Chapter 12 Deep Time How Old Is Old
- Slides: 68
Chapter 12 Deep Time: How Old Is Old? LECTURE OUTLINE earth Portrait of a Planet Third Edition © 2008 W. W. Norton & Company, Inc. Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 12: Deep Time: How Old Is Old?
Deep Time: How Old Is Old? Prepared by Ronald Parker Earlham College Department of Geosciences Richmond, Indiana
Geologic Time Discovering the magnitude of the Earth’s past was a momentous discovery in the history of humanity. < This discovery forever altered our perception of ourselves within nature and the Universe. < Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 12: Deep Time: How Old Is Old?
Geologic Time < Understanding time permits assigning an age to… = Rocks. = Fossils. = Geologic structures. = Landscapes. = Tectonic events. Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 12: Deep Time: How Old Is Old?
Geologic Time Deep time – The immense span of geologic time. < It is so vast that it is difficult for people to grasp. < = We think of time in terms of our lives… 4 The lives of our parents and grandparents. 4 The lives of our children or grandchildren. < Human history is tiny compared to geologic time. Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 12: Deep Time: How Old Is Old?
Geologic Time < Prior to the late 17 th century, geologic time was thought to be the same as historical time. = Archbishop James Ussher of Armagh, Ireland, 1654. 4 He added up generations from the Old Testament. 4 He determined that Earth formed on October 23, 4004 BCE. Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 12: Deep Time: How Old Is Old?
Geologic Time < Scientists began to find clues to an ancient Earth. = Nicolaus Steno (1638– 1686) – Danish physician. 4 Observed marine fossils high in the Apennines. 4 Deduced that these were ancient animals in loose sediment. 4 Lithification and uplift suggested long periods of time. Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 12: Deep Time: How Old Is Old?
Geologic Time < James Hutton (1726 -1797) – Scottish physician. = Called “the Father of Modern Geology. ” = First to articulate the “Principle of Uniformitarianism. ” = Of the abyss of time, Hutton wrote: “we find no vestige of a beginning; no prospect of an end. ” Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 12: Deep Time: How Old Is Old?
Geologic Time < James Hutton’s Principle of Uniformitarianism. = “The present is the key to the past. ” 4 Processes seen today are the same as those of the past. a. Ancient mudcracks formed as mudcracks do today. 4 Geologic change is slow; large changes require large times. Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 12: Deep Time: How Old Is Old?
Geologic Time < There are two ways of dating geological materials. = Relative ages – Based upon order of formation. 4 Qualitative method developed 100 s of years ago. 4 Permit determination of older vs. younger relationships. = Numerical ages – Actual number of years since an event. 4 Quantitative method developed recently. 4 Age is given a number. Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 12: Deep Time: How Old Is Old?
Relative vs. Absolute Relative ages assign order to events. < Numerical ages assign exact dates to events. < Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 12: Deep Time: How Old Is Old?
Relative Age < Logical tools are useful for defining relative age. = Principle of uniformitarianism. = Principle of superposition. = Principle of original horizontality. = Principle of original continuity. = Principle of cross-cutting relationships. = Principle of inclusions. = Principle of baked contacts. Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 12: Deep Time: How Old Is Old?
Geologic Time < Uniformitarianism – The present is key to the past. = Physical processes that we observe today operated in the same way in the geological past. = Modern processes help us understand ancient events. Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 12: Deep Time: How Old Is Old?
Defining Relative Age < Superposition. = In an undeformed sequence of layered rocks… 4 Each bed is older than the one above and… 4 Younger than the one below. = Younger strata are on top; older strata below. Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 12: Deep Time: How Old Is Old?
Relative Age < Horizontality and continuity. = Strata often form laterally extensive horizontal sheets. = Subsequent erosion dissects once-continuous layers. = Flat-lying rock layers are unlikely to have been disturbed. Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 12: Deep Time: How Old Is Old?
Relative Age < Cross-cutting relations. = Younger features truncate (cut across) older features. = Faults, dikes, erosion, etc. , must be younger than the material that is faulted, intruded, or eroded. = A volcano cannot intrude rocks that aren’t there yet. Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 12: Deep Time: How Old Is Old?
Relative Age < Inclusions – A rock fragment within another. = Igneous xenoliths – Country rock that fell into magma. = Weathering rubble – Debris from pre-existing rocks. < The inclusion is older than the material enclosing it. Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 12: Deep Time: How Old Is Old?
Relative Age < Baked contacts. = Thermal metamorphism occurs when country rock is invaded by a plutonic igneous intrusion. = The baked rock must have been there first (it is older). Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 12: Deep Time: How Old Is Old?
Relative Age < Determining relative ages empowers geologists to easily unravel complicated geologic histories. Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 12: Deep Time: How Old Is Old?
Geologic History < Deposition of horizontal strata below sea level in order 1, 2, 3, 4, 5, 6, 7, and 8 (oldest to youngest). Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 12: Deep Time: How Old Is Old?
