Presentation on theme: "Unit 2 Lecture: Geologic Time Concepts for Relative & Absolute Dating of Geologic Strata."— Presentation transcript:
Unit 2 Lecture: Geologic Time Concepts for Relative & Absolute Dating of Geologic Strata
How do we measure time? Easiest solution – look at the rate of a convenient natural process. If the rate is constant, then use it as a timer. Examples: Revolution of the Earth (years) Rotation of the Earth (days) Rotation of the Earth (days) But what about geologic time? Same answer.
Useful “Timing” Processes : Biological – Use tree rings “dendrochronology” Bristle-Cone Pine Tree: ~ years Geological – Counting “varves” (annual sedimentation layers (Baron de Geer, 1878; Bradley, 1929). Geophysical- Measuring cooling rates for magmas, then extrapolating for the entire Earth. (Lord Kelvin, 1899) Geochemical - Measuring thickness of sedimentary layers and estimating erosion rates Nuclear – Measure rates of radioactive decay and proportions of parent & daughter isotopes.
Two Kinds of Ages Relative - Know Order of Events But Not Dates Civil War Happened Before W.W.II Bedrock in Wisconsin Formed Before The Glaciers Came Absolute - Know Dates Civil War World War II Glaciers Left Wisconsin About 11,000 Years Ago
18.2 The beginnings of geology In 1666, Nicholas Steno, a Danish anatomist, studied a shark’s head and noticed that the shark’s teeth resembled mysterious stones called “tonguestones”.
18.2 The beginnings of geology Steno theorized that tonguestones looked like shark’s teeth because they actually were shark’s teeth that had been buried and became fossils.
Relative Dating - placing the geologic occurrence in the proper sequence Which came first and WHY----- To establish a “relative” time scale, rules were discovered (principles of relative dating) – Nicholas Steno ( ) o Principle of Original Horizontality o Law of Superposition o Principle of Cross-Cutting relations o Principle of Inclusions
RELATIVE DATING & AGE RELATIVE DATING & AGE Relative Dating: putting rocks and geological events in correct chronological order Relative Age: how old something is in comparison to something else HOW? Use of sedimentary rocks Use of fossils Study of strata
Let’s unravel some geologic history from observations of various formations and their contacts Nicholas Steno – 1669 proposed the following relative dating principles The principle of Original Horizontality: Sedimentary rock layers are deposited as horizontal strata Any observed non-horizontal strata has been disturbed basin Sediment input A B C
LAW OF HORIZONTALITY LAW OF HORIZONTALITY Sediments are originally deposited in horizontal layers Folds or inclines: layers must have been deformed after they were deposited
The principle of Superposition In any undisturbed sequence of strata, the oldest stratum is at the bottom of the sequence and the youngest stratum is on top Unit 1 = old Unit 5 = young
LAW OF SUPERPOSITION LAW OF SUPERPOSITION For undisturbed rocks, the oldest layer is on the bottom and the youngest is on top (Supai is oldest)
Superposition : Mindoro Cut, Wisconsin
The principle of Cross Cutting relationships Any geologic feature that cuts across another geologic feature is younger Unit 1 = older Unit 6 = youngest Which came first unit 5 or Unit 6 ?
The principle of Inclusions A piece of rock (clast) that has become “included” in another rock body is older than the rock body it has become part of – why? Rock body A Intrusion of pluton B A A A Older (rock A was there first)
LAW OF INCLUSIONS If a rock body (Rock B) contained fragments of another rock body (Rock A), then Rock B must be younger than the fragments of rock it contained
Which “granites” are older and younger? OLDER YOUNGER
Youngest Oldest Superposition Original Horizontality Cross-cutting relationship ABCAsp Vn principle of inclusions Which granite is older? Older Younger
The principle of Unconformities Rock surface that represents a period of erosion or non-deposition Often represent a “gap” in time Three major types of unconformities o Disconformity o Angular unconformity o Non-conformity Unconformity Disconformity – unconformity in non-disturbed sedimentary layers Angular unconformity – unconformity lies between angled strata and overlying horizontal strata Non-conformity – sedimentary strata overlies crystalline rocks (ign and meta) Igneous or metamorphic rock
Formation of an unconformity
Layers are formed according to superposition.
