Lecture 11: Rock deformation The movements of Earth's tectonic plates creates stresses rocks subject to stress will deform (change in volume or shape of a body of rock) ways in which rocks deform are varied; which style a particular rock mass adopts depends on the nature of the stress applied to it and properties of the rock itself.
Types of deformation ELASTIC (temporary deformation) -Small deformations are elastic, meaning the rock will return to its original shape when the stress is removed. DUCTILE & BRITTLE - Larger deformations, the deformations that produce mountains and faults, are permanent. Stress- force acting on a body; not applied uniformly in all directions Types of stress: Compressional - rocks are squeezed along the direction of stress Tensional - rocks are pulled apart Shear - cause slippage and translation within the rock
Measuring deformation in rocks Strike is the azimuth (degrees east of north) of the line formed by the intersection of a layer interface or bedding plane with the horizontal Dip is the angle between the layer interface or bedding plane and the horizontal measured perpendicular to the strike direction
Plastic deformation Compressional stress may cause rocks to be deformed into a series of wrinkles or folds (from few inches to hundreds of kilometers across) Parts of a fold Axial plane as the imaginary surface that divides a fold as symmetrically as possible, one limb on each side. Fold axis is the line made by the length-wise intersection of the axial plane with beds in the fold Limbs correspond to the two sides of an anticline or syncline Types of folds Symmetrical folds – axial plane is vertical Asymmetrical folds – beds in one limb deep more steeply than those in the other limb Overturned folds – both limbs tilted beyond the vertical in the same direction Recumbent folds – horizontal axial plane
Assymetrical Fold
Recumbent fold (upper right)
Fold Nomenclature Monocline – bend in a gently dipping horizontal strata Anticline – an arch in the form of an inverted letter U Syncline – an arch shaped like the letterU (where is the anticline? syncline?)
Brittle deformation Rocks under surface conditions also deform plastically but once elastic limit is exceeded, the rocks will behave like a brittle solid and fracture
Joint = break in rock mass in which sections on each side of the break DO NOT move relative to each other Fault = break in rock mass in which sections on either side of the break move relative to each other Fault Nomenclature hanging wall, foot wall (Where is the fault? What type of fault is it? Where is the hanging wall? footwall?)
Classification of faults Strike-slip fault – horizontal movement (can be right lateral or left lateral) Dip-slip fault – vertical movement (can be normal or reverse/thrust) Normal- tensional stress occur; hanging wall moves down Reverse- compressional stress occur; hanging wall moves up Thrust- same as reverse but small angle of inclination Oblique-slip fault – both horizontal and vertical Summary Type of Stress Compressional Extensional Shearing
Ductile Deformation Folding Thinning Shearing
Brittle Deformation Reverse Fault Normal Fault Strike Slip Fault
What is an active fault? There is evidence to show that it has moved in the past 10,000years. PHILIPPINE FAULT is a left lateral fault
Lecture 12: Earthquakes What is an earthquake? intense ground shaking caused by sudden release of energy can be generated by bomb blasts, volcanic eruptions and sudden slippage along faults a geologic hazard for those living in earthquake-prone areas also provided valuable information about the Earth’s interior Elastic Rebound Theory Before an earthquake, areas on opposite sides of a fault are subjected to force and accumulate energy. They undergo deformation until such time that their internal strength is exceeded, which will cause a sudden release in energy (earthquake). Parts of an earthquake: Fault hypocenter/focus- source of earthquake; point in a fault plane where slippage occur Epicenter- point on the surface directly above focus
Seismology – study of behavior of seismic waves Seismometer – instrument that records ground motion Seismograph – instrument that records seismic waves
Kinds of Seismic Waves Body waves – emanate from the focus and emanate in all directions through the Earth’s interior
P-wave – compressional (primary wave); fastest waves; can travel through solids and liquids S-wave – movement is perpendicular to propagation (secondary or shear wave); can travel through solids only Surface waves – travel along paths nearly parallel to the Earth’s surface but not through the interior Love wave – horizontal motion that is perpendicular to propagation Rayleigh wave – rolling surface wave that moves the ground up and down Describing the strength of an earthquake Intensity is the degree of ground shaking at a given locale based on the amount of damage (Modified Mercalli Intensity Scale; Philippine Earthquake Intensity Scale) Magnitude is calculated from seismic records and estimates the amount of energy released at the source (Richter Scale) Locating the epicenter: at least 3 seismic graph is needed, their intersection is the epicenter of the earthquake. Structural damage due to earthquake vibrations depends on: Wave amplitudes Duration of vibrations Nature of material upon which the structure rests ( ground shaking on bedrock is lesser than shaking in sandstone) Design of structure Secondary effects of earthquakes Tsunamis – offshore fault’s movement causes the water to displace, this displacement causes the formations of waves and waves accumulates to bigger waves as it approaches the shore landslides fire ground subsidence liquefaction – soil shakes as a liquid (i.e earthquake in sediment) material: thixotrophic Recent destructive earthquakes in the Philippines July 16, 1990, Luzon (7.7) Nov. 15, 1994, Mindoro (7.8) Mar. 6, 2002, Sultan Kudarat (6.8) Feb. 15, 2003, Masbate (6.2) Earthquake prediction: short term Monitor to look for patterns of recurrence Strange animal behavior
