ESC 102 Historical Geology 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 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?

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 universe Determine 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: Next 200 million years: 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 cloud of gases and dust formed a rotating disk condensed and collapsed due to gravity forming solar nebula with an embryonic Sun surrounded by a rotating cloud

Embryonic Sun and Rotating Cloud Planetesimals have formed in the inner solar system, and large eddies of gas and dust remain far from the protosun

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

The Planets Terrestrial Jovian Mercury Venus Earth Mars Asteroid Belt 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: no longer has planet status

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

Most of the lunar material came from the mantle of the colliding planetesimal The material cooled and crystallized into lunar layers Moon is smaller than Earth and cooled quickly. Light colored surface areas are lunar Highlands – heavily cratered. Evidence of massive meteorite Bombardment Mare 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 broken into plates that move over the asthenosphere Asthenosphere part of upper mantle behaves plastically and slowly flows

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 At plate boundaries plates diverge plates converge plates slide sideways past each other At plate boundaries Volcanic activity occurs Earthquakes occur

Modern Plate Map Active 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 systems Rapid 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