Tracks Through Tracks Through TimeTime

Metamorphic RocksMetamorphic Rocks

• rocks formed when older, pre-existing rocks have been altered by heat and/or pressure

• two groups are recognised• Contact • Regional

Contact Metamorphic RocksContact Metamorphic Rocks

• these rocks form around igneous intrusions

• as the molten rock cools, the heat alters minerals in the surrounding rocks causing recrystallisation and the development of new minerals

• examples include marble, quartzite and hornfels

White MarbleWhite Marble

HornfelsHornfels

Regional Metamorphic RocksRegional Metamorphic Rocks

• form due to the effects of both heat and pressure on pre-existing rocks

• this change occurs in the solid phase - no rock melting occurs

• the characteristic feature of these rocks is their pronounced foliation – minerals are forced into layers by extreme pressure

Regional Metamorphic RocksRegional Metamorphic Rocks

• slate - very fine grained• used as a building stone. e.g. slate floor tiles

• schist - medium grained

• gneiss - coarse grained, obvious banding• often incorrectly called “granite” and used as

a building stone for floor tiles and bench tops

Slate tiles

Banded gneiss

Geological TimeGeological Time

• Relative time – placing rocks and geologic events in their proper sequence

• Absolute time – define the actual age of a particular geologic event

Relative Time – Tracks on the BeachRelative Time – Tracks on the Beach

Principles of relative datingPrinciples of relative dating

• Law of Superposition• Developed by Nicolaus Steno in 1669• in an undisturbed sequence of

sedimentary rock layers, each layer of rock is older than the layer above it and younger than the rock layer below it.

Superposition illustrated by Superposition illustrated by strata in the Grand Canyonstrata in the Grand Canyon

Younger

Older

Principles of relative datingPrinciples of relative dating

• Law of Original Horizontality• sediments like sand, gravel and mud, are

deposited on the earths surface as flat, continuous sheets

• if they are no longer flat or discontinuous, then they necessarily have had something happen to them after they formed.

Principles of relative datingPrinciples of relative dating

For example, in the Grand Canyon, about ¾ of the way below the rim, towards the river, we find a series of tilted rock layers, you can see these tilted rock layers at view points like Point Imperial and Cape Royal.

The Law of Original Horizontality says that those layers were first deposited flat, and later tilted.

Principles of relative datingPrinciples of relative dating• Law of Cross-cutting Relationships

• Described by Scotsman James Hutton (1726 - 1997)• the rock layers that are cut by another geologic

feature formed before the feature that cuts through them

• an igneous intrusion (which is when an underground body of molten rock pushes its way up through hard rock) would have to be younger than the hard rock that was already there

• if a rock is cut by a fault, that rock had to be there first and then cut later

• erosion into a rock layer (like erosion of a valley or gulley) would have to occur after the rock layers the erosion cuts into had been formed

Law of Cross-cutting RelationshipsLaw of Cross-cutting Relationships

volcanic dyke

Law of Cross-cutting RelationshipsLaw of Cross-cutting Relationships

fault

Principles of relative datingPrinciples of relative dating

• Unconformity• a sequence of sedimentary strata may cut off

the bedding surfaces of other sedimentary strata

• this indicates there have been two periods of deposition separated by a period of time where no deposition occurred

• an unconformity is a break in the rock record produced by erosion and/or nondeposition

UnconformityUnconformity

1 billion year old granite

500 million year old sandstone

unconformity

UnconformityUnconformity

Formation of an angular unconformityFormation of an angular unconformity

Formation of an UnconformityFormation of an Unconformity

The Rock CycleThe Rock Cycle

Correlation of rock layersCorrelation of rock layers

• Matching strata of similar ages in different regions is

called correlationhttp://www.uwsp.edu/geo/faculty/ozsvath/images/stratigraphy.jpg

Correlation of rock layers with Correlation of rock layers with fossilsfossils

• Correlation often relies upon fossils• Principle of fossil succession (Wm. Smith)