Something happens to uplift the area folding faulting, etc.
Erosion wears away the uppermost layers
Area submerges and deposition begins again. Here’s the unconformity
What happened here? Deciphering Earth’s rock record…
Start by listing the events,such as deposition of..,erosion, intrusion of.., faulting of, etc. in order to piece together the story.. 1. Deposition of rock layer O 2. Deposition of rock layer N 3. Deposition of rock layer L 4. Intrusion of M (law of inclusions) 5. Erosion of surface(unconformity) 6. Depositionof H,I,J 7. Erosion (unconformity ) above J 8. Deposition of K 9. Erosion to present day surface
Let’s practice “Reading “ the rocks!! Determine the sequence of events in this geologic cross section:
The sequence of events is as follows: 1. Deposition of sedimentary rocks D 2. Fault B 3. Intrusion of igneous rock C 4. Erosion, forming the unconformity 5. Deposition of sedimentary rocks E
Fossils are the remains or traces of prehistoric life. They are important components of sediment and sedimentary rocks.
Specific conditions are needed for fossilization. Only a tiny percentage of living things became fossils.
Rocks can tell where fossils were made and when Rocks can tell when mass extinctions happened
the study of fossils remains of ancient life Body fossils vs. trace fossils Body = remain of organism, like bones; Trace = evidence of organism, like footprints
Hard Parts of Organisms: Bones Shells Hard Parts of Insects Woody Material
Soft or Easily Decayed Parts of Organisms: Internal Organs Skin Hair Feathers
Permineralization occurs when minerals carried by water are deposited around a hard structure.
The remains of an organism are likely to be changed over time. Molds and casts are another common type of fossil. Carbonization is particularly effective in preserving leaves and delicate animals. It occurs when an organism is buried under fine sediment.
A natural cast forms when flowing water removes all of the original tissue, leaving an impression.
Amber-preserved fossils are organisms that become trapped in tree resin that hardens after the tree is buried.
Petrified Wood Dinosaur Tracks Impressions Carbon Film Amber Casts
Fossil that defines and identifies geologic periods; often in only one layer of rock Easily recognizable Short-lived (found only in a few layers of rock worldwide) Wide distribution (geographic range)
Ammonite fossils are found worldwide, but they existed for only a very specific period of time this means ammonites are found in very specific layers of rock when an index fossil is found, the age of the rocks it is preserved in can be determined
The principle of fossil succession means that fossils can be used to identify the relative age of the layers of a rock formation. The organisms found in the top layers appeared after the organisms found in the layers below them.
Fossils are found in a predictable sequence Fossils in rock B are older then fossils in rock A
What kind of rocks are these fossils in? Which layer is oldest? Which layer is youngest? How do you know?
GEOLOGIC TIME SCALE a series of time intervals that divides Earth ’ s history Each layer of rock represents specific interval of time Index fossils help determine specific period Time periods divided by specific events like mass extinctions
Absolute Time Absolute Time - Numerical age determination of strata, events, and geologic structures from radiometric dating techniques.
Absolute Time Hourglass Think of an Hourglass timer (the term used by Arthur Holmes). Some initial quantity reduces, while its product accumulates at a constant rate. NO “sand” can be added or removed at any point in the process (closed system). Knowing the rate, and measuring quantities allows us to calculate the TIME duration for the process.
Absolute Time Early Attempts Bishop James Ussher (Prelate of Ireland) (1600s) Used O.T. biblical chronologies to date the “creation” October 22, 4004 B.C. (Sunday) Georges Buffon (1700s) Used a measured cooling rate from metal & non-metal balls to estimate the age for a molten Earth to cool. Earth’s Age = 75,000 yrs.
