Relative & Absolute Dating PDF

Relative Dating Process of determining if one rock or geologic event is older or younger than another, without knowing their specific ages—i.e., how many years ago the object was formed Relative Dating Principle Principle of Superposition In an otherwise undisturbed sequence of sedimen...

Relative Dating Process of determining if one rock or geologic event is older or younger than another, without knowing their specific ages—i.e., how many years ago the object was formed Relative Dating Principle Principle of Superposition In an otherwise undisturbed sequence of sedimentary strata, or rock layers, the layers on the bottom are the oldest and layers above them are younger. Relative Dating Principle Principle of Original Horizontality Layers of rocks deposited from above, such as sediments and lava flows, are originally laid down horizontally. The exception to this principle is at https://cdn.britannica.com/23/151223-050-CDE6FC90/Steno-laws-stratigraphy.jpg the margins of basins, where the strata can slope slightly downward into the basin. Relative Dating Principle Principle of Lateral Continuity Within the depositional basin, strata are continuous in all directions until they thin out at the edge of that basin. Of course, all strata eventually end, either by hitting a geographic barrier, such as a ridge, or when the depositional process extends too far from its source, either a sediment source or a volcano. Strata that are cut by a canyon later remain continuous on either side of the canyon. https://cdn.britannica.com/23/151223-050-CDE6FC90/Steno-laws-stratigraphy.jpg Relative Dating Principle Principle of Cross-Cutting Relationships Deformation events like folds, faults and igneous intrusions that cut across rocks are younger than the rocks they cut across. Relative Dating Principle Principle of Inclusions When one rock formation contains pieces or inclusions of another rock, the included rock is older than the host rock. https://i.ytimg.com/vi/pBQ-l6X6MMk/maxresdefault.jpg Relative Dating Principle https://cdn.britannica.com/23/151223-050-CDE6FC90/Steno-laws-stratigraphy.jpg Unconformities A break in time in an otherwise continuous rock record. Unconformities are a type of geologic contact—a boundary between rocks— caused by a period of erosion or a pause in sediment https://geologyscience.com/wp-content/uploads/2018/06/main-qimg-a3bc6b80cbcbe824095e6a1532b5bb47- accumulation, followed by c.jpg the deposition of sediments anew Unconformities Disconformity Where is a break or stratigraphic absence between strata in an otherwise parallel sequence of strata. Unconformities Angular Unconformity Where sedimentary strata are deposited on a terrain developed on sedimentary strata that have been deformed by tilting, folding, and/or faulting so that they are no longer horizontal. Relative Dating Principle Absolute Dating Radioisotopic Dating Discovery of radioactivity in the late 1800s provided scientists with a new scientific tool Assign specific time units, in this case years, to mineral grains within a rock https://pixfeeds.com/images/32/608610/1200-608610-relative-vs-absolute-dating.jpg Radioactive Decay Isotope An atom of an element with a different number of neutrons Radioactive Decay Radioactive Isotopes Unstable isotopes 1H and 2H are stable, but 3H is unstable Radioactive Decay Spontaneously decay over time releasing subatomic particles or energy in a process Radioactive Decay Half-life Time it takes for half of the atoms in a substance to decay The half-life of an isotope is the amount of time it takes for half of a group of unstable isotopes to decay to a stable isotope Radioactive Decay Half-life The half-life is constant and measurable for a given radioactive isotope, so it can be used to calculate the age of a rock The half-life uranium-238 (238U) is 4.5 billion years and the half-life of 14C is 5,730 years. Radioactive Decay Assumptions The mineral grains containing the isotope formed at the same time as the rock, such as minerals in an igneous rock that crystallized from magma. The mineral crystals remain a closed system, meaning they are not subsequently altered by elements moving in or out of them. Radioactive Decay These requirements place some constraints on the kinds of rock suitable for dating, with igneous rock being the best. Metamorphic rocks are crystalline, but the processes of metamorphism may reset the clock and derived ages may represent a smear of different metamorphic events rather than the age of original crystallization. Radioactive Decay Alpha Decay When an alpha particle, which consists of two protons and two neutrons, is emitted from the nucleus of an atom 238U – 234Th Loss of four particles Radioactive Decay Alpha Decay 238U – 234Th The radioactive decay product of an element is called its daughter isotope, and the original element is called the parent isotope The half-life of 238U is 4.5 billion years, i.e., the time it takes for half of the parent isotope atoms to decay into the daughter