Continuation of chapter 17
Continuation of chapter 17 GEO 100
Popular in Physical Geology
Popular in Geology
This 12 page Class Notes was uploaded by samantha Flavell on Friday March 18, 2016. The Class Notes belongs to GEO 100 at State University of New York at Oswego taught by Rachel Lee (P) in Fall 2015. Since its upload, it has received 23 views. For similar materials see Physical Geology in Geology at State University of New York at Oswego.
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Date Created: 03/18/16
Bedding *Sedimentary rocks are usually layered or “stratified” *Arranged in planar, horizontal “beds” *Bedding is often laterally continuous for long distances *Bedding caused by changing conditions during deposition *Energy conditions and hence grain size *Source Area *Sedimentation Rate *Change in sea level, climate, etc. *Bedding may also reflect nondeposition or erosion *A series of beds are referred to as STRATA *A sequence of strata that’s sufficiently unique to be recognized on a regional scale is termed a FORMATION Sedimentary Structures *Provide additional information about depositional environment RIPPLES: On the bedding plane CROSS BEDDING: Seen in Crosssection GRADED BEDDING: Seen in CrossSection MUDCRACKS: On the Bedding Plane SCOUR MARKS: On the Bedding Plane Asymmetric Ripples: CURRENTS Short, steep down current slip face Long, gentle up current ramp Symmetric Ripples: WAVE OSCILLATION Sharp ridges and concave troughs From back and forth swish Graded Beds: bedding layers that fine upward Pulse of turbid water (turbidity current) As water slows, it loses velocity Coarsest material settles first, medium next, then fine Absolute v. Relative Age *With the principle of superposition we are able to look at sediments and determine when layers are older, which are younger. This is RELATIVE AGE DETERMINATION. *It tells us nothing about how old the sediments are *The purpose of absolute age of a sample and the number of years since it formed Geologic Time Relative Age: A chronological sequence of events Assigns an order to events Absolute Age The exact time when something happened A numerical age is assigned to an event. Determining Relative Age *Logical principles are used to determine a sequence of events Principle of uniformitarianism Principle of original horizontality Principle of CrossCutting relationships Principle of Inclusions Principle of baked Contacts Law of faunal succession The Principle of Uniformitarianism Charles Lyell *Processes observed today were the same in the past *Mud cracks in old sediments formed like mud cracks today Principle of CrossCutting Relationships Charles Lyell *Igneous rock bodies and geologic structures may cut through preexisting rocks *Rocks being cut through must be older than the structure that cuts them Principle of Inclusions Charles Lyell *Fragments of rock may be incorporated or included in another rock *The fragments must be older than the host rock Principle of Baked Contact Charles Lyell *An igneous intrusion “cooks” the invaded country rock *The baked rock must have been there first (is older) *A chill margin is formed at the contact from rapid cooling Sequence of Events *Determining relative age allows geologists to reconstruct the geologic history of an area. Unconformities *Unconformity: An erosional surface that indicates a gap in the rock sequence Geologic time is continuous The rock record at a given location doesn’t record all of geologic time *Unconformities indicate changes in conditions that stopped the deposition of sediments or eroded strata. Change from a depositional to an erosional environment Crustal uplift or mountain building ANGULAR UNCONFORMITY *unconformities mark significant time gaps in the geological column *It was among the evidence that Hutton used to require and old Earth Disconformity *Strata above and below the erosional surface are parallel *Disconformities: erosional or nondepositional time gaps Identifying Disconformities *No stratigraphic method functions alone. Information from all gives the most complete description of the rock record *Although sedimentary strata are laterally continuous, there are breaks in the record *Disconformities are hard to find with lithostratigraphy alone. One way of locating them is using BIOSTRATIGRAPHY. Principles of Faunal Succession *Fossils are often preserved in sedimentary rocks *Fossils as time markers are useful for relative age dating *Fossils speak of past depositional environments *specific fossils are only found within limited time range *Species evolve, exist for a time, and then disappear *The first appearance, range, and extinction used for dating *Fossils succeed