Geologic Time

Geologic Time Introduction The Geologic Time Scale Relative Dating Fossils Radiometric Dating

GEOL131: Geologic Time Introduction

GEOL131: Geologic Time: Introduction Deep Time Current evidence indicates Earth is about 4.6 billion yrs old Dating of Earth minerals using radioactivity Dating of meteorites and moon rocks Rocks and fossils form the record of geologic time

Deep Time Two ways to date rocks GEOL131: Geologic Time: Introduction Deep Time Two ways to date rocks Relative to other rocks Numerically: age in years before present Relative dating principles are among oldest in geology Established in 1600s and 1700s Numerical (absolute) dating is one of geology’s youngest branches Post-World War II

The Geologic Time Scale GEOL131: Geologic Time The Geologic Time Scale

570 million to 4.6 billion yrs ago GEOL131: Geologic Time: The Time Scale, basic structure PRESENT Precambrian 570 million to 4.6 billion yrs ago EARTH FORMS

GEOL131: Geologic Time: The time scale shown to correct scale

The Precambrian 4.6 billion to 570 million years ago GEOL131: Geologic Time: The Time Scale The Precambrian 4.6 billion to 570 million years ago 88% of Earth history First single-celled organisms First multi-celled organisms

The Precambrian Very little known compared to more recent past GEOL131: Geologic Time: The Time Scale The Precambrian Very little known compared to more recent past Older rocks have been destroyed No organisms with hard parts, so no fossils

The Paleozoic Era First fish First land plants First land animals GEOL131: Geologic Time: The Time Scale The Paleozoic Era First fish First land plants First land animals Ended with largest mass extinction in Earth history 96% species mortality rate

Species existing before GEOL131: Geologic Time: The Time Scale “Adaptive radiation” Post-mass extinction, ecological niches are mostly empty Few surviving species give rise to many new species to fill these niches younger TIME MASS EXTINCTION older Species existing before mass extinction

Few extinction survivors GEOL131: Geologic Time: The Time Scale “Adaptive radiation” Post-mass extinction, ecological niches are mostly empty Few surviving species give rise to many new species to fill these niches younger TIME MASS EXTINCTION Few extinction survivors older

Many new species evolve from few extinction survivors GEOL131: Geologic Time: The Time Scale “Adaptive radiation” Post-mass extinction, ecological niches are mostly empty Few surviving species give rise to many new species to fill these niches younger TIME MASS EXTINCTION Many new species evolve from few extinction survivors older

The Mesozoic Era Dinosaurs dominant First flowering plants GEOL131: Geologic Time: The Time Scale The Mesozoic Era Dinosaurs dominant First flowering plants Ended by another mass extinction Likely caused by asteroid impact Mass extinction & adaptive radiation

The Cenozoic Era Mammals, including humans Most recent Ice Age GEOL131: Geologic Time: The Time Scale The Cenozoic Era Mammals, including humans Most recent Ice Age 2 mya – 10,000 yrs ago Mass extinction & adaptive radiation

GEOL131: Geologic Time Relative Dating

Principles Superposition Original Horizontality GEOL131: Geologic Time: Relative Dating Principles Superposition Original Horizontality Cross-cutting Relationships Inclusions

Superposition If Layer A is above Layer B, Layer A is younger GEOL131: Geologic Time: Relative Dating Superposition If Layer A is above Layer B, Layer A is younger Assumes layers are not overturned

Original Horizontality GEOL131: Geologic Time: Relative Dating Original Horizontality If a sedimentary rock layer is not horizontal, tectonic forces pushed it into its current position

Cross-cutting Relationships GEOL131: Geologic Time: Relative Dating Cross-cutting Relationships If A crosscuts B, A is younger Usually used with igneous intrusions and faults The igneous dike is the youngest feature, since it crosscuts everything else.

Cross-cutting Relationships GEOL131: Geologic Time: Relative Dating Cross-cutting Relationships

Inclusions If pieces of B are included in A, A is younger GEOL131: Geologic Time: Relative Dating Inclusions If pieces of B are included in A, A is younger Sandstone contains inclusions of granite, so sandstone is younger. Granite is younger than sandstone.

