4_Time - Deep Time: How Old Is Old? James Hutton...

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Unformatted text preview: Deep Time: How Old Is Old? James Hutton (1726-1797) Scottish physician and gentleman-farmer Scottish Called “the Father of Modern Geology” Called First to articulate the “Principle of Uniformitarianism” Of the abyss of time, Hutton wrote: “…we find no vestige of Of a beginning; no prospect of an end.” beginning; Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 10: Deep Time: How Old is Old? Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 10: Deep Time: How Old is Old? Principle of Uniformitarianism “The present is the key to the past” Physical processes that we observe today operated in the Physical same way in the geological past (does not apply to rates!) (does Modern processes help us understand ancient events Modern Ancient mudcracks formed as mudcracks do today Geologic change is mostly slow; requires a long time Paleozoic mudcracks: this is solid rock. Present day mudcracks form in clay­rich sediment. Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 10: Deep Time: How Old is Old? Two ways of dating geological materials: Relative ages – Based upon sequence of events Relative Based Qualitative method developed hundreds of years ago Older vs. younger relationships Older relationships Numerical / absolute ages – Actual number of years Numerical since an event since Quantitative method Developed more recently Based on statistics of Based radioactive decay radioactive Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 10: Deep Time: How Old is Old? Relative Age Logical tools are useful for defining relative age: Principle of Uniformitarianism Principle of Original Horizontality Principle of Superposition Principle of Original Continuity Principle of Cross-Cutting Relationships Principle of Inclusions Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 10: Deep Time: How Old is Old? Principle of Original Horizontality Strata often form laterally extensive horizontal sheets Subsequent erosion dissects once continuous layers Flat-lying rock layers are unlikely to have been disturbed Flat-lying Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 10: Deep Time: How Old is Old? Principle of Superposition In an undeformed sequence of layered rocks … In Each bed is older than the one above, and … Younger than the one below >> Younger strata are on top; older strata below Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 10: Deep Time: How Old is Old? Cross-Cutting Relationships Younger features truncate (“cut across”) older features Faults, dikes, erosion, etc., must be younger than the Faults, must be material that is faulted, intruded, or eroded material A a pluton cannot intrude / a volcano cannot extrude pluton through rocks that aren’t there yet Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 10: Deep Time: How Old is Old? Cross-Cutting Relationships Baked contacts: Contact metamorphism occurs when country rock is Contact invaded by a plutonic igneous intrusion invaded The baked rock must have been there first (it is older) The Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 10: Deep Time: How Old is Old? Principle of Inclusions Inclusion – A rock fragment within another Igneous xenoliths – Country rock that fell into magma Weathering rubble – Debris from preexisting rocks The inclusion is older than the material enclosing it Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 10: Deep Time: How Old is Old? Geologic History Determining relative ages empowers geologists Determining relative to easily unravel complicated geologic histories to Simple rules permit one to decipher this diagram Simple Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 10: Deep Time: How Old is Old? Geologic History Deposition of horizontal strata below sea level Deposition in order 1, 2, 3, 4, 5, 6, 7 and 8 (oldest to youngest) in Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 10: Deep Time: How Old is Old? Geologic History An igneous sill intrudes Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 10: Deep Time: How Old is Old? Geologic History Folding, uplift, and erosion take place Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 10: Deep Time: How Old is Old? Geologic History An igneous pluton cuts older rock Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 10: Deep Time: How Old is Old? Geologic History Faulting cuts the strata and the pluton Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 10: Deep Time: How Old is Old? Geologic History A dike intrudes Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 10: Deep Time: How Old is Old? Geologic History Erosion forms the present land surface Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 10: Deep Time: How Old is Old? Fossil Succession Fossil remnants or traces of once-living organisms Fossil are often preserved in sedimentary rocks Fossil are useful for relative age determination Several types of fossils will occur as an assemblage Fossils are useful time markers – index fossils Fossils index Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 10: Deep Time: How Old is Old? Fossil Succession Species evolve, exist for a time, and then go extinct A time period is recognized by its fossil content Fossil range: First and last appearance Each fossil has a unique range Overlapping ranges provide Overlapping distinctive time markers distinctive Fossils succeed one another Fossils in a known order in Permit correlation of strata Locally Regionally Globally Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 10: Deep Time: How Old is Old? No, dinosaurs did not co-evolve with humans! But, their heirs are still around … Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 10: Deep Time: How Old is Old? Unconformities A time gap in the rock record due to non-deposition or time erosion erosion Three types of unconformity: Three 1. Angular unconformity 1. 2. Nonconformity 3. Disconformity Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 10: Deep Time: How Old is Old? 1. Angular Unconformity 1. 