Presentation on theme: "Ways to tell the age of a rock"— Presentation transcript:
1 Ways to tell the age of a rock Relative Dating:Places events in geologic history in the proper order.The basis for the geologic time scaleMainly Sedimentary RocksDoes not provide a true “age”
2 Ways to tell the age of a rock Absolute Dating:All you need is a tiny sample of material (mineral, bone) no larger than a grain of rice.Gives us the true “age” of a fossil or rockMainly organic tissue or igneous crystalsMeasure the amount of unstable isotopes that have “decayed” to figure out age
3 Dating Rocks (determining their age, that is) Relative DatingSuperposition - The youngest rocks are on the top, oldest at the bottom.
5 Dating Rocks (determining their age, that is) Relative DatingSuperpositionCross-cutting relationships - Geologic features that cut through and across rocks are younger than those rocks.Mostly Faults and Igneous intrusions
7 Dating Rocks (determining their age, that is) Relative DatingSuperpositionCross-cutting relationshipsLaw of Inclusions - Rocks embedded in other rocks are older than those rocks they are embedded in.
12 What are the relative age relationships shown here? How can you tell a sill from a lava flow?M&W4 Fig. 17.4; M&W5 Fig. 17.4
13 Dating Rocks (determining their age, that is) Relative DatingSuperpositionCross-cutting relationshipsLaw of InclusionsLaw of Original Horizontality (and Lateral continuity)Law of Unconformities
14 A DISCONFORMITY is a boundary between two layers of non-continuous ages. This boundary is usually markedby an erosionalsurface and isoften irregular.M&W4 Fig. 17.8; M&W5 Fig. 17.8
15 An ANGULAR UNCONFORMITY is a disconformity between layers of different angles. The underlying layers are first tilted, then erosion scours away a new, horizontal surface.New, horizontal layers form on top
16 An NONCONFORMITY is a disconformity between different rock types, one of them sedimentary.
18 STRATIGRAPIC PRINCIPLES: FAUNAL SUCCESSION & CORRELATION Do three meters of strata at place A record the same amount of time as three meters at place B?How do we correlate events and the passage of time from one outcrop of rock to another and even around the world?Fossils!the main tool for correlating strata (and intervals of time represented by strata) from one rock outcrop to another outcrop.
19 STRATIGRAPIC PRINCIPLES: FAUNAL SUCCESSION & CORRELATION Different kinds of organisms have lived during different periods in Earth's history and then died off (or went extinct). This is called faunal succession.If a strata in different outcrops contain the same fossil assemblages, then the outcrops represent the same interval of time. These strata correlate.Figure 17.6: This generalized diagram shows how geologists use the principle of fossil succession to identify strata of the same age in different areas. The rocks in the three sections filled in with dashed lines contain similar fossils and are thus the same age. Note that the youngest rocks in this region are in section B, whereas the oldest rocks are in section C.M&W4 Fig. 17.6; M&W5 Fig. 17.6
20 STRATIGRAPIC PRINCIPLES: FAUNAL SUCCESSION & CORRELATION Formations: The fundamental stratigraphic units that are used to correlate stratifed rocks are called formations. Formations have between one (and preferably all) of the following characteristics:a distinctive set of physical properties (sedimentary rock type, bedding, grain size)a distinctive fossil assemblagehave a widespread (map scale) geographic distributionGeologists can thus construct a regional stratigraphy that represents much more geologic time than any single outcrop in any single location.
21 STRATIGRAPIC PRINCIPLES: FAUNAL SUCCESSION & CORRELATION From correlation of formations from different locations, the history of the entire region can be deciphered.Sequences of layers (from differente places) overlap, like when you create a panoramic photo from individual shots.M&W4 Fig ; M&W5 Fig
24 GEOLOGIC DATING: ABSOLUTE AGE DETERMINATION Radioactivity was first discovered by Henri Becquerel in 1896 and Polish-French chemist Marie Curie discovered that radioactivity produced new elements (radioactive decay).Ernest Rutherford first formulated the law of radioactive decay and was the first person to determine the age of a rock using radioactive decay methods.Marie CurieErnest Rutherford
25 GEOLOGIC DATING: ABSOLUTE AGE DETERMINATION The number of protons (the atomic number) is fixed for any element and is unique for each element but the number of neutrons in atoms of different elements can vary. Atoms of an element having different numbers of neutrons are referred to as the isotopes (of that element).Figure 3.3: Schematic representation of the isotopes of carbon. Carbon has an atomic number of 6 and an atomic mass number of 12, 13, or 14, depending on the number of neutrons in its nucleus.M&W4 Fig. 3.3; M&W5 Fig. 3.425
26 GEOLOGIC DATING: ABSOLUTE AGE DETERMINATION Radioactive decay occurs when an isotope of one element is transformed into a different element by changes in the nucleus. There are three different decay mechanisms:“Parent”“Daughter”Figure 17.18: Three types of radioactive decay. (a) Alpha decay, in which an unstable parent nucleus emits 2 protons and 2 neutrons. (b) Beta decay, in which an electron is emitted from the nucleus. (c) Electron capture, in which a proton captures an electron and is thereby converted to a neutron.M&W4 Fig ; M&W5 Fig
27 How can we tell age based on the number of parent isotopes? Radioactive isotopes “decay” at a particular rate. We express this rate as the “HALF-LIFE”, which is the time it takes for HALF of the parent isotopes to decay.
28 GEOLOGIC DATING: ABSOLUTE AGE DETERMINATION For radioactivity dating we use igneous rocks and minerals. The clock starts when radioactive atoms that are present in the magma get incorporated in the crystalline structure of certain minerals in the rocks.The crystals containing the parent atoms form and so we then have a “container” with parents that can begin decaying to form daughters.We can then use measure the parent-daughter ratio. This is our “atomic clock” that records the time since the rock crystallized.Figure 17.21: (a) Magma contains both radioactive and stable atoms. (b) As magma cools and begins to crystallize, some radioactive atoms are incorporated into certain minerals because they are the right size and can fit into the crystal structure. Therefore, at the time of crystallization, the mineral will contain 100% radioactive parent atoms and 0% stable daughter atoms. (c) After one half-life, 50% of the radioactive parent atoms will have decayed to stable daughter atoms.M&W4 Fig ; M&W5 Fig
29 GEOLOGIC DATING: ABSOLUTE AGE DETERMINATION To the oldest materials ever dated by the radioactive method are found in the Jack Hills of western Australia and are tiny zircon grains contained in sandstones and conglomerates. The zircons are 4.4 billion years old.The very remote “outback” of western Australia--the Jack HillsScanning electron microscope image of a Jack Hills zircon. Scale bar is 0.1 mm
30 The Half-Life of C14 is 5,730 years. C14 is an isotope of carbon that forms from Nitrogen in the atmosphere. Living things consume this radioactive carbon.Once dead, no new carbon is absorbed, and C14 turns back into Nitrogen.The Half-Life of C14 is 5,730 years.This method works best for fossils younger than 50,000 years. Why?(end)M&W4 Fig ; M&W5 Fig
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