Unfinished Earth

History and modern continuation of planetary accretion andThe Origin of Crust

Summary: Nebular Hypothesis

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

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 Earth’s lithosphere (crust +uppermost (crust +uppermost mantle) is divided into mantle) is divided into platesplates

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 andthe Origin of Crust

• Direct link with formation of oceanic crust (sea floor spreading)

• Less obvious connections with continental crust (continental accretion)

Plate Tectonics andthe 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