Geological History of Scotland Flashcards
- Lewisian Gneiss
Formed:
-3 to 1.7 Billion years ago
-oldest rocks in UK, one of the oldest on Earth
- throughout whole time period that these rocks were being deposited, only single celled organisms present
Currently found in:
-NW highland coastal region
-Outer Hebrides
Protoliths:
-Igemous felsic rock eg grsaite with mafic intrusion
1. intruded into seep sub surface (10s kms below surface)
2. before reaching surface, regionally metamorphozed into gneiss
3. Uplift and erosion expose them to surface
Everything metamorphuzed so many times makes it difficult to determine protlith
Some protliths re sedimentary, some igneous, but majority granitic/felsic
meta-sedimentary protolith?
Form basement rock for nw of scotland
-Zircon mineral grains found
- Grenville Orogeny
1.2 - 0.9 billion years ago
Lewisian Gneiss has made its way to the surface and the continent/mass of land its found on gets involved in mountain building event–> continent -continent collision associated with formation of supercontinent Rodinia
Proof/remnents of this collision?:
-UKs only eclogite facies found in NW highlands
Grenville orogeny: subduction or intense collision?
Intense collision
Eclogites formed are associated with Granulites–> granulites are formed at high temperatures (high T and P) Subduction zones = low T high P
- Erosion and Deposition (Torridonian Sandstone+Glaciation)
After Grenville orogeny, huge amounts of sandy sediments were produced
Erosion forming a large area od sediment, most of layer is sediment, lewisian gneiss involved cuz at the basement some clases of lewisian gneiss, so it was involved in greenville orogney
Torridian sandstone deposited near equator, subtroppical warm temp
Himalayan Ganges Plain
Similar envoronemtn to Torridian Sandstone: flat topographic lowland, but MUCH lerss extensive, torridnoian sandstone were up to 7km vertically
- Impact
During torridnoina sandsotne deposition environemnt, meteorite hits the region
Evidenced by Stac fada in NW coast of scotladn an impaxct deposit originally have thought to be a bolvanic deposit
Evidence in scottish rock:
-Accretionary lappilli whic chemically could not have been volcnaic
-angular breccia
Where is the crater of the impact event?
Grvaity anomly 65km to east of stac fada, no evidence topographically under surface rock because happened so long ago eroded, sedimentation into crater, collision events etc
based on data around 40km crater
stac fada thickness and location correspoinds very well to crater
- Moine Deposition and metamorphism
Around 1 billion years ago, happening around the same time as Torridonian sandstone was being deposited–> Moine supergroup being deposited.
Mostly sandstones, but deposited in a rifting environment – fluvial (rivers) and shallow sea sedimentation
Inovlved in 2 orogneic events, being regionally metmorphozed twice (though smaller and later than grenville orogney)
- Iapetus Ocean
Around 670 Million years ago
Proto scotland was sitting at edge of little cointine tcalled Laurentia:
1. Lewisian Gneiss
2. covered by torridonian sandstone
3. and the moine supergroup
England and wales sat on he other side of iapetus ocean, on margins of gondwana continent
At ridting margin
te from Scotland and join around 425 million years ago
Lewisian Gneiss: south pole
Torridonian: eqautor
Dalradian Supergroup
While Iapetus ocean was present, manynsediments that make up rocks of central scotland werre depostited, this deposit is called the dlrdian supergroup
Incluldes shale, linestone ad sandsotne protlith deposition from 800-500 million years ago off ther coast of laurentia–> shallow and deep nwater sediemnts
Towards the end limestone deposied in warm shallow water whwere fossil shells and sponges can be found because life was starting to evolve past single celled organissm
- Caledonian Orogeny
Subductive plate marhgin developpos off the coast of laurentia as a result, landmasses eventually collide
Avalonia (landmass contianig England and Wales), laurentia and baltica collide.