Increase in seismic tremors (mini-quakes) Seismic gaps Gas emissions Electromagnetic signals
Earthquake prediction: earthquake cycles Seismic waves and the interior of the earth Much of what we know about the interior of the Earth comes from knowledge of seismic wave velocities and their variation with depth in the Earth. Body wave velocities are as follows:
( )µ Vp = Vs = 1/ [µ/ρ]2 where K = incompressibility µ = rigidity ρ = density Higher density, higher velocity If the earth were homogeneous, it is possible to predict when a seismic signal will travel any given distance. If the seismic wave velocity in the rock above an interface is less than the seismic wave velocity in the rock below the interface, the waves will be refracted or bent upward relative to their original path. Propagation of seismic waves through the earth: P waves are refracted S waves do not propagate through a certain depth Layers of the Earth Crust Mantle – seismic wave velocities increase rapidly at the Moho Core – P wave velocities suddenly decrease and S wave velocities go to zero (outer core); at depth of ~4800 km, P wave velocities suddenly increase (inner core) Boundaries/discontinuities Mohorovicic discontinuity- between lithosphere and astenosphere Guttenberg discontinuity- between mantle and outer core Lehmann discontinuity- between outer core and inner core
Lecture 13: Plate Tectonics Continental Drift Theory Introduced by Alfred L. Wegener in his book “The Origin of Continents and Oceans” in 1915 200 mya, all the continents were joined into a supercontinent (Pangaea) and started to drift apart (first into Laurasia and Gondwana) until their present position today Evidence for continental drift: fit of the continents (especially when joined at the continental shelf) fossils (Mesosaurus, Lystrosaurus, Cynognathus, Glossopteris) rock type (rocks found in one continenet closely match those rocks found in the matching continent); structures and mountain belts paleoclimate (layers of glacial deposits found in S. Africa and S. America, India and Australia and there are coal deposits in Antarctica. Why? Ans. Antarctica must have been situated closer to the equator, in a more temperate climate where lush, swampy vegetation could grow) Sea Floor Spreading - process in which the ocean floor is extended when two plates move apart in mid-oceanic ridges Habang lumalayo sa mid-ocean ridges, tumatanda ang rocks New material is being formed along mid-oceanic ridges If new crust is being created along mid-oceanic ridges, does this mean that the Earth is expanding? (Recall Wilson Cycle) Wilson cycle- oceanic crust subducts and destroyed in subducting zones Paleomagnetism Magnetic minerals in rocks align themselves in the direction of the existing magnetic field at the time they were formed Rocks formed at the same time - record of magnetic field should be the same Evidence for continental drift: rocks of the same age at different places point to different locations of magnetic north; rocks of different ages in the same place shows that the magnetic north have moved through time (polar wandering); It would make more sense if the magnetic north did not move but rather, the continents have moved!
The concepts of continental drift, sea floor spreading and paleomagnetism gave rise to the plate tectonics concept
Plate Tectonics Unifying theory of geology All geological features and processes are related Concepts were drawn together in 1968
Lithosphere is made up moderately rigid plates (may consist of oceanic or continental lithosphere) 7 major plates (N America, S America, Antarctica, Eurasia, Africa, Australia, Pacific) Plate boundaries convergent – plates move toward each other; oceanic-continental (volcanic arc) continental-continental (mountain range) (orogenesis) oceanic-oceanic (island arc) i.e. Philippines divergent – plates move away from each other; mid-oceanic ridge transform – plates slide past each other (strike-slip faults) *Plate boundaries are locations of volcanism and earthquakes (Why do you think so?) What causes the plates to move? (Read on this!!!) Convection currents (one layer and two layers) Slab pull Mantle plumes Additional evidence for plate tectonics: hotspots and Global Positioning System (GPS) Philippine Tectonics (where are these features located?) Plates – Sundalan/Eurasia Plate; Philippine Sea Plate (where is the Palawan Microcontinental Block?) Trenches – Manila, Negros, Sulu, Cotabato; East Luzon Trough, Philippine Trench Philippine Fault – what type of fault? (left lateral fault) Sea – South China Sea, Sulu Sea, Celebes, Philippine Sea (where is the Pacific Ocean?)
