Geologic TimeChapter 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 onesCambrian 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 areincomplete

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)

QuickTime™ and a Sorenson Video decompressor are needed to see this picture.

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 nucleiTypes 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?

1. Collect sample (geologist as pack animal)

2. Process for minerals by crushing, sieve, separate magnetically and/or with heavy liquids

3. 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

1) Take out a piece of paper and put your name on it to obtain credit2) Assume a half-life of 5730 years.3) Determine the age of the sample after three half-lives4) Determine the amount of parent material in the rock after three half lives

5) Determine the age of a Precambrian igneous rock using Uranium 238.6) Assume a half life of 2.25 Billion years7) Assume the rock is 4.5 Billion years old8) Determine the relative amount of parent and daughter isotopes (or ratio

relative to one another.

End of Chapter 8