Historical Geology: Evolution of the Earth and Life Through Time 6th edition Reed Wicander and James S. Monroe

The Dynamic and Evolving Earth Chapter 1 The Dynamic and Evolving Earth

The Movie of Earth’s History What kind of movie would we have 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 incredible special effects a cast of trillions a plot with twists and turns and an ending that is still a mystery! 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 its surface ocean basins opened and closed mountain ranges formed along continental margins or where continents collided The oceans and atmospheric circulation patterns would shift in response to moving continents causing massive ice sheets to form, 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 Images from left to right: Changes in its surface Artist’s rendition of how Earth is thought to have appeared about 4.6 billion years ago. Paleogeography of the world for the Late Cambrian Period. Apollo 17 view of Earth. Africa, Arabian Peninsula, Madagascar, Antarctica, South Atlantic Ocean , Indian Ocean. Changes in life Possible precursor of life: bacterium-like proteinoid. Reconstruction of Middle Devonian reef from the Great Lakes area Shown are corals, ammonoids, trilobites and brachiopods. Cretaceous Pteranodon.

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-over says, “To be continued.”

The Movie’s Theme Every good movie has a theme, and The History of Earth is no exception. The major theme is that Earth is complex and dynamic Three interrelated themes sub-themes run throughout this epic: 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 Movie’s Theme The second is that Earth’s biota has evolved or changed throughout its history Organic evolution The third is that physical and biological changes have occurred over long periods of time Geologic or Deep Time These three interrelated themes are central to our understanding and appreciation of our planet’s history.

Earth’s Very Early History About 4.6 billion years ago, early Earth was probably cool with uniform composition/density Composed mostly of silicates, and iron and magnesium oxides The temperature increased because of meteorite impacts gravitational compression radioactive decay Iron and nickel melted and Earth’s homogeneous composition disappeared

Earth’s Differentiation Differentiation = segregated into a series of concentric layers of differing composition and density Molten iron and nickel sank to form the core Lighter silicates flowed up to form mantle and crust

Earth—Dynamic Planet Earth is a dynamic planet The size, shape, and geographic distribution of continents and ocean basins have changed through time The composition of the atmosphere has evolved Life-forms existing today differ from those that lived in the past

Geologic Time: Concepts and Principles Chapter 4 Geologic Time: Concepts and Principles

Grand Canyon When looking down into the Grand Canyon, we are really looking at the early history of Earth

Grand Canyon More than 1 billion years of history are preserved, like pages of a book, in the rock layers of the Grand Canyon Reading this rock book we learn that the area underwent episodes of mountain building advancing and retreating shallow seas We know these things by applying the principles of relative dating to the rocks and recognizing that present-day processes have operated throughout Earth history

What is time? We are obsessed with time, and organize our lives around it. Most of us feel we don’t have enough of it. Our common time units are seconds hours days weeks months years Ancient history involves hundreds of years thousands of years But geologic time involves millions of years even billions of years

Concept of Geologic Time Geologists use two different frames of reference when discussing geologic time Relative dating involves placing geologic events in a sequential order as determined from their position in the geologic record It does not tell us how long ago a particular event occurred, only that one event preceded another For hundreds of years geologists have been using relative dating to establish a relative geologic time scale

Relative Geologic Time Scale The relative geologic time scale has a sequence of eons eras periods epochs

Concept of Geologic Time The second frame of reference for geologic time is absolute dating Absolute dating results in specific dates for rock units or events expressed in years before the present It tells us how long ago a particular event occurred giving us numerical information about time Radiometric dating is the most common method of obtaining absolute ages Such dates are calculated from the natural rates of decay of various natural radioactive elements present in trace amounts in some rocks

Geologic Time Scale The discovery of radioactivity near the end of the 19th century allowed absolute ages to be accurately applied to the relative geologic time scale The geologic time scale is a dual scale a relative scale and an absolute scale

Changes in the Concept of Geologic Time The concept and measurement of geologic time have changed throughout human history Early Christian theologians conceived of time as linear rather than circular James Ussher (1581-1665) in Ireland calculated the age of Earth based on Old Testament genealogy He announced that Earth was created on October 22, 4004 B.C. For nearly a century, it was considered heresy to say Earth was more than about 6000 years old.

Changes in the Concept of Geologic Time During the 1700s and 1800s Earth’s age was estimated scientifically Georges Louis de Buffon (1707-1788) calculated how long Earth took to cool gradually from a molten beginning using melted iron balls of various diameters. Extrapolating their cooling rate to an Earth-sized ball, he estimated Earth was 75,000 years old

Changes in the Concept of Geologic Time Others used different techniques Scholars using rates of deposition of various sediments and total thickness of sedimentary rock in the crust produced estimates of less than 1 million to more than 2 billion years. John Joly used the amount of salt carried by rivers to the ocean and the salinity of seawater and obtained a minimum age of 90 million years