Lecture 14: Historical Geology Historical geology Deals with the origin of the Earth and its development through time strives to establish an orderly chronological arrangement of the physical and biological changes and events that have occurred in the geologic past. Previous estimates of the age of the Earth: • Cooling through conduction and radiation (Lord Kelvin, 1897): ~24 – 40 m.y. • Rate of delivery of salt to the oceans (John Joly, 1899-1901): ~90 – 100 m.y. • Thickness of total sedimentary record divided by average sedimentation rates (1910): ~1.6 b.y. Oldest rocks on Earth found so far: 1. Acasta Gneisses in northwestern Canada near Great Slave Lake (4.03 Ga) 2. Isua Supracrustal rocks in West Greenland (3.7 to 3.8 Ga) 3. rocks found in the Minnesota River Valley and northern Michigan (3.5-3.7 billion years), in Swaziland (3.4-3.5 billion years), and in Western Australia (3.4-3.6 billion years) Oldest materials to be found on Earth: Zircon grains found in sedimentary rocks in west-central Australia = 4.4 b.y. 70 well-dated meteorites using different dating methods (e.g. Rb-Sr, Sm-Nd, Ar-Ar) =4.4-4.6 b.y. Iron meteorite (Canyon Diablo meteorite) = 4.54 b.y. Age of the Earth: Most accepted age for the Earth and the rest of the solar system: ~4.55 b.y. old (+ ~1%) “Best” age of the Universe: 14 – 17 b.y. Evidence: rate of evolution of stars and age of elements in the galaxy based on the production ratios of Os isotopes in supernovae) Relative dating Putting rocks and events in their proper sequence of formation Dating of rocks and rock units with the use of fossils and correlation of different strata Does not require numerical ages of rocks or fossils or events Principles used in relative dating Principle of Uniformitarianism “The present is the key to the past.” Former changes of the earth’s surface may be explained by reference to causes in operation
The history of the earth may be deciphered in terms of present observations, on the assumption that physical and chemical laws are invariant with time.
Steno’s Laws Law of Superposition When examining an undisturbed sequence of stratified rocks, the oldest strata will be at the bottom and the youngest strata will be on the top of the sequence. Law of Original Horizontality Most layers are deposited horizontally or subhorizontally Sedimentary beds which are inclined at an angle must have undergone deformation after they had been deposited and lithified Law of Lateral Continuity Sediments would spread out until they thin out at the edge of the depositional basin, stop at a depositional barrier or grade into another type of sediment (indicative of a change in the depositional environment) Cross-cutting relationships When a fault or intrusion cuts through another rock, the fault or intrusion is younger that the rocks which it cuts. Principle of Inclusions The rock mass containing the inclusion is younger than the rock that provided the inclusion. Unconformity Any significant break in time within a stratigraphic column. Gaps in the rock record representing a long period during which deposition ceased, erosion removed previously formed rocks and then deposition resumed a period of non-deposition. Types: Angular Unconformity - Tilted or folded sedimentary rocks that are overlain by younger, more flat-lying strata. Disconformity - Strata on either side of the unconformity are essentially parallel with a distinctly recognizable surface Paraconformity - Beds above and below are parallel and the unconformity is identified by some evidence such as lack of certain diagnostic zone fossils in some horizon Nonconformity - Older metamorphic or igneous rocks are overlain by younger sedimentary strata
Principle of Faunal Succession Fossil organisms succeed one another in a definite and determinable order. Thus, any time period can be recognized by its fossil content. Correlation To show correspondence in character and in stratigraphic position (International Stratigraphic Guide) To demonstrate correspondence between geographically separated parts of a geologic unit (North American Stratigraphic Code) Based on similarity of lithologic and paleontologic features Key: E, Erosion, G, L, C, Tilting, H, Erosion, M, D, J, A, Erosion, N, K, B, Tilting, Erosion, F, Erosion http://facweb.bhc.edu/academics/science/harwoo dr/Geol101/labs/dating
Fossils and fossilization What are fossils? Remains or traces of prehistoric life preserved in sedimentary rocks Important time indicators and play a key role in the correlation of rocks; Include both the remains of organisms (bones or shells) and traces of organisms (trails, burrows or imprints) Requirements for preservation Rapid burial to prevent decomposition; Presence of protective cover or preserving medium; Possession of hard parts or durable tissues such as shells, bones, teeth and woody tissue Types of fossilization 1. Preservation of unaltered body parts: a. Hard parts – usually shells, bone, teeth or pollen b. Soft tissue – by mummification or freezing 2. Chemical alteration of hard parts: a. Carbonization – soft tissues preserved as thin carbon film b. Recrystallization – conversion of a mineral polymorph to another (e.g. aragonite → calcite)
