Week 2: When the world was young... EPSC Earth & Life History (Fall 2002)

Recommended reading: STANLEY “Earth System History” Chapter 11. Keywords: Archean eon ( 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

What did not grow into planets became asteroids (rocky)... Or comets (dirty snowballs)

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 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.

Manicouagan crater ~100 km diam (200 m.y. ago). The Earth, bigger, is cooling more slowly and remains geologically active. Crust showing scars of early meteoritic impacts has been recycled.

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.