Tracks Through Time. Metamorphic Rocks rocks formed when older, pre-existing rocks have been altered...
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![Page 1: Tracks Through Time. Metamorphic Rocks rocks formed when older, pre-existing rocks have been altered by heat and/or pressure two groups are recognised.](https://reader036.fdocuments.us/reader036/viewer/2022062519/56649db35503460f94aa378f/html5/thumbnails/1.jpg)
Tracks Through Tracks Through TimeTime
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Metamorphic RocksMetamorphic Rocks
• rocks formed when older, pre-existing rocks have been altered by heat and/or pressure
• two groups are recognised• Contact • Regional
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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
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White MarbleWhite Marble
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HornfelsHornfels
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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
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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
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Slate tiles
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Banded gneiss
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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
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Relative Time – Tracks on the BeachRelative Time – Tracks on the Beach
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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.
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Superposition illustrated by Superposition illustrated by strata in the Grand Canyonstrata in the Grand Canyon
Younger
Older
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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.
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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.
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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
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Law of Cross-cutting RelationshipsLaw of Cross-cutting Relationships
volcanic dyke
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Law of Cross-cutting RelationshipsLaw of Cross-cutting Relationships
fault
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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
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UnconformityUnconformity
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1 billion year old granite
500 million year old sandstone
unconformity
UnconformityUnconformity
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Formation of an angular unconformityFormation of an angular unconformity
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Formation of an UnconformityFormation of an Unconformity
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The Rock CycleThe Rock Cycle
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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
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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
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Correlation of strata in southwestern United States
Sections are incompleteMatch with fossils and lithology
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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)
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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
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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
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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
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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
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Decay CurveDecay Curve
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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?
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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
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Using Radioactivity in DatingUsing Radioactivity in Dating
Igneous rocks • the isotopic age is usually the time elapsed
since crystallisation of the magma
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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
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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
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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
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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
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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
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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
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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
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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
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Geologic Timescale
Divisions based on fossilsEon, Era, Period, Epoch
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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
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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
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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
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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
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Origin of Period Names
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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
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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
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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
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http://www.bio.uu.nl/~palaeo/people/Hanneke/index.html
Hanneke Bos
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End of Geologic End of Geologic Time LectureTime Lecture
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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)
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8_9(a)
(b)
(c)
Layeredsedimentaryrocks
NonconformityMetamorphicrock
Igneousintrusive rock
Youngersedimentaryrocks
Angularunconformity
Older, foldedsedimentaryrocks
Disconformity
Brachiopod(290 million years old)
Trilobite (490 million years old)
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Horizontal younger sediments over tilted older sedimentsCambrian Tapeats sandstone over Precambrian Unkar Group
What type of unconformity is this?
Grand Canyon in Arizona
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Development of a Nonconformity
Pennsylvanian sandstone over Precambrian granite is a nonconformity
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Nonconformity in the Grand Canyon - Sediments deposited over Schist
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Cross Cutting Relationships in strataZoroaster Granite across Vishnu Schist
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Determining the ages of Determining the ages of rocks using overlap of fossilsrocks using overlap of fossils
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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
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Correlation in spite of DisconformitiesCorrelation in spite of Disconformities
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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
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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
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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
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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
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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
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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
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Igneous RocksIgneous Rocks
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Igneous RocksIgneous Rocks
Kilauea Volcano in Hawaii
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Sedimentary RocksSedimentary Rocks
• accumulate in horizontal layers in sedimentary basins
• three groups are recognised• clastic• organic• chemical
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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.
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Sedimentary rocks in WA
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The Three Sisters, Katoumba, Blue Mountains NSW
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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)
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Coal Seam
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Limestone Cliffs
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The 12 Apostles
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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
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Ironstones in the Pilbara WA
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Lake Eyre