one another in a known order *A time period is recognized by its fossil content FOSSIL RANGE: the first and last appearance Each fossil has a unique range Range overlap narrows time *Index fossils are diagnostic of a particular geologic time *Fossils correlate Strata locally Regionally Globally *A composite stratigraphic column can be constructed Assembled from incomplete sections across the globe It brackets almost all of Earth history Divisions of Geologic Time *Eonssubdivided into… Erassubdivided into… Periodsalso subdivided… *Written as a stratigraphic sequence with oldest divisions at the bottom ABSOLUTE GEOLOGIC TIME *Geochronology is the determination of the absolute (numeral) age of a material Absolute v. Relative Ages *With the principle of superposition, we are able to look at sediments and determine which layers are older, which are younger. This is RELATIE AGE DETERMINATION. IT tells us nothing about how old the sediments are. *The purpose of ABSOLUTE AGE DETERMINATION is to be able to define the chronological age of a sample: the number of years since it formed. Lord Kelvin’s Absolute Age of Earth *In the mid. 19 Century, Lord Kelvin used basic physics to estimate the age of the Earth. He reasoned that if the early hot Earth had cooled by conduction. It would be simple enough to determine how much time had passes by measuring the geothermal gradient. KELVIN’S AGE OF THE EARTH *Depending on the numbers used, Kelvin’s estimates were in the range 25100 Million years. *A ~50myr old Earth was accepted by many scientists until the 20 Century, and was at serious odds to the concept of uniformitarianism which suggested an infinitely Old Earth. *The Kelvin estimate neglected to consider that convection was critical to the cooling of Earth over time and disregarded heating from radioactive decay. *It is somewhat impressive to consider that the framework for the geologic time table of the last ~550myr was in place long before we had the ability to put numerical (absolute) dates on any of these rocks. Radioactivity and HalfLife *In 1896 Bequerel discovered radioactivity *After this, a series of scientists mapped out what would become known as radioisotope geochronology *The determination of absolute ages of geological materials by understanding the decay of radioactive atoms *Recall that radioactive atoms decay into stable atoms at a constant rate. This rate is related to the halflife: the amount of time it takes for half of a number of radioactive atoms to decay Isotopes *Carbon (atomic #6) has three natural isotopes with atomic weights of 12, 13, and 14 *Remember: Most isotopes in nature are stable, but some combinations of p&n are unstable. *Unstable nuclei must shed mass in order to become stable ~This release of mass is RADIOACTIVITY! ~ Radioactive Decay *Atoms spontaneously change into another element Reactions in the nucleus can change the number of protons Reaction products are expelled from the nucleus, heat is produced Parent isotope Daughter isotope Parent Daughter 50% After 1 halflife 50% 25% After 2 Halflives 75% 12.5% After 3 Halflives 87.5% Radiometric Age Determination *Abundance of parent and daughter isotopes used to calculate age. *Relative amounts of parent and daughter isotopes determine the number of halflies *Halflife gives the rate at which decay occurred. Geochronology *To effectively (accurately and precisely) date geological materials, there are some requirements Abundant parent Atoms No loss/gain or daughter atoms *An ideal geochronometer is a mineral that, when it forms. Contains lots of a radioactive parent, but no preexisting daughter. *The mineral is chemically and physically resistant, so it will not leak parent or daughter *To be widely useful, it must be a relatively common mineral. What is Radiometric Date? *The time (years before present) that a mineral crystal formed Requires that crystal cooled below a “blocking temperature” If rock is reheated, clock can be reset *Beat for igneous rocks Crystals form at the same time that the rock forms Radiometric Age Determination *Metamorphic Rock Most isotope systems will give an age of metamorphism Recrystallization and heat will reset the “clock” *Sedimentary Rock Mineral Crystals in a clastic rock are older than the rock Carbonate and evaporate rocks do not have the “right” composition Dating Sediments: Ash Layers *Since major volcanic eruptions are geologically instantaneous events, their ash layers identify a point in time. Minerals like feldspar or zirconin volcanic ash air fall a material allows absolute constraints on the timing of these events. Problems *Some rocks don’t contain easily dated minerals *Isotopes may move