Module 24Historical Geology

The Earth started out coolProbably with uniform composition/density mostly silicate compounds iron and magnesium oxidesTemperature increased with heat sources:meteorite impactsgravitational compressionradioactive decayHeated up enough to melt iron and nickelEARTHS VERY EARLY HISTORYAbout 4.6 billion years ago . . .

Differentiation = segregated into layers of differing composition and densityEarly Earth was probably uniformMolten iron and nickel sank to form the coreLighter silicates flowed up to form mantle and crust

EARTHS VERY EARLY HISTORYEarths differentiation

4.6 to 4.4 billion years agothe young Earth was impacted by Mars-sized or larger planetesimalejected large quantity of hot materialand . . . formed the Moon

EARTHS VERY EARLY HISTORYForming the Earth-Moon system

EARTHS VERY EARLY HISTORYForming the Earth-Moon system

Most of the lunar material came from the mantle of the colliding planetesimalThe material cooled and crystallized into lunar layersEARTHS VERY EARLY HISTORYMoon

EARTHS VERY EARLY HISTORYMoonLight-colored areas are lunar highlandsHeavily cratered provide striking evidence of massive meteorite bombardment

EARTHS VERY EARLY HISTORYEarthAtmosphereHydrosphereCMC-sphereBiosphere

CRUST continental-crust: granitic (sialic) composition oceanic-crust: basaltic (mafic) compositionMANTELcomposed largely of peridotitedark, dense igneous rockrich in iron and magnesiumCOREcomposed largely of iron and a small amount of nickelEARTHS VERY EARLY HISTORYCMC-sphere

From the human perspective, time units are inseconds, hours, days, yearsAncient human historyhundreds or even thousands of yearsGeologic historymillions, hundreds of millions, billions of yearsGEOLOGIC TIME

Resulted from the work of many 19th century geologists who pieced together information from numerous rock exposures,constructed a sequential chronology based on changes in Earths biota through timeThe time scale was subsequently dated in years using radiometric dating techniquesGEOLOGIC TIMEGeologic Time Scale

GEOLOGIC TIMEGeologic Time Scale

GEOLOGIC TIMEGeologic Time Scale and football field

100 million years ago60 million years ago30 million years ago5 million years agoThe history of plate movement during the last 200 million years has been reconstructed from all available geologic and geophysical data. These maps show the general directions of movement from the time Pangaea began to break up until the continents moved to their present positions150 million years agoThe history of plate movement

The history of organism

Universe began with a big bang 15 billion years agoSolar system formed 4.6 billion years ago by condensation and gravitational collapse of a rotating interstellar cloudEarth formed 4.6 billion years ago as a swirling eddy in the solar system nebulaMoon may have formed when a planetesimal collided with Earth 4.6 to 4.4 billion years agoEarth probably started solid then differentiated into layers as it heated and melted SUMMARY

Earths layers mostly solidified into the core, mantle and crust, with the upper mantle and crust making up the soft asthenosphere and the solid lithosphereLithosphere is broken into plates that diverge, converge and slide sideways past each otherPlate tectonics is a unifying theorythat helps explain features and events including volcanic eruptions, earthquakes and mountain formingSUMMARY

An appreciation of the immensity of geologic time is central to understanding Earths evolutionUniformitarianism holds that the laws of nature have been constant through timeGeology is part of our lives and our standard of living depends on our use of natural resources that formed over billions of yearsSUMMARY