GEOL131: Geologic Time Unconformities

Unconformities A surface between layers that represents missing rock GEOL131: Geologic Time: Relative Dating Unconformities A surface between layers that represents missing rock Means there was an extended period of erosion

Unconformities: Three types GEOL131: Geologic Time: Relative Dating Unconformities: Three types Disconformity Angular unconformity Nonconformity Separates two horizontal layers Separates horizontal from tilted layers Separates sedimentary from non-sedimentary

Unconformities Unconformities in the Grand Canyon GEOL131: Geologic Time: Relative Dating: Unconformities Unconformities Unconformities in the Grand Canyon

GEOL131: Geologic Time Fossils

What Are Fossils? Any trace of past life preserved in rock GEOL131: Geologic Time: Fossils What Are Fossils? Any trace of past life preserved in rock Body (shells, bones, etc.) Trace (footprints, burrows, etc.)

GEOL131: Geologic Time: Fossils Fossil Ranges Each species has an age of earliest appearance in the rock record

GEOL131: Geologic Time: Fossils Fossil Ranges Most also have an age of final disappearance (extinction) Species don’t reappear after extinction

Fossils Ranges Fossil range: time between appearance and disappearance GEOL131: Geologic Time: Fossils Fossils Ranges Fossil range: time between appearance and disappearance Fossil ranges of multiple species can overlap

Fossils Ranges and Age Bracketing GEOL131: Geologic Time: Fossils Fossils Ranges and Age Bracketing Overlapping fossil ranges can be used to “bracket” the age of a rock layer

GEOL131: Geologic Time Radiometric Dating

Atomic Structure - review GEOL131: Geologic Time: Radiometric Dating Atomic Structure - review

Radioactivity Spontaneous changes in atomic nuclei GEOL131: Geologic Time: Radiometric Dating Radioactivity Spontaneous changes in atomic nuclei Loss or gain of protons or neutrons Occurs at known rates Each element’s decay rate is different

Radioactivity Parent: The element that changes GEOL131: Geologic Time: Radiometric Dating Radioactivity Parent: The element that changes Daughter: The product of the change Daughter can be A different element A different version (isotope)of the parent element

Radioactivity Some parent-daughter pairs commonly used in geology: GEOL131: Geologic Time: Radiometric Dating Radioactivity Some parent-daughter pairs commonly used in geology:

GEOL131: Geologic Time: Radiometric Dating Radioactivity Because rates are known and don’t change, radioactive decay can be used as a “clock” to measure geologic time

GEOL131: Geologic Time: Radiometric Dating Radioactivity Mineral crystallizes and incorporates radioactive parent atoms Parent atoms decay into daughter atoms at known rate Percentage of remaining parent can give age of mineral

GEOL131: Geologic Time: Radiometric Dating Half-life Time required for one-half of remaining parent atoms to decay

Example of radiometric dating GEOL131: Geologic Time: Radiometric Dating Example of radiometric dating If a mineral contains 50% K-40 and 50% Ar-40, how old is the mineral? How many half-lives of K-Ar have elapsed since the mineral formed? How many years is that equivalent to? - This is the age of the mineral

Example of radiometric dating GEOL131: Geologic Time: Radiometric Dating Example of radiometric dating How many half-lives of K-Ar have elapsed since the mineral formed? Mineral contains 50% parent and 50% daughter So, one K-Ar half-life has elapsed since mineral formed Percentage of remaining parent element

Example of radiometric dating GEOL131: Geologic Time: Radiometric Dating Example of radiometric dating 2) How many years is that? 1 half-life x 1.3 billion years = 1.3 billion years old

Half-life If 25% parent, 2 half-lives have passed GEOL131: Geologic Time: Radiometric Dating Half-life If 25% parent, 2 half-lives have passed If 12.5%, 3 half-lives Etc…

Applicability of radiometric dating GEOL131: Geologic Time: Radiometric Dating Applicability of radiometric dating Igneous rocks Minerals in rock did not come from an older rock So, mineral’s age is rock’s age Metamorphic rocks OK if mineral being analyzed formed during metamorphism

Applicability of radiometric dating GEOL131: Geologic Time: Radiometric Dating Applicability of radiometric dating Sedimentary rocks Very difficult Mineral grains usually derived from older rocks Weathering and erosion can “reset” clock by allowing daughter product to escape

GEOL131: Geologic Time End of Chapter