Geologic Time Chapter 8

Determining geological ages Relative age dates – placing rocks and geologic events in their proper sequence Numerical dates – define the actual age of a particular geologic event (termed absolute age dating)

First principle of relative dating Law of superposition Developed by Nicolaus Steno in 1669 In an undeformed sequence of sedimentary or volcanic rocks, oldest rocks at base; youngest at top

Superposition illustrated by strata in the Grand Canyon

2nd, 3rd principles of relative dating Principle of original horizontality Layers of sediment are originally deposited horizontally (flat strata have not been disturbed by folding, faulting) Principle of cross-cutting relationships Younger features cut across older ones

Cross Cutting Relationships in strata

Grand Canyon younger strata cutting across older ones Cambrian Tapeats sandstone over Precambrian Unkar Group

Unconformities (loss of rock record) An unconformity is a break in the rock record produced by erosion and/or nondeposition Types of unconformities Angular unconformity – tilted rocks overlain by flat-lying rocks Disconformity – strata on either side of the unconformity are parallel (but time is lost) Nonconformity – sedimentary rocks deposited above metamorphic or igneous rocks (basement)

Formation of an angular unconformity

An angular unconformity at Siccar Point, England

Development of a Nonconformity The basement-cover contact near Boulder (Pennsylvanian sandstone over Precambrian granite) is a nonconformity (visible on Flagstaff Road near bouldering area)

Nonconformity in the Grand Canyon - Strata deposited over Schist

Correlation of rock layers Matching strata of similar ages in different regions is called correlation

Correlation of strata in southwestern United States Sections are incomplete

Correlation of rock layers with fossils Correlation relies upon fossils Principle of fossil succession – fossil organisms succeed one another in a recognizable order - thus any time period is defined by the type of fossils in it

Determining the ages of rocks using fossils

Geologic time scale The geologic time scale – a “calendar” of Earth history Subdivides geologic history into units Originally created using relative dates Structure of the geologic time scale Eon – the greatest expanse of time

Geologic Timescale Divisions based on fossils

Geologic time scale Structure of the geologic time scale Names of the eons Phanerozoic (“visible life”) – the most recent eon, began about 540 million years ago Proterozoic Archean Hadean – the oldest eon

This, believe it or not, is the rock with the oldest known minerals ever found. From NW Australia, the rock (a conglomerate) is about 3.0 Billion years old. The rock contains detrital grains of zircon (a mineral formed in granite in the crust) that is 4.4 Billion years old. Age of the Earth is 4.54 Billion (sample and age date courtesy of Steve Mojzsis)

Geologic time scale Structure of the geologic time scale Era – subdivision of an eon Eras of the Phanerozoic eon Cenozoic (“recent life”) Mesozoic (“middle life”) Paleozoic (“ancient life”) Eras are subdivided into periods Periods are subdivided into epochs

Geologic time scale Precambrian time Nearly 4 billion years prior to the Cambrian period Not divided into small time units because the events of Precambrian history are not know in detail Immense space of time (Earth is ~ 4.5 Ga)

Radioactivity (Used to age date rocks) Spontaneous changes (decay) in structure of atomic nuclei Types of radioactive decay Alpha emission Emission of 2 protons and 2 neutrons (an alpha particle) Beta emission An electron (beta particle) is ejected from the nucleus Electron capture An electron is captured by the nucleus The electron combines with a proton to form a neutron

Neutron capture (A) and Beta emission (B)

Using radioactivity in dating Parent – an unstable radioactive isotope Daughter product – isotopes resulting from decay of parent Half-life – time required for one-half of the parent isotope in a sample to decay

A radioactive decay curve

Dating with carbon-14 (radiocarbon dating) Half-life only 5730 years Used to date very young rocks Carbon-14 is produced in the upper atmosphere Useful tool for geologists who study very recent Earth history (for me this is the history of earthquakes).

Using radioactivity in dating Importance of radiometric dating Allows us to calibrate geologic timescale Determines geologic history Confirms idea that geologic time is immense

How do we actually “date” a rock? Collect sample (geologist as pack animal) Process for minerals by crushing, sieve, separate magnetically and/or with heavy liquids Measure parent/daughter ratio of mineral separates with a mass spectrometer

Dating sediments without fossils

In-class exercise: Determining the age of a young sedimentary rock with carbon-14 Take out a piece of paper and put your name on it to obtain credit Assume a half-life of 5730 years. Determine the age of the sample after three half-lives Determine the amount of parent material in the rock after three half lives Determine the age of a Precambrian igneous rock using Uranium 238. Assume a half life of 2.25 Billion years Assume the rock is 4.5 Billion years old Determine the relative amount of parent and daughter isotopes (or ratio relative to one another.

End of Chapter 8