2. 3. Horizontal marine sediments deformed by orogenesis Horizontal orogenesis High mountains are eroded away to below sea level High below Sediments deposited horizontally on the erosion Sediments Mountains form and layers surface fold, then erosion removes the highland. Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 10: Deep Time: How Old is Old? 1. Angular Unconformity “Hutton’s Unconformity” on Siccar Point, Scotland, Hutton’s is a common destination for geologists is Sequence of Events: Sequence deposition > deformation > erosion > deposition Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 10: Deep Time: How Old is Old? 2. Nonconformity 2. Non Metamorphic or igneous rocks overlain by sedimentary Metamorphic strata strata Non-sedimentary rocks were exposed by erosion Sediment was deposited Sediment on this eroded surface on Not to be confused with intrusion of a pluton into sedimentary rocks! Essentials of Geology, 3rd edition, by Stephen Marshak Erosion removes cover, so basement lies exposed at the Earth’s surface. Chapter 10: Deep Time: How Old is Old? Nonconformity Cambrian Sawatch sandstone. Pre­Cambrian Pikes Peak granite near Manitou Springs, Colorado. Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 10: Deep Time: How Old is Old? 3. Disconformity Parallel strata bracket non-deposition or absent layers Due to an interruption in sedimentation Erosion or non-deposition May be difficult to recognize (parallel contacts) May contacts Sea level drops and flat­ lying strata are eroded. Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 10: Deep Time: How Old is Old? Types of Unconformity Try that animation … Try animation Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 10: Deep Time: How Old is Old? Unconformities – Unconformities Missing pages in the Earth’s diary Missing Earth history is recorded in strata Missing strata = missing history Missing The Grand Canyon: Thick layers of strata Numerous gaps A partial record of partial geological history Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 10: Deep Time: How Old is Old? Stratigraphic Correlation Stratigraphic columns depict strata in a region Stratigraphic Drawn to scale to accurately portray relative thicknesses Rock types are depicted by graphical fill patterns Rock Divided into formations Divided formations Mappable rock units Formations are Formations separated by contacts Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 10: Deep Time: How Old is Old? Stratigraphic Correlation In 1793, William “Strata” Smith was the first to note In that strata could be matched across distances Similar rock types in a similar order Similar Rock layers contained the same distinctive fossils After years of work, he made the 1st geologic map Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 10: Deep Time: How Old is Old? Bill Smith’s Maps of Great Britain 1815 1815 Essentials of Geology, 3rd edition, by Stephen Marshak (80,000 mi2) Recent Chapter 10: Deep Time: How Old is Old? Stratigraphic Correlation Lithologic correlation - Based on rock type Sequence – The relative order in which the rocks occur Limited to correlation between nearby regions Fossil correlation – Based on fossils within rocks Applicable to much broader areas Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 10: Deep Time: How Old is Old? Key Beds & Index Fossils Best Key Beds: Distinct Deposited during short time range Wide extension Volcanic Ash deposits are great! Iridium Anomaly Best index fossils: Short-lived (as a species, not an invidual) Wide-spread Planktonic organisms ideal Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 10: Deep Time: How Old is Old? Stratigraphic Correlation National Parks of Arizona and Utah Formations can be traced long distances Overlap is seen in the sequences of rock types Overlapping rock columns are used to build a composite Overlapping Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 10: Deep Time: How Old is Old? Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 10: Deep Time: How Old is Old? The Geologic Column A composite stratigraphic column can be constructed Assembled from incomplete sections across the globe It brackets almost the entirety of Earth’s history Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 10: Deep Time: How Old is Old? Geologic Time Scale Geologic The composite column is divided into time units Eons – The largest subdivision of time (100s to 1000s Ma) Eons The Eras – Subdivisions of an eon (65 to 100s Ma) Eras Subdivisions Periods – Subdivisions of an era (2 to 70 Ma) Periods Subdivisions Epochs – Subdivisions of a period (0.011 to 22 Ma) Epochs Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 10: Deep Time: How Old is Old? The Geologic Time The Scale Scale Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 10: Deep Time: How Old is Old? Numerical Age Many relative ages can now be assigned actual dates Based on radioactive decay of atoms in minerals Radioactive decay proceeds at a known, fixed rate Radioactive elements act as internal clocks Numerical dating is also called geochronology Numerical geochronology Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 10: Deep Time: How Old is Old? Radioactive Decay Isotopes – Elements that have varying #s of neutrons Isotopes Isotopes have similar but different mass numbers Isotopes 13 Stable – Isotopes that never change (i.e., 13C) Stable 14 Radioactive – Isotopes that spontaneously decay (i.e., 14C) Radioactive Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 10: Deep Time: How Old is Old? Radioactive Decay Radioactive decay progresses along a decay chain Decay creates new unstable elements that also decay Decay proceeds to a stable element endpoint Decay Parent isotope – The isotope that undergoes decay Parent The Daughter isotope – The product of this decay Daughter The Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 10: Deep Time: How Old is Old? Radioactive Decay Half-life (t½) – Time for ½ unstable nuclei to decay After one t½, one half of the original parent