Geologic History < Igneous intrusion of a sill. Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 12: Deep Time: How Old Is Old?
Geologic History < Tectonic compression and folding. = Beds < had to be present to be folded. Uplift above sea level and erosion. Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 12: Deep Time: How Old Is Old?
Geologic History < Intrusion of a granitic igneous pluton. Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 12: Deep Time: How Old Is Old?
Geologic History < Extensional normal faulting. = Faulting cross-cuts the older granitic pluton. Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 12: Deep Time: How Old Is Old?
Geologic History < Intrusion of a dike. = Dike cross-cuts the normal fault. Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 12: Deep Time: How Old Is Old?
Geologic History < Erosion to present landscape configuration. = Erosion removed the volcano and cross-cuts the dike. Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 12: Deep Time: How Old Is Old?
Geologic History Relative ages help to unravel a complicated history. < Simple rules permit one to decipher this diagram. < Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 12: Deep Time: How Old Is Old?
Fossil Succession Fossil remnants, or traces of once living organisms, are often preserved in sedimentary rocks. < Fossil are useful for relative age determination. < = Several types of fossils will occur as an assemblage. = Fossils are time markers. Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 12: Deep Time: How Old Is Old?
Fossil Succession Species evolve, exist for a time, and then go extinct. < First appearance, range, and extinction dates rocks. < Fossils succeed one another in a known order. < A time period is recognized by its fossil content. < Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 12: Deep Time: How Old Is Old?
Fossil Succession < Fossil range – First and last appearance. = Each fossil has a unique range. = Overlapping ranges provide distinctive time markers. < Permit correlation of strata. = Locally. = Regionally. = Globally. Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 12: Deep Time: How Old Is Old?
Unconformities < An unconformity is a time gap in the rock record. = Nondeposition. = Erosion. < There are three types of unconformity. Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 12: Deep Time: How Old Is Old?
Unconformities < Three unconformity types: = Disconformity – Parallel strata bracketing nondeposition. 4 Due to an interruption in sedimentation. 4 May be difficult to recognize. Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 12: Deep Time: How Old Is Old?
Disconformities
Unconformities < Three unconformity types: = Nonconformity – Metamorphic or igneous rocks overlain by sedimentary strata. 4 Crystalline ig/met rocks were exposed by erosion. 4 Sediment was deposited on this eroded surface. Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 12: Deep Time: How Old Is Old?
Nonconformity
Unconformities < Three unconformity types: = Angular unconformity – Represents a huge gulf in time. 4 Horizontal marine sediments deformed by orogenesis. 4 High mountains are eroded away to below sea level. 4 Sediments deposited horizontally on the erosion surface. Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 12: Deep Time: How Old Is Old?
Angular Unconformity < James Hutton was the 1 st to realize the enormous time-significance of angular unconformities. = Mountains created. = Mountains completely erased. = New sediment deposition. < Incomprehensible time. Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 12: Deep Time: How Old Is Old?
Angular Unconformity < “Hutton’s Unconformity” on Siccar Point, Scotland, is a common destination for geologists. Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 12: Deep Time: How Old Is Old?
Unconformities Earth history is recorded in strata. < Missing strata = missing history. < The Grand Canyon. < = Thick layers of strata. = Numerous gaps. = A partial record of geological history. Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 12: Deep Time: How Old Is Old?
Stratigraphic Correlation < Stratigraphic columns depict strata in a region. = Drawn to scale to accurately portray relative thicknesses. = Rock types are depicted by graphical fill patterns. = Divided into formations. 4 Mapable rock units. = Formations are separated by contacts. Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 12: Deep Time: How Old Is Old?
Stratigraphic Correlation < In 1793, William “Strata” Smith was the first to note that strata could be matched across distances. = Similar rock types in a similar order. = Rock layers contained the same distinctive fossils. < After years of work, he made the 1 st geologic map. Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 12: Deep Time: How Old Is Old?
Stratigraphic Correlation < Lithologic correlation is 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. Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 12: 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. Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 12: Deep Time: How Old Is Old?
Correlation among rock strata in 3 national parks.
The Geologic Column < A composite stratigraphic column exists. = Constructed from incomplete sections across the globe. = It brackets almost the entirety of Earth history. Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 12: Deep Time: How Old Is Old?
Geologic Time The composite column is divided into time blocks. < This is the geologic time scale, Earth’s “calendar. ” < = The structure of the geologic time scale. 4 Eons – The largest subdivision of time (100 s to 1000 s Ma). 4 Eras – Subdivisions of an eon (65 to 100 s Ma). 4 Periods – Subdivisions of an era (2 to 70 Ma). 4 Epochs – Subdivisions of a period (0. 011 to 22 Ma). Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 12: Deep Time: How Old Is Old?
Geologic Time < Time scale subdivisions are variously named. = The nature of life (“zoic” means life); i. e. , Proterozoic. = A characteristic of the time period; i. e. , Carboniferous. = A specific locality; i. e. , Devonian. Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 12: Deep Time: How Old Is Old?
The Geologic Time Scale Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 12: Deep Time: How Old Is Old?