Relative-Dating Principles Six fundamental geologic principles are used in relative dating Principle of superposition Nicolas Steno (1638-1686) In an undisturbed succession of sedimentary rock layers, the oldest layer is at the bottom and the youngest layer is at the top This method is used for determining the relative age of rock layers (strata) and the fossils they contain

Relative-Dating Principles Principle of original horizontality Nicolas Steno Sediment is deposited in essentially horizontal layers Therefore, a sequence of sedimentary rock layers that is steeply inclined from horizontal must have been tilted after deposition and lithification

Principle of Superposition Illustration of the principles of superposition Superposition: The youngest rocks are at the top of the outcrop and the oldest rocks are at the bottom

Principle of Original Horizontality Horizontality: These sediments were originally deposited horizontally in a marine environment

Relative-Dating Principles Principle of lateral continuity Nicolas Steno’s third principle Sediment extends laterally in all direction until it thins and pinches out or terminates against the edges of the depositional basin Principle of cross-cutting relationships James Hutton (1726-1797) An igneous intrusion or a fault must be younger than the rocks it intrudes or displaces

Cross-cutting Relationships North shore of Lake Superior, Ontario Canada A dark-colored dike has intruded into older light colored granite. The dike is younger than the granite.

Cross-cutting Relationships Templin Highway, Castaic, California A small fault displaces tilted beds. The fault is younger than the beds.

Relative-Dating Principles Other principles of relative dating Principle of inclusions Principle of fossil succession are discussed later in the text

Neptunism Neptunism Werner was an excellent mineralogist, All rocks, including granite and basalt, were precipitated in an orderly sequence from a primeval, worldwide ocean. proposed in 1787 by Abraham Werner (1749-1817) Werner was an excellent mineralogist, but is best remembered for his incorrect interpretation of Earth history

Neptunism Werner’s geologic column was widely accepted Alluvial rocks unconsolidated sediments, youngest Secondary rocks rocks such as sandstones, limestones, coal, basalt Transition rocks chemical and detrital rocks, some fossiliferous rocks Primitive rocks oldest including igneous and metamorphic

Catastrophism Catastrophism concept proposed by Georges Cuvier (1769-1832) dominated European geologic thinking The physical and biological history of Earth resulted from a series of sudden widespread catastrophes which accounted for significant and rapid changes in Earth and exterminated existing life in the affected area Six major catastrophes occurred, corresponding to the six days of biblical creation The last one was the biblical deluge

Neptunism and Catastrophism These hypotheses were abandoned because they were not supported by field evidence Basalt was shown to be of igneous origin Volcanic rocks interbedded with sedimentary and primitive rocks showed that igneous activity had occurred throughout geologic time More than 6 catastrophes were needed to explain field observations The principle of uniformitarianism became the guiding philosophy of geology

Uniformitarianism Principle of uniformitarianism Present-day processes have operated throughout geologic time. Developed by James Hutton (1726-1797), advocated by Charles Lyell (1797-1875) William Whewell coined the term “uniformitarianism” in 1832 Hutton applied the principle of uniformitarianism when interpreting rocks at Siccar Point, Scotland We now call what Hutton observed an unconformity, but he properly interpreted its formation

Unconformity at Siccar Point Hutton explained that the tilted, lower rocks resulted from severe upheavals that formed mountains these were then worn away and covered by younger flat-lying rocks the erosional surface represents a gap in the rock record

Uniformitarianism Hutton viewed Earth history as cyclical erosion erosion Hutton viewed Earth history as cyclical deposition uplift He also understood that geologic processes operate over a vast amount of time Modern view of uniformitarianism Today, geologists assume that the principles or laws of nature are constant but the rates and intensities of change have varied through time Some geologists prefer the term “actualism”

Crisis in Geology Lord Kelvin (1824-1907) knew about high temperatures inside of deep mines and reasoned that Earth was losing heat from its interior Assuming Earth was once molten, he used the melting temperature of rocks the size of Earth and the rate of heat loss to calculate the age of Earth as between 400 and 20 million years

Crisis in Geology This age was too young for the geologic processes envisioned by other geologists at that time, leading to a crisis in geology Kelvin did not know about radioactivity as a heat source within the Earth

Absolute-Dating Methods The discovery of radioactivity destroyed Kelvin’s argument for the age of Earth and provided a clock to measure Earth’s age Radioactivity is the spontaneous decay of an element to a more stable isotope The heat from radioactivity helps explain why the Earth is still warm inside Radioactivity provides geologists with a powerful tool to measure absolute ages of rocks and past geologic events

Atoms: A Review Understanding absolute dating requires knowledge of atoms and isotopes All matter is made up of atoms The nucleus of an atom is composed of protons – particles with a positive electrical charge neutrons – electrically neutral particles with electrons – negatively charged particles – outside the nucleus The number of protons (= the atomic number) helps determine the atom’s chemical properties and the element to which it belongs

Isotopes: A Review Atomic mass number = number of protons + number of neutrons The different forms of an element’s atoms with varying numbers of neutrons are called isotopes Different isotopes of the same element have different atomic mass numbers but behave the same chemically Most isotopes are stable, but some are unstable Geologists use decay rates of unstable isotopes to determine absolute ages of rocks

