Geology

By Mr. Joydeep Saha

Geological Time - Motion pictures are generally projected at 32 frames per second. Therefore, each frame (image) is on the screen for only split second- let each frame represent 100 years. Start movie at present and go back in time. •The Declaration of Independence would show up 1/16 of a second into the movie. •The Christian era (BC-AD boundary) would be 3/4 of a second into the movie. •The most recent Ice Age would be 7 seconds into it.•The movie would run about 6 hours before we got to the end of the Mesozoic era (extinction of the dinosaurs).•We'd have to watch the movie for about 2 days to see the beginning of the Paleozoic era (macroscopic life). •The whole movie (to the beginning of geologic time on Earth) would be approximately 16 days long!

Motion pictures are generally projected at 32 frames per second. Therefore, each frame (image) is on the screen for only split second- let each frame represent 100 years. Start movie at present and go back in time. •The Declaration of Independence would show up 1/16 of a second into the movie. •The Christian era (BC-AD boundary) would be 3/4 of a second into the movie. •The most recent Ice Age would be 7 seconds into it.•The movie would run about 6 hours before we got to the end of the Mesozoic era (extinction of the dinosaurs).•We'd have to watch the movie for about 2 days to see the beginning of the Paleozoic era (macroscopic life). •The whole movie (to the beginning of geologic time on Earth) would be approximately 16 days long!

•

> Relative: Placing events in asequence based on their positionsin the geologic record.

> Chronologic

sample.

• Two ways to relate time in geology: Two ways to relate time in geology:

> Relative: Placing events in asequence based on their positionsin the geologic record.

> Chronologic: Placing a specific: Placing a specificnumber of years on an event or rocknumber of years on an event or rock

sample.

Geologic TimeGeologic Time

Geologic Time ScaleGeologic Time Scale

•• A combination of the two types of age` A combination of the two types of age`determinationsdeterminations>> A A relativerelative sequence of lithologic units sequence of lithologic units

-- established using logical principles established using logical principles>> Measured against a framework of Measured against a framework ofchronologicchronologic dates. dates.

Geologic Time and the "geologic column"• Developed using logical rules to establish relative

sequences of events

- - - -

•

- •

-

Geologic Time and the "geologic column"•

- superposition- cross-cutting relationships- original horizontality - lateral continuity

• Added to as new information is obtained and data is refined

- Use of fossils for correlation and age determination• Numerical Dates attached to strata after the

development of Radiometric techniques-

The Geologic Time Scale (1:2)The Geologic Time Scale (1:2)

The Geologic Time Scale (2:2)The Geologic Time Scale (2:2)

Relative Dating MethodsRelative Dating Methods

•• Determines the relative sequence of events.Determines the relative sequence of events.>> which came first, which came last. which came first, which came last.>> no numeric age assigned no numeric age assigned

•• 6 Relative age principles: 6 Relative age principles: >> SuperpositionSuperposition >> Original Horizontality, Original Horizontality,>> Lateral continuity Lateral continuity > > Cross-cutting RelationshipsCross-cutting Relationships>> Inclusions Inclusions >> Fossil succession. Fossil succession.

Those in yellow are most useful

History of Historical Geology

- Fundamental Principles of Relative Time> Principle of Superposition> Principle of Original Horizontality> Principle of Original Lateral Continuity

Law of SuperpositionLaw of Superposition• In undisturbed strata, the layer on the bottom is•

oldest, those above are younger.oldest, those above are younger.

Original HorizontalityOriginal Horizontality

•••• Sediments are generally deposited as Sediments are generally deposited ashorizontal layers.horizontal layers.

Lateral ContinuityLateral Continuity•••• Sediment layers extend laterally in all Sediment layers extend laterally in all

direction until they thin & pinch out asdirection until they thin & pinch out asthey meet the edge of the depositionalbasin. they meet the edge of the depositionalbasin.

included description and use ofincluded description and use of

Charles Lyell

•

-

> principles of cross-cutting relationships

> principles of inclusions

• relative time tools

• 1st Principles of Geology text1st Principles of Geology text

-

> principles of cross-cutting relationships> principles of inclusions

• relative time tools

Cross-cutting RelationshipsCross-cutting Relationships

That which cuts through is younger than the Object that is cut

dike cuts through

granite is cut

Relative Ages of Lava Flows and SillsRelative Ages of Lava Flows and Sills

Principle of InclusionsPrinciple of Inclusions• Inclusions (one rock type contained in another rock type) are

older than the rock they are embedded in. That is, the younger rock contains the inclusions

Principle of InclusionsPrinciple of Inclusions

Faunal/Floral Succession•• Fossil assemblages (groupings of fossils) Fossil assemblages (groupings of fossils)

succeed one another through time.succeed one another through time.

