Week 6 How to Build a Planet: The Archean Earth

Question 1

What is the upper and lower ages of the boundaries to the Archaean?

Answer 1

4000-2500Ma

Question 2

What defines the start of the Archaean? Why are rocks formed before 3.85 Ga so rare?

Answer 2

the rock record begins at 4030Ma

Erosion and plate tectonics has destroyed all of the solid rocks that were older than 3.8 billion years.

Question 3

What key events took place during the Archaean?

Answer 3

The rock record begins at 4030 Ma
protectonics
presence of early organisms

Question 3

What is the age and name of the oldest rocks?

Answer 3

Acasta Gneisses, 4030Ma

Question 3

What proportion of earth history does the Archaean represent?

Answer 3

1500Ma

Question 4

What is the age and name of the oldest sedimentary rocks?

Answer 4

isua supracrustal belt off greenland, 3.8ga

Question 5

What is the age of the oldest macroscopic fossils?

Answer 5

Stromatolites, 3490Ma

Question 6

What is a craton? Where are the Archaean cratons located?

Answer 6

craton=large stable regions of the lithosphere
Canada, Western Australia

Question 7

When was the period of most rapid growth of the crust?

Answer 7

during the archaean, generally towards the end, e.g. 3ga-2.5ga

Question 8

What are the key differences in tectonics from the end of the Hadean to the end of the Archaean?

Answer 8

Hadean earth had thick immobile basaltic crust overlaying convecting mantle (mantle much hotter due to impacts and radioactivity) End of Archaean has continental and oceanic crust.

Question 9

What was the tectonic style of the early Earth before plate tectonics began? What role did mantle plumes play?

Answer 9

Stagnent lid tectonics

mantle plumes cause partial melting to create mafic lavas and mafic intrusions

Question 10

What is a mantle plume? What is a hot spot? How can we identify a hot spot? What is an example
of a modern hot spot?

Answer 10

mantle plume= An upwelling of hot material from the Earth’s interior that is cylindrical in shape
hotspot=a volcanically active area of Earth’s surface far from a tectonic plate boundary
e.g. Hawaiian islands

Question 11

What evidence is used to indicate the presence of plate tectonics?

Answer 11

common on other planets in the solar system

Question 12

How did the first protocontinents form in the early Archaean? What was the crust made of?

Answer 12

1. rising mantle plumes cause partial melting
2. partial melting froms mafic lavas and mafic intrusions
3. oceanic plates rise above surface sea level
4. melt to give felsic intrusions
   - crust made of TTG (quartz-feldspar rich granite rock)

Question 13

What process drove continental collision and accretion of land masses in the mid Archean in the absence of subduction?

Answer 13

archaean lithosphere not rigid enough to survive deep subduction, outflow from upwelling mantle plumes pushed on the cratonic keels causing them to drift

Question 14

What evidence is used to indicate the onset of subduction?

Answer 14

-slow down in rate continental crust formation (increase in rate of destruction)

-crustal recycling (Rb-Sr, Hf, Oxygen isotope ratios of magmatic zircons)

-change in compositional of continental crust from mafic to intermediate

Question 15

How did early subduction differ to subduction today?

Answer 15

no deep subduction, mantle circulation bottom-up

Question 16

When did the first supercontinent form in the Archaean and what is it called?

Answer 16

Protopangaea - 2700ma

Question 17

What is an ironstone?

Answer 17

sedimentary rock with 15% iron

Question 18

How old are Banded Iron formations?

Answer 18

3500ma-1900ma

Question 19

What conditions does the formation of Banded Iron Formations (BIFs) require?

Answer 19

huge quantities of dissolved Fe to be in oceans.+ lack of oxygen

Question 20

What are two proposed sources of reduced iron in the oceans?

Answer 20

1.hydrothermal vents on sea floor
2.ddeeping weathering of continental rock

Question 21

What is the relationship between the formation of BIFs, and changes in oxygen levels in the oceans and in the atmosphere? What is the cause of these changes?

Answer 21

-Higher CO2 content in the atmosphere lowered the ph of water and led to greater efficiency in iron leaching and transportation

-Oxygen-poor atmosphere means Fe2+ from continental weathering is transported to oceans

Question 22

What is the most common style of tectonics in the solar system?

Answer 22

stagnent lid

Question 23

What other planet is the Archaean Earth said to be most like in terms of its geodynamics?

Answer 23

Venus