over time Weathering, metamorphism or other alterations may ‘reset’ the clock *Radioactive elements are usually found in trace concentrations Requires precise measurements of concentration Other Numerical Ages *Magnetostratigraphy Remnant magnetism in sediments and sedimentary rocks Sequence of normal and reverse polarity compared to a reference column Cosmogenic Isotopes *Unlike most of the longlies isotope system, there are some radioelements that are produced in nature as a result of reactions with cosmic particles. These are called: cosmogenic isotopes *They’re produced at constant rates, so they can be useful chronometers of relatively young materials (<~10myr depending on system) Having some exposure time at the Earth’s surface *The most widely used cosmogenic isotope system is Carbon 14 (C14) Metamorphic Rocks *Metamorphism The solid state change of one geological material to another in response to change in temperature, pressure or chemical composition of the environment is called metamorphism. *The rocks produced by this process are recrystallized versions of sedimentary, igneous or other metamorphic rocks. *Most metamorphism is regional, it is a large scale process whose specific effects vary spatially. METAMORPHISM CHANGES TEXTURE *Red Shale: Quartz, clay and iron oxide *Gneiss: Quartz, feldspar, biotite and garnet METAMORPHIC PROCESS *Recrystallization: minerals change size and shape *Mineral identity does not change I.e. limestone Marble *Recrystallization makes stable minerals under new conditions of temperature and pressure. *At high pressure, denser mineral forms are made PHASE CHANGE *Change in crystal structure but not composition Same chemical formula Different crystal structure Different mineral name NEOCRYSTALLIZATION *New minerals form as initial minerals become unstable Original mineral ‘decomposes’ Elements diffuse slowly through solid rock Chemical reaction of elements form new minerals PRESSURE SOLUTION *Mineral grain partially dissolves (and sometimes reprecipitates to change shape) PLASTIC DEFORMATION *Mineral grains soften and deform Requires elevated temperature and pressure Rock is squeezed or sheared Minerals change shape without breaking, behave plastically Metamorphic Process *Recrystallization: same mineral but larger *Phase Change: Some composition but different structure (polymorph) *Neocrystallization: New Composition *Pressure Solution: Minerals don’t change but dissolve *Deformation: Change in shape not mineralogy Cases of Metamorphism *The agents of metamorphism Heat (T) Pressure (P) Compression and shear Hot Water *Not all agents are required, they often do cooccur *Rocks may be overprinted by multiple events Hydrothermal Fluid Metamorphism *Hot water with dissolved ions and volatiles *Hydrothermal fluids facilitate metamorphism Accelerate chemical reactions Alter rocks by adding or subtracting elements *Hydrothermal alteration is called metasomatism Metamorphism due to Heat (T) *One cause of metamorphism is heat *Most metamorphism occurs between 250 degrees C and 850 degrees C *Between diagenesis (low T) and melting (up to 1200 degrees C) Causes of Metamorphism *Increase in Pressure Most metamorphism occurs in 212 kilobar range *Pressure increases with depth in the crust 1Kb increase with every 3km Intensity of Metamorphism *Metamorphic grade is a measure of intensity low grade less intense high grade maximum intensity *Depends on pressure AND temperature Directed Stress *We know that as you go deeper in the crust, the pressure increases, as a result of the load of rock above *Metamorphic rocks, however, will commonly be affected by directed pressure, or stress *When pressure is applied inhomogenously, one dimension receives greater stress than the others. *Minerals recrystallizing in this uneven stress field will always respond by minimizing energy and growing perpendicular to the direction of maximum stress. Metamorphism via. Differential Stress *Compression and shear combine with elevated T and P Cause rocks to change shape without breaking *Internal textures of deforming rocks can also change Minerals rotate into preferred orientations Minerals grow in preferred directions relative to stretching Foliation and Lineation *As a result of this dynamic recrystallization minerals in rocks take on planar and linear patterns *The pattern is called a fabric *A planar fabric is called a foliation *A linear fabric is called a lineation *Minerals that tend to define foliation planes are platy minerals like micas. Minerals that tend to define lineations are elongate minerals, like amphiboles and sometimes quartz and feldspar.
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