remains t½ is a characteristic of each isotope After three t½, one eighth of the original parent remains As the parent disappears, the daughter “grows in” Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 10: Deep Time: How Old is Old? Radiometric Dating The age of a mineral can be determined by … Measuring the ratio of parent isotopes to daughter isotopes Calculating the amount of time by using the known t ½ Must pick the right mineral and the right isotope Must Geochronology requires analytical precision Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 10: Deep Time: How Old is Old? What is a Radiometric Date? Radiometric dates give the time a mineral began to Radiometric preserve all atoms of parent and daughter isotopes preserve Requires cooling below a “closure temperature” Requires If rock is reheated, the radiometric clock can be reset Igneous & Metamorphic rocks best for geochronology Igneous Sedimentary rocks cannot be directly dated – WHY? Sedimentary WHY? Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 10: Deep Time: How Old is Old? Dating the Geologic Column Geochronology is less useful for sedimentary deposits Sediments can be bracketed by numerical dates Yields age ranges that narrow as data accumulate Defines major boundaries in the geologic column Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 10: Deep Time: How Old is Old? The Age of the Earth Before radiometric dating, Before age estimates varied widely age 20 Ma – From Earth cooling 90 Ma – Ocean salinization If oceans started as fresh Unchanging mass of dissolved Unchanging material added by rivers material Uniformitarianism and Uniformitarianism evolution indicated an Earth much older than ~100 Ma much Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 10: Deep Time: How Old is Old? Other Numerical Dating Methods Growth rings – Annual layers from trees or shells Growth Rhythmic layering – Annual layers in sediments or ice Rhythmic Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 10: Deep Time: How Old is Old? The Geologic Time The Scale Scale Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 10: Deep Time: How Old is Old? Geologic Time and Life - Eons Hadean (“Hell”): 4.6 – 3.8 Ga (“Hell”): Internal differentiation of Earth Formation of oceans and secondary atmosphere Origin of Origin of Life Life Complex life Complex life appears appears Archean (”Ancient”): 3.8 – 2.5 Ga (”Ancient”): ~ 3.8 Ga Life first appears; anaerobic and single-celled Birth of continents Cambrian Explosion: Shells Cambrian Explosion: Shells appear appear Age of Age of dinosaurs dinosaurs Age of Age of mammals mammals Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 10: Deep Time: How Old is Old? Geologic Time and Life - Eons Proterozoic (“Before Life”): 2.5 – 0.542 Ga (“Before O2 from cyanobacteria built up in atmosphere by 2 Ga up ~ 700 Ma, multicellular life evolved Development of tectonic plates like those of today Origin of Origin of Life Life Complex life Complex life appears appears Phanerozoic (“Visible Life”): 542 Ma to the present Precambrian / Cambrian ~ 542 Ma marks the 1st appearance of hard shells appearance Increased fossil preservation Life diversified rapidly: “Cambrian Explosion” Essentials of Geology, 3rd edition, by Stephen Marshak Cambrian Explosion: Cambrian Explosion: Shells appear Shells appear Age of Age of dinosaurs dinosaurs Age of Age of mammals mammals Chapter 10: Deep Time: How Old is Old? The Geologic Time Scale - Eras Paleozoic – “Ancient Life” 542 to 251 Ma Life diversified rapidly Mesozoic – “Middle Life” 251 to 65.5 Ma The “Age of Dinosaurs” Cenozoic – “Recent life” Cenozoic 65.5 Ma to present The “Age of Mammals” Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 10: Deep Time: How Old is Old? Major subdivisions of time Major based on mass extinctions based “The stratigraphic record The is like the life of a soldier – is Long periods of boredom separated by brief periods of terror” Paraphrased after D.V. Ager (1993) Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 10: Deep Time: How Old is Old? The Age of the Earth The oldest rocks on Earth’s surface date to 3.96 Ga Zircons in ancient sandstones date to between 4.1 and 4.2 Ga Age of Earth is 4.57 Ga based on correlation with meteorites Age and Moon rocks and WHY NOT CORRELATED WITH EARTH ROCKS? Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 10: Deep Time: How Old is Old? Geologic Time The immensity of time is beyond comprehension Metaphors often needed to illustrate the scale of time The age of Earth (4.6 Ga) can be compared to pennies Lined up, 4.6 billion pennies would be 87,400 km long More than twice around the Earth Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 10: Deep Time: How Old is Old? What to Remember: Difference between Relative and Numerical Dating Important Principles for Relative Dating Can you explain what they mean? 3 Types of unconformities: Can you tell them apart Fundamental rock unit is the formation Fundamental formation Fundamental time unit is the period (eon > era > period Fundamental period > epoch) epoch) What eon are we living in? What is Precambrian? How much time does it What Precambrian? represent? represent? What makes a good key bed, or a useful index fossil? What key index What is correlation? What correlation? Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 10: Deep Time: How Old is Old? What to Remember: Numerical or Absolute Dating What is radioactivity? How does radiometric dating work? What is half-life? What half-life How does the parent-to-daughter ratio change over How time? time? Does numerical dating work for all rocks? What is Radiocarbon Dating used for? Be prepared to calculate rock ages, half-lives, parent or Be daughter materials daughter Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 10: Deep Time: How Old is Old? ...
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This note was uploaded on 02/25/2012 for the course EARTH 2 taught by Professor Thomas during the Summer '11 term at UCSB.

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