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Eduqas GCSE Geology (9-1) > 2.3 Geochronological Principles > Flashcards

2.3 Geochronological Principles Flashcards

1
Q

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

A
  • relative dating

* absolute dating

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2
Q

What is relative dating?

A

Stating whether one rock or geological event is older than another

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3
Q

What is absolute dating?

A

Giving a precise age to a rock or geological event

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4
Q

What are orgoenies?

A

A time of mountain building

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5
Q

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

A

The geological time scale

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6
Q

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

A

3

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7
Q

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

A
  • Alpine
  • Variscan
  • Caledonian
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8
Q

What are orogenies caused by?

A

Plate tectonics

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9
Q

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

A
  • superposition
  • fossils and correlation
  • unconformities
  • cross-cutting relationships
  • included fragments
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10
Q

What does superposition state?

A

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

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11
Q

What is correlation?

A

When fossils are used to find the relative ages of rocks

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12
Q

What are zones?

A

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

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13
Q

What can a zone fossil occur in?

A

Any sedimentary rock type

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14
Q

What are qualities that a good zone fossil needs?

A
  • 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
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15
Q

What are the best fossils for zoning?

A

Floating forms e.g. ammonites and graptolites

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16
Q

What is a diachronous rock?

A

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

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17
Q

What group are ammonoids part of?

A

A group of molluscs called the cephalopods

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18
Q

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

A

By looking at the modern cephalopod Nautilus

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19
Q

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

A

They evolved rapidly into many different forms

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20
Q

What is the ammonoid shell divided into?

A

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

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21
Q

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

A

Suture lines

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22
Q

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

A

The suture lines became more complex

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23
Q

What are the three main types of ammonoid?

A
  • ammonites
  • ceratites
  • goniatites
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24
Q

What suture patterns do goniatites have?

A

Zig zag

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25
Q

When are goniatites found?

A

In the Upper Paleozoic only

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26
Q

What suture patterns do ceratites have?

A

Partly ‘frilled’

27
Q

When are ceratites found?

A

In the Triassic only

28
Q

What suture pattern do ammonites have?

A

Very complicated pattern of ‘frills’ on all parts of the suture line

29
Q

When did ammonites live?

A

Jurassic and Cretaceous

30
Q

How long, on average, are ammonite zones?

A

900,000 years

31
Q

Where did graptolites live?

A

In colonies

32
Q

What are graptolites?

A

The remains of extinct marine organisms that lived in colonies

33
Q

What is a theca?

A

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

34
Q

What is a stipe?

A

A ‘branch’ that thecae were arranged along

35
Q

Why were graptolites widespread?

A

Due to their floating and planktonic mode of life

36
Q

Why are graptolites good zone fossils?

A

They floated and had a planktonic mode of life

37
Q

When did graptolites evolve from?

A

The Cambrian to the Silurian

38
Q

Which period are graptolites most important for zoning?

A

The Ordovician and Silurian period

39
Q

How long, on average, are graptolite zones?

A

4 million years

40
Q

What is uniformitarianism?

A

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

41
Q

What are unconformities?

A

Markings showing breaks in the rock record

42
Q

What events might result in an unconformity?

A
  • no deposition
  • erosion
  • orogeny (mountain building)
43
Q

How can unconformities usually be recognised?

A

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

44
Q

What do angular conformities look like?

A

A change in the dip of the two sequences of rocks

45
Q

What are the steps that might make an unconformity form?

A
  • deposition
  • folding and uplifting
  • erosion
  • subsidence and renewed deposition
46
Q

What is a cross-cutting relationship?

A

Where one geological feature cuts across another

47
Q

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

A

The one that cuts across the other

48
Q

Where can cross-cutting relationships be seen?

A
  • 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
49
Q

What are included fragments?

A

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

50
Q

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

A

The rock which contains the included fragments

51
Q

Where can included fragments be found?

A

Above unconformities

52
Q

What can included fragments found above unconformities form?

A

Conglomerates

53
Q

What are the steps in conglomerate forming from included fragments?

A
  • intrusive igneous rock is uplifted and erosion occurs to rocks above
  • fragments of granite eroded
  • deposition above and rock combines with fragments
54
Q

What does absolute time mean?

A

The precise age of the rocks of the Earth

55
Q

How is absolute time found?

A

Using radiometric dating

56
Q

What is radiometric dating based on?

A

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

57
Q

What is half life?

A

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

58
Q

What are the most useful radioactive isotopes to geologists?

A
  • Uranium 238
  • Uranium 235
  • Thorium 232
  • Potassium 40
59
Q

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

A

A downwards curve, as x ↑ y ↓

60
Q

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

A

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

61
Q

What is a difficulty with radiometric dating?

A

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)

62
Q

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

A
  • cephalods (goniatites, ceratites, ammonites - suture line)

* graptolites (stipes, thecae)

63
Q

Roughly, what is the age of the earth?

A

4.6 billion years

Eduqas GCSE Geology (9-1) (13 decks)

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