2.3 Geochronological Principles Flashcards
What two methods are used to calculate the age of rocks?
- relative dating
* absolute dating
What is relative dating?
Stating whether one rock or geological event is older than another
What is absolute dating?
Giving a precise age to a rock or geological event
What are orgoenies?
A time of mountain building
What must the dating of rocks and events always be related to?
The geological time scale
How many major orogenies have affected the British isles over the last 600Ma?
3
Which major orogenies have affected the British isles over the last 600Ma?
- Alpine
- Variscan
- Caledonian
What are orogenies caused by?
Plate tectonics
What are the ways of dating one rock relative to another?
- superposition
- fossils and correlation
- unconformities
- cross-cutting relationships
- included fragments
What does superposition state?
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
What is correlation?
When fossils are used to find the relative ages of rocks
What are zones?
What rocks are divided into, based on the fossil or fossil they contain
What can a zone fossil occur in?
Any sedimentary rock type
What are qualities that a good zone fossil needs?
- 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
What are the best fossils for zoning?
Floating forms e.g. ammonites and graptolites
What is a diachronous rock?
When different zone fossils are found in the same rock, showing that the rock was formed at different times in different places
What group are ammonoids part of?
A group of molluscs called the cephalopods
How can an ammonoid’s shell form best be understood?
By looking at the modern cephalopod Nautilus
Why can geologists allocate very specific portions of time to each time zone on ammonoids?
They evolved rapidly into many different forms
What is the ammonoid shell divided into?
Many chambers, with the dividing partitions between chambers known as septa
On an ammonoid shell, what is it called where the septa meet the shell?
Suture lines
What happened to suture lines on ammonoid shells further on in time?
The suture lines became more complex
What are the three main types of ammonoid?
- ammonites
- ceratites
- goniatites
What suture patterns do goniatites have?
Zig zag
When are goniatites found?
In the Upper Paleozoic only
What suture patterns do ceratites have?
Partly ‘frilled’
When are ceratites found?
In the Triassic only
What suture pattern do ammonites have?
Very complicated pattern of ‘frills’ on all parts of the suture line
When did ammonites live?
Jurassic and Cretaceous
How long, on average, are ammonite zones?
900,000 years
Where did graptolites live?
In colonies
What are graptolites?
The remains of extinct marine organisms that lived in colonies
What is a theca?
The tiny cup-like feature that each individual graptolite in the colony used to live in
What is a stipe?
A ‘branch’ that thecae were arranged along
Why were graptolites widespread?
Due to their floating and planktonic mode of life
Why are graptolites good zone fossils?
They floated and had a planktonic mode of life
When did graptolites evolve from?
The Cambrian to the Silurian
Which period are graptolites most important for zoning?
The Ordovician and Silurian period
How long, on average, are graptolite zones?
4 million years
What is uniformitarianism?
The assumption that the same processes that operate in our scientific observations now have always operated in the past
What are unconformities?
Markings showing breaks in the rock record
What events might result in an unconformity?
- no deposition
- erosion
- orogeny (mountain building)
How can unconformities usually be recognised?
By a change in the dip of the two sequences of rocks
What do angular conformities look like?
A change in the dip of the two sequences of rocks
What are the steps that might make an unconformity form?
- deposition
- folding and uplifting
- erosion
- subsidence and renewed deposition
What is a cross-cutting relationship?
Where one geological feature cuts across another
In a cross-cutting relationship, which rock is younger?
The one that cuts across the other
Where can cross-cutting relationships be seen?
- 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
What are included fragments?
Where one layer of rock is eroded and it may supply fragments for the next layer of rock deposited above
When looking at included fragments, which rock is the youngest?
The rock which contains the included fragments
Where can included fragments be found?
Above unconformities
What can included fragments found above unconformities form?
Conglomerates
What are the steps in conglomerate forming from included fragments?
- intrusive igneous rock is uplifted and erosion occurs to rocks above
- fragments of granite eroded
- deposition above and rock combines with fragments
What does absolute time mean?
The precise age of the rocks of the Earth
How is absolute time found?
Using radiometric dating
What is radiometric dating based on?
The decay of radioactive elements in the minerals of the Earth’s crust
What is half life?
The time taken for half of the nuclei of an element to decay
What are the most useful radioactive isotopes to geologists?
- Uranium 238
- Uranium 235
- Thorium 232
- Potassium 40
In a graph of number of half-lives against % of radioactive isotopes remaining, what shape is the graph?
A downwards curve, as x ↑ y ↓
As well as rocks, what can be dated by radiometric dating?
Major geological events e.g. beginning of volcanic activity or regional metamorphism
What is a difficulty with radiometric dating?
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)
What are the zone fossil groups that can be used in dating?
- cephalods (goniatites, ceratites, ammonites - suture line)
* graptolites (stipes, thecae)
Roughly, what is the age of the earth?
4.6 billion years
