ESC 102HistoricalGeology

Fall 2010

Spheres of the Earth

• When we view the Earth from space what Earth systems are observable?

• What is most obvious?• Are these systems independent or do they

interact with one another?

Spheres of the Earth

• Lithosphere: Earth’s solid rocky mass• Hydrosphere: All of earth’s water• Atmosphere: The thin gaseous layer

above Earth’s surface• Biosphere: All of earth’s life forms

Historical Geology: A study of the dynamic and evolving Earth

• Changes in its surface

• Changes in life

Three Themes dominate the story of the evolving Earth

• Solid Earth is composed of plates that move over Earth’s surface over time. This is explained by the Theory of Plate Tectonics

• Earth’s biota – all of living things – has evolved or changed through history and is explained by the Theory of Organic Evolution

• All of the geologic processes take place within an extensive geologic time scale spanning 4.6 billion years of Earth’s history

Historical Geologyapplies geologic principles to

help predict and explain Earth’s materials

• William Smith was an English surveyor who realized that rock types and fossils occur in repeated patterns. He was able to predict rock sequences that would be encountered in constructing canals

• Smith mapped the geology of much of England. (1815)

This map took many years to complete and helped establish the geologic time scale.

Slide 12 Fig 1-9, p. 12

Hypothesis or Theory?

• The scientific method brings an orderly and logical approach to decoding geologic evidence.

• A hypothesis is a tentative explanation for observations

• Scientists make predictions using hypotheses – then they are tested

• After repeated testing, a theory may be proposed• Some phenomena cannot be tested or explained

A theory is formed

• A theory is more than an “educated guess”• A theory explains natural phenomena and

may relate several observations• A theory is well-tested and well-supported

by objective evidence• Examples include the Plate Tectonics

Theory and the Theory of Organic Evolution

Where do scientists look for evidence for the following?

• The origin and age of the universe

• The origin and age of the solar system

• The origin and age of the Earth and Moon

• The origin of life on Earth

• Evidence of plate movement on Earth

• Explanation for large scale extinctions on Earth

How old is the universe?• When? Scientists believe the universe was

formed about 15 billion years ago • How? The Big Bang is a model for the

“beginning” of the universe• “Show me”! What is the evidence?

Evidence of the Big Bang• Pervasive background radiation of 2.7o

above absolute zero is observed in space(-273o C or -460o F)--Afterglow of the Big Bang

Galaxies moving away – expanding universeDetermine the Age of the

Universe*Determine rate of expansion*Backmodel to a time when the galaxies would be together in space

Big Bang hypothesis

• Initial state: NO time, NO matter, NO space• Universe was pure ENERGY• During the FIRST second of time:

--very dense matter came into existence--The four basic forces separated:

gravity, electromagnetic force, strong and weak nuclear forces

--Enormous expansion occurred

Big Bang Model

• 300,000 years later:– Atoms of hydrogen and helium formed– Light (photons) burst forth for the first time

• Next 200 million years:– Continued expansion– Stars and galaxies began to form– Elements heavier than hydrogen and helium began

to form with stars by nuclear fusion

Origin of Our Solar System

Solar nebula theory

• formed a rotating disk

• condensed and collapsed due to gravity

• forming solar nebula – with an embryonic Sun – surrounded by a rotating cloud

• cloud of gases and dust

• Planetesimals have formed – in the inner solar system,

– and large eddies of gas and dust – remain far from the protosun

Embryonic Sun and Rotating Cloud

The planets formed with distinctorbits around the sun.Some planets have satellites which orbit individual planets.

The Planets

• Terrestrial• Mercury• Venus• Earth• MarsSmall in size.Composed of rock.Metallic cores.

Asteroid Belt

• Jovian• Jupiter• Saturn• Uranus• Neptune• Large in size.• Composed of hydrogen,

helium, ammonia, methane

• Small rocky cores• Pluto: no longer

has planet status

Relative Sizes of the Sun and Planets

Earth’s Very Early History

• Started out cool about 4.6 billion years ago– probably with uniform composition/density

• Mostly:– silicate compounds– iron and magnesium oxides

• Temperature increased. Heat sources:– meteorite impacts– gravitational compression– radioactive decay

• Heated up enough to melt iron and nickel

The earth and moon were heavily bombarded in Earth’s early history.