isotope. Radioactive Decay Beta Decay When a neutron in its nucleus splits into an electron and a proton. The electron is emitted from the nucleus as a beta ray. The new proton increases the element’s atomic number by one, forming a new element with the same atomic mass as the parent isotope. Radioactive Decay Beta Decay Decay chain The decay process of radioactive elements like uranium keeps producing radioactive parents and daughters until a stable, or non-radioactive, daughter is formed. Radioactive Decay Electron Capture When a proton in the nucleus captures an electron from one of the electron shells and becomes a neutron. Radioactive Decay Electron Capture 1) an electron jumps in to fill the missing spot of the departed electron and emits an X-ray 2) in what is called the Auger process, another electron is released and changes the atom into an ion Radioactive Decay Radioactive Decay The parent and daughter isotopes are separated out of the mineral using chemical extraction. In the case of uranium, 238U and 235U isotopes are separated out together, as are the 206Pb and 207Pb with an instrument called a mass spectrometer. Radioactive Decay Age calculation using the daughter-to-parent ratio of isotopes. Age of the Earth Clair Patterson – 1950s Determine the age of the Earth using radioactive isotopes from meteorites, which he considered to be early solar system remnants that were present at the time Earth was forming. Other Dating Techniques Luminescence/Thermolumi- nescence Measures the time elapsed since some silicate minerals, such as coarse-sediments of silicate minerals, were last exposed to light or heat at the surface of Earth Other Dating Techniques Fission Track Relies on damage to the crystal lattice produced when unstable 238U decays to the daughter product 234Th and releases an alpha particle. These two decay products move in opposite directions from each other through the crystal lattice leaving a visible track of damage. Fossils and Evolution Fossils Evidence of past life preserved in rocks Actual remains of body parts (rare), impressions of soft body parts, casts and molds of body parts (more common), body parts replaced by mineral (common) or evidence of animal behavior such as footprints and burrows Types of Preservation Actual preservation Rare form of fossilization where the original materials or hard parts of the organism are preserved. Preservation of soft-tissue is very rare since these organic materials easily disappear because of bacterial decay. Types of Preservation Permineralization Occurs when an organism is buried, and then elements in groundwater completely impregnate all spaces within the body, even cells. Soft body structures can be preserved in great detail, but stronger materials like bone and teeth are the most likely to be preserved Types of Preservation Molds and casts Form when the original material of the organism dissolves and leaves a cavity in the surrounding rock Mold -The shape of this cavity is an external Cast - If the mold is subsequently filled with sediments or a mineral precipitate, the organism’s external shape is preserved Types of Preservation Carbonization Occurs when the organic tissues of an organism are compressed, the volatiles are driven out, and everything but the carbon disappears leaving a carbon silhouette of the original organism Types of Preservation Trace Fossil Indirect evidence left behind by an organism, such as burrows and footprints, as it lived its life Ichnology is specifically the study of prehistoric animal tracks Stress in Earth’s Crust Stress Force applied to an object In geology, stress is the force per unit area that is placed on a rock Stress in Earth’s Crust Stress Types Confining stress A deeply buried rock is pushed down by the weight of all the material above it. Since the rock cannot move, it cannot deform https://static.hindawi.com/articles/mpe/volume-2019/5432470/figures/5432470.fig.003.jpg Stress in Earth’s Crust Stress Types Compression Squeezes rocks together, causing rocks to fold or https://scontent.fmnl10-1.fna.fbcdn.net/v/t1.6435-9/182680321_966783470793156_477237650343521750_n.jpg?_nc_cat=109&ccb=1-7&_nc_sid=8bfeb9&_nc_ohc=zfk9nwdLfGIAX-r7-jS&_nc_ht=scontent.fmnl10-1.fna&oh=00_AfDoixNfdcOZDak8YSE5c- 6GXQ_g_SoWYtfbm_Pbe1qQSA&oe=651F78CE fracture (break) Most common stress at convergent plate https://images.slideplayer.com/23/6584055/slides/slide_14.jpg boundaries Stress in Earth’s Crust Stress Types Tension Rocks under tension lengthen or break apart https://scontent.fmnl10-1.fna.fbcdn.net/v/t1.6435-9/182680321_966783470793156_477237650343521750_n.jpg?_nc_cat=109&ccb=1-7&_nc_sid=8bfeb9&_nc_ohc=zfk9nwdLfGIAX-r7-jS&_nc_ht=scontent.fmnl10-1.fna&oh=00_AfDoixNfdcOZDak8YSE5c- 6GXQ_g_SoWYtfbm_Pbe1qQSA&oe=651F78CE Tension is the major type of stress at divergent plate boundaries. https://images.slideplayer.com/23/6584055/slides/slide_14.jpg Stress in Earth’s Crust Stress Types Shear When forces are parallel but moving in opposite https://scontent.fmnl10-1.fna.fbcdn.net/v/t1.6435-9/182680321_966783470793156_477237650343521750_n.jpg?