Fossils, Chronology, and Geologic TimeHow Fossils Help Explain Earth’s History
The most straightforward method of correlating sedimentary rocks is to compare their fossils.• Fossils = the remains or traces of organisms preserved in rocks.• Fossil = preserved from a past geologic age.• Fossil = an animal or plant that lived many thousands or millions of years ago that has been preserved, or the shape of one of these organisms, in rock.• Fossil = any remains, trace, or imprint of a plant or animal that has been preserved in the earth’s crust since some past geologic or prehistoric time.
Fossils can be vertebrates, invertebrates, plants or even footprints.
Becoming a Fossil Is Not Easy• The process of making something into a fossil is not an easy one.• It takes fairly rapid burial in fine-grained sediment to stop the three major agents of destruction: decay, dissolution and disarticulation.• This is why hard skeletons in the fossil record are more likely to have been preserved form marine environments than from on land.
Similar rocks of the same age typically contain similar fossils.• Fossils hold the key to predicting the sequence of layers in a given location and to matching outcrops of similar rocks between different locations.• Geologists can make more precise correlations using index fossils--species that existed for relatively short periods of time and are found over large geographic areas.• Index fossils are useful because their appearance in the rock record represents a specific time interval that scientists can use to identify and correlate rocks chronologically between different regions around the world.
Lack of Continuityof SedimentaryStrataWhen geologists try tomatch rocks fromdifferent areas of anycontinent, they canrarely follow the bestexposures of layers ofsedimentary rock formore than a few tensof kilometers beforethey disappearunderground or areremoved by erosion.(Note gaps in dates forGrand Canyon strata)
Grand Canyon Fossils Match Fossils Elsewhere in North America• The Grand Canyon’s Redwall Limestone layer, about 350 million years old, contains an assemblage of fossils that includes extinct corals, cephalopods and crinoids (all sea bottom creatures).• Similar fossil assemblages are found in limestone quarries in Indiana and in deposits surrounding Kentucky’s Mammoth Cave.• The same fossils are even found in the Rundle Limestone formation in the Rocky Mountains way up in Banff, Canada.• All of these strata were formed at the same time when much of N. America was covered by a shallow sea.
Fossils similar to those in the Grand Canyon’s Redwall Limestone strata are found in Indiana quarries and around Kentucky’s Mammoth Cave.
Examine the following illustration and predict whichrock strata in the Grand Canyon is most likely to have formed in a depositional environment like the one pictured.
Geologic Time: KEY POINTS• Earth’s “history” is divided into 3 long spans of time know as eons (Archean, Proterozoic, and Phanerozoic).• The Archean and Proterozoic were once commonly called the Precambrian—a span of over 4 billion years, from the time the Earth first formed to when fossils become common in rocks.
KEY POINTS (Cont’d)• The most recent eon, the Phanerozoic, is characterized by abundant fossils, especially thoses with shells or hard skeleton parts.• The Phanerozoic is divided into three eras (Paleozoic, Mesozoic, and Cenozoic), which in turn are divided into 13 periods (see Figure 8.14 and Table 8.1).• The fossil record has been interpreted to record the changes in the biosphere over time.
Early Earth at 4.3 Billion Year Ago• Much hotter from:1. Left over heat from planet’s formation period.2. More volcanic activity than today.• Regular bombardment of asteroids and comets.• Little or no oxygen.• Life arose in hot, acrid conditions like those in the scorching, acidic, hot springs in Yellowstone Park (some extremophile bacteria live in such conditions today).
“Firsts”First bacteria-likefossils (below)composed of smallrods and spheres(right), occur in 3.5billion year oldrocks.
First MulticellularLife and First“Animals”About 2 billion yearsago, oxygen began toaccumulate in theatmosphere as theresult of O2 productionby blue-green algae inthe seas, and lifeevolved beyondprimitive bacteriaaround 1.7 b.y.a.The earliest creaturesthat might be labeled“animals” appearedsome 580 million yearsago (R).
CambrianPeriod:Explosion of fossilswith shells or hardskeletons in theseas.Many of the majoranimal groups canbe found in theCambrian period,even though theydid not lookanything like theirdescendents dotoday.
The fossil record of marine animals demonstrates theslow pace of evolution, with new families appearing at a rate of roughly one per million years.
On average, the number of species represented by fossilsincreases from the Cambrian onward. All major phyla had appeared by the Cambrian.
“The Great Dying”• The Permian-Triassic extinction (or P-T event) killed off an estimated 96% of marine species and 70% of land species.• The P-T extinctions may have taken a few million years.• Two geologic events were ongoing at the time:1. The supercontinent Pangaea was assembled during the Period, crating a single worldwide ocean—reducing the area of continental shelf, the shallow ocean floor around continents that’s home to a majority of marine species.2. And, thousands of eruptions, starting about 251 m.y.a., took place over 1 million years to form the Siberian Traps (lava plateaus).
