Early Earth: Hadean and Archean Eons

What did the surface of Earth probably look like during the Hadean Eon?

Molten and heavily bombarded by asteroids
Time of Earth’s early evolution, High interior and surface temperatures, Moon's formation

No rock record survives from this time

What did the surface of Earth probably look like during the Archean Eon?

Continents forming and oceans taking shape

What is differentiation?

Formation of different material zones within Earth

Is differentiation thought to have happened early or late in the Hadean Eon?

Early in Hadean Eon

How old is the Moon?

Younger than the earth, 4.5 billiion years old

What is the current thinking on how the Moon formed?

Giant Impact Theory

How old is the oldest known terrestrial geologic material?

Mineral, 4.4 billion years

Which eon do the oldest known rocks belong too?

Hadean

When did Earth develop an atmosphere (which Eon)?

Hadean

What is the primary difference between the atmosphere today and the early atmosphere?

Early atmosphere had high levels of methane and ammonia

Why did the atmosphere change?

Addition of oxygen
Biological activity of early life

When did oceans form (which Eon)?

Hadean

How old are the oldest fossils (which Eon)?

3.5-3.7 Ga

What did life look like at the end of the Archean Eon?

all living organisms are prokaryotes

singled cell organisms without a nucleus

Hadean Eon (4.6 - 4.0 GA) General Characterization

Earth’s early evolution

High interior and surface temperatures

Moon's formation by giant impact theory

No rock record survives from this time

Hadean Eon (4.6 - 4.0 G) Atmosphere Status

Early Atmosphere came from volatile gases

Today’s atmosphere has changed but still has common compounds

Hadean Eon (4.6 - 4.0) Ocean Status

Outgassed water vapor condenses in atmosphere and creates rain

Most of current ocean volume accumulated by end of Hadean Eon

Hadean Eon (4.6-4.0) Life Status

No evidence of life

Archean Eon (4.0 - 2.5 Ga) General Characteristics

Surface has mostly cooled and solidified

Evidence of the earliest continents

Have Oceans and Atmosphere

Marine Sedimentary rocks

Earliest Fossils

Life gets started

Archean Eon (4.0 - 2.5 Ga) Geological Events

Thin crusts forms from cooling magma ocean by volcanoes, granite forms

Plate Tectonism (continental and oceanic plates converge) 3.8 Ga

Significant fraction of continental provinces had been formed by the end

Oldest known rocks are formed: Greenstone belts, Gneiss belts, Banded Iron Formations (BIFs)

Craton

Geologically stable part of continental crust

Consists of Shield and Stable Platform regions

Shield

Part of craton that is exposed on the surface

Stable Platform

Part of craton that is covered with younger sediments or sedimentary rocks

Archean Eon (4.0 - 2.5 Ga) Atmosphere Status

Early Archean atmosphere has little or no free oxygen

Oxygen levels gradually increase thanks to BIFs (Photodissociation of water vapor & Photosynthesis)

Atmosphere composition significantly different

Archean Eon (4.0 - 2.5 Ga) Ocean Status

Rain continues to accumulate in low regions (ocean basins)

Early rain water would have been acidic

Weathered rock materials eroded and carried into oceans

Water absorbs atmosphere CO2

Organisms would later use CO2 to form shells and skeletons

Archean Eon (4.0 - 2.5 Ga) Life Status

Microfossils provide evidence of early life

By the end of the Archean Eon, all living organisms are prokaryotes

o Single-celled organisms

o Cells do not have a nucleus

Essential chemical elements for life

- Carbon (C)

- Hydrogen (H)

- Oxygen (O)

- Nitrogen (N)

- Phosphorous (P)

- Sulfur (S)

Hadean Earth had most of these elements in the atmospheric gases

Essential components of all life forms

Proteins - Provide structure and do most of the functional work in cells

Nucleic Acids - Provide storage and means of expression of genetic information(DNA and RNA)

Organic phosphorus compounds - Convert light or chemical "fuel" into energy

Cell membrane - Keep all of the above components together