Dalradian sediments –> dragged down into deep crust, heavily regionallly metamoprhoyed 8schisits and quartzite)
Sediments deposited in deep iapetus ocean exposed at surface because ccreted ono laurentian continental marginn that formed accreitonary wedge
3 major faults formed within scotland
Highland Boundary fault (Cal. Orogeny)
Formed when initial continetal accretiona began, at beginning of calednanprgeny
Largely strike slip fault, thoguh movemtn of this fault has changed over time
Seprates Dalradian group from younger sedientary rocks in South
The Great Glen Fault (Cal. Orogeny)
Formed towards ened of caledonina orgeoney when Baltica collide with laurentia (avalnoia colided w lurentia first)
Seprates metamorphozed Dalradian with older Mine supergroup
Originally strike slip fault (dispolaced of 64km witb aoccasionalseismic activity along the line)
Moine thrust zone (Cal. Orogeny)
Series or zone of thrsut faults formed near end of calednoina orgoeny
laces the Moine supergroup on top of younger rocks, such as the Durness
limestones, and the Durness limestone group on top of the Torridonian sandstones and Lewisian gniess
- Glen Coe
5 million yeards after formation of UK, Glen Coe (supervolcano) becomes active
Magna chamber evcentually collapses and becomes caldera
Erupt through dalradian country rock
avas were initially andesitic, developing to very viscous rhyolite
- Pangea
Supercontinent Pangea has formed and UK is somewhere in the middle of it, a little South of the equator
Between dalradian supergroup and the southern uplands MIDLAND VALLEY is forming. Erosion coming from both sides where midland valley is acting liek basin colledting mixed pebbles nand erosion
Midland Valley
Erosion coming from both Dalradian Supergrouop to the north and Southern Uplands in the south, the midland valley acts like a basin collecting mixed pebbles and erosion from both highlands.
Dalradian: mostly quartzite units (other rocks like schist are softer and have eroded away already)
Southern uplands: mudstone and sanstone
Flat laying basin that has sediments busilding layer upon layer
4 types of climate scotlands experienced during Pangeas existence?
1) slightly SOUTH IF EQUATOR:
desert climate with sanddunes, some fluvial processes/erosion
2) 300 million years ago: Scotland sits AT EQUATOR
hot, tropical jungle climate–> this land has been populated by the plants, including the first trees
This explains why the UK has coal deposits: lots of organic material falling into wet environemt where it gets compressed and accumulate
3) Still part of Pangea but Scotland moves NORTH OF EQUATOR:
Desert environment similar to Sahara desert, lots of wind blown sand and dunes deposited
4) 170 million years ago Agtlantic ocean starts to form, splittung up Pangea, though little evidence of this time period in Scotland’s rocks
One area these rocks are preserved is Skye and Rassay, where 165 Ma sedimentary rocks preserve a river estuary environment with dinosaur fossils and footprints
- Time gap?
After depositioin of sediments in Skye, there is about a 100 million year where there is a gap in the scottish rock record, probably due to wind, water and ice erosion
However it is likley that a lot of Scotland would have been under water in this time period. There are large chalk deposits in England and a house wsized block of the same chalk has been found on Arran suggeating a similar environment in scotland.
Chalk = fine grained carbonate, deposited in deep sea environments
Fits with the fact that global sea levels were 300m higher at this time
- Volcanoes
About 65 million years ago, the Atalantic Ocean is widening. This means that the crust around the UK was in an extensional regime –>thinning , allowing magma to penetrate more easily, giving access to the formation of volanoes
Skye, Mull, Rum, Eigg, Canna, Arran and Ailsa Craig owe thei existence to the N. Atlantic rifting
- Ice and Erosion
From about 24 million years ago, volcanic activity mostly stopped
Scotland sat pretty much where it is now
Glacial age causes erosion: erosion of huge areas of land into U shaped valley. This is also the reaosn why the scottish highlands are no longer high compared to the European Alps and why older rocks are exposed at the surface today
