Transcript Lecture7

Lecture 7. The Hadean Earth, heavy bombardment,
origin of the atmosphere and oceans
announcement: schedule for next week
reading: Chapter 4
The Hadean Eon
origin of the
solar system
oldest rocks
on Earth - end of
heavy bombardment
Hadean
billions of
years ago:
4.56
rise in
oxygen first multiplate
tectonics?
cellular fossils
Archean
3.8
Proterozoic
2.5
Cambrian
Explosion
Phanerozoic
0.55
present
Distribution of Hadean Rocks on the Earth
Slave Province,
northern Canada
3.96 - 4.0 Ga gneiss
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Porpoise Cove,
Canada
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3.825 Ga
volcanic and sedimentary rocks
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Western Greenland
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3.8 Ga Isua Greenstone Belt, southern West Greenland
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2.991 Ga felsic (Si-rich) gneiss
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calc-silicate rocks, once thought
to be sedimentary, now thought
to be metamorphosed basalt
Greenland, cont.
felsic dikes in gneisses
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pillow basalt - formed when
basalt erupts underwater
(water 3.8 Ga)
Pilbara, Western Australia
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komatiite, Mg-rich form of basalt
very hot eruptions
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3.5 Ga stromatolites - may be
the oldest microbial mats
Jack Hills, Western Australia
rock contains 4.4 Ga zircon grains oldest dated material on Earth
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banded iron formation
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What about Earlier Hadean?
Little or no direct evidence.
Moon formed by 4.4 Ga.
Know the Earth was differentiated by then.
Isotopic evidence suggests Earth’s core formed very early - 45 Ma
after accretion began.
Moon age 4.3-4.4 Ga.
Zircon grains have oxygen isotopes that indicate the presence of
liquid water - oceans.
Vigorous Volcanic Eruptions
Higher (~5x) heat flux due to abundant heat sources.
Abundant magma, recycling and remelting of crust.
Earliest rocks are either volcanic rocks or sedimentary rocks that
are heavily metamorphosed.
How does heat escape from the Earth?
Hydrothermal vents
Mid-ocean ridges
Volcanoes and
hot springs
Interplanetary Dust
10 µm diameter
Impacts are Common
Small impacts - 20,000 tons of interplanetary dust every year
Giant impacts - rare
Over 100 craters identified.
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Chicxulub Crater
Yucatan Peninsula
~180 km diameter
65 Ma
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Manicouagan Crater, Quebec
70 km diameter
206-214 Ma
Meteor Crater, Arizona
1 km diameter
~50,000 years old
Aorounga Impact Crater, Chad
17 km diameter
several hundred Ma
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Heavy Bombardment
Cratering rate drops with time.
Cratering rate determined from the Moon.
But Earth is larger - so it makes a bigger target.
Earth has 6x gravity field of Moon - attracts
more impactors.
Cratering rate of the Earth ~ 10x the Moon.
Older surfaces of the Moon - Highlands
saturated with craters.
Cratering rate actually difficult to measure.
Younger surfaces - Mare/maria have
few craters.
Radiometric dating of maria basalts
indicate they are 3-3.9 Ga.
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# of events N greater than mass m per year
Size Distribution of Craters
microscopic Moon craters
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modern Earth
Size Really Does Matter
Meteor Crater impactor 50-60 m.
Chicxulub Crater impactor 10-15 km.
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350-400 km diameter:
- could vaporize entire oceans
- raise surface temperatures by 2000˚C
- sterilize Earth’s surface
- could have been a couple
Source of temperature increase:
- heat of impact
- vaporization of rock (radiates heat into the atmosphere)
- vaporized water absorbs sunlight and heats the atmosphere
150-190 km diameter:
- vaporized top few hundred meters of the oceans
- killed most things except those in deep oceans or deep subsurface
- probably several/many
Large Impactors Through Time
High energy impactors
likely hit during the Hadean
era.
Early Atmosphere and Oceans
If there was an early atmosphere, it was likely lost from
the large number of early impacts.
Planet too small to capture H and He gas from the nebula.
Formed in a region with few volatiles.
Planetesimals and comets from farther out of the solar system
also impacted Earth.
Some gases mixed in with the mantle.
Impact Delivery of Volatiles
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Most comets originate from the Oort cloud.
Highly elliptical orbits.
Composition:
water, dust, methane, ammonia, CO2.
Small percent of organic carbon.
Stardust mission:
caught comet dust from Wild2 in aerogel
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Comet West, 1975
Outgassing of Volatiles
Magmas nearing the surface undergo degassing.
Gases: water, CO2, CH4, N2, NH3, H2S, SO2.
Hydrothermal vents and hot springs also contain magmatic gases.
Water vapor cooled, then formed the oceans.
Difficult to determine how much came from comets, how much from
outgassing.
Also don’t know what early atmosphere was like - no oxygen.
Faint Young Sun
Sun has been getting brighter with time.
Sun today 30% brighter than when solar system formed.
Early Earth received less sunlight.
Atmosphere not warmed as much as it is today.
Faint Young Sun Problem:
Faint Sun yet had early liquid oceans.
Have some form of crust.
Thought plate tectonics were much different.
Lecture 8. The Archean Earth, the Greenhouse Effect
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reading: Chapter 4