More genera were alive in the very recent past thanever before (at least until the 6th and current, ongoing, human-caused mass extinction).
Euthycarcinoidsand First LandPlantsTracks from theprimordial sea (p.224): Euthycarcinoidsseem to be the firstsea creatures to haveinvaded the land some510 m.y.a. (in theCambrian)The first plants toinvade the land fromthe sea occurredabout 440 m.y.a. (inthe Silurian Period).
Numerical Time: KEY POINTS• Radioactive decay occurs when a radioactive parent isotope undergoes a change to its nucleus and is converted to a daughter atom and releases energy.• Radioactive decay of unstable isotopes can be used to determine the age of igneous and metamorphic rocks.• The half-life is the length of time it takes for half of the radioactive material to decay.
What are isotopes?• Isotopes are varieties of the same element that have different mass numbers (their nuclei contain the same number of protons but a different number of neutrons).• In other words, isotopes are variants of atoms of a particular element, which have differing numbers of neutrons.
What are isotopes?• Atoms of a particular element by definition must contain the same number of protons but may have a distinct number of neutrons, which differs from atom to atom, without changing the designation of the atom as a particular element.• The number of protons and neutrons in the nucleus, known as the mass number, is not the same for two isotopes of any element.
What are radioactive isotopes?• Radioactive isotopes, also called radioisotopes, are atoms with a different number of neutrons than a usual atom• It has an unstable nucleus that decays, emitting alpha, beta and gamma rays until the isotope reaches stability.• Once its stable, the isotope becomes another element entirely.
What is radioactive decay?• The process by which unstable (radioactive) isotopes transform to new elements by a change in the number of protons (and neutrons) in the nucleus.• In other words, radioactive decay is when an unstable isotope changes to a new element.
Radioactive decay is the process by which an unstable atomic nucleus loses energy by emitting radiation inthe form of particles or electromagnetic waves, thereby transitioning toward a more stable state.
PotassiumArgon Dating Technique• Geologists have used this method to date rocks as much as 4 billion years old.• It is based on the fact that some of the radioactive isotope of Potassium, Potassium- 40, decays to the gas Argon as Argon-40.• By comparing the proportion of K-40 to Ar-40 in a sample of volcanic rock, and knowing the decay rate of K-40, the date that the rock formed can be determined.
PotassiumArgon Dating Technique• As the K-40 in the rock decays into Ar-40, the gas is trapped in the rock.• When rocks are heated to the melting point, any Ar-40 contained in them is released into the atmosphere.• The technique works well for almost any igneous or volcanic rock, provided that the rock gives no evidence of having gone through a heating-recrystallization process after its initial formation.
Carbon-14 DatingTechniqueRadiocarbon dating(sometimes simplyknown as carbondating) is aradiometric datingmethod that uses thenaturally occurringradioisotope carbon-14 (14C) to estimatethe age of once livingmaterials from todayup to about 58,000 to62,000 years in thepast.
Radioisotopes provide numerical dating here. 1. Place fossilsin correct order according to relative ages, oldest to youngest.2. How would you estimate the age ranges of C, G & K fossils?
Catastrophism, Unifromitarianism, and now a Combination of Both• Catastrophism is the idea that Earth’s features have remained fairly static until dramatic changes were wrought by sudden, short-lived, violent events (catastrophes) that were occasionally worldwide in scope.• By contrast, during most of the 1800s & 1900s, the dominant paradigm of geology has been uniformitarianism.• Uniformitarianism, also known as gradualism, according to which Earths features have been gradually but continually changing, eroding and reforming at a roughly constant rate.• Recently, however, the scientific consensus has been changing toward a more inclusive and integrated view of geologic events, reflecting acceptance of some catastrophic events along with gradual changes
Georges Cuvier(August 1769-May1832Cuvier was a major figurein natural science researchin the early 1800s.He was a proponent ofcatastrophism --that manyof the geological featuresof the earth and the pasthistory of life could beexplained by short-livedcatastrophic global eventsthat had caused theextinction of many speciesof animals.Cuvier came to believethat there had not been asingle catastrophe butseveral.
Sir Charles Lyell(November 1797 –February 1875)Lyell was the foremostgeologist of his day.He is best known asthe author ofPrinciples of Geology,which popularizedJames Huttonsconcepts ofuniformitarianism(slides 12-15) – theidea that the earthwas shaped by slow-moving forces still inoperation today.
Uniformitarianism These (a) mud cracks formed recently, while (b) the mud cracks preserved in rocks are millions of year old. The concept of uniformitarianism holds that the ancient mud cracks formed under the same conditions necessary for the formation of modern mud cracks.
Uniformitarianism or gradualism holds that geologicprocesses are typically very slow and take immense periods of time.
Now a Combo: Uniformitarianism is the main process,intermittently interrupted by Catastrophism (e.g. asteroid impacts or super-volcanoes or mega-earthquakes).