- - - -

- -

- -

• Correlation-• Correlation-relating rocks in one location to those inrelating rocks in one location to those in

another using relative age stratigraphicanother using relative age stratigraphicprinciplesprinciples

- - Superposition Superposition - -

Lateral Continuity Lateral Continuity - -

Faunal SuccessionFaunal Succession

- - Cross-cuttingCross-cutting

••UnconformitiesUnconformities

surfacessurfacesrepresent a long time.represent a long time.

a time when rocks were not a time when rocks were notdeposited or deposited or a time when rocks were a time when rocks were

eroded eroded

HiatusHiatusthe gap in time represented the gap in time represented in the rocks by an uncon-in the rocks by an uncon-formity formity

3 kinds3 kinds Angular Unconformity Angular Unconformity Nonconformity Nonconformity Disconformity Disconformity

DisconformitiesDisconformities A surface of erosion or non-deposition between A surface of erosion or non-deposition betweenParallel sedimentary rock beds of different agesParallel sedimentary rock beds of different ages

Angular UnconformitiesAngular Unconformities• An angular unconformity is an erosional surface on tilted

or folded strata, over which younger strata have been deposited.

NonconformitiesNonconformitiesA nonconformity is an erosional surface on igneous or

metamorphic rocks which are overlain by sedimentary rocks.

Breakout in to groups and discuss the sequence observed here

Age Estimates of EarthAge Estimates of Earth Counting lifetimes in the Bible Counting lifetimes in the Bible

Comparing cooling rates of iron pellets. Comparing cooling rates of iron pellets.

Determine sedimentation rates & compare Determine sedimentation rates & compare

Estimate age based on salinity of the ocean. Estimate age based on salinity of the ocean.

all age estimates were off by billions of years

some were more off than others! some were more off than others!

>

+

Absolute Dating MethodsAbsolute Dating MethodsRadioactiveRadioactive Decay sequencesDecay sequences acts as an atomic clock acts as an atomic clock

we see the clock at the end of its cyclewe see the clock at the end of its cycle

analogous to starting a stopwatchanalogous to starting a stopwatch

allows assignment of numerical dates to allows assignment of numerical dates torocks.rocks.

>

+

decaydecay) into) into Radioactive isotopes change ( Radioactive isotopes change (daughter isotopes at known rates.daughter isotopes at known rates.

rates vary with the isotoperates vary with the isotope

e.g., U , K , C, etc. e.g., U , K , C, etc. 235235 4040 1414

•

DecayDecay unstable nuclei in parent isotope emits unstable nuclei in parent isotope emitssubatomic particles and transform intosubatomic particles and transform intoanother isotopic element (daughter).another isotopic element (daughter).

does so at a known rate, measured in thedoes so at a known rate, measured in thelablab

Half-lifeHalf-life The amount of time needed for one-half of a The amount of time needed for one-half of a

radioactive parent to decay into daughterradioactive parent to decay into daughterisotope.isotope.

Rate of DecayRate of Decaytt00

tt 11

tt 33

All atoms are parent isotope or someAll atoms are parent isotope or someknown ratio of parent to daughterknown ratio of parent to daughter

1 half-life period has elapsed, half of the1 half-life period has elapsed, half of thematerial has changed to a daughtermaterial has changed to a daughterisotope (6 parent: 6 daughter)isotope (6 parent: 6 daughter)

tt222 half-lives elapsed, half of the parent2 half-lives elapsed, half of the parentremaining is transformed into a daughterremaining is transformed into a daughterisotope (3 parent: 9 daughter)isotope (3 parent: 9 daughter)

3 half-lives elapsed, half of the parent3 half-lives elapsed, half of the parentremaining is transformed into a daughterremaining is transformed into a daughterisotope (1.5 parent: 10.5 daughter)isotope (1.5 parent: 10.5 daughter)

We would see the rock at this point.We would see the rock at this point.

Radioactive Isotopes

• analogous to sand in an hour glass- we measure how much sand there is

> represents the mass of elements- we measure the ratio of sand in the bottom to sand in the top - at the end (present)

> daughter (b) and parent (t)- we know at what rate the sand falls into the bottom

> the half life of the radioactive element- how long would it take to get the amount sand in the observed

ratio starting with all of it in the top?

