Copyright (c) 2005 Pearson Education Canada, Inc.8-1 PowerPoint Presentation Stan Hatfield. Southwestern Illinois College Ken Pinzke. Southwestern Illinois College Charles Henderson. University of Calgary Chapter 8 Geologic Time

Copyright (c) 2005 Pearson Education Canada Inc.8-2 Geologic Time 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) Geologic time scale – Earth’s history is exceedingly long

Copyright (c) 2005 Pearson Education Canada Inc.8-3 Relative Dating – Key Principles Law of Superposition Developed by the physician Nicolaus Steno in 1669 In an undeformed sequence of sedimentary rocks (or layered igneous rocks), the oldest rocks are on the bottom

Copyright (c) 2005 Pearson Education Canada Inc.8-4 Relative Dating – Key Principles Applying the Law of Superposition to Grand Canyon formations.

Copyright (c) 2005 Pearson Education Canada Inc.8-5 Principle of Original Horizontality Layers of sediment are generally deposited in a horizontal position Rock layers that are flat have not been disturbed Principle of Cross-Cutting Relationships Younger features cut across older features Relative Dating – Key Principles

Copyright (c) 2005 Pearson Education Canada Inc.8-6 Relative Dating – Key Principles Cross-cutting relationships represent one principle used in relative dating.

Copyright (c) 2005 Pearson Education Canada Inc.8-7 Inclusions An inclusion is a piece of rock that is enclosed within another rock Rock containing the inclusion is younger Unconformities An unconformity is a break in the rock record produced by erosion and/or nondeposition of rock units When there is no break the rocks are considered conformable Relative Dating – Key Principles

Copyright (c) 2005 Pearson Education Canada Inc.8-8 Unconformities Types of Unconformities –Angular unconformity – tilted rocks are overlain by flat-lying rocks –Disconformity – strata on either side of the unconformity are parallel –Nonconformity – metamorphic or igneous rocks in contact with sedimentary strata Relative Dating – Key Principles

Copyright (c) 2005 Pearson Education Canada Inc.8-9 Relative Dating – Key Principles This Grand Canyon cross-section illustrates the three basic types of unconformities.

Copyright (c) 2005 Pearson Education Canada Inc.8-10 Relative Dating – Key Principles Formation of an angular unconformity.

Copyright (c) 2005 Pearson Education Canada Inc.8-11 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 and early 1800s) noted that sedimentary strata in widely separated areas could be identified and correlated by their distinctive fossil content

Copyright (c) 2005 Pearson Education Canada Inc.8-12 Correlation often relies upon fossils Principle of Fossil Succession – fossil organisms succeed one another in a definite and determinable order that documents the evolution of life; therefore any time period can be recognized by its fossil content Index fossils – represent best fossils for correlation; they are widespread geographically and are limited to a short time span (i.e., they evolved rapidly) Correlation of Rock Layers

Copyright (c) 2005 Pearson Education Canada Inc.8-13 Correlation of Rock Layers Overlapping ranges of fossils held date rocks more exactly than using a single fossil.

Copyright (c) 2005 Pearson Education Canada Inc.8-14 Dating with Radioactivity Reviewing Basic Atomic Structure Nucleus –Protons – positively charged particles with mass –Neutrons – neutral particles with mass –Electrons – negatively charged particles that orbit the nucleus

Copyright (c) 2005 Pearson Education Canada Inc.8-15 Reviewing Basic Atomic Structure Atomic number –An element’s identifying number –Equal to the number of protons in the atom’s nucleus Mass number –Sum of the number of protons and neutrons in an atom’s nucleus Dating with Radioactivity

Copyright (c) 2005 Pearson Education Canada Inc.8-16 Reviewing Basic Atomic Structure Isotope –Variant of the same parent atom –Differs in the number of neutrons –Results in a different mass number than the parent atom Dating with Radioactivity

Copyright (c) 2005 Pearson Education Canada Inc.8-17 Radioactivity Spontaneous changes (decay) in the structure of atomic nuclei Types of radioactive decay Alpha particle 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 Dating with Radioactivity

Copyright (c) 2005 Pearson Education Canada Inc.8-18 Types of radioactive decay Beta particle emission –An electron (beta particle) is ejected from the nucleus –Mass number remains unchanged and the atomic number increases by 1 (remember a neutron is a combination of a proton and an electron) Dating with Radioactivity

Copyright (c) 2005 Pearson Education Canada Inc.8-19 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 Dating with Radioactivity

Copyright (c) 2005 Pearson Education Canada Inc.8-20 Dating with Radioactivity Common types of radioactive decay.

