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 Geologic Time Scale Relative age dating principles 3) Unconformities 4) Correlation of rock units 5) Absolute dating using radioactivity 6) Development of the Geologic Time Scale  

Built up over time by correlation and relative dating of rocks Where did the geologic time scale come from? Built up over time by correlation and relative dating of rocks from around the world!

Telling time in Geology: 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

Law of superposition: In a sedimentary sequence Relative Dating: Main Principles A bed of rock is older than that above, younger than that below Law of superposition: In a sedimentary sequence that has not been overturned, the oldest rock units are always at the bottom. Original lateral continuity Original horizontality Cross-cutting relationships Rule of inclusions

Fig. 12.04c W. W. Norton

Relative Dating: Main Principles Youngest Oldest Principle of Superposition

Law of superposition 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 Principles of Relative Dating

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.

are originally deposited lying flat (i.e. ~parallel to Law of superposition Principle of original lateral continuity Principle of original horizontality: Sedimentary layers are originally deposited lying flat (i.e. ~parallel to the Earth’s surface. Principle of cross-cutting relationships Rule of Inclusions Principles of relative dating

Sedimentary units deposited ~ flat and parallel to the earth’s surface. Original horizontality

Principle of Original Horizontality So, what happened here? Principle of Original Horizontality

What happened here? Principle of Original Horizontality

Principle of cross-cutting relationships: Principles of Relative Dating Law of superposition Lateral of original continuity Principle of original horizontality Principle of cross-cutting relationships: If one rock unit cuts across another, it is younger! Rule of Inclusions

Fig. 12.04h W. W. Norton If something cuts something else, it has to be younger than what it cuts. Fig. 12.04h W. W. Norton Principle of cross-cutting relationships.

Which vein is youngest, ‘A’ or ‘B’? Principle of Cross-cutting relationships

Rule of Inclusions: If a unit contains a clast, Principles of Relative Dating Law of superposition Lateral of original continuity Principle of original horizontality Principle of cross-cutting relationships Rule of Inclusions: If a unit contains a clast, or inclusion of another rock unit, the clast or inclusion is older.

Fig. 12.04i W. W. Norton If a rock unit contains a piece of another rock unit, the piece is older. Fig. 12.04i W. W. Norton Rule of Inclusions

Principles of Relative Dating Rule of Inclusions: Fragments of one rock included in another. Example from Papago Park fieldtrip…

Unconformities are buried surfaces of erosion or non-deposition… Modern erosion surface Fig. 12.04e 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.

Fig. 12.11b W. W. Norton 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

Unconformities Three types: Angular unconformity Disconformity Relative Dating: Key principles Three types: Angular unconformity Disconformity Nonconformity

Principles of Relative Dating Conformable layers: Layers in a sequence that were deposited w/out noticeable interruption

Principles of Relative Dating Conformable layers:Groups of conformable layers define packages of sedimentary units bounded by unconformities.

Unconformities Angular unconformity Principles of Relative Dating Tilted and/or folded sedimentary rocks, overlain by younger layers That are more flat-lying.

Unconformities Three types… Angular unconformity Disconformity Principles of Relative Dating Three types… Angular unconformity Disconformity Nonconformity Tilted and/or folded sedimentary rocks, overlain by younger layers that are more flat-lying.

Stephen Marshak W. W. Norton Angular Unconformities Fig. 12.08ab Stephen Marshak W. W. Norton

Recipe for an angular unconformity Deposition Folding/Uplift Erosion Subsidence/more erosion

Fig. 12.09a W. W. Norton

Unconformities Three types… Angular unconformity Disconformity Principles of Relative Dating Three types… Angular unconformity Disconformity Nonconformity Layers on either side of unconformity are parallel. No tilting, just erosion.

Fig. 12.09c W. W. Norton

Fig. 12.04j W. W. Norton Instrusion of magma and contact metamorphism… Step 1 in the creation of a nonconformity.

Making a nonconformity… Fig. 12.09b W. W. Norton

Unconformities Three types Angular unconformity Disconformity Relative Dating: Key principles Three types Angular unconformity Disconformity Nonconformity Erosion surface separating sedimentary layers and metamorphic or plutonic igneous rocks.

