Chapter 9: Geological Time

Relative dating

determines the order of geological events without a date

Absolute dating

determines actual ages of rocks and events; uses dates

What does absolute dating use to determine the ages of rocks?

isotopes, radiometric dating

stratification

layering of rock

The layers of sedimentary rocks reflect the

deposition of original sediment

Two ways that layers end:

thinning at margin of a basin; grading into different sediment

What does lateral continuity tell us?

the layers were once continuous across a canyon

Correlation

matching a rock layer in one place with its counterpart in a different place

Are rock units/layers always continuous? What happens to them?

no; they get buried or eroded after separation

What can be constructed from geological correlation?

geologic maps

What can correlation determine?

how far rock types extend geographically

Fossils

remains of ancient life in rocks

Where are fossils found?

sediment, sedimentary rock

What is the study of fossils called?

paleontology

Why are fossils relevant in geology?

help interpret the past, help correlate rocks

What conditions favor preservation?

low oxygen, rapid burial, hard parts (teeth, skeleton)

Anoxic

little to no oxygen

Depression or cavity in a rock that takes on the shape of the original fossil

mold

Void space in rock that is filled with secondary material

cast

Permineralization

mineral water penetrates wood and turns it to stone

Gas and liquid components squeezed by pressure during burial, leaving a film in the shape of the organism

carbonization

Markings and objects left behind by ancient life

trace fossils

Examples of trace fossils

burrows, tracks, coprolites

Gastrolith

stomach stones

Principle of Fossil Succession

fossils succeed each other in definite order

True/False: Any time-period can be recognized by its fossil content

true

In fossil succession, where can you find the oldest fossils? Where can you find the youngest?

oldest at the bottom, youngest at the top

Fossil range

time interval a species lived

True/False: fossils can be used to date the rocks that contain them

true

How long may a fossil range span?

millions of years

When does fossil range begin? When does it end?

emergence of a new species; extinction

What can overlapping fossil ranges do?

constrain age of rock layer containing fossil

Stratigraphy

study of strata; organization of rock record

Lithostratigraphy

organize rocks from their physical properties

Biostratigraphy

fossils determine relative age of rocks

Chronostratigraphy

age of strata in relation to time

Basic unit of stratigraphy

formation

How are rock units in a formation grouped?

lithology (rock type), color, structure

Unconformities

gaps in earth’s record (missing strata in rock sequence)

Lithostratigraphy is distinguished by:

mineral composition, texture, color

Younger rock sits on top of older rocks that have been tilted, folded or eroded

angular unconformity

Geological contact where younger sedimentary rock layers lie directly on top of older, eroded igneous or metamorphic rock (different rock family)

nonconformity

An erosional surface or a gap in the rock record that separates two parallel sets of sedimentary (or volcanic) rock layers

disconformity

Law of superposition

top rock is younger, bottom is older

Principle of original horizontality

rock is first laid horizontally, folding occurs after

How is sediment originally deposited?

horizontally

Principle of lateral continuity

Rock layers are continuous until thinning at basin edge

Principle of cross-cutting relationships: if a fault cuts rock or magma intrudes rocks

the fault/magma are younger than the rocks they cut

Inclusions

rock trapped inside another rock

In a rock inclusion, the one inside is

older

What does absolute dating do to rocks and geological events?

assign actual dates

Does the construction of the geologic time scale use relative or absolute dating?

absolute

Is the nucleus stable or unstable in most atoms?

stable

This type of element has an unstable nucleus

radioactive element

Unstable element decays to a more _______ form and becomes a _________

stable; new element

The original unstable, radioactive isotope

parent

The new, more stable isotope formed from radioactive decay of parent

daughter

The rate the parent isotope decays at to become a stable daughter isotope is called

half life

Alpha emission results in

loss of 2 protons and 2 neutrons

During alpha emission, what happens to the atomic mass? What happens to the atomic number?

decreases by 4, decreases by 2

Beta emission changes a:

neutron into a proton

In beta emissions, what happens to the atomic number? What about the atomic mass?

atomic number increases by 1, mass stays the same

Electron capture changes a:

proton into a neutron

In electron capture, what happens to the atomic number? What about the atomic mass?

atomic number decreases by 1, no change in mass

True/false:

When uranium- 238 decays, a series of intermediate daughter isotopes form and undergo further decay until ultimately stable lead-206 results

true

The time required for half the number of radioactive nuclei in a sample to decay to a new element

half life

How do you find the parent/daughter percentages after X half-lives?

keep cutting the parent value in half

What fraction of the parent isotope remains after 2 half lives? How much of the daughter isotope do you have now?

1/4 ; 3/4

What fraction of the parent isotope remains after 3 half lives?

1/8

Parent isotope + daughter isotope aways =

100 percent

What needs to be known in order to do radiometric dating (the age of a sample)?

half life; parent-daughter ratio

What is the age of a sample if the ratio is 25% parent and 75% daughter. The half-life is 5 million years

10 million

What can be used to measure parent-daughter isotope ratios?

mass spectrometer

When temperatures become high enough for daughter isotopes to escape the minerals where they accumulate…

the isotopic clock resets

What is produced in the atmosphere via bombardment of nitrogen atoms with neutrons?

carbon 14

True'/False: Living matter absorbs carbon dioxide containing carbon-14

true

After death, what happens to carbon 14?

decays but is not replaced

Why is the Carbon-12/Carbon-14 ratio in living matter constant?

decaying C14 is continuously replaced with new C14

If a rock has experienced more than one episode of metamorphism, then only

the most recent event can be dated

Igneous rocks can provide an approximate crystallization age using

whole rock analysis

Why don’t detrital (clastic) sedimentary rocks give meaningful ages?

Minerals only give the same age as parent rock

The mixture of different ages in a basin is _________ in dating ages

meaningless

Isotopic dating of a sedimentary rock is possible if it contains __________ that formed during burial/diagenesis in the sedimentary basin

authigenic minerals

How do you date sedimentary rock packages (since you can't radiometrically date them directly)?

bracket them using surrounding features (rocks above, below, cross-cutting dikes, etc.)

A “calendar” of earth’s history; was created using relative dating

geologic time scale

Largest unit of time (500 million - 2 billion years)

eon

Second largest unit of time (tens to hundreds of millions of years)

era

Third largest unit of time (tens of millions of years)

period

Smallest unit of time

epoch

Most recent eon; visible life (541 million years ago - present day)

Phanerozoic

After Phanerozoic, name the eons from youngest to oldest

protozoic, archaen, hadean

cenozoic means

recent life

mesozoic means

middle life

paleozoic means

ancient life