Radioactive Decay Radioactive decay is the process whereby an unstable atomic nucleus spontaneously transforms into an atomic nucleus of a different element

Half-Lives The half-life of a radioactive isotope is the time it takes for one half of the atoms of the original unstable parent isotope to decay to atoms of a new more stable daughter isotope The half-life of a specific radioactive isotope is constant and can be precisely measured

Half-Lives The length of half-lives for different isotopes of different elements can vary from less than one billionth of a second to 49 billion years! Radioactive decay is geometric, NOT linear, and produces a curved graph

Uniform Linear Change In this example of uniform linear change, water is dripping into a glass at a constant rate

Geometric Radioactive Decay In radioactive decay, during each equal time unit half-life the proportion of parent atoms decreases by 1/2

Determining Age By measuring the parent/daughter ratio and knowing the half-life of the parent which has been determined in the laboratory geologists can calculate the age of a sample containing the radioactive element The parent/daughter ratio is usually determined by a mass spectrometer an instrument that measures the proportions of atoms with different masses

Determining Age Example: how old is the rock? If a rock has a parent/daughter ratio of 1:3 or a ratio of (parent)/(parent + daughter) = 1:4 or 25%, and the half-live is 57 million years, how old is the rock? 25% means it is 2 half-lives old. the rock is 57my x 2 =114 million years old.

What Materials Can Be Dated? Most radiometric dates are obtained from igneous rocks As magma cools and crystallizes, radioactive parent atoms separate from previously formed daughter atoms Because they are the right size some radioactive parents are included in the crystal structure of cooling minerals

What Materials Can Be Dated? The daughter atoms are different elements with different sizes and, therefore, do not generally fit into the same minerals as the parents Geologists can use the crystals containing the parent atoms to date the time of crystallization

Igneous Crystallization Crystallization of magma separates parent atoms from previously formed daughters This resets the radiometric clock to zero. Then the parents gradually decay.

Sedimentary Rocks Generally, sedimentary rocks can NOT be radiometrically dated The date obtained would correspond to the time of crystallization of the mineral, when it formed in an igneous or metamorphic rock, and NOT the time that it was deposited as a sedimentary particle Exception: The mineral glauconite can be dated because it forms in certain marine environments as a reaction with clay minerals during the formation of the sedimentary rock

Dating Metamorphism a. A mineral has just crystallized from magma. b. As time passes, parent atoms decay to daughters. c. Metamorphism drives the daughters out of the mineral as it recrystallizes. d. Dating the mineral today yields a date of 350 million years = time of metamorphism, provided the system remains closed during that time. Dating the whole rock yields a date of 700 million years = time of crystallization.

Long-Lived Radioactive Isotope Pairs Used in Dating The isotopes used in radiometric dating need to be sufficiently long-lived so the amount of parent material left is measurable Such isotopes include: Parents Daughters Half-Life (years) Most of these are useful for dating older rocks Uranium 238 Lead 206 4.5 billion Uranium 234 Lead 207 704 million Thorium 232 Lead 208 14 billion Rubidium 87 Strontium 87 48.8 billion Potassium 40 Argon 40 1.3 billion

Theory of Organic Evolution Provides a framework for understanding the history of life Charles 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 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 seconds, hours, days, years Ancient human history hundreds or thousands of years ago Geologic history millions, hundreds of millions, billions of years

Geologic Time Scale Resulted from the work of many 19th century geologists who gathered information from numerous rock exposures, and constructed a sequential chronology based on changes in Earth’s biota through time Ages subsequently were assigned to the time scale using radiometric dating techniques

Geologic Time Scale

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 By studying what has happened in the past on a global scale, and try to 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

Earth’s Interior Layers Crust Continental (20-90 km thick) Oceanic (5-10 km thick) Mantle 83% volume composed largely of peridotite dark, dense igneous rock, rich in iron and magnesium Core Solid inner region, liquid outer region iron and a small amount of nickel

Earth’s Interior Layers Lithosphere solid upper mantle and crust Crust Continental (20-90 km thick) Oceanic (5-10 km thick) Mantle 83% volume composed largely of peridotite dark, dense igneous rock, rich in iron and magnesium Asthenosphere part of upper mantle behaves plastically and slowly flows Core Solid inner region, liquid outer region iron and a small amount of nickel

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

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 and gabbro

Plate Tectonic Theory Lithosphere is broken into individual pieces or plates Plates move over the asthenosphere as a result of underlying convection cells

Modern Plate Map

Plate Tectonic Theory Plate boundaries are marked by Volcanic activity Earthquake activity At plate boundaries plates diverge, plates converge, plates slide sideways past each other

Plate Tectonic Theory Types of plate boundaries

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 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 Atmosphere

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 Movement of continents creates corridors or barriers to migration, the creation of ecological niches, and transport of habitats into more or less favorable climates Biosphere

Next time: Chapter 3: Plate Tectonics