c. Replacement – dissolution of original material and precipitation of new mineral d. Permineralization – porous material filled with secondary materials e. Petrification – replacement of wood 3. Imprints of hard parts in sediment or trace fossils: a. Mold – dissolution of shell b. Cast – filling of mold c. Borings and burrows – worms, clams and other invertebrates burrow into rocks and sediments d. Coprolites – fossil excrement e. Gastroliths – smooth, polished stones found in the abdominal cavities of dinosaur skeletons Oldest human fossil - complete skeleton of a 3-year-old female; remains found in Africa are 3.3 million years old, making this the oldest known skeleton of such a youthful human ancestor (Australopithecus afarensis) (Reported in Nature by Zeresenay Alemseged (Max Planck Institute for Evolutionary Anthropology) and Fred Spoor (University College London) and others Oldest fossils in the Philippines? a. oldest human fossil - skull cap of the “Tabon Man”; ~22,000 years old; discovered by Dr. Robert B. Fox, American anthropologist of the National Museum, inside Tabon Cave, Palawan, on May 28, 1962. b. fusulinids – Permian; found in Calamian Islands, Palawan Uses of fossils? tracing the evolutionary history of extinct as well as living organisms; reconstructing paleoclimates and paleoenvironments ; providing the source of energy resources (e.g. oil, gas, coal) Absolute dating Numerical dating of rocks, minerals and fossils; Utilizing radioactive isotopes Radioactive isotopes variants of the same atom but with different mass numbers (number of protons? neutrons? electrons?) Undergo spontaneous breaking apart (decay) of certain unstable atomic nuclei Unstable parent isotope decay into stable daughter isotope Half – life – the length of time required for one-half of the nuclei of a radioactive isotope to decay
* After 1 half life, 50% of the parent and 50% of the daughter isotope is present. *After 2 half-lives, 25% of the parent and 75% of the daughter is present and so on… In dating rocks, the ratio of the parent to the daughter isotope is measured. With knowledge of the half-life of the isotope, the age of the rock can be computed based on how much parent isotope remained versus how much daughter isotope was formed. (Assumption: there were no amount of daughter isotope present in the original mineral being analyzed; no amount of parent isotope escaped to the environment) Radioactive Parent
Stable Daughter
Half life
Dating Range
Rubidium 87
Strontium 87
48.8 b.y.
10 m.y. – 4.6 b.y.
Thorium 232
Lead 208
14 b.y.
10 m.y. – 4.6 b.y.
Uranium 238
Lead 206
4.47 b.y.
10 m.y. – 4.6 b.y.
Potassium 40
Argon 40
1.25 b.y.
50,000 – 4.6 b.y.
Uranium 235
Lead 207
704 m.y.
10 m.y. – 4.6 b.y.
Carbon 14
Nitrogen 14
5730 yrs.
100 – 70,000
C-14 is used for dating younger materials. The older dating methods (e.g. U-Pb) cannot be used for young materials because only a small amount of parent has decayed and therefore negligible because of poor resolution. Most minerals which contain radioactive isotopes (except C14) are in igneous and metamorphic rocks. K40 is usually found in potassium feldspar, muscovite and amphibole. Uranium may be found in zircon, uraninite, apatite and sphene. Geologic Time Scale The history of the earth is broken up into a hierarchical set of divisions for describing geologic time. Units of time include eon, era, period, epoch, age (arranged from largest division ).
based on the type of organisms that were abundant at the time (from the fossil record) Early efforts to develop the time scale 1. Giovanni Arduino – applied Steno’s Laws and classified rocks in Italian mountain exposures into: Primary, Secondary and Tertiary groups 2. Abraham G. Werner – saw similar transitions in Germany 3. William Smith – saw same subdivisions in Great Britain Modern Time Scale Carboniferous System (1822) – coal-rich interval in Northern Europe Cretaceous System (1822) – chalk-rich rocks in France (“creta” = chalk in Latin) Tertiary System (1831) – subdivided by Charles Lyell based on % of fossil species still living today Cambrian System (1835) – defined by Adam Sedgwick for the fossil-poor strata in NW Wales Silurian System (1835) – defined by R.I. Murchison based on the rocks in SE Wales; contained fossils Ordovician System (1879) – named by Charles Lapworth based on the presence of a distinct fossil assemblage Jurassic System (1839) – named after the strata in the Jura Mountains in France and Switzerland Devonian System (1840) – named after sandstones in Devonshire, SE England Relative Geologic Time Scale No numbers to indicate how long ago each of these times occurred; Absolute dating allowed the numerical dating of each time division International Commission on Stratigraphy – provided a standard Geologic Time Scale
