Week 2 - Precambrian (Hadean --> Mesoproterozoic) Flashcards
When did the Earth form?
4.6 Ga
What is the principle of uniformitarianism?
The present is the key to the past.
Name the techniques used to establish stratigraphic information.
1) Biostratigraphy (fossils).
2) Lithostratigraphy (sequence of rocks).
3) Chronostratigraphy (radiometric dating).
4) Chemostratigraphy (chemical composition).
5) Magnetostratigraphy (magnetic properties).
6) Cyclostratigraphy (Milankovitch cycles).
7) Tephra stratigraphy (ash layers).
Which techniques are used for dating Precambrian rocks?
Chemostratigraphy and Chronostratigraphy.
Why are there no golden spikes in the Precambrian?
Rocks are linked by numerical ages using radiometric dating and chemostratigraphy.
What was the nature of the early Earth’s crust?
1) Poor in silicon dioxide, thin, dense, and basaltic.
2) Re-melting produced buoyant, silicon-rich upper crust.
What are cratons?
Silicon-rich crusts formed from early continental production.
When might the first subduction zones have started?
Around 3 Ga.
When and where did early life likely form?
Around 4 - 3.7 Ga in a warm/hot environment.
What are stromatolites?
Structures formed by communities of micro-organisms.
Where were the earliest stromatolites found?
Greenland, 3.7 Ga.
When was photosynthesis first invented?
Around 3 Ga.
What marked the Earth’s surface oxygenation?
The appearance of banded iron formations.
How did the rise of oxygen affect the atmosphere?
Reduced greenhouse gases like methane, leading to global cooling.
What was the Lomagundi Event?
A rise in organic productivity causing increased burial of light carbon.
How did the Huronian and Transvaal supergroups show changing conditions?
Huronian: reducing conditions.
Transvaal: oxidizing conditions.
What is the “Boring Billion”?
A stable period from 1.8 to 0.8 Ga with little surface perturbation.
What evolutionary progress occurred during this period?
The subdivision of eukaryotes into five major groups.
Where in the UK are Precambrian rocks found?
England, Wales, Northern Ireland, and Scotland.
Name the main Precambrian rock groups in Scotland.
1) Lewisian gneiss (3.0 – 1.7 Ga).
2) Torridonian sedimentary rocks (1.2 – 1.0 Ga).
3) Moine supergroup (1.0 – 0.87 Ga).
What are the Hadean, Archean, and Proterozoic?
Hadean: Time before 4.0 Ga with no preserved rocks.
Archean: Rocks older than 2.5 Ga.
Proterozoic: After the Archean.
What is the Giant Impact Hypothesis?
The Moon formed from debris after a collision with a Mars-sized body.
Describe the early Earth’s crust.
Mafic, thin, and constantly growing.
What created magma oceans?
The Moon-forming event melted large parts of the mantle.
Name two early supercontinents.
Rodinia (1.1 – 1.0 Ga) and Nuna (1.9 – 1.8 Ga).
When did the Great Oxygenation Event (GOE) occur?
Around 2.2 Ga.
What caused the GOE?
Cyanobacteria producing oxygen through photosynthesis.
What event followed the GOE?
The Lomagundi Event, marked by organic carbon burial.
How did oxygen increase during the Late Proterozoic?
Photosynthetic organisms evolved, contributing to oxygen buildup.
What was the Earth’s mantle like during its early formation?
The mantle was hotter than today due to radioactive decay, core differentiation, and the Moon-forming event.
What tectonic process dominated early Earth?
Vertical tectonic processes, due to limited subduction or flatter subduction compared to modern processes.
What was the state of the Earth after the Moon-forming event?
Earth retained enough heat to melt large parts of the mantle, forming a “magma ocean” for 10 million years.
How did the rapid rotation of early Earth affect its tectonics?
The fast rotation helped retain heat, resulting in a “magma ocean” and possibly influencing early mantle convection patterns.
How did the magma ocean crystallization affect the mantle?
It led to the formation of low shear wave velocity provinces (LLSVPs) in the mantle.
What evidence suggests the start of plate tectonics on early Earth?
The presence of eclogitic diamond compositions around 3 Ga, which can only form through subduction and continental collision.
What was the significance of Rb/Sr ratios in early Earth’s crust?
An increase in Rb/Sr ratios in juvenile continental crust indicated it became more silica-rich and thicker, with a maximum thickness of 40 km at the time of Rodinia assembly.