– fossil organisms succeed one another in a recognizable order - thus any time period is defined by the type of fossils in it

• Index Fossils - useful for correlation– Existed for a relatively brief time– Were widespread and common

http://www.csun.edu/~psk17793/ES9CP/ES9%20fossils.htm

•Most fossils are just impressions. A few may have small amounts of some original tissue

Correlation of strata in southwestern United States

Sections are incompleteMatch with fossils and lithology

8_10

Rock brokento reveal external moldof shell

Rock brokento revealfossil cast

Shellsburied insediment

Mold, or cavity,forms when originalshell materialis dissolved

Cast forms when moldis filled in with mineralwater

Shellssettle onoceanfloor

How impression fossils form (the most common type)

Recall Isotopes

• the number of protons in an atom's nucleus is called its atomic number –defines “element”

• protons + neutrons called mass number

• the number of neutrons can vary

• atoms of the same element with different numbers of neutrons are called isotopes

• radioactive isotopes have unstable nuclei and decay to gain stability

Using Radioactivity in DatingUsing Radioactivity in Dating

• Importance of radiometric dating• allows us to calibrate geologic timescale

• determines geologic history

• confirms idea that geologic time is immense

p

p

p

Atomic mass not changedmuch; atomic numberincreases by 1 becauseNeutron becomes proton

(b) Beta decayBeta particle

Radioactiveparent nucleus

Decay process Daughternucleus

Atomic mass unchanged;atomic numberdecreases by 1

(c) Electron capture

Beta particle

Atomic mass decreasesby 4; atomic numberdecreases by 2

(a) Alpha decay

Alpha particle

ProtonNeutron

pp

pp

pp

pp

pp

ppp

p

pp

ppp

p

pp

pp

pp

pp

ppp

pp

pp

ppp

pp

p

pp

p

p

ppp

p

pp

Emission of 2 protons and 2 neutrons (alpha particle)

An electron (beta particle) is ejected from the nucleus

electron combines with a proton to form a neutron

Using Radioactivity in DatingUsing Radioactivity in Dating• parent – an unstable radioactive

isotope

• daughter product – stable isotopes resulting from decay of parent

• Half-life – time required for one-half of the parent isotope in a sample to decay into stable daughter product

Decay CurveDecay Curve

1. Collect sample

2. Process for minerals by crushing, sieving and using a variety of separation methods

3. Measure parent/daughter ratio of target isotopes - mass spectrometer

How do we actually “date” a rock?

Mass SpectrometryMass Spectrometry

Magnetic field separates ions on their charge/mass ratio – ions of the same charge are separated into different isotopic masses

Can measure how much of each isotope in sample

Using Radioactivity in DatingUsing Radioactivity in Dating

Igneous rocks • the isotopic age is usually the time elapsed

since crystallisation of the magma

Mineralcrystal

Mineral crystalformed in igneousrock

Parentatoms

Daughteratoms

1

Igneous rock

buried beneathyounger rocks;daughter atomsformed bynormal decay

2

Dating a crystal

(3) We calculate age based on half-life

Using Radioactivity in DatingUsing Radioactivity in Dating

Sedimentary rocks – need care• may tell the age of formation of parent rock

not necessarily sedimentary rock• some minerals only form during sedimentary

processes so can tell time of formation of sediment

Aging Sedimentary RocksAging Sedimentary Rocks

Basalt Lava flow 2200 mya

Lava flow 1209 mya

We can bracket thislimestone’s age between 209 and 200 mya

If a layer we need to date is between two datable beds - have an upper and lower boundary on the age of this limestone

Using Radioactivity in DatingUsing Radioactivity in Dating

Metamorphic rocks• during metamorphism all of the daughter

product may be released – age gives time at which metamorphism occurred

• if only some of daughter product released – gives age older than metamorphism but younger than parent

• age determinations are used along with other geological evidence

8_22bDeep burial andmetamorphismduring mountainbuilding causesdaughter atomsto escape fromcrystal

