Geologic time and dating. Geologic time refers to the ages relevant to Earth s history

Transcription

1 Geologic time and dating Most figures and tables contained here are from course text: Understanding Earth Fourth Edition by Frank Press, Raymond Siever, John Grotzinger, and Thomas H. Jordan Geologic time refers to the ages relevant to Earth s history This shows major events of Earth s history on a logarithmic scale In a logarithmic scale numbers are plotted by the exponent used to raise ten to in order to get to the respective number. (i.e. 10 plots a 1, 0.1 plots as -1, 1000 plots as 3) Earth changes on scales much larger than human experience 1

2 The following slides discuss basic principles used in relative age dating. Relative age dating is where geologist analyze relationships in the rock to describe a sequence of events as to how rocks were formed and deposited through time without regard to absolute age. Absolute dating is where a specific age (in years or millions or of years old are assigned to the rock) Absolute dating was not possible until radiometric age dating became available. Sedimentary rock records clues as to Earth s history Later tectonic uplift and erosion uncovers these rock. Geologist study rock to understand Earth s history Original horizontality the concept that rock units were originally deposited in relatively horizontal planes. New deposits Older folded and eroded rocks 2

3 Superposition the concept that in layered rock the oldest units are at the bottom and as we move vertically through the rock column we are looking at successively younger rock. Faunal succession is the observation that fossils (and therefore the life forms that left them behind) change throughout geologic time. As a result fossils can be used to correlate and calibrate rocks of different ages Index fossils are fossils used to identify rocks of similar ages. Index fossils have been calibrated using radiometric age dating and can also be used to identify the age of the rock that contains them. A good index fossil must: 1) Be short lived in geologic time (as a species) 2) Geographically wide spread 3) Have parts that are easily preserved (hard parts like teeth or shells) 4) Easily recognized (by an expert) 3

4 Trilobites are a good index fossil for much of the Paleozoic These are one species but the trilobite changed much throughout geologic time. Ammonites are also important index fossils. Their species ca me in many sizes and shapes and the shells are easily identified by the suture patterns. sutures Fossil assemblages (groups of fossils from a time period can be used to narrow ages more than one fossil alone) 4

5 Cross cutting relationships: a geological unit that cuts across and other unit will be the younger of the two. dike E D C B A batholith In this example: Young Igneous intrusion Dep E Dep D Dep C Dep B Dep A Old Cross cutting relationships: In this example: Young D D Dep D C B A C B A Old Fault Dep C Dep B Dep A fault 5

6 Sill vs. lava flow D Basalt C B A Vesicular texture in the basalt would indicate this is a lava and therefore older than unit D. Contact metamo rphism in unit D would indicate this is a sill and therefore younger than unit D Contact metamo rphism Inclusions: If fragments of one rock unit (A) are contained in an other (B) then the unit providing the rock fragments (A) must be older. Unconformities periods of erosion where the rock sequence has been lost. This must usually be preceded by uplift. Uplift will change a location from a depositional to an erosional environment 6

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9 Example of the development of a region and how to interpret the relative ages of events and units 9

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11 Seismic data is an important to to look at subsurface geology 11

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15 The geologic time scale is a description of rock units ordered in time. When these units were first describe radiometric age dating was not possible but using the geological principles discussed above there was a recognition that these rock units represented a progression of Earth history. 15

16 This time scale w as produced by correlating rocks from distant locations from one another and then continuing the description as a progression through time in younger rocks at the new locations 16

17 Absolute age dating by radiometric methods Types of nuclear decay: Beta decay: An electron escapes fro m a neutron and the neutron becomes a proton (e.g. Rb 87 -Sr 87 ). Atomic weight does not change atomic number does. Electron capture: a proton captures an electron and becomes a neutron (e.g. K 40 Ar 40 ). Atomic weight does not change atomic number does. Alpha- An alpha particle is emitted. An alpha particle is equivalent to a helium nucleus with an atomic weight of 4 (2 protons and 2 neutrons). This is part of the uranium lead decay sequence. 17

18 Beta decay Half life is the time it takes for ½ the parent product to decay to the daughter product 18

19 For example: At t=0 we will only have parent product. At t= 1 half-life there will be an equal amount of parent and daughter atoms. Hypothetically speaking if we start with 64 parent atoms of an element with 5,000 year half life, the following will be true: At t= parent atom daughter atoms number of half-lifes , , , , , Assumptions in the above plan for radiometric age dating are: 1) There was no daughter when the rock formed. There are in fact methods using many samples that enable us to estimate and correct for the amount of daughter product when the rock formed. 2) The system remained closed: did not loose daughter or parent product with time. There are methods to identify samples with that leaked. If a system leaks it is unlikely to leak in a regular way. If many minerals fro m the same rock are dated it is unlikely a leaky system will yield many mineral with the same age. Therefore this can be overcome by repeated measurements. There are other methods using multiple dating techniques that can also overcome this problem 19

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