- Archean Eon is about 33% of Earth’s history
- Only 22% of exposed continental crust
- Rocks are difficult to interpret
- Commonly metamorphosed and deformed
- Most rocks have been buried deeply
- Few contain any clear fossils, since life was microscopic with no hard parts
- Archean rocks in North America
- Acasta Gneiss
- Oldest known exposed rocks on Earth about 4 billion years old
- Northwest Territories, Canada
- Only meteorites and a few single zircon grains are older
- Most Archean rocks in North America are underneath newer rocks
- Some uplifted and exposed by later mountain building events
- Greenstone Belts
- greenstone belts and granite-gneiss-complexes are most common Archean rocks
- Belts > 2.8 billion years old are basaltic over ultramafic
- Younger belts are more intermediate, andesite-rhyolite over basaltic
- Upper sedimentary unit
- Synclinal structures
- North American Greenstone Belts
- Canadian Shield
- Superior and Slave cratons
- 2.7-2.5 billion years ago
- Low-grade metamorphism
- Often intruded by granitic magmas
- Faulted
- Sedimentary rocks
- Sandstones and conglomerates
- Mudstones
- Carbonates
- Banded iron formations
- Fluctuating atmospheric oxygen levels
- More common in Proterozoic
- Archean Plate Tectonics and the origin of cratons
- Some kind of plate tectonics probably occurred during the Archean
- Small cratons were present
- Grew by accretion, lateral growth by island arc collision
- Also, underplating with rising magmas accumulating below or within continental crust
- By end of Archean (2.5 billion years ago)
- 30-40% of the present volume of continental curst formed
- Plates moved faster than today, more heat in Earth caused faster magma convection
- The atmosphere and hydrosphere
- Early atmosphere quite different than modern
- Hydrogen and helium, solar winds
- Formation of core allowed atmospheric gases to accumulate
- Magnetic field shielded atmosphere from solar wind
- Outgassing from volcanic eruptions
- Gases from Earth’s interior into the atmosphere
- Water, Carbon dioxide, hydrogen, nitrogen
- Methane and ammonia from chemical reactions
- Very little free Oxygen in “oases”
- Faint Young Sun paradox
- The sun was only 70% as bright as today early in Earth’s history
- Less energy to warm the Earth
- All else equal, Earth should have been glaciated globally for the first half of its history
- Evidence for liquid water
- Requires larger greenhouse effect
- Origin of the hydrosphere
- Sources of surface water include
- Outgassing from earths interior
- Bombardment by meteorites and icy comets
- Not known which is more important
- Archean oceans existed
- Evidence in sedimentary rocks and pillow basalts
- Likely deeper than today
- possibly saltier than today
- Archean Origin of life
- Oldest fossils
- 3.5 billion years ago for the oldest “biologic” fossils in Australia
- Chemical evidence indicates photosynthesis existed at 3.8 billion years ago
- Controversial isotopic evidence from zircons may suggest life by 4.1 billion years ago (Hadean)
- 3.7 billion years ago “stromatolites” in Greenland