Earth’s Differentiation

• Differentiation = segregated into layers of differing composition and density

• Early Earth was probably uniform

• After heating, molten iron and nickel sank to form the core

• Lighter silicates flowed up to form mantle and crust

Forming the Moon

• Impact by Mars-sized planetesimal with early Earth

• 4.6 to 4.4 billion years ago

• Ejected large quantity of hot material

• Formed the moon

Slide 8 Fig 1-5, p. 9

Most of the lunar materialcame from the mantle of the colliding planetesimal

The material cooled andcrystallized into lunar layers

Moon is smaller than Earth and cooled quickly.Light colored surface areas are lunarHighlands – heavily cratered.Evidence of massive meteorite BombardmentMare are areas of lava flows, more likely due to impact than tectonics

Earth—Dynamic Planet

• Earth was also subjected – to the same meteorite barrage – that pock-marked the Moon

• Why isn’t Earth’s surface also densely cratered?

Earth—Dynamic Planet

• Earth was also subjected – to the same meteorite barrage – that pock-marked the Moon

• Why isn’t Earth’s surface also densely cratered?– Because Earth is a dynamic and evolving planet– Craters have long since been worn away

Earth’s Interior Layers• Lithosphere

– solid upper mantle and crust

• Asthenosphere– part of upper

mantle– behaves plastically

and slowly flows

– broken into plates that move over the asthenosphere

Plate Tectonic Theory• Lithosphere is broken into individual pieces

called plates

• Plates move over the asthenosphere – as a result of underlying convection cells

Plate Tectonic Theory

• Movement at plate boundaries – plates diverge– plates converge– plates slide sideways past each other

• At plate boundaries– Volcanic activity occurs– Earthquakes occur

Modern Plate MapActive tectonic boundaries are in red

Plate Tectonic Theory

After decades of puzzling evidence, the theory was developed in the1960s

• Provides a framework for – interpreting many aspects of Earth on a global scale– relating many seemingly unrelated phenomena– interpreting Earth history

The “unifying theory of geology”

Plate Tectonics and Earth Systems

Mechanism: Plate tectonics is driven by convection in the mantle

and in turn drives mountain building and associated igneous and metamorphic activity

Global effects of plate movement:Arrangement of continents affects

solar heating and cooling, winds and weather systemsRapid plate spreading and hot-spot activity may release volcanic carbon dioxide and affect global climate

History of Earth

• The history of the early earth through the present is revealed mainly in the rock and fossil records.

• By applying principles of formation and determining environments from life forms, early interpretations about Earth’s land masses and oceans have been made

Theory of Organic Evolution• Provides a framework for understanding the

history of life• Darwin’s

– On the Origin of Species by Means of Natural Selection, published in 1859,

– revolutionized biology– Provided the mechanism of natural selection

Central Thesis of Evolution

• All present-day organisms – are related and descended from organisms that

lived during the past• Natural selection is the mechanism

– that accounts for evolution • Natural selection results in the survival

– to reproductive age of those organisms – best adapted to their environment

History of Life• The fossil record provides perhaps

– the most compelling evidence – in favor of evolution

• Fossils are the remains or traces – of once-living organisms

• Fossils demonstrate that Earth – has a history of life

Geologic Time

• From the human perspective time units are in– seconds, hours, days, years

• Ancient human history– hundreds or even thousands of years

• Geologic history– millions, hundreds of millions, billions of years

Geologic Time Scale

• 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 Earth’s biota through time

• The time scale was subsequently dated in years – using radiometric dating techniques

Geologic Time Scale

Uniformitarianism: The Present is the key to the past• Uniformitarianism is a cornerstone of geology

– is based on the premise that present-day processes – have operated throughout geologic time

• The physical and chemical laws of nature – have remained the same through time

• To interpret geologic events – from evidence preserved in rocks – we must first understand present-day processes – and their results– Rates and intensities of geologic processes – may have changed with time