Presentation on theme: "Geologic Time. Figure 7.1 How Old is the Earth? Historical Records, written word –5000 years Bishop Usher (1664) 9:00 AM, Oct. 23, 4004 BC Earth is ~6000."— Presentation transcript:
How Old is the Earth? Historical Records, written word –5000 years Bishop Usher (1664) 9:00 AM, Oct. 23, 4004 BC Earth is ~6000 years old Geologic events catastrophism Hutton (1795) Uniformitarianism “deep” time very old Earth Modern View, ~ 4.6 billion years old
How Old is the Earth Relative Dating –Determines how old a rock is in relation to its surrounding Absolute (Numerical) Dating –Determines actual age in years Geochronology is the study of time in relation to earth’s existence
Geologic Time and the Rock Record Rocks record the processes and events and help us measure geologic time. By studying outcrops with the scientific method, we can figure out the relative order of events! Even absolute ages! How? Geometric relationships Stratigraphy. Fossils Biostratigraphy. Radiometric dating Geochronology.
Stratification (Strata) Layering of Sedimentary Rocks Sedimentary Rock Distinct layering Beds Bed 1 Bed 2 Different thicknesses Color, & other characteristics
Relative Age Inferences Original Horizontality If layers are inclined at an angle, then something tilted them - they didn’t form that way Sedimentary rocks are formed in layers (strata) which were originally horizontal.
Relative Age Inferences Superposition If one layer is on top of another, then it came later (it’s younger). Note that layers can be completely upside down, and you need something like ripple marks to tell which way the layers are “facing” Youngest Oldest Youngest Oldest
Relative Age Inferences Cross-cutting relationships Crosscutting igneous rocks are younger than what they intrude. Faults are younger than what they cut.
Relative Age Inferences Inclusion Units that include bits of another came later (are younger) younger older younger older
Relative Age Inferences Assumptions / Geometric Principles: 1.Sediments deposited horizontally 2.Younger sediments on top of older 3.Units that cross-cut (e.g. faults or intrusions) came after (i.e., are younger than) those that they cut 4.Units that include bits of another came later (are younger)
Let’s practice List events from oldest to youngest (including faulting and erosion) Deposition of Abo Formation, Yeso Formation, Moenkopi Formation, Agua Zarco Formation Fault (covered) offsets the four sedimentary units Erosion (especially of Moenkopi) Emplacement of Bandelier Rhyolite (as hot ash flow) Erosion
Missing Time-Gaps Happen Buried and tilted erosional surface
Conformable Contact Layers of rock that have been deposited without any interruption. No gaps in time. No missing record due to erosion, non- deposition, etc.
Unconformity 3 types of break in the rock record. Such surfaces represent: A hiatus in deposition and/or… A period of erosion. “Missing time” Significant events. Popostosa Fm. Playa deposits & post-Santa Fe Group (Pl) alluvium. N of San Lorenzo Cyn (Socorro, NM) – P. A. Scholle (1999).
Angular Unconformity A sharp discontinuity in the rock record separating strata that are not parallel. Indicates that during the break, a period of deformation occurred.
Disconformity A break in the rock record across which there is little change in orientation of strata. Often just a pause in deposition (subtle). May also be obvious erosion surface. River Road
Nonconformity Horizontal sedimentary rocks on top of eroded crystalline rocks (metamorphic or igneous). Requires erosion to bring crystalline rocks to the surface.
Missing Time: Unconformities
practice relative dating
Fossils as Timepieces Fossils remains of ancient organisms trilobites
Recent Older Very Old Fossil Record
Relative Dating faunal succession trilobites dinosaurs – groups of fossil animals and plants occur in the geologic record in a definite chronological order – periods of time recognized by characteristic fossils
Correlation of Rock Units: Index Fossils Common occurrence Wide geographic distribution Very restricted age range
Correlation Match rocks between different areas: Key Beds & Index Fossils Fossil successions. Unique minerals. Unique rock sequence. Can extend relative age sequence elsewhere.
Correlation Grand Canyon Zion Bryce
The Geologic Column and the Geologic Time Scale In 19 TH Cent., geologists began to assemble a geologic column – composite column containing, in chronological order –the succession of known strata, fitted together on the basis of their fossils or other evidence of relative age. The corresponding column of time is the geologic time scale.
Geologic Column Catalog of all known strata Not one physical locality but a chronological compilation of all localities
Geologic Time Scale Precambrian
Eon: largest interval into which geologic time is divided. – Hadean Eon Some moon samples were formed during the Hadean Eon. –Archean Eon Archean rocks, which contain primitive microscopic life forms are the oldest rocks we know of on the Earth. –Proterozoic Eon –Phanerozoic Eon
Relative Time Scale Worldwide changes in fossils give break points When did dinosaurs go mostly extinct?
Relative Time Scale Worldwide changes in fossils give break points The relative time scale doesn’t give us numerical ages. Where do these numbers come from?
Early Attempts to Measure Geologic Time Numerically Time = Quantity of Something Rate Quantity changes with time For example, Rates of sedimentation & thickness of sedimentary rocks Problem: did not account for past erosion differences in sedimentation rates
Early Attempts to Measure Geologic Time Numerically Saltiness of Seawater (date the ocean) Oceans Salt rivers Edmund Halley (1715) John Joly (1889) Answer: ~ 90 million years Salts are added both by erosion and by submarine volcanism, but salts are also removed by solution. Incorrect!!!