_nc_cat=109&ccb=1-7&_nc_sid=8bfeb9&_nc_ohc=zfk9nwdLfGIAX-r7-jS&_nc_ht=scontent.fmnl10-1.fna&oh=00_AfDoixNfdcOZDak8YSE5c- 6GXQ_g_SoWYtfbm_Pbe1qQSA&oe=651F78CE directions Most common stress at transform plate boundaries https://images.slideplayer.com/23/6584055/slides/slide_14.jpg Stress in Earth’s Crust Strain/Deformation When stress causes a material to change shape, it has undergone strain or deformation Deformed rocks are common in geologically https://enigmaticscience.files.wordpress.com/2018/07/slide_3.jpg?w=900 active areas. Stress in Earth’s Crust A rock’s response to stress depends on the rock type, the surrounding temperature, and pressure conditions the rock is under, the length of time the rock is under stress, and the type of https://enigmaticscience.files.wordpress.com/2018/07/slide_3.jpg?w=900 stress Stress in Earth’s Crust Elastic deformation The rock returns to its original shape when the stress is removed. Stress in Earth’s Crust Plastic deformation The rock does not return to its original shape when the stress is removed. https://media.springernature.com/lw685/springer-static/image/prt%3A978-3-540-31080-8%2F16/MediaObjects/978-3-540-31080-8_16_Part_Fig1_HTML.jpg Stress in Earth’s Crust Fracture The rock breaks https://4.bp.blogspot.com/-AwWfCuRhMzM/VcEdq1UPBLI/AAAAAAAABZo/KhEBz06Rt5o/s1600/7.6.jpg Stress in Earth’s Crust Stress in Earth’s Crust At the Earth’s surface, rocks usually break quite quickly, but deeper in the crust, where temperatures and pressures are higher, rocks are more likely to deform plastically. Sudden stress, such as a hit with a hammer, is more likely to make a rock break. Stress applied over time often leads to plastic deformation. Geologic Structure Sedimentary rocks are formed with the oldest layers on the bottom and the youngest on top. Sediments are deposited horizontally, so sedimentary rock layers are originally horizontal, as are some volcanic rocks, such as ash falls. Sedimentary rock layers that are not horizontal are deformed. Geologic Structure Folds Rocks deforming plastically under compressive stresses crumple into folds. They do not return to their original shape. If the rocks experience more stress, they may undergo more folding or even fracture Geologic Structure Folds A monocline is a simple bend in the rock layers so that they are no longer horizontal Geologic Structure Folds An anticline is a fold that arches upward. The rocks dip away from the center of the fold The oldest rocks are at the center of an anticline and the youngest are draped over them Dome – Structure Geologic Structure Folds Syncline: A syncline is a fold that bends downward. The youngest rocks are at the center and the oldest are at the outside Basin - structure Geologic Structure Faults A rock under enough stress will fracture. If there is no movement on either side of a fracture, the fracture is called a joint Geologic Structure Faults If the blocks of rock on one or both sides of a fracture move, the fracture is called a fault Sudden motions along faults cause rocks to break and move suddenly. The energy released is an earthquake. Geologic Structure Faults Dip Faults lie at an angle to the horizontal surface of the Earth Dip-slip fault Fault’s dip is inclined relative to the horizontal Geologic Structure Faults Thrust fault Type of reverse fault in which the fault plane angle is nearly horizontal. Rocks can slip many miles along thrust faults Geologic Structure Faults Normal fault Responsible for uplifting mountain ranges in regions experiencing tensional stress Geologic Structure Faults Strike-slip fault Dip-slip fault in which the dip of the fault plane is vertical. Strike-slip faults result from shear stresses Geologic Structure Faults Strike-slip fault Dip-slip fault in which the dip of the fault plane is vertical. Strike-slip faults result from shear stresses Stress and Mountain Building The world’s highest mountain range, the Himalayas, is growing from the collision between the Indian and the Eurasian plates The crumpling of the Indian and Eurasian plates of continental crust creates the Himalayas. Stress and Mountain Building Subduction Oceanic lithosphere at convergent plate boundaries also builds mountain ranges The Andes Mountains are a chain of continental arc volcanoes that build up as the Nazca Plate subducts beneath the South American Plate Stress and Mountain Building Basin-and-range When tensional stresses pull crust apart, it breaks into blocks that slide up and drop down along normal faults. The result is alternating mountains and valleys References https://opengeology.org/textbook/7-geologic-time/ https://geo.libretexts.org/Bookshelves/Geology/Fundamentals_of_Geology_(Schulte)/07%3A_Cr ustal_Deformation/7.03%3A_Stress_in_Earth's_Crust https://courses.lumenlearning.com/geo/chapter/reading-continental-drift-2/ https://courses.lumenlearning.com/geo/chapter/reading-theory-of-plate-tectonics-2/ https://courses.lumenlearning.com/geo/chapter/reading-supercontinents/