Week 2 - Precambrian (Hadean --> Mesoproterozoic)

Question 1

When did the Earth form?

Answer 1

4.6 Ga

Question 2

What is the principle of uniformitarianism?

Answer 2

The present is the key to the past.

Question 3

Name the techniques used to establish stratigraphic information.

Answer 3

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).

Question 4

Which techniques are used for dating Precambrian rocks?

Answer 4

Chemostratigraphy and Chronostratigraphy.

Question 5

Why are there no golden spikes in the Precambrian?

Answer 5

Rocks are linked by numerical ages using radiometric dating and chemostratigraphy.

Question 6

What was the nature of the early Earth’s crust?

Answer 6

1) Poor in silicon dioxide, thin, dense, and basaltic.
2) Re-melting produced buoyant, silicon-rich upper crust.

Question 7

What are cratons?

Answer 7

Silicon-rich crusts formed from early continental production.

Question 8

When might the first subduction zones have started?

Answer 8

Around 3 Ga.

Question 9

When and where did early life likely form?

Answer 9

Around 4 - 3.7 Ga in a warm/hot environment.

Question 10

What are stromatolites?

Answer 10

Structures formed by communities of micro-organisms.

Question 11

Where were the earliest stromatolites found?

Answer 11

Greenland, 3.7 Ga.

Question 12

When was photosynthesis first invented?

Answer 12

Around 3 Ga.

Question 13

What marked the Earth’s surface oxygenation?

Answer 13

The appearance of banded iron formations.

Question 14

How did the rise of oxygen affect the atmosphere?

Answer 14

Reduced greenhouse gases like methane, leading to global cooling.

Question 15

What was the Lomagundi Event?

Answer 15

A rise in organic productivity causing increased burial of light carbon.

Question 16

How did the Huronian and Transvaal supergroups show changing conditions?

Answer 16

Huronian: reducing conditions.
Transvaal: oxidizing conditions.

Question 17

What is the “Boring Billion”?

Answer 17

A stable period from 1.8 to 0.8 Ga with little surface perturbation.

Question 18

What evolutionary progress occurred during this period?

Answer 18

The subdivision of eukaryotes into five major groups.

Question 19

Where in the UK are Precambrian rocks found?

Answer 19

England, Wales, Northern Ireland, and Scotland.

Question 20

Name the main Precambrian rock groups in Scotland.

Answer 20

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).

Question 21

What are the Hadean, Archean, and Proterozoic?

Answer 21

Hadean: Time before 4.0 Ga with no preserved rocks.
Archean: Rocks older than 2.5 Ga.
Proterozoic: After the Archean.

Question 22

What is the Giant Impact Hypothesis?

Answer 22

The Moon formed from debris after a collision with a Mars-sized body.

Question 23

Describe the early Earth’s crust.

Answer 23

Mafic, thin, and constantly growing.

Question 24

What created magma oceans?

Answer 24

The Moon-forming event melted large parts of the mantle.

Question 25

Name two early supercontinents.

Answer 25

Rodinia (1.1 – 1.0 Ga) and Nuna (1.9 – 1.8 Ga).

Question 26

When did the Great Oxygenation Event (GOE) occur?

Answer 26

Around 2.2 Ga.

Question 27

What caused the GOE?

Answer 27

Cyanobacteria producing oxygen through photosynthesis.

Question 28

What event followed the GOE?

Answer 28

The Lomagundi Event, marked by organic carbon burial.

Question 29

How did oxygen increase during the Late Proterozoic?

Answer 29

Photosynthetic organisms evolved, contributing to oxygen buildup.

Question 30

What was the Earth's mantle like during its early formation?

Answer 30

The mantle was hotter than today due to radioactive decay, core differentiation, and the Moon-forming event.

Question 31

What tectonic process dominated early Earth?

Answer 31

Vertical tectonic processes, due to limited subduction or flatter subduction compared to modern processes.

Question 32

What was the state of the Earth after the Moon-forming event?

Answer 32

Earth retained enough heat to melt large parts of the mantle, forming a "magma ocean" for 10 million years.

Question 33

How did the rapid rotation of early Earth affect its tectonics?

Answer 33

The fast rotation helped retain heat, resulting in a "magma ocean" and possibly influencing early mantle convection patterns.

Question 34

How did the magma ocean crystallization affect the mantle?

Answer 34

It led to the formation of low shear wave velocity provinces (LLSVPs) in the mantle.

Question 35

What evidence suggests the start of plate tectonics on early Earth?

Answer 35

The presence of eclogitic diamond compositions around 3 Ga, which can only form through subduction and continental collision.

Question 36

What was the significance of Rb/Sr ratios in early Earth's crust?

Answer 36

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.