Unfinished Earth History and modern continuation of planetary accretion and The Origin of Crust

Earth’s Early History Hadean Eon – Earth’s first 500 Ma Intense bombardment by bolides (meteors, comets, etc.) No permanent crust – destroyed by impacts Little direct evidence on Earth Excellent evidence on Moon, Mars and other terrestrial planets

Craters on the Moon – Hadean and later bombardment

Earth – the water planet

Presently, Oceans cover 71% of Earth’s surface Odds of impact on land are 29% Water and oxygen weather rocks and destroy evidence of impacts

Crater Preservation Arid climate is more likely to preserve craters than humid climate Less weathering Less erosion Australian “Outback” Quebec, Canada

Location of Craters on Earth

Craters in North America

Gene and Carolyn Shoemaker Gene Shoemaker, geologist Pioneer in study of impact structures worldwide Studied Berringer Crater, AZ Well-respected in geology, but not in astronomy Until Shoemaker-Levy 9!

Berringer Crater, Arizona Young impact structure Approximately 40,000 years old Arid climate – excellent preservation Remnants of meteorite found in crater and in ejecta

Planetary Accretion Today: Shoemaker-Levy 9

Significance of Shoemaker-Levy 9 Convinced astronomers that such events were still possible Focused attention on near-Earth objects Showed how little we know of such objects Showed how completely unprepared we are

A Focus on Earth

Why the Gap: 4.6 – 3.98 Ga? Asteroids, meteorites, comets, planetesimals collide as Earth accretes to form homogeneous protoplanet Intense bombardment during continued accretion destroyed primeval crust (if there was any) Inertial heating from impacts and radioactive decay may have liquified the entire planet This afforded mobility for the start of density differentiation

Earth’s Layered Structure Gravity caused Earth’s interior to differentiate into layers Layers are arranged according to density Most dense material sank to the center – core Density decreases outward Density layering includes the hydrosphere (oceans) and atmosphere

Evidence for Earth’s Internal Structure Refraction of seismic waves Density of the entire planet Gravity (mass) of entire planet Composition of meteorites Existence of magnetic field

Earth: The Unfinished Planet Earth continues to lose heat Volcanism brings material to Earth’s surface Other processes (subduction) return more dense material to interior Conclusion: Earth is still under construction!

Theory of Plate Tectonics Plates move as a result of heat inside the Earth Plates interact to cause: –Earthquakes –Volcanos –Mountain systems Earth’s lithosphere (crust +uppermost mantle) is divided into plates Earth’s lithosphere (crust +uppermost mantle) is divided into plates

What makes the plates move? Uneven distribution of heat in the upper mantle (Asthenosphere) causes heat to rise in some places (e.g., Mid-ocean ridges) Differences in density cause colder, more dense Lithosphere to sink back into the asthenosphere (subduction) This Convection drives the motion of the plates

Plate Tectonics and the Origin of Crust Direct link with formation of oceanic crust (sea floor spreading) Less obvious connections with continental crust (continental accretion)

Plate Tectonics and the Origin of Crust Archean convection may have caused earliest crust to float to surface as “scum” Subduction returned more dense material to interior Less dense components were scraped off or returned to surface via volcanism Plate interactions added continental crust to margins of continents Quantity of continental crust has increased through time