Text: Historical Geology

Evolution of Earth and Life Through Time4th edition

by Wicander and Monroe

The Dynamic and Evolving Earth

Chapter 1

The Movie of Earth’s History• What kind of movie would we see

– if it were possible to travel back in time – and film Earth’s History– from its beginning 4.6 billion years ago?

• It would certainly be a story of epic proportions– with great special effect – and a cast of trillions– twists and turns in its plot– with an unknown ending

• Although we cannot travel back in time, – the Earth’s History is still preserved – in the geologic record

Subplot: Landscape History• In this movie we would see

– a planet undergoing remarkable change as – continents moved about– ocean basins opened– mountain ranges grew along continental margins – and where continents collided

• The oceans and atmosphere would – form and grow– change circulation patterns– cause massive ice sheets to form and grow – and then melt away

• Extensive swamps or vast interior deserts – would sweep across the landscape

Subplot: Life’s History

• We would also witness – the first living cells evolving– from a primordial organic soup – between 4.6 and 3.6 billion years ago

• Cell nuclei would evolve, – then multicelled soft-bodied animals– followed by animals with skeletons and then

backbones

• The barren landscape would come to life as– plants and animals moved from their watery home– insects, amphibians, reptiles, birds and mammals – would eventually evolve

Earth is a Dynamic and Evolving Planet

• Changes in its surface

• Changes in life

At the End of the Movie

• The movie’s final image is of Earth, – a shimmering blue-green oasis – in the black void of space– and a voice says,

• “To be continued.”

The Movie’s Theme

• Every good movie has a theme, – and “The History of Earth” is no exception

• Three interrelated themes run throughout it• The first is that Earth’s outermost part

– is composed of a series of moving plates• Plate tectonics

– whose interactions have affected its physical and biological history

• The second is that Earth’s biota – has evolved or changed throughout its history

• organic evolution

Earth is a System of Interconnected Subsystems

• Atmosphere (air and gases)• Hydrosphere (water and oceans)• Biosphere (plants and animals)• Lithosphere (Earth’s rocky surface)• Interior (mantle and core)

Interactions in Earth’s Subsystems

Gases from respiration

Transport of seeds and

spores

Atmosphere

Biosphere

Interactions in Earth’s Subsystems

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

Interactions in Earth’s Subsystems

HydrosphereLithosphere

This class is about historical geology

What is Geology?

• From the Greek – geo (Earth) logos (reason)

• Geology is the study of Earth

• Physical geology studies Earth materials,– such as minerals and rocks– as well as the processes operating within– and on Earth’s surface

Historical Geology

• In historical geology we study– changes in our dynamic planet– how and why past events happened– implication for today’s global ecosystems

• Principles of historical geology – not only aid in interpreting Earth’s history– but also have practical applications

• William Smith, an English surveyor/engineer– used study of rock sequences – to help predict the difficulty of excavation – in constructing canals

Scientific Method

• The scientific method – an orderly and logical approach– Gather and analyze facts or data

• A hypothesis is a tentative explanation – to explain observed phenomena

• Scientists make predictions using hypotheses– then they test the predictions

• After repeated tests, – if one hypothesis continues – to explain the phenomena, – scientists propose it as a theory

Formulation of Theories

Theory

• colloquial usage - speculation or conjecture

• scientific usage – coherent explanation for one or several related

natural phenomena – supported by a large body of objective evidence

Origin of the Universe

• The Big Bang – occurred 15 billion years ago– and is a model for the beginning of the universe

Evidence for the Big Bang

• Universe is expanding

• How do we determine the age?– measure the rate of expansion – backtrack to a time when the galaxies – were all together at a single point

• Pervasive background radiation of 2.7º above absolute zero – is the afterglow of the Big Bang

Big Bang Model

• Initial state:– No time, matter or space existed

• There is no “before the Big Bang”

– Universe consisted of pure energy

• During 1st second:– Very dense matter came into existence– The four basic forces separated

• gravity, electromagnetic force, 2 nuclear forces

– Enormous expansion occurred

Big Bang Model (cont.)

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

• During the next 200 million years:– Continued expansion and cooling– Stars and galaxies began to form– Elements heavier than hydrogen and helium – began to form within stars by nuclear fusion

Features of Our Solar System

• In a spiral arm of the Milky Way Galaxy• Sun• 9 planets• 101 known moons (satellites)• a tremendous number of asteroids

– most orbit the Sun between the orbits of Mars and Jupiter

• millions of comets and meteorites• interplanetary dust and gases

Relative Sizes of the Sun and Planets

Solar System Configuration

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 Planets– Mercury – Venus– Earth– Mars

• small, composed of rock, with metal cores

• Jovian Planets– Jupiter – Saturn– Uranus– Neptune

• large, composed of hydrogen, helium, ammonia, methane, relatively small rocky cores

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 or larger planetesimal with young Earth– 4.6 to 4.4 billion

years ago

– ejected large quantity of hot material,

– and formed the Moon

Forming the Earth-Moon System

• Most of the lunar material – came from the

mantle of the colliding planetesimal

• The material cooled – and crystallized – into lunar

layers

Forming the Earth-Moon System

• Most of the lunar material – came from the

mantle of the colliding planetesimal

• The material cooled – and crystallized – into lunar

layers

Moon

• Light-colored areas are lunar highlands

– Heavily cratered

• Provide striking evidence – of massive

meteorite bombardment

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• 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

How Does the Study of Historical Geology Benefit Us?

• Survival of the human species – depends on understanding – how Earth’s various subsystems – work and interact

• Study what has happened in the past, – on a global scale, – to try and determine how our actions – might affect the balance of subsystems in the future

We “Live” Geology

• Our standard of living depends directly on – our consumption of natural resources– resources that formed millions and billions of

years ago

• How we consume natural resources – and interact with the environment determines– our ability to pass on this standard of living – to the next generation

Summary

• Earth is a system – of interconnected subsystems

• Geology is the study of Earth• Historical geology is the study

– of the origin and evolution of Earth• Scientific method is

– an orderly, logical approach – to explain phenomena, – using data, – formulating and testing hypotheses and theories

• Universe began with – a big bang 15 billion years ago

Summary

• Solar system formed 4.6 billion years ago – by condensation and gravitational collapse – of a rotating interstellar cloud

• Earth formed 4.6 billion years ago – as a swirling eddy in the solar system nebula

• Moon may have formed – when a planetesimal collided with Earth – 4.6 to 4.4 billion years ago

• Earth probably started solid – then differentiated into layers – as it heated and melted

Summary

• Earth’s layers mostly solidified – into the core, mantle and crust, – with the upper mantle and crust – making up the soft asthenosphere – and the solid lithosphere

• Lithosphere is broken into plates – that diverge, converge and – slide sideways past each other

• Plate tectonics is a unifying theory– that helps explain features and events – including volcanic eruptions, – earthquakes and mountain forming

Summary

• Central thesis of organic evolution is – that all living organisms evolved – from organisms that existed in the past

• An appreciation – of the immensity of geologic time – is central to understanding Earth’s evolution

• Uniformitarianism holds that the laws – of nature have been constant through time

• Geology is part of our lives – and our standard of living depends – on our use of natural resources – that formed over billions of years