Copyright (c) 2005 Pearson Education Canada Inc.8-21 Parent – an unstable radioactive isotope Daughter product – the isotopes that result from the decay of a parent Half-Life – the time required for one-half of the radioactive nuclei in a sample to decay Dating with Radioactivity

Copyright (c) 2005 Pearson Education Canada Inc.8-22 Dating with Radioactivity The radioactive-decay curve shows change that is exponential.

Copyright (c) 2005 Pearson Education Canada Inc.8-23 Radiometric Dating Principle of Radioactive Dating –The percentage of radioactive atoms that decay during one half-life is always the same (50 percent) –However, the actual number of atoms that decay continually decreases –Comparing the ratio of parent to daughter yields the age of the sample Dating with Radioactivity

Copyright (c) 2005 Pearson Education Canada Inc.8-24 Radiometric dating Useful radioactive isotopes for providing radiometric ages –Rubidium-87 –Thorium-232 –Two isotopes of uranium (235 and 238) –Potassium-40 Dating with Radioactivity

Copyright (c) 2005 Pearson Education Canada Inc.8-25 Dating with Radioactivity

Copyright (c) 2005 Pearson Education Canada Inc.8-26 Radiometric dating Sources of error –A closed system is required –If temperature is too high, daughter products may be lost –To avoid potential problems, only fresh, unweathered rock samples should be used Dating with Radioactivity

Copyright (c) 2005 Pearson Education Canada Inc.8-27 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, archaeologists, historians, and geologists who study very recent Earth history Dating with Radioactivity

Copyright (c) 2005 Pearson Education Canada Inc.8-28 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 Dating with Radioactivity

Copyright (c) 2005 Pearson Education Canada Inc.8-29 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

Copyright (c) 2005 Pearson Education Canada Inc.8-30 Structure of the geologic time scale Names of the eons –Phanerozoic (“visible life”) – the most recent eon, began just over 540 million years ago –Proterozoic –Archean –Hadean – the oldest eon Geologic Time Scale

Copyright (c) 2005 Pearson Education Canada Inc.8-31 Structure of the geologic time scale Era – subdivision of an eon Eras of the Phanerozoic eon –Cenozoic (“recent life”) –Mesozoic (“middle life”) –Paleozoic (“ancient life”) Eras are subdivided into periods Periods are subdivided into epochs Geologic Time Scale

Copyright (c) 2005 Pearson Education Canada Inc.8-32 Geologic Time Scale

Copyright (c) 2005 Pearson Education Canada Inc.8-33 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 known in great enough detail –Also, first abundant fossil evidence does not appear until the beginning of the Cambrian Geologic Time Scale

Copyright (c) 2005 Pearson Education Canada Inc.8-34 Difficulties in dating the geologic time scale Not all rocks can be dated by radiometric methods –Grains comprising detrital sedimentary rocks are not the same age as the rock in which they formed –The age of a particular mineral in a metamorphic rock may not necessarily represent the time when the rock formed Geologic Time Scale

Copyright (c) 2005 Pearson Education Canada Inc.8-35 Difficulties in dating the geologic time scale Datable materials (such as volcanic ash beds and igneous intrusions) are often used to bracket various episodes in Earth history and arrive at ages Dates change as brackets become narrower and methods refined; e.g., base of Triassic is now 252 Ma, base of Permian is now 299 Ma, base of Cambrian is 543 Ma… Geologic Time Scale

Copyright (c) 2005 Pearson Education Canada Inc.8-36 End of Chapter 8