The rest of the Archean (midterm 2) Flashcards
What % of the geologic time scale does the Precambrian take up? What is the Precambrian split into?
88% of geologic time
3 Eons:
- Hadean
- Archean
- Proterozoic
Why are Archean rocks difficult to interpret?
- many metamorphosed and complexly deformed
- most buried beneath younger rocks
- contain few fossils > little use for age determination
What is a craton? What are the two parts of cratons?
It is a large stable block of the earth’s crust forming the nucleus of a continent
Shield: Exposed Precambrian rocks
Platform: Covered Precambrian rocks
When had large parts of the continents formed by?
By the end of the Precambrian
What are two examples of Archean rocks we have in North America?
Acasta Gneiss: oldest known rocks on Earth (4 Ga) NWT Canada
Archean granite (2.9 Ga), Wyoming
*precambrian rocks are found everywhere though
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)
Continental crust:
- about 35 km thick
- granitic composition (felsic / higher in silica) and less dense than today
- older
Oceanic crust:
- about 7 km thick
- basaltic composition
- younger
Describe Archean plate tectonics
What percent of our continental crust had formed?
- some kind of plate tectonics probably occurred during the Archean > plates moved faster, more heat, more volcanic activity
- small cratons present
- 30-40% of the present volume of continental crust had formed by the end of Archean (2.5 Ga)
How did cratons grow in the Archean?
Grew by accretion and by underplating
- magmatic underplating from partial melting underneath
What 2 types of terranes (provinces) are Archean rocks divided into?
- Greenstone belts (GSB)
- Granite-gneiss complexes (GGC)
Explain the steps of greenstone belt formation in a back-arc basin
- 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 creates magmas under the rift
- island arc lavas & sediment fill back-arc basin
- back-arc basin closes
- compression, deformation
- syncline-like (valley) structure intruded by granitic magma from partial melting
*this gives us the ingredients to form our greenstone belt
*you should probably look at the diagram pictures of this
Explain the significance of pillow basalts from this time
Pillow basalts are a feature from rocks at the time
- Type of texture that forms when basaltic magma erupts under water > happens lots at mid-ocean ridges
- outer shell rapidly cools because of the water - inside pillows cool a little bit slower → this is why there are two different textures
How would a craton (nucleus for building continents) begin?
Partial melting from the mantle would start to increase the composition of silica - buildup of granitic material
> 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
Explain the significance of Komatiites
- 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 > left a very THIN veneer of komatiite lava
- 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!