3

After mountainbuilding ends,accumulation ofdaughter atomsin crystalresumes

4

Heat

Resets the clock

But IF:

Easily recognized,useful in studyingmetamorphism

Rock looks as if it just formed: it looks young

Age found dates from metamorphic event

Dating sediments without fossils

Radiometric Dating with Igneous RocksOr Bracket between fossiliferous layers

Morrison Fm older than 160 my

Wasatch Fm. younger than 66 myMancos Shale and Mesa Verde Fm.

older than 66 my

Radiometric Age DeterminationsRadiometric Age Determinations of the Earth of the Earth

• the age of the Earth is thought to be about 4.6 billion years

• based on the dates obtained from meteorites and samples collected on the moon, assumed to have formed at the same time

Geologic Time ScaleGeologic 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, Era, Period, Epoch

Geologic Timescale

Divisions based on fossilsEon, Era, Period, Epoch

Dating with carbon-14

(Carbon 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

8_24Atoms split intosmaller particles,among them neutrons

Neutrons strikenitrogen atoms

Nitrogen atoms lose aproton and becomescarbon-14

C-14 mixes with atmospheric oxygento produce CO2

CO2 taken upby plants

C-14 absorbedby livingorganisms

CO2 dissolvedin water

C-14 intake ceases when organismdies; C-14 concentration decreases

Cosmic raysbombardatmospheric atoms

Carbon-14

8_27

Sediment layerswith tree logs tobe collected fordendrochronology

Annual-ring similaritiesshow correlation Current year

Years of age

50 100 150 200400

500

Buried treelogs

Treegrowthrings

A

A

B

B

C

C

D

D

Tree Rings

8_28

Turbid water

Heavyrunoffintolake

Very little or norunoff

Summer layer(coarse, thick, andlight-colored)

Clear water

Winter layer(fine, thin, and dark-colored)

Summer Winter

Ice

Dating with Lake Varves

Modern Lakes, just count back from present. Fossil pollen track climate.

Southern lakes track glaciation

Origin of Period Names

Geologic time scaleGeologic time scale• Structure of the geologic time scale

• Names of the eons– Phanerozoic (“visible life”) – the most recent

eon, began about 545 million years ago– PreCambrian (Cryptozoic)

• PreCambrian subdivisions:• Proterozoic – begins 2.5 billion years ago• Archean – begins 3.8 bya• Hadean – the oldest eon begins 4.6 bya

Read from bottom to top – Oldest to Youngest

Geologic time scaleGeologic time scale

• Precambrian time• Nearly 4 billion years prior to the Cambrian period• Long time units because the events of Precambrian

history are not know in detail – few fossils, most rock modified

• Immense space of time (Earth is ~ 4.6 Ga)• PreCambrian spans about 88% of Earth’s history

Geologic time scaleGeologic time scale

• Structure of the geologic time scale• Era – subdivision of an eon• Eras of the Phanerozoic eon

– Cenozoic (“recent life”) begins ~ 65 mya

– Mesozoic (“middle life”) begins ~ 248 mya

– Paleozoic (“ancient life”) begins ~ 540 mya

• Eras are subdivided into periods• Periods are subdivided into epochs

http://bcornet.tripod.com/Cornet94/Cornet94.htm

http://www.bio.uu.nl/~palaeo/people/Hanneke/index.html

Hanneke Bos

End of Geologic End of Geologic Time LectureTime Lecture

2nd principle of relative dating2nd principle of relative dating• Principle of original

horizontality• Layers of sediment are originally deposited

horizontally (flat strata have not been disturbed by folding, faulting)

8_9(a)

(b)

(c)

Layeredsedimentaryrocks

NonconformityMetamorphicrock

Igneousintrusive rock

Youngersedimentaryrocks

Angularunconformity

Older, foldedsedimentaryrocks

Disconformity

Brachiopod(290 million years old)

Trilobite (490 million years old)

Horizontal younger sediments over tilted older sedimentsCambrian Tapeats sandstone over Precambrian Unkar Group

What type of unconformity is this?

