1. Evidence for the Northward Drift of Britain Through the Phanerozoic.
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Transcript of 1. Evidence for the Northward Drift of Britain Through the Phanerozoic.
1. Evidence for the Northward Drift of Britain Through the Phanerozoic
Pre-Cambrian ~600 Ma
Rondinia
Supercontinent
Cambrian ~550 Ma
Early Ordovician ~490 Ma
Late Ordovician-Early Silurian ~450-440 Ma
Mid-Devonian ~400 Ma
Old Red Sandstone Continent
Pre-Cambrian ~ 850 MaCarboniferous ~300 Ma
Pangaea
Supercontinent
Triassic ~237 Ma
Jurassic ~195 Ma
Tertiary ~65 Ma
What is the rate (mm/year) Southern Britain has moved northwards over the last 500Ma?
• 60°S to 55°N• 1° = 111km
Explain how magnetic inclination preserved in rocks and apparent polar wandering curves provide evidence of changes in palaeolatitude. (25)
Palaeomagnetism
1. Palaeomagnetism2. Earth’s magnetic field3. Magnetite in basalts4. Erupt at surface & cool5. Curie point6. Align with magnetic field7. Crystallisation8. Magnetic inclination9. Remnant magnetism10.Palaeolatitude
Period Age Rocks Magnetic Inclination
Palaeolatitude
tan I = 2 tan LI = angle of inclination
L = latitude
Quaternary
Tertiary
Cretaceous
Jurassic
Triassic
Permian
Carboniferous
0
65
140
200
250
2
290
Ice age deposits
Crocodiles, lotus plant & laterites
Chalk – Chilterns, North & South DownsOolitic Lst - Cotswolds
Desert sst - Eden Valley
Desert sst - Eden Valley
Carb lst – PenninesCoal - Wakefield
69º
61º
57º
50º
50º
23º
0º
52º
42º
38º
31º
31º
12º
0º
tan I = tan L
2
“Palaeomagnetic data is a reliable indicator of palaeolatitude.”Evaluate this statement.
• igneous rocks with magnetite needed (very rare in sedimentary rocks)• doesn’t indicate which hemisphere
• assumes Earth’s magnetic field remained same over time (dipolar)• doesn’t tell lines of longitude
• remanent magnetism very weak so need lots of readings for average• re-melting/metamorphism destroys remanent magnetism
• inaccuracies caused by problems of radiometric dating of rocks
• very useful if backed up by study of sediments
Palaeomagnetic evidence suggests that during the Late Palaeozoic (Devonian, Carboniferous and Permian), Britain drifted across the equator.
a). Describe the evidence from sedimentary rocks and fossils which suggests an equatorial climate in Britain in the Late Palaeozoic.b). Describe and evaluate the palaeomagnetic evidence. (25)
With the aid of a labelled diagram, describe and account for the variation in heat flow across an ocean basin and an adjoining arc system.
(25)
• Read & dissect the question (doing words & key words)
Define meanings & give figures. Say what
you see!
Explain & give reasons for
How heat is transferred from inside the Earth to the outside
• Plan an outline & write out the essay question 1. Introduction
2. Main Body•Paragraph 1•Paragraph 2•Paragraph 3•Paragraph 4•Paragraph 5•Paragraph 6
3. Conclusion
• Use paragraphs as basic structuring unit in the essay
Topic sentence
• supporting evidence• supporting evidence• supporting evidence
Concluding sentence
Theme of paragraph & should link with concluding sentence of paragraph above.
Say what you are going to do by defining your version of the title. Show you have understood every significant word of it. Define key words.
Briefly summarise the essay by referring to the title
• Presentation
Each paragraph should be on a different aspect of the topic.Put the reader first.
Make it easy for them to read.
Clear & well presented.
Spelling & punctuation.
Permo-Triassic 290 – 208 MYA
Permo-Triassic 290 – 208 MYA
• evaporites/inter dune areas
• desiccation cracks
• playa lakes
• Zechstein Sea
A facies (a group of rocks and structures that indicate a particular environment. e.g. cross bedded desert sst, brecio-conglomerates, evaporites etc)
Burrells Quarry, Appleby, Vale of Eden
Pangaea 20ºN – semi-arid/desert
Hoff Quarry, near Appleby, Vale of Eden
Eden Shales, Vale of Eden
Desert sst
• Grain size – medium (fine)
• Grain sorting – very well sorted
• Grain Shape – well rounded
• Grain surface - quartz frosting
• quartz (sand)
• haematite cement
• barchan sand dunes
• cross bedding
Breccio-conglomerate “brockrams”
•wadis
• alluvial fans
• poorly sorted/angular to sub-rounded
• Carboniferous lst clasts
Calcite (CaCO3) – least solubleGypsum/Anhydrite (CaSO4)Halite (NaCl)Potassium saltsMagnesium salts – most soluble
Order of Precipitation
In Permo-Triassic times (~250 mya) Britain was about 20ºN of the equator in a latitude similar to that of present day North Africa. Consequently, Britain experienced a hot arid climate, which was very different from that of today. In this essay I will show how sedimentary structures, textures, fossils and mineralogy can be used as evidence to prove this.
