Lecture 23 geologic history and time

Chapter 12: History Before History (Part 1) Class Announcements:  Midterm exam results should be ready on Wednesday Updated grade record will be posted then  Exam key posted in glass case outside lecture hall  Field trips this week Be sure to read instructions on website and bring a pencil and eraser!  Extra Credit exercise on “The Core” is due Friday  An updated course schedule on website after class today TODAY’s LECTURE: Chapter 12: Intro to Historical Geology 1) 2) 3) 4) 5) Geologic Time Scale Relative age dating principles Unconformities Correlation of rock units Absolute dating using radioactivity 6) Development of the Geologic Time Scale Where did the geologic time scale come from? Built up over time by correlation and relative dating of rocks from around the world! Relative Dating: Key principles Telling time in Geology: ❖ Earth’s history is recorded in rocks of the crust… ❖ Geology seeks to unravel this history! ❖Two basic time keeping methods: Relative dating - Place events in their proper order Absolute dating - Determine, in years before the present, when event actually occurred Relative Dating: Main Principles Law of superposition Principle of lateral continuity Principle of original horizontality Principle of cross-cutting relationships Rule of inclusions Relative Dating: Main Principles Law of superposition: In a sedimentary sequence that has notisbeen oldest rock A bed of rock older overturned, than that above,the younger than that units below are always at the bottom Original lateral continuity Original horizontality Cross-cutting relationships Rule of inclusions Fig 12.04c W W Norton Relative Dating: Dating: Main Main Principles Principles Relative Youngest Oldest Principle of Superposition Law of superposition Principles of Relative Dating Principle of original lateral continuity: Most sedimentary rock units were originally deposited over a large area, but later became isolated by erosion or faulting Principle of original horizontality: Principle of cross-cutting relationships Rule of Inclusions Absolute vs relative age dating Fig 12.03 W W Norton Absolute Age Dating Using Radioactivity ◆ Chemistry review Model of an atom: el s n o r t c e # protrons = atomic #, defines the element nucleus protrons neutrons # neutrons can vary: “isotopes” Absolute Age Dating Using Radioactivity Overheads Half-life Concept: The half life of a radioactive isotope is the time it takes for half of the parent isotope to decay to the daughter product Radioactive decay occurs at a constant rate… Fig 12.17a W W Norton Absolute Dating Using Radioactivity Example: An isotope with a half life of year… Number of atoms Parent Now 1000 Next year 500 yrs from now 250 yrs from now 125 10 yrs from now Daughter 500 750 875 999 Errors in age dates can arise if the rock gains or loses parent of daughter isotopes… Fig 12.19 W W Norton Absolute Dating Using Radioactivity These systems are useful for dating very old rocks For some systems, the half-life exceeds even the age of the Universe Absolute dating using density of fission tracks… Fig 12.22b Defects in crystals formed by fission Particles Fig 12.21 W W Norton Correlation and dating using magnetic reversal patterns Where did the geologic time scale come from? Built up over time by correlation and relative dating of rocks from around the world! Fig 12.14a W W Norton Geologic Time Scale Absolute ages have been added to the time scale mostly by dating igneous rocks that bracket unit boundaries Eons Phanerozoic - Last 570 million years Eras Cenozoic - recent life 0-66.4 m.y.a (mammals, humans) Mesozoic - age of middle life 66.4-245 m.y.a (dinosaurs, 1st scrawny mammals) 245-570 m.y.a (fish, trilobites, clams, corals, ferns ) Paleozoic - ancient life Precambrian - from birth of Earth up to before complex life forms developed (algae, bacteria, some fossils without shells like jellyfish) Next time: Chapter 14: Resources! [...]... Stephen Marshak Unconformities dominate the geologic record… Most of the rock record consists of unconformities, or time gaps in the record That is… most of geologic history was unrecorded, or was recorded and later lost by erosion Example of an unconformity… Local removal of Rufus Lms by erosion Fig 12.11b W W Norton Rock sections at three different localities: A, B and C Relative Dating: Key principles... sedimentary units bounded by unconformities Principles of Relative Dating Unconformities ❖ Angular unconformity Tilted and/ or folded sedimentary rocks, overlain by younger layers That are more flat-lying Principles of Relative Dating Unconformities Three types… ❖ Angular unconformity Tilted and/ or folded sedimentary rocks, overlain by younger layers that are more flat-lying ❖ Disconformity ❖ Nonconformity...Fig 12.04fg W W Norton Fig 12.11c W W Norton Note that geologic units can also disappear between localities because of a lateral transition in environments if deposition Law of superposition Principle of original lateral continuity Principles of relative... horizontality: Sedimentary layers are originally deposited lying flat (i.e ~parallel to the Earth’s surface Principle of cross-cutting relationships Rule of Inclusions Sedimentary units deposited ~ flat and parallel to the earth’s surface Original horizontality So, what happened here? Principle of Original Horizontality What happened here? Principle of Original Horizontality Principles of Relative Dating