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HotE > W2+3 - Precambrian > Flashcards

W2+3 - Precambrian Flashcards

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1
Q

When was the Hadean eon?

A

4.54-4bya

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2
Q

What were the conditions on early earth?

A

Lots of unknowns about conditions - best guess
Almost entire Earth was likely molten magma (magma ‘ocean’)
Planet began to cool over a few hundred years - rocks formed from cooling magma
Atmosphere had little to no oxygen

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3
Q

Why was early Earth much hotter?

A

Due to residual heat from planetary accretion
Planet is continuously cooling down, even now - has slowed as radioactive isotopes release heat in core

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4
Q

What is komatitte?

A

Type of ultramafic basalt
Rich in iron and magnesium
Quite dense
Forms at temps >1100C
Suspected to have made the earliest oceanic crust (4.5bya)

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5
Q

What is the average surface temperature of the Earth?

A

12-15C

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6
Q

How old is the earliest discovered?

A

340my

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7
Q

What were conditions like inside the early Earth?

A

Convection currents were driven by the heat of the core
Not fully understood
Some of earliest core may have sunk to form outer core

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8
Q

What type of crust was the first to form?

A

Oceanic

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9
Q

What is the density of oceanic and continental crust?

A

Oceanic = 3g/cm^3
Continental = 2.7g/cm^3

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10
Q

What type of rock is the oceanic and continental crust made of?

A

Oceanic = basalt (mafic)
Continental = granodiorite (felsic)

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11
Q

Who was Norman Bowen?

A

Early 1900s Canadian geologist
Experimentally melted igneous rocks and tested minerals formed at different temperatures
Developed Bowen’s reaction series

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12
Q

What is Bowen’s reaction series

A

Idealized progression of minerals produced by cooling basaltic magma (what temp do minerals crystalize at)
Low end (700C) consists of K-spar, muscovite, and quartz
Upper end (1250C) consists of olivine, pyroxene, amphibole, plagioclase feldspar, and biotite

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13
Q

How does the sequence of mineral crystallization affect magmas?

A

Lavas erupted first from a magma body may be rich in iron, magnesium, and calcium - produce basalts (mafic)
Lavas erupted later may be richer in sodium, potassium, and silicon - produce andesites (felsic)

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14
Q

How were early oceans formed?

A

Formed underwater sometime before 4bya
As water was added to lavas, the melting of the wet crust at subduction zones formed andesitic magmas

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15
Q

How was continental crust developed?

A

Continued subduction of sediment and water with basaltic oceanic crust creates more felsic magmas over time
Low density lavas don’t subduct and form continental crust
Patches by 4.4bya

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16
Q

What is the earliest evidence of water?

A

4.4bya zircon has stable oxygen isotope ratios indicating that zircon reacts with water and that deep oceans had formed by 3.8bya

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17
Q

Where did water come from?

A

1) Outgassing from magmas
2) Comets - have water ingrained in minerals (as it hit the Earth, water was unlocked as minerals)
More debris (comets) was present at this time in the solar system
Comet water has similar isotopic composition to ocean water
Contain carbonaceous chondrites - contain up to 20% water that is locked up in clay minerals

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18
Q

What was the early atmospheric composition

A

Abundant hydrogen, no free oxygen (O2), anoxic or reducing, high CO2 levels

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19
Q

How does today’s atmospheric composition differ from that in the Hadean eon?

A

More oxygen (0 vs 21%)
Mainly composed of nitrogen
Contains argon, water vapour, and other gases
0.03% CO2

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20
Q

How did oxygen concentrations build up?

A

1) Photochemical dissociation (minor contributor) = splitting of water molecules by UV radiation in the upper atmosphere
2) Photosynthesis by early photosynthetic organisms (more important process - great oxidation even)

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21
Q

What were the earliest photosynthetic organisms?

A

Cyanobacteria and other algae
Take up CO2 for energy and release oxygen

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22
Q

What evidence is there of early life forms?

A

1) Stromatolites (build of algal mats) are found in 3.5by Archean rock
2) Banded Iron Formations (BIFs) + microfossil tubes found in BIF deposits in Quebec
3) 3.5by Apex Chert fossils of blue-green algae

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23
Q

What are banded iron formations (BIFs)?

