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 • 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 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 matteris 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 • Three types of radioactive decay: • In alpha decay, two protons and two neutrons • (alpha particle) are emitted from the nucleus.
Radioactive Decay • In beta decay, a neutron emits a fast moving electron (beta particle) and becomes a proton. • In electron capture decay, a proton captures an electron and converts to a neutron.
Radioactive Decay • Some isotopes undergo only one decay step before they become stable. • Examples: • rubidium 87 decays to strontium 87 by a single beta emission • potassium 40 decays to argon 40 by a single electron capture • But other isotopes undergo several decay steps • Examples: • uranium 235 decays to lead 207 by 7 alpha steps and 6 beta steps • uranium 238 decays to lead 206 by 8 alpha steps and 6 beta steps
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: • 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
Sources of Uncertainty • In glauconite, potassium 40 decays to argon 40 • Because argon is a gas, • it can easily escape from a mineral • A closed system is needed for an accurate date! • Neither parent nor daughter atoms • can have been added or removed • from the sample since crystallization • If leakage of daughters has occurred, • this partially resets the radiometric clock • and the age of the rock will show to be too young • If parents escape, the date obtained will be too old. • The most reliable dates use multiple methods.
Sources of Uncertainty • During metamorphism, some of the daughter or parent atoms may escape • leading to a date that is inaccurate. • However, if all of the daughters are forced out during metamorphism, • then the date obtained would be the time of metamorphism—a useful piece of information. • Dating techniques are always improving. • Presently measurement error is typically <0.5% of the age, and in some cases, better than 0.1% • A date of 540 million might have an error of ±2.7 million years, or as low as ±0.54 million
Dating Metamorphism b. As time passes, parent atoms decay to daughters. a. A mineral has just crystallized from magma. 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
Fission Track Dating • Atomic particles in uranium • will damage crystal structure as uranium decays • The damage can be seen as fission tracks • under a microscope after etching the mineral • The age of the sample is related to • the number of fission tracks • and the amount of uranium • with older samples having more tracks • This method is useful for samples between 40,000 years and 1.5 million years old