Geology By Mr. Joydeep Saha

Geological Time - Start movie at present and go back in 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!

Geologic Time • • Two ways to relate time in geology: > > Relative Relative : Placing events in a : Placing events in a sequence based on their positions sequence based on their positions in the geologic record. in the geologic record. > > Chronologic Chronologic : Placing a specific number of years on an event or rock sample. sample.

Geologic Time Scale • A combination of the two types of age` determinations > A relative sequence of lithologic units - established using logical principles > Measured against a framework of chronologic dates.

Geologic Time and the "geologic column" • • Developed using logical rules to establish relative sequences of events - - 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 (2:2)

Relative Dating Methods • Determines the relative sequence of events. > which came first, which came last. > no numeric age assigned • 6 Relative age principles: > Superposition > Original Horizontality, > Lateral continuity > Cross-cutting Relationships > Inclusions > 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 Superposition • • In undisturbed strata, the layer on the bottom is oldest, those above are younger.

Original Horizontality • • Sediments are generally deposited as horizontal layers. Lateral Continuity • • Sediment layers extend laterally in all direction until they thin & pinch out as they meet the edge of the depositional basin.

Charles Lyell included description and use of • • 1st Principles of Geology text - - included description and use of > > principles of cross-cutting relationships principles of cross-cutting relationships > > principles of inclusions principles of inclusions • • relative time tools relative time tools

Cross-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 Sills

Principle 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 Inclusions

Faunal/Floral Succession • • Fossil assemblages (groupings of fossils) succeed one another through time.

relating rocks in one location to those in • Correlation- relating rocks in one location to those in another using relative age stratigraphic principles - - Faunal Succession - - Superposition Lateral Continuity - - - - Cross-cutting

Unconformities • • surfaces represent a long time. Hiatus a time when rocks were not deposited or a time when rocks were eroded Hiatus the gap in time represented in the rocks by an uncon- formity 3 kinds Angular Unconformity Nonconformity Disconformity

Disconformities A surface of erosion or non-deposition between Parallel sedimentary rock beds of different ages

Angular Unconformities Angular Unconformities • An angular unconformity is an erosional surface on tilted or folded strata, over which younger strata have been deposited.

Nonconformities A 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 Earth Counting lifetimes in the Bible Comparing cooling rates of iron pellets. Determine sedimentation rates & compare Estimate age based on salinity of the ocean. all age estimates were off by billions of years some were more off than others!

Absolute Dating Methods Radioactive Decay sequences acts as an atomic clock we see the clock at the end of its cycle analogous to starting a stopwatch allows assignment of numerical dates to rocks. > > decay ) into Radioactive isotopes change ( daughter isotopes at known rates. rates vary with the isotope e.g., U , K , C, etc. + + 235 40 14

Decay unstable nuclei in parent isotope emits subatomic particles and transform into another isotopic element (daughter). does so at a known rate, measured in the lab Half-life The amount of time needed for one-half of a radioactive parent to decay into daughter isotope. •

Rate of Decay t All atoms are parent isotope or some 1 3 All atoms are parent isotope or some known ratio of parent to daughter 1 half-life period has elapsed, half of the material has changed to a daughter isotope (6 parent: 6 daughter) 2 2 half-lives elapsed, half of the parent remaining is transformed into a daughter isotope (3 parent: 9 daughter) 3 half-lives elapsed, half of the parent isotope (1.5 parent: 10.5 daughter) We would see the rock at this point.

100 % parent remaining Parent Parent Daughter Daughter 50 25 13 Radioactive Isotopes Radioactive Isotopes • • analogous to sand in an hour glass analogous to sand in an hour glass - - we measure how much sand there is we measure how much sand there is > > represents the represents the mass of elements mass of elements - - we measure the ratio of sand in the bottom to sand in the top we measure the ratio of sand in the bottom to sand in the top - - at the end (present) at the end (present) > > daughter (b) and parent (t) daughter (b) and parent (t) - - we know at what rate the sand falls into the bottom we know at what rate the sand falls into the bottom > > the half life of the radioactive element the half life of the radioactive element - - how long would it take to get the amount sand in the observed how long would it take to get the amount sand in the observed ratio starting with all of it in the top? ratio starting with all of it in the top? 100 Parent Parent % parent remaining Daughter Daughter 50 25 13 time----------->

Five Radioactive Isotope Pairs Five Radioactive Isotope Pairs Effective Dating Range Minerals and Isotopes Half-Life of Parent (Years) Rocks That Can Parent Daughter (Years) Be Dated Uranium 238 Lead 206 4.5 billion 10 million to Zircon 4.6 billion Uraninite Uranium 235 Lead 207 704 million Muscovite Thorium 232 Lead 208 14 billion 48.8 billion Biotite Potassium feldspar Rubidium 87 Strontium 87 4.6 billion 10 million to Whole metamorphic 4.6 billion or igneous rock Potassium 40 Argon 40 1.3 billion 100,000 to Glauconite 4.6 billion Muscovite Biotite Hornblende Whole volcanic rock

Carbon-14 dating is based on the Carbon-14 dating is based on the Radiocarbon and Tree- Ring Dating Methods Carbon-14 dating is based on the Carbon-14 dating is based on the • • ratio of C-14 to C-12 ratio of C-14 to C-12 in an organic sample. sample. > > Valid only for samples less than 70,000 Valid only for samples less than 70,000 years old. years old. > > Living things take in both isotopes of Living things take in both isotopes of carbon. carbon. > > When the organism dies, the "clock" starts. When the organism dies, the "clock" starts. Method can be validated by cross-checking with tree rings

Carbon 14 Cycle

Recognizing Patterns of change Walther's Law • The vertical sequence is repeated by the horizontal sequence - walking from A to B to C to the Coast you would encounter the rocks that would be encountered by drilling a core into the earth at any point (A, B, or C)

Facies Diagram • distribution of lithofacies (rock-types) • these are associated with their respective EOD • biofacies are similar but refer to fossils rather than rock types

Eustasy, relative sea-level, and relative position of 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 of lithologies for better production of petroleum resources

Rock designations • Rock units called Lithostratigraphic units - described in terms of Group, Formation, & Member > each term has specific meanings in geological parlance Formation a mappable lithostratigraphic unit has a location for identifying the type-section has a rock designation describing the lithology sometimes not all the same lithology in which case the term "Formation" takes the place of lithologic type Groups are composed of several formations Members are distinctive units within a formation group is largest and contains formations and members formations 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 dynamo 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 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 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 Theory Tectonic cycle

Plate Tectonics and Earthquakes Most earthquakes can be explained by plate tectonics: Divergent plate boundaries Divergent motion and high temperatures cause rocks to fail easily in tension Earthquakes are small and generally non-threatening Transform 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 Convergent 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 Earthquakes Examine 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, 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

Question????

Test 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?