Lecture 1: A brief history of the Earth Flashcards
What does ‘a’ stand for in Geological time?
Annum
How long is an annum?
1 Year
What does ‘ka’ stand for in Geological time?
Kilo-annum
How long is a kilo-annum
1000 Years
What does ‘Ma’ stand for in Geological time?
Mega-annum
How long is a mega-annum
1 million years (1000 ka)
What does ‘Ga’ stand for in Geological time?
Giga-annum
How long is a giga-annum
1 billion years (1000 Ma)
What were the four eons?
Phanerozoic
Proterozoic
Archean
Hadean
How long was the Hadean eon and how long ago?
- 7 Ga in length
4. 6 Ga from present
How long was the Archean eon and how long ago?
- 4 Ga in length
3. 9 Ga from present
How long was the Proterozoic eon and how long ago?
- 96 Ga in length
2. 5 Ga from present
How long was the Phanerozoic eon and how long ago?
0.54 Ga in length
We’re currently in this eon
What eons were in the precambrian eon?
Proterozoic
Archean
Hadean
How long ago was the big bang?
Approx 13.8 Ga ago
How long ago was the solar system created?
Approx 4.5 Ga ago
What theory describes the creation of the solar system?
Nebula Hypothesis
What does the nebula hypothesis suggest?
the origin of the solar system can be traced to a rotating cloud of gases and fine dust
What is the process of the nebula hypothesis?
rotating cloud of gases and fine dust - stars form with the coalescence(joining) of molecular hydrogen → gravitationally unstable GMCs (giant molecular clouds) → stars
What does GMC stand for?
Giant molecular clouds
How can GMCs form solar systems?
coalesce further (after forming a star) → matter → gravitationally attracted to these stars → solar systems
How can planetary accretion create a planet?
little rocks condensing out of the solar nebula as it cools forming bigger rocks. Process continues building up over time eventually coalesces into a planet
How is energy created via planetary accretion?
kinetic energy → thermal energy
Radioactive decay creates additional thermal energy
What different layers in planets did gravitational differentiation form?
High density core: mainly iron (Fe)
Medium density mantle: ultramafic silicate rock
Low density crust: felsic silicate rock
When was differentiation of the lithosphere complete?
4.4 Ga ago
What was the crust like 4.4 Ga ago and why?
Entirely igneous (because it solidified from a molten state)
Lithosphere could have been recycled in an early form of plate tectonics - true or false?
True
What was the primitive atmosphere composed of and how were these gases released?
Mainly Nitrogen, Water vapour and Carbon Dioxide
released via volcanic outgassing
What compensated for the weak sun in the primitive atmosphere?
High concentrations of greenhouse gases
How much dimmer was the sun compared to now in the primitive atmosphere?
30% dimmer
How long ago did oceans form?
Between 4.5-3.8 Ga ago
How were oceans formed?
water outgassed by volcanoes and brought to earth by comets
What could have been the earliest organisms and where would they have been?
Hyperthemophilic chemoautotrophs at hot springs or deep hydrothermal vents
hyperthemophilic definition
can tolerate very hot environments (more than 80 degrees)
chemoautotrophs definition
primary producers of their energy from chemical reactions (don’t have to eat another animal/thing)
Microfossil definition
a fossil or fossil fragment that can only be seen with a microscope
Stromatolite definition
a calcareous mound built up of layers of lime-secreting cyanobacteria and trapped sediment, found in Precambrian rocks as the earliest known fossils
What is the oldest evidence for photosynthesis?
Oldest chemofossils (organic residue) provide evidence of photosynthesis by 2.7 Ga
Chemofossil definition
A fossil that consists only of chemicals remaining from the decomposition of a living organism.
What was oxygen initially produced by?
Photosynthetic cyanobacteria
Cyanobacteria definition
a division of microorganisms that are related to the bacteria but are capable of photosynthesis
How were iron bands formed in the ocean?
Iron is soluble in water when oxygen concentrations are low (early oceans)
As oxygen levels rose – iron oxide minerals precipitated from seawater forming bands
What factor in the oceans alternated and what was the atmosphere like then?
Oceans alternately oxic (O2 rich)/anoxic (O2 deprived), atmosphere anoxic at this time (doesn’t contain ‘free’ oxygen, not in the form O2 or O3)
When did oxygen accumulate in the atmosphere?
Approx 2.4 Ga ago
What evidence is there for O2 levels plateauing at 1% of the atmosphere?
rocks that were laid down as sediment in rivers 2 Ga ago, suggesting the atmosphere had enough oxygen to oxidise Fe2+ into Fe3+
When did oxygen rise significantly to normal levels and why was this?
580 Ma ago
Probably due to mountain building, sediment production, burial of organic Carbon (preventing reactions with O2, causing a build up)
What did the cambrian explosion (and the increase in oxygen) cause?
First multicellular animals (circa 575 Ma)
Rapid evol. of shells/skeletons → first fossils
542 Ma marks start of the Cambrian period of Phanerozoic eon
All major animal groups appeared within 10 Ma (arthropoda, chordata, mollusca, etc.)
How many mass extinctions have there been in the Phanerozoic fossil record?
5, and several smaller ones
What is the current mass extinction caused by?
Humans
What are mass extinctions followed by?
evolutionary radiations - life evolves to fill the gaps left by dead creatures
What causes Mass extinctions?
rapid environmental change (meteorite impact, volcanic eruptions, climate change)
When’s the earliest plants on land?
850 Ma ago
When’s the earliest hominids?
6 Ma ago
When did the first homo sapiens evolve?
0.3 Ma ago (300 ka)
