Presentation on theme: "Geologic Time. How much time ? Determining geological ages Relative age dates – placing rocks and events in their proper sequence of formation Numerical."— Presentation transcript:
Determining geological ages Relative age dates – placing rocks and events in their proper sequence of formation Numerical dates – specifying the actual number of years that have passed since an event occurred (known as absolute age dating) BUT number of years is just an arbitrary time scale we use
Early attempts to calculate the Earths Age: Rate sediments are deposited Determine rate of sedimentation determine total sediment thickness ---> get the age of the Earth BUT values ranged from 3 million years to 1.5 billion!
Correlation of rock layers Matching of rocks of similar ages in different regions is known as correlation Correlation often relies upon fossils William Smith (late 1700s) noted that sedimentary strata in widely separated area could be identified and correlated by their distinctive fossil content
So, how old is the Earth? Up to now we only discussed the relative age of rock units Using: the principle of superposition (oldest is always on the bottom) Fossils that existed during a certain time and then disappeared BUT: how do we know the absolute age??
Time in the most general sense is a measure of how long something takes (measure of duration). Time itself does not make things happen. Its not a force. Its the processes that operate over some duration that result in changes We decided to use time as a measure of duration We decided that one year is how long it takes for the Earth to rotate around the sun. We decided that one day is the time it takes the Earth to rotate around its axis once.
So, to figure out the duration the Earth has been existing, We need to find a natural process that occurs throughout the Earths history and that we can observe. We need an absolute clock that started ticking when the Earth was formed and is still ticking now.
Radioactive decay of elements is the smoking gun This process occurs over a long duration and we can measure the products that result from this process. Parent atomDaughter atom decay If we can measure the rate of decay (or how long it takes for a parent atom to change into the daughter atom) we have a clock.
We know that radioactive decay occurs because of the release of harmful radioactive emissions that can destroy organic cells and destroy life. The radioactive emissions are a product of the decay process
Marie Sklodowska Curie was one of the first woman scientists to win worldwide fame, and indeed, one of the great scientists of the 20th century. Winner of two Nobel Prizes (for Physics in 1903 and for Chemistry in 1911), she performed pioneering studies with radium and contributed profoundly to the understanding of radioactivity...carried out an extensive test of all chemical elements and their compounds for radioactivity, and found that thorium emits radiation similar to that of uranium. Comparing radioactivity of uranium ores with that of metallic uranium, she noticed that ores are about five times more radioactive than would be expected from their uranium content. This indicated that the ores must contain small amounts of some other radioactive substances much more active than uranium itself. she separated polonium and radium it was two million times more radioactive than uranium.
Using radioactivity in dating Reviewing basic atomic structure Nucleus –Protons – positively charged particles with mass –Neutrons – neutral particles with mass –Electrons – negatively charged particles that orbit the nucleus
Using radioactivity in dating Reviewing basic atomic structure Atomic number –An elements identifying number –Equal to the number of protons in the atoms nucleus Mass number –Sum of the number of protons and neutrons in an atoms nucleus
Using radioactivity in dating Reviewing basic atomic structure Isotope –Variant of the same parent atom –Differs in the number of neutrons –Has the same number of protons –Results in a different mass number than the parent atom
An Example of Isotopes: lets look at Hydrogen (H) Has mass number 1 -> it has one proton in the nucleus and one electron in the shell -> has zero neutrons in the nucleus An isotope of Hydrogen is Deuterium (D) Has mass number 2 -> it has one proton and one electron -> it has one neutron It is twice as heavy as Hydrogen but has the same charge 1+0 = 1 1+1 = 2
An Example of Isotopes: lets look at Carbon (C) Has mass number 12 -> it has six protons in the nucleus and six electron in the shell -> has six neutrons in the nucleus An isotope of Carbon is Carbon-14 (C-14) Has mass number 14 -> it has six protons and six electrons -> it has eight neutrons It is heavier than C-12 but has the same charge It is also unstable and decays to Nitrogen-14 6+6 = 12 6+8 = 14
C 6 12.0107 Carbon Atomic number: number of protons Mass of Carbon (g/mol) This mass includes contributions from all carbon isotopes (C-12, C-13, C14…) that occur in nature You see: mass is very close to 12. So, C-12 is by far the most abundant isotope in nature Name
Why are isotopes important to understand? Radioactive decay can produce a different isotope from a parent atom 238 92 U Mass number Atomic number Uranium
U 238.0289 Uranium Atomic number: number of protons Mass of Uranium (g/mol) Occurring in nature 92
Using radioactivity in dating Radioactivity Spontaneous changes (decay) in the structure of atomic nuclei Types of radioactive decay Alpha emission –Emission of 2 protons and 2 neutrons (an alpha particle) –Mass number is reduced by 4 and the atomic number is lowered by 2
Alpha Decay 238 92 U 234 90 Th + alpha particle The alpha particle consists of 2 protons and 2 neutrons (it is a Helium atom) Mass number Atomic number
Using radioactivity in dating Types of radioactive decay Beta emission –An electron (beta particle) is ejected from the nucleus –Mass number remains unchanged and the atomic number increases by 1 –This produces radioactive gamma rays
40 19 K 40 20 Ca Mass number Atomic number + beta particle The beta particle is an electron (e - ) Fermi showed in 1934 that beta decay is the transformation of a Neutron into a proton and an electron.
