Early Earth

Question 1

Hadean

Answer 1

Huh-dee’-un
up to 4 billion years ago
oldest eon in ages of the earth

Question 2

Old theory of earth hadean surface

Answer 2

lava oceans

Question 3

New theory of earth hadean surface

Answer 3

water, stratovolcanoes, stromatolites

Question 4

stromatolite

Answer 4

sedimentary rocks formed from layers of microbial reefs
oldest dated rocks

Question 5

Earliest crust

Answer 5

komatiites

Question 6

komatiite

Answer 6

ultramafic extrusive igneous rock
forms in extremely hot conditions

Question 7

how did felsic crust form

Answer 7

after some cooling of earth surface
recycling of komatiites due to fractional crystallization

Question 8

fractional crystallization

Answer 8

composition of magma changes as certain elements solidify into minerals (crystallize) in specific temperature ranges
(Fe, Mg crytallize first, carbon and silicon later)

Question 9

characteristics of zircon

Answer 9

durable
chemically inert
persist as detrital (sand) and reincorporated over and over in sedimentary rock

Question 10

Jack Hills zircon

Answer 10

oldest dated zircon ~4.4 Gya, found in conglomerates and metamorphic (so must be recycled) rocks in the Jacks Hills, mountain range in Australia (rocks dated to 3.8Gya)
G= billion

Question 11

2 pieces of evidence from Jack Hills zircon and what that tells us

Answer 11

1) crust existed 4.4 Gya - the zircons had to have come from an older crust and been recycled into these rocks (those rocks likely did not survive Heavy Bombardment of 3.9Gya and were recycled as sedimentary/metamorphic rocks)
2) oxygen isotopes in the zircon suggest liquid water was present at the formation of zircon

Question 12

What does Jack Hills zircon mean for life on earth?

Answer 12

If crust and water was present, the environment was potentially hospitable earlier than previous thought

Question 13

Did life exist in Hadean?

Answer 13

There is no evidence to suggest it

Question 14

If life existed during Hadean, how could it have survived bombardment?

Answer 14

Hydrothermal vents could have provided some
(maybe - its just a thought experiment)

Question 15

Archean

Answer 15

2nd oldest eon
4Gya -> 2.5 Gya (has 4 eras)

Question 16

Mantle temperature during Archean

Answer 16

hotter than any other known eon, believed to not be as hot as Hadean
(no rocks remaining from Hadean to confirm)

Question 17

What makes measuring temperature of mantle difficult?

Answer 17

mantle eruptions come from a plume, not the bulk or ambient mantle

Question 18

How can we infer the ambient mantle temperature?

Answer 18

empirical evidence links MgO weight % to temperature at time of basalt formation
(still measuring plumes, not ambient mantle, so an inference, not measurable fact)

Question 19

Two theories on regarding HOT Archean CRUST

Answer 19

Hot because greater abundance of radioactive species in crust (generating heat as they decay)
1) so hot it was thermally unstable
2) mechanisms in play that provided effective enough cooling to prevent instability

Question 20

Thermal Instabailty

Answer 20

Temperatures so hot and variable that it induces rapid changes in pressure and density

Question 21

Rocks that help understand Archean environment

Answer 21

Acasta Granitic Gneiss (4.03Gy)
Greenland conglomerate (3.8Gy)
Coonterunah/Warrawoona (Australia) unconformity (3.5Gy)

Question 22

Acasta Granitic Gneiss

Answer 22

oldest known exposed rock (most well accepted proposed)

Question 23

What does gneiss tell us about environment?

Answer 23

Contain mica = hydrated minerals = liquid water had to be present

Question 24

What does conglomerate tell us about the environment?

Answer 24

conglomerate has pebbles with rounded edges = shallow flowing liquid water had to be present on surface to erode weathered rocks

Question 25

What does an unconformity tell us about the environment?

Answer 25

It is subaerial - not sure why this matters

Question 26

How does plate tectonics help life?

Answer 26

provides planetary homeostasis by regulating the environment

Question 27

2 theories about Archean plate tectonics

Answer 27

hotter mantle (evidence from MgO weight % in basalts) 1) plates were smaller and moved faster 2) thicker oceanic crust due to hotter mantle having more melt, would have caused problems for subduction and therefore larger and slower plates

Question 28

Bonninites

Answer 28

extrusive igneous rock containing magnesium and silica, thought to form in the early stages of subduction in volcanic arc systems; Drew's definition = primitive andesite

Question 29

oldest bonninites

Answer 29

2.7 to 2.8, Finland and Canada

Question 30

Ophiolites

Answer 30

rock formed from sequence of formations and deposits on the ocean floor that have been uplifted and exposed in continental crust

Question 31

oldest ophiolite

Answer 31

2.0 Ga, Canada

Question 32

rocks that help define history of plate movement via tectonics

Answer 32

bonninites ophiolites rocks that constrain pressure/temp conditions that are found in subduction zones (blueschist, eclogites)

Question 33

blueschist

Answer 33

formed in subduction zones and indicate high pressure, low temperature (200-500 Celsius, 15-30 km depth)

Question 34

eclogites

Answer 34

formed in subduction zones and indicate high pressure, mid-range temperature (500-1000 Celcius, 30-50 km depth)

Question 35

oldest subduction zone rocks

Answer 35

~ 1 Ga (many locations)

Question 36

Timeline of early earth (9 points)

Answer 36

4.54 Ga - earth formed 4 Ga - Moon forming impact cooling 4.4 Ga water present/crust present? ~4 Ga water/crust exposed? 3.8 Ga water definitely present 3.5 Ga crust definitely exposed ~ 2.8 Ga plate tectonics? 1 Ga, definitely plate tectonics

Question 37

Why is crust important for early life?

Answer 37

provided stable platform, diverse environments, opportunity for heating and cooling

Question 38

why is temperature change important for origin of life?

Answer 38

causes disequilibrium that could encourage specific chemical reactions

Question 39

what are indications that the earth had an environment friendly for life early in its history?

Answer 39

presence of liquid water and crust