A

Red streaks in rocks formed by the combination of oxygen and dissolved iron to form insoluble iron oxide which precipitated out, forming thin layer of ocean floor
Formed in early oceans as a result of oxygen produced by cyanobacteria
Common in rocks 3-1.8by but absent in younger rocks (iron was more prevalent during Archean eon - released from abundant hydrothermal vents at mid-ocean ridges)

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24
Q

Why did CO2 levels decrease over time?

A

Weathering of early rocks cause CO2 in atmosphere to be drawn down
Weathering of igneous rocks (calcium-silicates) produced calcium carbonate and silica - locks the carbon dioxide away

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25
What elements are needed for life to evolve?
Carbon, oxygen, hydrogen, nitrogen, phosphorus, and sulfur All of these are abundant in our solar system
26
What is the Miller-Urey Experiment?
1950-60s experiments that produced amino acids from atmospheric gases, water, electricity, and heat Demonstrated that amino acids can be easily synthesized abiotically in an early Earth Experiment produced 6 types of amino acids and common organic compounds (ex: cyanide and formaldehyde)
27
Alternatively to abiotic synthesis, where else might organic materials have originated?
Organic materials are also present in comets and meteorites
28
What is a chain molecule?
Hydrophilic phosphate molecule attached to a pair of fatty acid chains Attracted to other fats Combine to form a sheet or ball (Michelle) - considered archaic cell wall
29
How did early organic molecules come together to form larger, more complex structures?
Concentration of reagents makes polymers (chains of amino acids or proteins) Clay or pyrite substances can also aid in polymerizing organic molecules Rewetting can cause formation of hollow bilayer balls (liposomes) - have hollow interior (vesicle) that can contain reagents and may have been the precursors (protocells) to the first cells Wetting and drying can concentrate reagents and protocells that can replicate may become self-selected for improvement
30
What are lithotropes?
Bacteria that live in and on rocks Consume hydrogen and geothermal energy Form huge biomass below the Earth's surface Found in granites >3600m below Earth's surface
31
Where on Earth did early life forms flourish?
Probably in oceans Salt, solvents, and elements mixed to create organic compounds Life forms (like archaea) may have first developed at hydrothermal vents (high heat) at mid-ocean ridges
32
What evidence is there of early life developing in hydrothermal vents?
Hyperthermophiles found at modern hydrothermal vents Live at temps > 100C, no light, and receive energy from chemosynthesis No actual fossils of archaebacteria from early spreading centres have been found
33
What were the earliest fossils?
Stromatolites = columnar organic structures formed by trapping sediment on sticky bacterial mats Formed by prokaryotes Prokaryotes = single-celled bacteria, microscopic, hypersalinic (suggests first bacteria was formed in high salt environments)
34
What is the oldest fossil?
Stromatolites from the Pilbara Craton, Australia
35
What is a craton?
Oldest parts of modern continents Stable parts of continental crust (have survived subduction - didn't melt) Composed of Archean and Proterozoic age rocks >600 million years Ex: North American Craton (Canadian Shield)
36
What is a shield?
Exposed parts of cratons Covered by younger rocks - referred to as platform or cover rocks Canada shield is an ancient erosional surface
37
What is a peneplain?
Ancient erosional surface Form as the original old mountain range is eroded through glaciation and weathering , with the remaining inside of the mountain becoming the peneplain Ex: Canadian Shield
38
What is the Great Unconformity?
Missing 1by from 1.5-0.5-4bya Unsure what happened in Ontario to produce erosional surface
39
What are geological provinces?
Areas of cratons with distinct geology Ancient microcontinents (terranes) brought together by plate tectonics Welded onto craton by suture zones (oldest in centre with younger at margins)
40
What is Arctica?
Earliest recognizable North American continent (proto-North America) Formed around 2.5bya Composed of Slave, Hearne, Wyoming, Rae, and Superior provinces
41
What is orogeny?
Mountain building feature Created between continental-ocean or continental-continental plates
42
When was the Southern province added to Arctica?
First to be added 2.5-2.2bya Consists of sedimentary rocks of the Huronian Supergroup that were deposited along its southern margin