Nonconformity Example from Papago Park fieldtrip… Erosion surface developed on igneous or metemorphic basement before sedimentary units deposited…. Example from Papago Park fieldtrip…

Chapter 12: History Before History (Part 1)

Class Announcements: Midterm exam results. Updated grade record will be posted today on website. Field trips this week. Be sure to read instructions on website and bring a pencil and eraser! Extra Credit exercise on “The Core” due Friday. Homework on Relative Dating due Friday. An updated course schedule on website today.

TODAY’s LECTURE: Chapter 12: Intro. to Historical Geology Animations to review principles of relative dating 2) Correlation of rock units 3) Absolute dating using radioactivity 4) Development of the Geologic Time Scale  

Reconstructing Geologic History… Dike B Dike A Fault B Fault A

In-Class Problem: Place each labeled rock unit in its proper place in a relative time sequence from oldest to youngest.

Fig. 12.05 W. W. Norton Name:___________________Student ID__________ Homework Problem: Place each labeled rock unit in its proper place in a relative time sequence, from oldest to youngest.

Youngest Event Rock Unit or Feature 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. Oldest Event

Correlation by: Rock type Paleoenvironment Age Physical criteria Correlation of Rock Units Correlation by: Rock type Paleoenvironment Age Types of Evidence Physical criteria Fossils

Position of a bed in sequence. Distinctive lithologies (rock types, Correlation of Rock Units Physical Criteria: Position of a bed in sequence. Distinctive lithologies (rock types, mineralogy, sedimentary structures, color, etc.). Unique paleoenvironment implied by above.

Correlation based on rock type…

Correlation using distinctive associations of sedimentary structures (paleoenvironments). Fig. 12.04b Stephen Marshak Ancient mudcracks

Modern mudcracks Fig. 12.04a Stephen Marshak

Correlation Fossils and Biostratigraphy Principle of Fossil Succession: In sedimentary sequences, fossil species succeed one another in and orderly, systematic fashion, reflecting cycles of evolution and Extinction. Fossils can be used to assign relative ages to rock units based on the fact that each fossil species lived for a only brief time and then died out.

“Index fossils” Geographically wide-spread. Lived form Correlation of Rock Units “Index fossils” Geographically wide-spread. Lived form a short time span.

Index zones and correlation using fossils. W. W. Norton

Principle of Fossil Succession Biostratigraphic correlation

correlated sequence 1 Kansas 2 Indiana 3 Ohio 4 Pennsylvania Mostly nonmarine limestone correlated sequence

Absolute vs. relative age dating Fig. 12.03 W. W. Norton

Chemistry review electrons nucleus Absolute Age Dating Using Radioactivity Chemistry review Model of an atom: electrons # protrons = atomic #, defines the element nucleus # neutrons can vary: “isotopes” protrons neutrons

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

Number of atoms Parent Daughter Now 1000 0 Next year 500 500 Absolute Dating Using Radioactivity Example: An isotope with a half life of 1 year… Number of atoms Parent Daughter Now 1000 0 Next year 500 500 2 yrs from now 250 750 3 yrs from now 125 875 10 yrs from now 1 999

Fig. 12.19 W. W. Norton 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… Defects in crystals formed by fission Particles. Fig. 12.22b

Fig. 12.21 W. W. Norton Correlation and dating using magnetic reversal patterns.

Built up over time by correlation and relative dating of rocks 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

0-66.4 m.y.a 66.4-245 m.y.a 245-570 m.y.a Geologic Time Scale Eons Absolute ages have been added to the time scale mostly by dating igneous rocks that bracket unit boundaries. Eons Phanerozoic - Last 570 million years Cenozoic - recent life Mesozoic - age of middle life Paleozoic - ancient life Precambrian - from birth of Earth up to before complex life forms developed Eras 0-66.4 m.y.a (mammals, humans) 66.4-245 m.y.a (dinosaurs, 1st scrawny mammals) 245-570 m.y.a (fish, trilobites, clams, corals, ferns ) (algae, bacteria, some fossils without shells like jellyfish)

Next time: Chapter 14: Resources!