Early Attempts to Measure Geologic Time Numerically Lord Kelvin (1870’s), a physicist, attempted to calculate the time Earth has been a solid body.
Early Attempts to Measure Geologic Time Numerically Lord Kelvin (1897’s), a physicist, attempted to calculate the time Earth has been a solid body. Earth molten Earth Solid Time=0 Time=Today Cooling off by conduction No more heating Theory of heat conduction Experimental data (melting temp. of rocks, size of Earth) Answer: million years Too Young for Geologists!
Radioactivity: A Little History H. Becquerel (1896) discovers radioactivity in Uranium Marie Curie (1900) discovers radium & heat is given off as byproduct of radioact. E. Rutherford (1905) Radioactive elements transform from one chemical element to another B. Boltwood (1907) Radiometric dating of minerals ( million years)
Radioactive Atoms Atoms contain Protons, Electrons & Neutrons Carbon: Atomic number =6 (6 protons) Isotope: atoms of the same element containing different # neutrons Stable Isotope Unstable Isotope
Radioactive Decay It turns out that some elements will spontaneously turn into other elements. This is called radioactivity
Half-life (T 1/2 ) Time needed for ½ of parent atoms to decay (rate of decay) Time# of Parent atoms # of daughter atoms hr500 2 hr hr T 1/2 = 1 hour
Decay rate is a non-linear process All radioactive elements follow the same law But, each element will have different decay rates (half-life) 1 nanosecond 49 billion years
B B BA Decay Rates Decay rates are unaffected by geological processes (mainly chemical) Once radioactive atoms are created they start to act like ticking clock A A A AA Know the decay rate Count the daughter atoms Count the Parent atoms Calculate the time since the atomic clock started ticking
Mass spectrometer A Minnesota Connection Alfred Nier
Potassium-Argon Dating 40 K- 40 Ar half-life = 1.3 billion years 40K 40Ar Magma Clock is ticking Open system 40K Crystallization K-mineral Closed system Rock clock is reset Age of crystallization
Closed system no leakage or addition of K or Ar 40K K-mineral Closed system Rock clock is reset Age of crystallization Geological processes can allow material to be added or lost date will be incorrect Cross-check with other radiometric systems using different minerals
Carbon-14 Method Surface CO 2 Water, Plants, & Animals After Death Time=0 14-C decays 14-N T 1/2 = 5730 yrs Atmosphere Neutron + 14-Nitrogen 14-Carbon CO 2
Isotopic Systems Used for Radiometric Dating Rubidium-Strontiumt 1/2 = 47 billion yr Uranium-leadt 1/2 =4.5 billion yr Potassium-Argont 1/2 =1.3 billion yr Carbon-14t 1/2 =5730 yr Long t 1/2 useful for dating old material Short t 1/2 useful for dating young material
Dating the Geologic Time Scale Sedimentary and Igneous Rocks Granite is older than OLD RED SANDSTONE Volcanic is younger than OLD RED SANDSTONE
Age of Earth Oldest dated rocks3.94 by Oldest dated material4.2 by Moon Rocks & Meteorites by
0:00 AM, Jan 1Formation of Earth Late JanuaryFormation of Core-Mantle-Crust Mid FebruaryLife Begins, Oldest Know Rocks Late MarchFirst Photosynthetic Organisms Mid JulyEvolution of Cells with Nucleus Mid NovemberFirst Organisms with Shells Late NovemberFirst Land Plants/Fish Mid DecemberDinosaurs became Dominant Dec 26Extinction of Dinosaurs Evening of Dec 31Human-like Animals 11:59:45-11:59:50Rome Ruled the Western World 11:59:59Modern Geology Started with Hutton A Year of Geologic Time 1second ≈ 200 years
Clair Patterson & the Age of the Earth In early 1950’s, Clair Patterson was a graduate student at the University of Chicago. Wanted to use lead isotope ratios to determine the Earth’s age, but the background level of lead contamination was too high Lead used in gasoline, paints, plumbing, solder (cans for food) and pesticides.
Clair Patterson & the Age of the Earth To accurately measure very low lead concentrations, Patterson created the modern laboratory ‘clean room’. in 1953, published estimate of Earth’s age as 4.55 BY (previously estimated at 3.3 BY)
By 1960’s, Patterson began to worry about the extent of lead contamination in our environment. Patterson discovered that modern humans had 700 to 1,200 times as much lead in their bones as pre- Columbian Incas. over 99% of the northern hemisphere atmospheric lead originated from human activity. The average atmospheric lead levels were 10 to 200 times higher than in pre- industrial times and up to 1,000 to 10,000 times higher in urban areas!
First recognition of the global scale and early history of lead pollution First recognition that essentially EVERYONE in 1950’s-60’s society suffered from low-level lead poisoning. Patterson campaigned extensively for lead removal, but was vigorously opposed by industry labs and some other scientists. Eventually, scientific data accumulated by Patterson and others led to the 1970 Clean Air Act By 1991, lead levels in Greenland snow had fallen by a factor of 7.5