Grand Canyon in Arizona

Development of a Nonconformity

Pennsylvanian sandstone over Precambrian granite is a nonconformity

Nonconformity in the Grand Canyon - Sediments deposited over Schist

Cross Cutting Relationships in strataZoroaster Granite across Vishnu Schist

Determining the ages of Determining the ages of rocks using overlap of fossilsrocks using overlap of fossils

Index Fossils

Fossils can be preserved in several rock types. Note the use of overlapping fossil ranges in two distant outcrops (one is turned on its side for this illustration),

even though the sediment facies are different.

Two Measured Sections

Correlation in spite of DisconformitiesCorrelation in spite of Disconformities

A radioactive decay curveA radioactive decay curve

1/2 = 50% parent: 1 half-life has passed1/2x1/2 = 1/4 = 25% parent: 2 half-lives have passed1/2x1/2x1/2 = 1/8 = 12.5% parent: 3-half-lives have passed

Methods of ScienceMethods of Science

Law• states a relationship that is always the

same under the same conditionsTheory• serves as a unifying principle that can be

used to explain the laws and the behaviour and facts about a phenomena

• if it fails to explain new facts it is revised or rejected

EvolutionEvolution

At least three distinct meanings:• Change over time - The fact of

evolution• Natural selection - The theory of

evolution• No Creator/God - The belief all

organisms descended from a single common ancestor produced by “natural” events

GalapagosIslands

Falkland Islands

The Voyage Of The BeagleThe Voyage Of The Beagle

The object of the expedition was to complete the survey of Patagonia and Tierra del Fuego . . . to survey the shores of Chile, Peru, and some islands in the Pacific-and to carry a chain of chronometrical measurements round the World.

Charles Darwin in The Voyage of The Beagle

SouthAmerica

Rio deJeneiro

England

Cape Verde Islands

AustraliaSydney

New Zealand

Mauritius

CocosIslandsTahiti

TIMETIME

• studying the Earth leads to the realisation of the immensity of time

• historical time deals with days, years and centuries

• geological time deals with thousands, millions and billions of years

Igneous RocksIgneous Rocks

• form from the cooling of molten rock• on the surface molten rock is called lava

• these rocks have very small mineral crystals due to rapid cooling e.g. basalt

• beneath the surface molten rock is called magma. • rocks formed from magma cool slowly so they

have large mineral crystals e.g. granite and pegmatite

Igneous RocksIgneous Rocks

Igneous RocksIgneous Rocks

Kilauea Volcano in Hawaii

Sedimentary RocksSedimentary Rocks

• accumulate in horizontal layers in sedimentary basins

• three groups are recognised• clastic• organic• chemical

Clastic Sedimentary RocksClastic Sedimentary Rocks

• these rocks formed when particles of other rocks were weathered, eroded, deposited, compacted and hardened

• lithification is the process by which sediments become compacted and cemented into a sedimentary rock

• examples include conglomerate, sandstone, shale, siltstone, mudstone, and claystone.

• fossils may be found in clastic sediments.

Sedimentary rocks in WA

The Three Sisters, Katoumba, Blue Mountains NSW

Organic Sedimentary RocksOrganic Sedimentary Rocks

• these rocks formed when once living things have been buried in sediments, compressed and altered over geological time to form rocks

• examples• coal (from ancient vegetation)• limestone (from ancient corals and shells)

Coal Seam

Limestone Cliffs

The 12 Apostles

Chemical Sedimentary RocksChemical Sedimentary Rocks

• these rocks are chemical precipitates

• minerals have been precipitated out of solutions

• for example• the banded iron formations of Western

Australia formed when iron oxide minerals precipitated out of ancient seas in the Proterozoic Eon

Ironstones in the Pilbara WA

Lake Eyre