ES 16 Historical Geology 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

Wind erosion, transport of water vapor for precipitation Mountains divert air movements Atmosphere Lithosphere

Source of sediment and dissolved material Water and glacial erosion, solution of minerals Hydrosphere Lithosphere

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 Geology applies 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 2.7 o 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 Mars Small 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 ?????????

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 material Came from the mantle of the colliding planetesimal The material cooled and Crystallized into lunar layers Light colored surface areas are lunar Highlands – heavily cratered. Evidence of massive meteorite Bombardment Mare 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 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 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/cm 3 –contains Si, Al oceanic (5-10 km thick) –density 3.0 g/cm 3 –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 Mid-oceanic ridge Transform 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 activity Solid Earth Atmosphere Arrangement of continents affects solar heating and cooling, and thus winds and weather systems Rapid plate spreading and hot-spot activity may release volcanic carbon dioxide and affect global climate

Plate Tectonics and Earth Systems Continental arrangement affects ocean currents Rate of spreading affects volume of mid-oceanic ridges and hence sea level Placement of continents may contribute to the onset of ice ages Hydrosphere Biosphere 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