And others… John Joly (1889) acting upon a suggestion from Edmund Halley, estimated the ocean’s salinity & its rate of increase. Age: 90 million years Lord Kelvin (1899) estimated the Earth’s thermal gradient. Comparing this to cooling rates for known materials he said: Age : 20 – 100 million years (max)
A few more… Various geologists (1800s) estimated sedimentation & erosion rates and compared these to sediment thicknesses. Age : ~ 3 million to 1.5 billion years Arthur Holmes (1900s) first to use Uranium decay techniques. Age of Earth: ~ 4 billion years pЄ boundary: ~ 600 million
Basic Atomic Structures Orbiting the nucleus are electrons, which are negative electrical charges. Atomic number is the number of protons in the atom’s nucleus. Mass number is the number of protons plus the number of neutrons in an atom’s nucleus
ISOTOPES: Isotopes are atoms of an element that differ in their number of neutrons. neutronsprotrons
RADIOMETRIC DATING Radioactivity is the spontaneous decay of certain unstable atomic nuclei. Radiometric dating provides an accurate way to estimate the age of fossils. Radiometric dating uses the decay of unstable isotopes.
Radiometric Dating Each radioactive isotope has been decaying at a constant rate since the formation of the rocks in which it occurs. Radiometric dating is the procedure of calculating the absolute ages of rocks and minerals that contain radioactive isotopes
Radiometric Dating Techniques Radioactive elements decay at constant rates. There are various decay processes. see chart → If we can measure: number of Parent & Daughter isotopes, and the decay rate, then we can calculate an age
Radiometric Dating As a radioactive isotope decays, atoms of the daughter product are formed and accumulate. Each radioactive isotope has its own unique half-life. A half-life is the time it takes for half of the parent radioactive element to decay to a daughter product.
The key is the radioactive “Half Life” The idea is: Parents decay into Daughters P radioactive → D stable The rate of this decay is constant. A period of time exists during which ½ of the P isotopes will decay into D’s. This is called the half life, t ½. Since the rate is constant, so is the t ½. Isotopes used for geologic dating are called: “Geochronometer Isotopes”
Okay, so does this work? Let’s not get too technical. What we do is use a radioactive isotope’s “half life”. If we know how long a half-life is, then all we need to do is measure the number of half-lives that have elapsed for a particular sample.
An example would be nice… Okay. We measure P & D in a rock sample. The ratio of P:D is 1:3. Or…25% of P remains Look at chart. 25% P corresponds to 2 half lives. If a half life is 200,000 yrs, then this sample is: 2 hl x 200,000 yr/hl 400,000 years old “hl” – half life
–A half-life is the amount of time it takes for half of the isotope to decay. Radiometric dating uses decay of unstable isotopes. –Isotopes are atoms of an element that differ in their number of neutrons.
Radiometric Dating: Half-Life
What if there’s been 2.4 or 1/3 of t ½ ? Okay. In the “real world” of geochronology things can get a bit more tricky. We have equations that we use to calculate ages that don’t really use the t ½ approach…directly. Like: Age = 1/λ ln (D/P +1) This is good because we can then use statistics to evaluate the reliability of the age we’ve found. If an age passes the test, its called an isochron age. If it fails, then it’s called a errorchron, and isn’t used.
Dating with Carbon-14 Radiocarbon dating is the method for determining age by comparing the amount of carbon-14 to the amount of carbon-12 in a sample. When an organism dies, the amount of carbon-14 it contains gradually decreases as it decays. By comparing the ratio of carbon-14 to carbon-12 in a sample, radiocarbon dates can be determined.
Present Radiometric Dating Methods Cosmogenic C Yr. Primordial K-Ar (K-40) 1.25 B.Y. Rb-Sr (Rb-87) 48.8 by U M.Y.
One last thing “Radiocarbon” dating is rarely used in geology. The t ½ of 14 C is only 5730 yrs. After t ½ ’s it’s reliability becomes questionable. Also, 14 C is created in the atmosphere at uneven rates. 14 C decays into 12 C leaving 14 N behind…so the P/D ratio only tells you the “age since death” for living things. It is useless for rocks…but absolutely great for archaeologists, who use it as far back as ~ 50,000 yrs Don’t confuse radiocarbon with geologic dating!
In practice, both relative and absolute dating are combined, following a procedure like this: Igneous rocks, such as lava flows, volcanic ash beds, and intrusions) are dated radiometrically. The dates of fossil-bearing sedimentary rocks are in a certain area are bracketed using the dates of associated igneous rocks which have been dated radiometrically. The fossil-bearing sedimentary rocks are correlated with sedimentary rocks in other areas which contain the same fossils. The age of the rocks in other areas is determined indirectly, from the ages of the fossils they contain..