Geologic time

Geologic Time Geologic time scale Relative dating principles Absolute dating w/ radioactivity Correlation of rock layers  

Relative Geologic Time Scale The relative geologic time scale has a sequence of eons eras periods epochs but no numbers indicating how long ago each of these times occurred

0-66.4 m.y.a 66.4-245 m.y.a 245-570 m.y.a Geologic time scale 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)

“Telling time geologically” Relative Dating: Key principles “Telling time geologically” Earth’s history concealed in rocks Goal of geology: unraveling Earth’s history Principle time keeping devices: Relative dating - putting rocks/events in proper order Absolute dating - determining event’s actual time

Relative Dating: Key principles A bed of rock is older than that above, younger than that below Law of superposition Principle of original horizontality Principle of cross-cutting relationships Inclusions Unconformities

Relative Dating: Key principles Law of superposition Principle of original horizontality Principle of cross-cutting relationships Inclusions Unconformities A bed of rock is older than that above, younger than that below Sedimentary layers deposited flat

Relative Dating: Key principles Law of superposition Principle of original horizontality Principle of cross-cutting relationships Inclusions Unconformities A bed of rock is older than that above, younger than that below Sedimentary layers deposited flat Faults & dikes are younger than beds they cross

Relative Dating: Key principles Inclusions Unconformities Fragments of one rock enclosed in another

Fragments of one rock enclosed in another

Relative Dating: Key principles Inclusions Unconformities Fragments of one rock enclosed in another Break in time in deposition • An unconformity represents a long period during which deposition stopped, erosion removed previously formed rocks, and then deposition resumed. (Conformable layers: deposited w/out interruption)

Unconformities 3 types Angular unconformity Disconformity Relative Dating: Key principles 3 types Angular unconformity Disconformity Nonconformity • An angular unconformity indicates that during the pause in deposition, a period of deformation (folding or tilting) and erosion occurred. • A disconformity is when two sedimentary rock layers are separated by an erosional surface. • A nonconformity is when the erosional surface separates older metamorphic or intrusive igneous rocks from younger sedimentary rocks.

Unconformities 3 types Angular unconformity Disconformity Relative Dating: Key principles 3 types Angular unconformity Disconformity Nonconformity Tilted/folded sedimentary rocks overlain by younger, more flat layers

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

Unconformities 3 types Angular unconformity Disconformity Relative Dating: Key principles 3 types Angular unconformity Disconformity Nonconformity Layers on either side of unconformity are parallel

Unconformities 3 types Angular unconformity Disconformity Relative Dating: Key principles 3 types Angular unconformity Disconformity Nonconformity Interface between sedimentary layers and metamorphic or igneous rock

Unconformity Types To view this animation, click “View” and then “Slide Show” on the top navigation bar.

Relative Dating Principles To view this animation, click “View” and then “Slide Show” on the top navigation bar.

Physical criteria Fossils Correlation of rock layers Matching up rocks from different places that are similar in age Physical criteria Fossils

position of a bed in sequence of beds distinct/uncommon minerals Correlation of rock layers Physical criteria (OK for short distances) position of a bed in sequence of beds distinct/uncommon minerals

“principle of fossil succession” Correlation of rock layers Fossils fossil organisms succeed each other in systematic fashion any time period can be determined by fossil content “principle of fossil succession”

“Index fossils” geographically wide-spread short span of time Correlation of rock layers “Index fossils” geographically wide-spread short span of time T&L Figure 8.10

Overlapping Ranges of Fossils Makes no sense without caption in book

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

Chemistry review electrons nucleus Absolute dating w/ radioactivity Atom model electrons # protrons = atomic #, defines the element nucleus # neutrons can vary: “isotopes” protrons neutrons

# of atoms Parent Daughter Now 1000 0 Next year 500 500 Absolute dating w/ radioactivity Example: an isotope with a half life of 1 year # 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

Radioactive Decay To view this animation, click “View” and then “Slide Show” on the top navigation bar.

Absolute dating w/ radioactivity