43
What and when were the earliest glacial deposists?
Gowganda Formation ~2.3bya (when ice sheets first formed) Canada has some of the earliest evidence of glaciation Consists of diamictites (very poorly sorted tills) Glaciomarine deposits Deposited by glaciers on basin margins
44
What are diamictites?
Analogous to tills Diamictites are the sediments, diamicts are the rocks
45
What created the Sudbury Impact crater?
Created ~1.85bya Caused by meteorite >4km diameter, traveling 15km/s 60x20km and 10km deep Second largest crater on Earth (largest is in Africa) Contains fragmented rocks and mineral resources (ex: nickel)
46
What evidence is there of the Sudbury Impact crater?
Fragmented rocks (ex: Sudbury breccia) Shatter cones - apices of cones point towards impact Shocked quartz grains - evidences intense shock due to high durability of quartz Pseudotachylites = rocks fused into glass by shock wave
47
What are breccia?
Angular rocks with varying clasts Formed through coalition
48
Why is the Sudbury Impact crater eliptical?
Deformed during the Penokean Orogeny (1.7bya) Original crater was circular and much larger
49
What mineral resources are found in the Sudbury Impact crater?
Impacted melted crust formed Sudbury Igneous Complex Rich in sulphide ores, including pentladite (35% nickel), pyrite, chalcopyrite Allows Canada to produce 5% of global nickel supply
50
What was the Grenville Orogeny?
1.7-1bya Collision of N and S America Final stages of craton formation Created Rodinia = first supercontinent Most continents were located in the Southern hemisphere at this time - imply conditions for glaciers to grow Caused formation of Canadian Shield - overlying orogenic belts have eroded to reveal folded and deformed shield rocks (ex: gneiss) beneath today
51
What is an ice age?
Relatively long period of time where the Earth's surface temperature and atmospheric temperature are reduced that allow for continental, polar ice sheets and alpine glaciers to grow Ice sheets present Five to six major ice ages in the past 3by
52
What is a glacial period?
Major ice sheets present on continents Lowered sea levels Ex: 20 000-25 000ya
53
What is an interglacial period?
Restricted ice volumes - only at low latitudes or high altitudes Raised sea levels Ex: present times
54
What is an interstadial period?
Warm intervals within a glacial period Still colder than interglacial periods
55
What are the requirements for glaciers to form?
Temperature low enough to retain snow year round (high latitude and altitude) Sufficient snow (some polar climates are too dry for glaciers to develop) Summer melting and ablation must be less than winter accumulation (aka snow must survive a summer melt to become firn)
56
What are Milankovitch cycles?
Regular variations in Earth's orbit control ice ages Calculate by astronomer in 1941 Occur in in 20, 40, 100, and 400 year orbital cycles Sometimes cycles constructively interfere and sometimes cancel each other out, so they can be somewhat irregular Change amount of exposure to solar radiation and energy Include changes in eccentricity (orbit shape), axial precession (wobble of axis), and changes in obliquity (angle of Earth on axis)
57
When was the Pongola Ice Age?
2.9-2.8bya Sometimes included in ice ages based on different sedimentological and oxygen isotope data Evidenced by diamictite (Pongola Supergroup) in South Africa
58
When was the Huronian Ice Age?
2.4-2.3bya Evidenced by diamictites of Gondwana Formation found in northern Ontario
59
When was the Neoproterozoic Ice Age?
715-547mya Composed of Sturtian glacial period (720-660mya) and Marinoan glacial period (654-625mya) Evidenced by glacial deposits found throughout the world Increase in Earth's coverage of snow/ice increase Earth's albedo and in turn result in a positive feedback loop
60
What is the snowball earth theory?
Theory that states that the entire Earth surface was covered in ice during the Neoproterozoic Ice Age This and other similar theories can't be proven, but it is known that some glaciation occurred
61
What evidence is there against the snowball earth theory?
Ice covering oceans would have blocked sunlight for photosynthesizing bacteria underwater - some say the layer over oceans was more slushy Find deltaic sediment deposits - need to have been terrestrial environments to create them
62
How was the Neoproterozoic Ice Age hypothesized to have ended?
i) Eruption of supervolcano ii) Reduction in greenhouse gases through chemical weathering of CO2 iii) Changes in solar energy output

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