Chapter 4 Geologic Time: Concepts and Principles Grand
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 dont 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 Earths 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 Stenos 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 Werners 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 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 Kelvins argument for the age of Earth and provided a clock to measure Earths 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 atoms 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 elements 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 Uranium 238 decay 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 halflives 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 metamorphisma 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 Dating the whole rock yields a date of 700 million years = time of crystallization. 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. 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) Uranium 238 Uranium 234 Thorium 232 Rubidium 87 Potassium 40
4.5 billion 704 million 14 billion 48.8 billion 1.3 billion Lead 206 Lead 207 Lead 208 Strontium 87 Argon 40 Most of these are useful for
dating older rocks 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 Radiocarbon Dating Method Carbon is found in all forms of life It has 3 isotopes carbon 12 and 13 are stable, but carbon 14 is not Carbon 14 has a half-life of 5730 years 30 years
Carbon 14 dating uses the carbon 14/carbon 12 ratio of material that was once living The short half-life of carbon 14 makes it suitable for dating material < 70,000 years old It is not useful for most rocks, but is useful for archaeology and young geologic materials Carbon 14 Carbon 14 is constantly forming in the upper atmosphere
When cosmic rays strike atoms of upper atmospheric gases, Splitting nuclei into protons and neutons When a neutron strikes a nitrogen 14 atom it may be absorbed by the nucleus and eject a proton changing it to carbon 14 Carbon 14 The carbon 14 becomes part of the natural carbon cycle
and becomes incorporated into organisms While the organism lives it continues to take in carbon 14, but when it dies the carbon 14 begins to decay without being replenished Thus, carbon 14 dating
measures the time of death Tree-Ring Dating Method The age of a tree can be determined by counting the annual growth rings in lower part of the stem (trunk) The pattern of wide and narrow rings can be correlated from tree to tree a procedure called cross-dating The tree-ring time scale now extends back 14,000 years Tree-Ring Dating Method
In cross-dating, tree-ring patterns are used from different trees, with overlapping life spans Geologic Time and Climate Change With current debates concerning global warming it is extremely important to reconstruct part regimes as accurately as possible Geologists must have an accurate and precise geologic calendar to model how Earths climate system has responded to past changes
Geologic Time and Climate Change Geologists use stalagmites from caves which are formed from calcium carbonate and rise from a cave floor Stalagmites record a layered history with older layers in the center at its base Geologic Time and Climate Change
Geologists can radiometrically date individual layers of stalagmites with Uranium 234-Thorium 230 methods Geologic Time and Climate Change History of stalagmites
from Crevice Cave, Missouri revealed a history of climatic and vegetation change in the midcontinent US 75,000 and 25.000 years ago These changes correlated with vegetation and average temperature fluctuations which were obtained from carbon 13 and oxygen 18 isotope profiles Geologic Time and Climate Change
Geologic Time and Climate Change Precise dating techniques Uranium 234Thorium 230 Allows geologists to model climate systems from the past Geologic Time and Climate Change By analyzing past environmental and climate changes and their duration
geologists hope to use data to predict and possibly modify regional climatic changes Summary Time is defined by the methods used to measure it. Relative dating places geologic events in sequential order as determined by their position in the geologic record Absolute dating provides specific dates for geologic rock units or events
expressed in years before present. Summary Early Christian theologians viewed time as linear and decided that Earth was very young (about 6000 years old) A variety of ages for Earth were estimated during the 18th and 19th centuries using scientific evidence, ages now known to be too young Neptunism and catastrophism were popular during the 17th, 18th and early 19th centuries because of their consistency with scripture,
but were not supported by evidence Summary James Hutton viewed Earth history as cyclical and very long. His observations were instrumental in establishing the principle of uniformitarianism Charles Lyell articulated uniformitarianism in a way that soon made it the guiding principle of geology According to uniformitarianism
the laws of nature have been constant through time and that the same processes operating today have operated in the past, although not necessarily at the same rates Summary The principles of superposition,
original horizontality, lateral continuity and cross-cutting relationships are basic for determining relative geologic ages and for interpreting Earth history Radioactivity was discovered during the late 19th century and lead to radiometric dating,
which allowed geologists to determine absolute ages for geologic events Summary Geologists determine how many half-lives of a radioactive parent isotope have elapsed since the sample crystallized Half-life is the length of time it takes for one-half of the radioactive parent isotope to decay to new, more stable daughter element Summary The most accurate radiometric dates
are obtained from long-lived radioactive isotope/daughter pairs in igneous rocks Common pairs include:
uranium 238 lead 206 uranium 235 lead 207 thorium 232 lead 208 rubidium 87 strontium 87 potassium 40 argon 40 Summary The most reliable radiometric ages are obtained using two different pairs in the same rock Carbon 14 dating can be used
only for organic matter such as wood, bones, and shells and is effective back to about 70,000 years Summary To reconstruct past climate changes, and link them to possible causes, geologists must have a geologic calendar that is precise and accurate They must be able to date geologic events and the onset and duration of climate changes
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