Geologic Time Geologic Time Introduction The Geologic Time

Geologic Time

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

GEOL 131: Geologic Time Introduction

GEOL 131: 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

GEOL 131: 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 1600 s and 1700 s • Numerical (absolute) dating is one of geology’s youngest branches – Post-World War II

GEOL 131: Geologic Time The Geologic Time Scale

GEOL 131: Geologic Time: The Time Scale, basic structure PRESENT EARTH FORMS Precambrian 570 million to 4. 6 billion yrs ago

GEOL 131: Geologic Time: The time scale shown to correct scale

GEOL 131: 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

GEOL 131: 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

GEOL 131: 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

GEOL 131: Geologic Time: The Time Scale “Adaptive radiation” • Post-mass extinction, ecological niches are mostly empty younger TIME • Few surviving species give rise to many new species to fill these niches MASS EXTINCTION Species existing before mass extinction older

GEOL 131: Geologic Time: The Time Scale “Adaptive radiation” • Post-mass extinction, ecological niches are mostly empty younger MASS EXTINCTION TIME • Few surviving species give rise to many new species to fill these niches Few extinction survivors older

GEOL 131: Geologic Time: The Time Scale “Adaptive radiation” • Post-mass extinction, ecological niches are mostly empty younger MASS EXTINCTION TIME • Few surviving species give rise to many new species to fill these niches Many new species evolve from few extinction survivors older

GEOL 131: Geologic Time: The Time Scale The Mesozoic Era • Dinosaurs dominant • First flowering plants Mass extinction & adaptive radiation • Ended by another mass extinction – Likely caused by asteroid impact

GEOL 131: 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

GEOL 131: Geologic Time Relative Dating

GEOL 131: Geologic Time: Relative Dating Principles • • Superposition Original Horizontality Cross-cutting Relationships Inclusions

GEOL 131: Geologic Time: Relative Dating Superposition • If Layer A is above Layer B, Layer A is younger • Assumes layers are not overturned

GEOL 131: Geologic Time: Relative Dating Original Horizontality • If a sedimentary rock layer is not horizontal, tectonic forces pushed it into its current position

GEOL 131: 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.

GEOL 131: Geologic Time: Relative Dating Cross-cutting Relationships

GEOL 131: 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.

GEOL 131: Geologic Time Unconformities

GEOL 131: Geologic Time: Relative Dating Unconformities • A surface between layers that represents missing rock • Means there was an extended period of erosion

GEOL 131: Geologic Time: Relative Dating Unconformities: Three types Disconformity Separates two horizontal layers Angular unconformity Separates horizontal from tilted layers Nonconformity Separates sedimentary from non-sedimentary

GEOL 131: Geologic Time: Relative Dating: Unconformities in the Grand Canyon

GEOL 131: Geologic Time Fossils

GEOL 131: Geologic Time: Fossils What Are Fossils? • Any trace of past life preserved in rock – Body (shells, bones, etc. ) – Trace (footprints, burrows, etc. )

GEOL 131: Geologic Time: Fossils Fossil Ranges • Each species has an age of earliest appearance in the rock record

GEOL 131: Geologic Time: Fossils Fossil Ranges • Most also have an age of final disappearance (extinction) • Species don’t reappear after extinction

GEOL 131: Geologic Time: Fossils Ranges • Fossil range: time between appearance and disappearance • Fossil ranges of multiple species can overlap

GEOL 131: Geologic Time: Fossils Ranges and Age Bracketing • Overlapping fossil ranges can be used to “bracket” the age of a rock layer

GEOL 131: Geologic Time Radiometric Dating

GEOL 131: Geologic Time: Radiometric Dating Atomic Structure - review

GEOL 131: 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

GEOL 131: 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

GEOL 131: Geologic Time: Radiometric Dating Radioactivity • Some parent-daughter pairs commonly used in geology:

GEOL 131: 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

GEOL 131: Geologic Time: Radiometric Dating Radioactivity 1) Mineral crystallizes and incorporates radioactive parent atoms 2) Parent atoms decay into daughter atoms at known rate 3) Percentage of remaining parent can give age of mineral

GEOL 131: Geologic Time: Radiometric Dating Half-life • Time required for one-half of remaining parent atoms to decay

GEOL 131: Geologic Time: Radiometric Dating Example of radiometric dating • If a mineral contains 50% K-40 and 50% Ar -40, how old is the mineral? 1) How many half-lives of K-Ar have elapsed since the mineral formed? 2) How many years is that equivalent to? - This is the age of the mineral

GEOL 131: Geologic Time: Radiometric Dating Example of radiometric dating Mineral contains 50% parent and 50% daughter So, one K-Ar half-life has elapsed since mineral formed Percentage of remaining parent element 1) How many half-lives of K-Ar have elapsed since the mineral formed?

GEOL 131: 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

GEOL 131: Geologic Time: Radiometric Dating Half-life • If 25% parent, 2 half-lives have passed • If 12. 5%, 3 half-lives • Etc…

GEOL 131: 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

GEOL 131: 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

GEOL 131: Geologic Time End of Chapter