The rest of the Archean (midterm 2)

Question 1

What % of the geologic time scale does the Precambrian take up? What is the Precambrian split into?

Answer 1

88% of geologic time

3 Eons:
- Hadean
- Archean
- Proterozoic

Question 2

Why are Archean rocks difficult to interpret?

Answer 2

• many metamorphosed and complexly deformed
• most buried beneath younger rocks
• contain few fossils > little use for age determination (wasn’t much alive back then and what was, was very hard to preserve)

Question 3

What is a craton? What are the two parts of cratons?

Answer 3

It is a large stable block of the earth’s crust forming the nucleus of a continent (part of the continental lithosphere - consists of the crust and uppermost mantle)

Shield: Exposed Precambrian rocks
ex. Canadian shield!

Platform: Covered Precambrian rocks

Question 4

When had large parts of the continents formed by?

Answer 4

By the end of the Precambrian

Question 5

What are two examples of Archean rocks we have in North America?

Answer 5

Acasta Gneiss: oldest known rocks on Earth (4 Ga) NWT Canada

Archean granite (2.9 Ga), Wyoming
> 2 diff rock types of two diff ages and a boundary between them called a contact

*precambrian rocks are found everywhere though

Question 6

Explain the composition and timeframe of the original continental crust and oceanic crust

(maybe look this over and look at the diagram a bit more)

Answer 6

Continental crust:
- about 35 km thick
- granitic composition (felsic / higher in silica) and less dense than today
- 0-4 billion years old (older)

Oceanic crust:
- about 7 km thick
- basaltic composition
- less than 180 million years old (younger)

Question 7

Describe Archean plate tectonics
What percent of our continental crust had formed?

Answer 7

• some kind of plate tectonics probably occurred during the Archean
• 30-40% of the present volume of continental crust had formed by the end of Archean (2.5 Ga)
• plates moved faster than today; more heat in earth caused faster magma convection (lots of volcanic activity)

Question 8

How did cratons grow in the Archean?

Answer 8

Grew by accretion and by underplating
- magmatic underplating from partial melting underneath

Question 9

What 2 types of terranes (provinces) are Archean rocks divided into?

Answer 9

1. Greenstone belts (GSB)
2. Granite-gneiss complexes (GGC)

Question 10

Explain the steps of greenstone belt formation in a back-arc basin

Answer 10

• we have a subduction zone - continental-oceanic crust (oceanic crust is being subducted)
• causes volcanic arc
• rifting on continent side (back-arc basin)
• partial melting of oceanic lithosphere supplies andesite & diorite magmas under the rift
• island arc basalt lavas & sediment fill back-arc basin
• back-arc basin closes
• compression, deformation
• syncline-like structure intruded by granitic magma

*this gives us the ingredients to form our greenstone belt
*Contain a whole range of sediments depending on the continental environment it’s coming from

*you should probably look at the diagram pictures of this

Question 11

Explain the significance of pillow basalts from this time

Answer 11

Pillow basalts are a feature from rocks at the time
- Type of texture that forms when you have the basaltic magma being erupted under water > happens lots at mid-ocean ridges
- outer shell rapidly cools because of the water - inside pillows had time to cool a little bit slower → this is why there are two different textures
- Great way to be able to tell which way was up - dome at top and point at bottom → point points down

Question 11

How would a craton (nucleus for building continents) begin?

Answer 11

Partial melting from the mantle would start to increase the composition of silica - buildup of granitic material (nucleus for building continents)
> from here would accrete more and have underplating (magma rising up and cooling underneath the surface) that would eventually solidify and then with erosion would be exposed

Question 12

Explain the significance of Komatiites

Answer 12

• only formed during Archean because the earth was so much hotter > can’t be formed anymore
• type of magma that has very high melting temps. > high in magnesium oxide > instead of thick layers of lava it left a very THIN veneer of komatiite
• when the thin layer cools there would be a rock with needle like crystals
• Not found in very many places on earth → but Manitoba is one of them!