Geologic Time and Life < < < Life first appears on Earth ~ 3. 8 Ga. Early life consisted of anaerobic single-celled organisms. Oxygen from cyanobacteria built up by 2 Ga. ~ 700 Ma, multicellular life evolved. ~ 542 Ma marks the 1 st appearance of hard shells. = = Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Shells increased fossil preservation. Life diversified rapidly – the “Cambrian Explosion. ” Chapter 12: Deep Time: How Old Is Old?
The Geologic Time Scale < Names of the eons. = Phanerozoic – “Visible life” (542 Ma to the present). 4 Started 542 Ma at the Precambrian – Cambrian boundary. 4 Marks the 1 st appearance of hard shells. 4 Life diversified rapidly afterward. = Proterozoic – “Before life” (2. 5 to 0. 542 Ga). 4 Development of tectonic plates like those of today. 4 Buildup of atmospheric O 2; multicellular life appears. = Archean – “Ancient” (3. 8 to 2. 5 Ga). 4 Birth of continents. 4 Appearance of the earliest life forms. = Hadean – “Hell” (4. 6 to 3. 8 Ga). 4 Internal differentiation. 4 Formation of the oceans and secondary atmosphere. Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 12: Deep Time: How Old Is Old?
The Geologic Time Scale < Names of the eras. = Cenozoic – “Recent life. ” 465. 5 Ma to present. 4 The “Age of Mammals. ” = Mesozoic – “Middle life. ” 4251 to 65. 5 Ma. 4 The “Age of Dinosaurs. ” = Paleozoic – “Ancient life. ” 4542 to 251 Ma. 4 Life diversified rapidly. Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 12: Deep Time: How Old Is Old?
Numerical Age The relative age of geologic events is established. < 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. Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 12: Deep Time: How Old Is Old?
Radioactive Decay Isotopes – Atoms with the same # of protons, different # of neutrons. < Isotopes have similar but different mass numbers. < = Stable – Isotopes that never change (i. e. , 13 C). = Radioactive – Isotopes that spontaneously decay. Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 12: 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. Parent isotope – The isotope that undergoes decay. < Daughter isotope – The product of this decay. < Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 12: Deep Time: How Old Is Old?
Radioactive Decay < Half-life (t½) – Time for ½ unstable nuclei to decay. = t½ is a characteristic of each isotope. = After one t½ half of the original parent remains. = After three t½ an eighth of the original parent remains. < As the parent disappears, the daughter “grows in”. Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 12: Deep Time: How Old Is Old?
Radiometric Dating < The age of a mineral can be determined by… = Measuring the ratio of parent to daughter isotopes. = Calculating the amount of time by using the known t ½. Must pick the right mineral and the right isotope. < Geochronology requires analytical precision. < Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 12: Deep Time: How Old Is Old?
What Is a Radiometric Date? < Radiometric dates give the time a mineral began to preserve all atoms of parent and daughter isotopes. = Requires cooling below a “blocking temperature. ” = If rock is reheated, the radiometric clock can be reset. Ig / Met rocks are best for geochronologic work. < Sedimentary rocks cannot be directly dated. < Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 12: Deep Time: How Old Is Old?
Other Numerical Ages < Numerical ages are possible without isotopes. = Growth rings – Annual layers from trees or shells. = Rhythmic layering – Annual layers in sediments or ice. Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 12: Deep Time: How Old Is Old?
Other Numerical Ages < Magnetostratigraphy – Magnetic signatures in strata are compared to the global reference column. Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 12: Deep Time: How Old Is Old?
Other Numerical Ages < Fission-track analysis – Measuring decay paths. = Radioactive decay particles scar crystals. = The number of scars is proportional to age. Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 12: Deep Time: How Old Is Old?
Dating the Geologic Column Geochronology less useful for sediment deposits. < It can, however, constrain these deposits. < Sediments can be bracketed by absolute dates. < = Yields age ranges that improve as data accumulates. = Defines major boundaries in the geologic column. Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 12: Deep Time: How Old Is Old?
The Age of the Earth < Before radioactivity-based dating methods… = 20 Ma – From Earth cooling. = 90 Ma –Ocean salinization. 4 Assumed oceans were initially freshwater. 4 Measured the mass of dissolved material in rivers. = Uniformitarianism and evolution indicated an Earth older than ~100 Ma. Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 12: 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 4. 1 -4. 2 Ga. < Age of Earth is 4. 57 Ga based on correlation with… < = Meteorites. = Moon rocks. Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 12: Deep Time: How Old Is Old?
Geologic Time The immensity of time is beyond comprehension. < Metaphors 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. 4 More than twice around Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 12: Deep Time: How Old Is Old?
Geologic Time Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 12: Deep Time: How Old Is Old?
This concludes the Chapter 12 Deep Time: How Old Is Old? LECTURE OUTLINE earth Portrait of a Planet Third Edition © 2008 W. W. Norton & Company, Inc. Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 12: Deep Time: How Old Is Old?
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