ES 16HistoricalGeology

Spring 2008

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

Gases from respiration

Transport of seeds and spores

Atmosphere

Biosphere

Slide 2 Fig. 1-1, p. 3

Wind erosion, transport of water vapor for

precipitation

Mountainsdivert air

movements

Atmosphere

Lithosphere

Source of sediment and dissolved

material

Water and glacial

erosion, solution of minerals

HydrosphereLithosphere

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.

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?

Edwin Hubble and Albert Einstein: contemporaries in time and space

•

Birth of a modern hypothesis

• Hubble, an Oxford Rhodes scholar, a former lawyer, and boxer, with a PhD in astronomy, was an infantry soldier in WW I.

• After the war he went to work at Mt. Wilson observatory using the 100” Hooker Telescope.

• Here he observed the existence of galaxies outside of the Milky Way

A new view of the Universe

• Hubble used light, which travels over finite time from one part of the universe to another, to measure distance to stars outside of the Milky Way.

• By studying variable stars with predictable cycles, he discovered that objects are moving uniformly away from our Galaxy

• He used the red shift of stellar spectra to measure distances and velocities of deep space objects

• Hubble concluded that the universe is expanding and that distant stars and galaxies are moving away from the Milky Way.

•

• A few years earlier, Einstein had predicted the universe would be found to be expanding.

• He proposed his theory of relativity and proposed that the objects in the universe are not moving apart; rather space is expanding and the objects are carried along

•

The effects of gravity on light

Hubble’s Constant quantifies the movement Using predictable variable stars, along with redshift observations,

Hubble predicted that a galaxy twice as far away from us is receding twice as fast.

• This was predicted by Einstein’s theory of relativity.

Hubble’s constant was tested and continues to be refined today as the ability to observe the outer reaches of the universe improves.

Other evidence of the Big Bang

• Pervasive background radiation of 2.7o

above absolute zero is observed in space

--Afterglow of the Big Bang

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

• 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 ?????????

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

Earth’s Differentiation

• Differentiation = segregated into layers of differing composition and density

• Early Earth was probably uniform

• Molten iron and nickel sank to form the core

• Lighter silicates flowed up to form mantle and crust

Forming the Earth-Moon System

• 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

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

The material cooled andCrystallized into lunar layers

Light colored surface areas are lunarHighlands – heavily cratered.Evidence of massive meteorite BombardmentMare are areas of lava flows

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

• Crust - 5-90 km thick– continental and

oceanic

• Mantle– composed largely

of peridotite– dark, dense

igneous rock – rich in iron and

magnesium• Core

– iron and a small amount of nickel

Earth’s Interior Layers

• Crust - 5-90 km thick– continental and

oceanic

• Mantle– composed largely

of peridotite– dark, dense

igneous rock – rich in iron and

magnesium• Core

– iron and a small amount of nickel

• Lithosphere– solid upper mantle

and crust

• Asthenosphere– part of upper

mantle

– behaves plastically and slowly flows

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

Earth’s Crust• outermost layer• continental (20-90 km thick)

– density 2.7 g/cm3 – contains Si, Al

• oceanic (5-10 km thick)– density 3.0 g/cm3 – composed of basalt

Plate Tectonic Theory

• Lithosphere is broken into individual pieces called plates

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

Modern Plate Map

Plate Tectonic Theory

• At plate boundaries– Volcanic activity occurs– Earthquakes occur

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

Plate Tectonic Theory

• Types of plate boundaries

Divergent plate boundary

Divergent plate boundaryMid-oceanic

ridgeTransform plate boundary

Continental-continental convergent plate boundary

Continental-oceanic convergent plate boundary

Oceanic-oceanic convergent plate boundary

Trench

Plate Tectonic Theory

Influence on geological sciences:

• Revolutionary concept– major milestone

• comparable to Darwin’s theory of evolution in biology

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

Plate Tectonics and Earth Systems

Plate tectonics is driven by convection in the mantle and in turn drives mountain building and associated igneous and metamorphic activityS

olid

Ear

thA

tmos

pher

e Arrangement of continents affects solar heating and cooling, and thus winds and weather systemsRapid plate spreading and hot-spot activity may release volcanic carbon dioxide and affect global climate

Plate Tectonics and Earth Systems

Continental arrangement affects ocean currentsRate of spreading affects volume

of mid-oceanic ridges and hence sea levelPlacement of continents may contribute

to the onset of ice agesHyd

rosp

here

Bio

sphe

re Movement of continents creates corridors or barriers to migration, the creation of ecological niches, and transport of habitats into

more or less favorable climates

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

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