Using radioactivity in dating Types of radioactive decay Electron capture –An electron is captured by the nucleus –The electron combines with a proton to form a neutron –Mass number remains unchanged and the atomic number decreases by 1
14 C N + beta particle 87 Rb 87 Sr + beta particle Mass number 238 U 207 Pb + alpha particles Series of decays 232 Th 208 Pb + alpha particles Important decays for dating geologic materials
40 K + e 40 Ar 11% of K 40 K Ca + beta particle remaining 89%
Why does radioactive decay offers a dependable means of keeping time? Average rate of decay is fixed Does not vary with any changes in chemical or physical conditions Once a quantity of a radioactive element is created somewhere in the universe, it starts to act like a balance wheel of a clock, steadily firing off one atom after another at a steady rate
To tell time we need: Some kind of reference (like the numbers of a digital watch) Numbers we use to read the radioactive clock are supplied in the form of new atoms (the daughter elements) that form from the parent elements. Need to count the daughter elements and if we know the rate of decay, we can work back to the time when there were no daughter elements but only parents. This assumes that the daughter elements are exclusively produced by radioactive decay. ( 40 Ar only produced by 40 K)
Rate of decay varies from one element to another Half Life: the time it takes for one half of the original number of radioactive atoms to decay 14 C 87 Rb 5570 years Half life 47 billion years Atom 40 K 1.3 billion years
After (half lives) Fraction of atoms left 11/25570 21/411,140 31/816,710 41/1622,280 51/3227,850 Years passed C-14 C-14 is usually used to date events that happened less that 30,000 years ago. After that, too few atoms are left to measure accurately.
After (half lives) Fraction of atoms left 11/21.3 billion 21/42.6 billion 31/83.9 billion 41/165.2 billion Years passed K-40 decays to Ar-40 SO: K-Ar method of dating rocks is very good. Lots of minerals have K (I.e. Feldspars). Half life is long enough so that we can still accurately measure the parent and daughter for geologically relevant processes
After (half lives) Fraction of atoms left 11/247 billion Years passed Rb-87 Rb-87 is used to date very old rocks. After 4 billion years only about 1/10 of a single half life has passed. SO: in the oldest rocks we can measure the amount of Rb-87 and the amount of Sr-87. We know the half life and can calculate the age of the rock.
Using radioactivity in dating Parent – an unstable radioactive isotope Daughter product – the isotopes resulting from the decay of a parent Half-life – the time required for one-half of the radioactive nuclei in a sample to decay
When does the clock start? It starts once the minerals in a rock are formed. Once minerals are formed the daughter elements cannot escape that mineral and are trapped in the mineral. U Pb Molten magma: daughter escapes U Pb Mineral: daughter trapped
Using radioactivity in dating Radiometric dating Sources of error –A closed system is required (no Ar escape) –To avoid potential problems, only fresh, unweathered rock samples should be used –No re-melting or metamorphism must have occurred. That would re-set the clock by allowing earlier formed daughters to escape.
Using radioactivity in dating Dating with carbon-14 (radiocarbon dating) Half-life of only 5730 years Used to date very recent events Carbon-14 is produced in the upper atmosphere Useful tool for anthropologists, archeologists, and geologists who study very recent Earth history
14 C Tree takes up C-14 to make organic material Tree dies: Does not take up C-14 anymore. C-14 decay replenished with New C-14 from atmosphere C-14 decay not replenished with C-14 from atmosphere -> clock starts
Using radioactivity in dating Importance of radiometric dating Radiometric dating is a complex procedure that requires precise measurement Rocks from several localities have been dated at more than 3 billion years Confirms the idea that geologic time is immense
Recent developments: Dating of Meteorites Big surprise: All Meteorites are of the same age (4.6 billion) No matter of their composition, or when they fell down on the Earth.
The fact that there are no meteorites of any other age suggests strongly that they originated in other bodies in the solar system and formed at the same time the Earth formed.
How old is the Earth? Oldest Earth rocks (Isua Stones)3.8 billion Oldest Moon rocks4.2 billion All Meteorites4.6 billion
Isua stones oldest rocks on Earth, 3.8 billion years old Greenland Godthaabfiord
Geologic time scale The geologic time scale – a calendar of Earth history Subdivides geologic history into units Originally created using relative dates Structure of the geologic time scale Eon – the greatest expanse of time
Geologic time scale Structure of the geologic time scale Names of the eons –Phanerozoic (visible life) – the most recent eon, began about 540 million years ago –Precambrian - everything before 540 million –-> 4600 million to 540 million
Geologic time scale Structure of the geologic time scale Eras of the Phanerozoic (540 Ma to now) –Cenozoic (recent life) –Mesozoic (middle life) –Paleozoic (ancient life) Eras are subdivided into periods
Geologic time scale Precambrian time Nearly 4 billion years prior to the Cambrian period Not divided into smaller time units because the events of Precambrian history are not know in great enough detail –First abundant fossil evidence does not appear until the beginning of the Cambrian
Time It is a measure of duration and tells us in a scale so that we understand how long something takes. Our time scale is designed so that it scales to our life-time and daily lives. Years, months, weeks, hours, minutes. Time scales outside of this customary scale are difficult for us to intuitively understand. Long time scales: 1 Billion years (1,000,000,000 years) Short time scales: 1 nano second (0.000000001 second)
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