Transcript EPSC233ArcheanPart1

Week 2:
When the world was young...
EPSC233-001 Earth & Life History (Fall 2002)
Recommended reading:
STANLEY “Earth System History”
Chapter 11.
Keywords: Archean eon (4.6-2.5 billion
years ago), nebula, planets, meteorites,
comets, differentiation, oceanic crust,
continental crust, plate tectonics, mantle
convection.
Geologists seeking evidence of the oldest
rocks on earth are up against large odds...
... our
planet is a
huge
recycling
engine.
Our solar system
started with the
explosion of a
supernova, the
dying stage of a
star.
Clouds of gas and
dust were
compressed by
the shock wave,
giving rise to a
solar nebula.
1) a slowly rotating cloud of
gases called a “nebula”
starts to collapse
(gravitational attraction);
2) the cloud continues to
condense and flattens, a
gaseous protostar and
solid planetesimals grow;
3) the sun lights up, solar
wind vapourizes dust and
blows it away
4) the nebula “clears up”: a
solar system is born.
Dust accreted into larger bodies.
Planets which grew from planetesimals which condensed
close to the sun are rocky, those far from the sun are
gas- and ice-rich
http://nssdc.gsfc.nasa.gov/planetary/image/near_mathilde3.jpg
What did not grow
into planets became
asteroids (rocky)...
Or comets (dirty
snowballs)
http://www.jpl.nasa.gov/comet/hyakutake/gif/burger2.jpg
Dates from about 70 meteorites
Allende meteorite (Mexico): 4.56 B.y. old,
sample of the stuff from the sun (- H, He).
Impacts and radioactive decay
led to melting & differentiation
Around 4.5-4.4 Ga, an impact with a Mars-sized body had a
great effect on Earth’s history. All volatiles were lost
from Earth, and the Moon
formed from the impactor
and terrestrial debris.
Lunar basalts on the moon
Apollo 17, oldest
lunar basalt samples
4.55  0.1 b.y. and
4.60  0.1 b.y.
Initially the Moon was
hot-- covered by a
magma ocean.
But, the Moon is small
and cooled rapidly.
It now has no volcanic
activity and is both
biologically and
geologically lifeless.
The Earth, bigger, is
cooling more slowly and
remains geologically active.
Crust showing scars of
early meteoritic impacts
has been recycled.
Manicouagan crater
~100 km diam (200 m.y. ago).
Why is there a difference?
Earth is a geologically active planet, the Moon is not.
Top few hundred km of Earth are broken into tectonic plates
which are constantly being created and destroyed.