question 24

What suture patterns do goniatites have?

Answer 24

Zig zag

Question 25

When are goniatites found?

Answer 25

In the Upper Paleozoic only

Question 26

What suture patterns do ceratites have?

Answer 26

Partly 'frilled'

Question 27

When are ceratites found?

Answer 27

In the Triassic only

Question 28

What suture pattern do ammonites have?

Answer 28

Very complicated pattern of 'frills' on all parts of the suture line

Question 29

When did ammonites live?

Answer 29

Jurassic and Cretaceous

Question 30

How long, on average, are ammonite zones?

Answer 30

900,000 years

Question 31

Where did graptolites live?

Answer 31

In colonies

Question 32

What are graptolites?

Answer 32

The remains of extinct marine organisms that lived in colonies

Question 33

What is a theca?

Answer 33

The tiny cup-like feature that each individual graptolite in the colony used to live in

Question 34

What is a stipe?

Answer 34

A 'branch' that thecae were arranged along

Question 35

Why were graptolites widespread?

Answer 35

Due to their floating and planktonic mode of life

Question 36

Why are graptolites good zone fossils?

Answer 36

They floated and had a planktonic mode of life

Question 37

When did graptolites evolve from?

Answer 37

The Cambrian to the Silurian

Question 38

Which period are graptolites most important for zoning?

Answer 38

The Ordovician and Silurian period

Question 39

How long, on average, are graptolite zones?

Answer 39

4 million years

Question 40

What is uniformitarianism?

Answer 40

The assumption that the same processes that operate in our scientific observations now have always operated in the past

Question 41

What are unconformities?

Answer 41

Markings showing breaks in the rock record

Question 42

What events might result in an unconformity?

Answer 42

* no deposition * erosion * orogeny (mountain building)

Question 43

How can unconformities usually be recognised?

Answer 43

By a change in the dip of the two sequences of rocks

Question 44

What do angular conformities look like?

Answer 44

A change in the dip of the two sequences of rocks

Question 45

What are the steps that might make an unconformity form?

Answer 45

* deposition * folding and uplifting * erosion * subsidence and renewed deposition

Question 46

What is a cross-cutting relationship?

Answer 46

Where one geological feature cuts across another

Question 47

In a cross-cutting relationship, which rock is younger?

Answer 47

The one that cuts across the other

Question 48

Where can cross-cutting relationships be seen?

Answer 48

* where rocks above an unconformity cut across those below * where a fault cuts through older rocks * where igneous features cut through older rocks * where magma, that will become an igneous rock, heats up and bakes older rocks

Question 49

What are included fragments?

Answer 49

Where one layer of rock is eroded and it may supply fragments for the next layer of rock deposited above

Question 50

When looking at included fragments, which rock is the youngest?

Answer 50

The rock which contains the included fragments

Question 51

Where can included fragments be found?

Answer 51

Above unconformities

Question 52

What can included fragments found above unconformities form?

Answer 52

Conglomerates

Question 53

What are the steps in conglomerate forming from included fragments?

Answer 53

* intrusive igneous rock is uplifted and erosion occurs to rocks above * fragments of granite eroded * deposition above and rock combines with fragments

Question 54

What does absolute time mean?

Answer 54

The precise age of the rocks of the Earth

Question 55

How is absolute time found?

Answer 55

Using radiometric dating

Question 56

What is radiometric dating based on?

Answer 56

The decay of radioactive elements in the minerals of the Earth's crust

Question 57

What is half life?

Answer 57

The time taken for half of the nuclei of an element to decay

Question 58

What are the most useful radioactive isotopes to geologists?

Answer 58

* Uranium 238 * Uranium 235 * Thorium 232 * Potassium 40

Question 59

In a graph of number of half-lives against % of radioactive isotopes remaining, what shape is the graph?

Answer 59

A downwards curve, as x ↑ y ↓

Question 60

As well as rocks, what can be dated by radiometric dating?

Answer 60

Major geological events e.g. beginning of volcanic activity or regional metamorphism

Question 61

What is a difficulty with radiometric dating?

Answer 61

Making sure that the mineral tested is the same age as the rock in which it is found (no problem in igneous rocks, however in sedimentary a mineral may be much older that the time at which sediment was deposited)

Question 62

What are the zone fossil groups that can be used in dating?

Answer 62

* cephalods (goniatites, ceratites, ammonites - suture line) | * graptolites (stipes, thecae)

Question 63

Roughly, what is the age of the earth?

Answer 63

4.6 billion years