Radioactive Isotopes

• analogous to sand in an hour glass- we measure how much sand there is

> represents the mass of elements- we measure the ratio of sand in the bottom to sand in the top - at the end (present)

> daughter (b) and parent (t)- we know at what rate the sand falls into the bottom

> the half life of the radioactive element- how long would it take to get the amount sand in the observed

ratio starting with all of it in the top?

50

100

2513

time----------->

ParentDaughterParentDaughter

% p

aren

t rem

aini

ng

Five Radioactive Isotope PairsFive Radioactive Isotope Pairs

Half-LifeEffective Minerals and

Isotopes of ParentDating Range

Rocks That Can Parent Daughter

(Years)Be Dated

Uranium 238 Lead 206 4.5 billion 10 million to Zircon 4.6 billion UraniniteUranium 235 Lead 207 704 million Thorium 232 Lead 208 14 billion 48.8 billion

Rubidium 87 Strontium 87 4.6 billion 10 million to

Muscovite

Biotite

Potassium feldspar

Whole metamorphic

or igneous rock

Potassium 40 Argon 40 1.3 billion 100,000 to Glauconite 4.6 billion Muscovite Biotite Hornblende Whole volcanic rock

(Years)

4.6 billion

Radiocarbon and Tree-Ring Dating Methods• Carbon-14 dating is based on theratio of C-14 to C-12sample.

> Valid only for samples less than 70,000years old.

> Living things take in both isotopes ofcarbon.

> When the organism dies, the "clock" starts.

• Carbon-14 dating is based on theratio of C-14 to C-12 in an organicsample.

> Valid only for samples less than 70,000years old.

> Living things take in both isotopes ofcarbon.

> When the organism dies, the "clock" starts.

Method can be validated by cross-checking with tree ringsMethod can be validated by cross-checking with tree rings

Carbon 14 CycleCarbon 14 Cycle

Recognizing Patterns of changeRecognizing Patterns of change

Walther's LawWalther's Law• • The vertical sequence is repeated by the horizontalThe vertical sequence is repeated by the horizontalsequencesequence

- - walking from A to B to C to the Coast you would encounter thewalking from A to B to C to the Coast you would encounter therocks that would be encountered by drilling a core into therocks that would be encountered by drilling a core into the

earth at any point (A, B, or C)earth at any point (A, B, or C)

Facies DiagramFacies Diagram• • distribution of lithofacies (rock-types)distribution of lithofacies (rock-types)

- - these are associated with their respective EODthese are associated with their respective EOD

• • biofacies are similar but refer to fossils rather thanbiofacies are similar but refer to fossils rather thanrock typesrock types

Eustasy, relative sea-level, and relative positionof lithofacies

• Eustasy= changes in volume of water in ocean• lithofacies depend on

- sea-level

- land level

- geometry of coast

- sediment supply

Vail Curve• an attempt at global• correlation oflithologies

- for better production

- of petroleum resources

Rock designationsRock designations• • Rock units called Lithostratigraphic unitsRock units called Lithostratigraphic units

- - described in terms of Group, Formation, & Memberdescribed in terms of Group, Formation, & Member> > each term has specific meanings in geological parlanceeach term has specific meanings in geological parlance

• • Formation Formation - - a mappable lithostratigraphic unita mappable lithostratigraphic unit

> > has a location for identifying the type-sectionhas a location for identifying the type-section> > has a rock designation describing the lithologyhas a rock designation describing the lithology

- - sometimes not all the same lithologysometimes not all the same lithology> > in which case the term "Formation" takes the place of lithologicin which case the term "Formation" takes the place of lithologic

typetype

• • Groups are composed of several formationsGroups are composed of several formations• • Members are distinctive units within a formationMembers are distinctive units within a formation

- - group is largest and contains formations and membersgroup is largest and contains formations and members- - formations are next and contain membersformations are next and contain members

Rock designationsRock designations• • Rock units called Lithostratigraphic unitsRock units called Lithostratigraphic units

- - described in terms of Group, Formation, & Memberdescribed in terms of Group, Formation, & Member> > each term has specific meanings in geological parlanceeach term has specific meanings in geological parlance

• • Formation Formation - - a mappable lithostratigraphic unita mappable lithostratigraphic unit

> > has a location for identifying the type-sectionhas a location for identifying the type-section> > has a rock designation describing the lithologyhas a rock designation describing the lithology

- - sometimes not all the same lithologysometimes not all the same lithology> > in which case the term "Formation" takes the place of lithologicin which case the term "Formation" takes the place of lithologic

typetype

• • Groups are composed of several formationsGroups are composed of several formations• • Members are distinctive units within a formationMembers are distinctive units within a formation

- - group is largest and contains formations and membersgroup is largest and contains formations and members- - formations are next and contain membersformations are next and contain members

Magnetization of Volcanic Rocks• Successive lava flows stack up one on top of

another, each lava flow recording the Earth’s polarity at the time at which it formed

• Each lava flow can also be dated using radioactive elements in the rock to give its age

Magnetization of Volcanic Rocks• Magnetic patterns of ocean floor • What does magnetic polarity of lava flows tell us?