1. Geological background/context of essay/definitions/set the scene
2. In this essay I will …. How/what going to do in essay
3. Outline content to be covered in essay
Describe the evidence from sedimentary rocks which shows that semi-arid and desert conditions existed in the British area during the Permo-Triassic period. (25)
Introduction:
In the Carboniferous period Britain was situated on the equator, and over the ensuing 300 million years has moved northwards to its present position of 50-60ºN of the equator. In this essay I will describe the evidence that proves this.
In the Carboniferous period Britain was situated on the equator, and over the ensuing 300 million years has moved northwards to its present position of 50-60ºN of the equator. In this essay I will describe how sedimentary and palaeomagnetic evidence can be used as evidence to proves this.
Describe the evidence for the northward drift of the British area from the beginning of the Carboniferous to the present day. (25)
Introduction:
Paragraphs:
4. Palaeomagnetism
1,2 & 3. Sedimentary rocks – Carboniferous, Permo-Triassic, Jurassic, Cretaceous, Tertiary & Quaternary
5. Fossils
During Late Palaeozoic times Britain moved from a tropical equatorial region in the Carboniferous (~350 mya) to a arid desert environment 20ºN of the equator in the Permian (~290 mya). In this essay I will show how palaeomagnetism, sedimentary rocks and fossils can be used as evidence to prove this.
Outline the evidence that Britain drifted north across the equator during the Late Palaeozoic. (25)
Introduction:
Paragraphs:
1. Palaeomagnetism
2. Sedimentary rocks – Carboniferous - limestones
4. Fossils
3. Sedimentary rocks – Permian – desert sst, brecio-conglomerates & evaporites
“Sedimentary rocks and palaeomagnetic data are reliable indicators of palaeolatitude.”
• Sedimentary rocks
• Palaeomagnetic data
Evaluate this statement.
• igneous rocks with magnetite needed (very rare in sedimentary rocks)• doesn’t indicate which hemisphere
• assumes Earth’s magnetic field remained same over time (dipolar)• doesn’t tell lines of longitude
• remanent magnetism very weak so need lots of readings for average• re-melting/metamorphism destroys remanent magnetism
• inaccuracies caused by problems of radiometric dating of rocks
• very useful if backed up by study of sediments
The rocks in Britain show evidence of climatic change throughout the Phanerozoic. Evaluate the evidence in the rock record for major climatic change in Great Britain from the Carboniferous to the present day. (25 marks)
Title:
Introduction:
Point ( Topic/first sentence) Evidence/Example/Extension Evaluation/Analysis of Evidence
1.
2.
3.
4.
5.
Conclusion:
Period/Age/Latitude Evidence Climate Evaluation
Carboniferous
0º Equator
Equatorial
300 Ma
Carboniferous lst – Pennines
Corals
Fossil soil
Coal – Wakefield, Yorkshire
Plant fossils - Lepidodendron
Amphibians
Tropical shallow seas
Tropical forest/swamps
UniformitarianismCoral speciesCold water lsts?
Indicates climate not latitude
Deciduous trees not evolved
Permo-Triassic
20º North
Desert
250 Ma
Desert sst (well sorted, well rounded, fine sand, haematite cement, frosted grains, cross bedding) – Eden Valley
Breccio-conglomerates
Mudcracks
Evaporites - Cheshire
Lack fossils
Beaconites (burrows) – Kingsand, Cornwall
Jurassic & Cretaceous
30º - 40º North
Tertiary
40º -60º North
Chalk – North & South DownsOolitic limestone – CotswoldsLand fossils – reptiles, dinosaurs Plant remains – Lotus flowers in SkyeRed laterite soils
Hot desert with dunes & wadis (flashflood canyons) leading out into alluvial fans.
Hypersaline conditions with playa lakes, inland seas & salt pans.
Tropical shallow seas
Tropical forest/swamps
Humid & sub-tropical
Principle of Uniformitarianism
Good evidence
Sedimentary facies – evidence needs to be taken as a whole
Implies high temp & tropical but mid latitude at time of high global temp
Implies sub-tropical but high latitude at time of high global temp
Why is it not possible to establish palaeo-longitude positions for Britain at different points through geological time?
• All places on same latitude will have similar climatic conditions
• Rock types do not vary with longitude
• Magnetic inclination is the same along every line of latitude
• Earth’s magnetic field is dipolar – aligned north to south due to spin of Earth
• One longitude could cross many climate types
Palaeomagnetic evidence suggests that during the Late Palaeozoic (Devonian, Carboniferous and Permian), Britain drifted across the equator.
a). Describe the evidence from sedimentary rocks and fossils which suggests an equatorial climate in Britain in the Late Palaeozoic.
b). Describe and evaluate the palaeomagnetic evidence.
(25)