– Plotting the polarity of different lava flows against their ages gives us a record of the Earth’s polarity at different times in the past

– Timing of polarity reversals (north to south; south to north) seems random

– Reversals probably caused by changes in the flow of iron-rich liquid in the Earth’s outer core

Earth’s Magnetic Field• Earth’s magnetic field acts like giant bar magnet, with

north end near the North Pole and south end near the South Pole

• Magnetic field axis is now tilted 11o from vertical (tilt has varied with time) so that magnetic poles do not coincide with geographic poles (but are always near each other)

• Inclination of magnetic lines can also be used to determine at what latitude the rock formed

• Magnetic field is caused by dynamodynamo in outer core:

– Movements of iron-rich fluid create electric currents that generate magnetic field

Magnetization Patterns on the Seafloors

• Atlantic Ocean floor is striped by parallel bands of magnetized rock with alternating polarities

• Stripes are parallel to mid ocean ridges, and pattern of stripes is symmetrical across mid ocean ridges (pattern on one side of ridge has mirror opposite on other side)

Magnetization Patterns on the Seafloors

• Magma is injected into the ocean ridges to cool and form new rock imprinted with the Earth’s magnetic field

• Seafloor is then pulled away from ocean ridge like two large conveyor belts going in opposite directions – seafloor spreading

Other Evidence of Plate Tectonics• Earthquake epicenters outline plate boundariesEarthquake epicenters outline plate boundaries

– Map of earthquake epicenters around the world shows not random pattern, but lines of earthquake activity that define the edges of the tectonic plates

Other Evidence of Plate Tectonics• Deep earthquakesDeep earthquakes

– Most earthquakes occur at depths less than 25 km

– Next to deep-ocean trenches, earthquakes occur along inclined planes to depths up to 700 km

– These earthquakes are occurring in subducting plates

The Grand Unifying TheoryTectonic cycleTectonic cycle

Plate Tectonics and EarthquakesMost earthquakes can be explained by plate tectonics:

• DivergentDivergent plate boundaries

– Divergent motion and high temperatures cause rocks to fail easily in tension

– Earthquakes are small and generally non-threatening

• TransformTransform plate boundaries

– Plates slide past each other in horizontal movement, retarded at irregularities in plate boundaries

– Energy required to move plates is released as large earthquakes

• ConvergentConvergent plate boundaries

– Great amounts of energy are required to pull a plate back into the mantle or push continents together

– Largest earthquakes are generated at convergent boundaries

Plate Tectonics and EarthquakesExamine example of Pacific plate:

• Created at spreading centers on eastern and southern edges, producing small earthquakes

• Slides past other plates on transform faults (Queen Charlotte fault, Canada; San Andreas fault, California; Alpine fault, New Zealand), generating large earthquakes

• Subducts along northern and western edges, generating enormous earthquakes

Subduction Zones• Tokyo, Japan, 1923Tokyo, Japan, 1923 – one of world’s most deadly

disasters (probably about 144,000 people killed)• Series of earthquakes, with principal one worst of

year globally• Tsunami 11 m high hit city• Fires raced through city for 2½ days, destroying

71% of Tokyo and all of Yokohama– 38,000 people were killed by fire, crowded into a park

that was consumed by fire from three sides

Continent-Continent Collisions• Collision of India into Asia

– India has moved 2,000 km north into Asia from initial contact

– Pre-collision, Indian and Asian crusts were 35 km thick

– Now crust under area of Tibetan plateau is 70 km thick and highest-standing continental area on Earth

– India continues to move 5 cm/year into Asia, along a 2,000 km front, affecting India, Pakistan, Afghanistan, Tibetan Plateau, eastern Russia, Mongolia and China with great earthquakes, and pushing parts of China to the east and southeastern Asia farther to the southeast

Write two ways to relate time in Geology. Write any two relative age principle. How age of earth can be estimated? What is Half Life? What does magnetic polarity of Lava flows tell us?