Physical Geology Course Notes
Physical Geology Course Notes GEO 101
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GEO 101 February 9, 2016 The convective flow of the iron alloy liquid outer core creates an electric current that in turn generates a magnetic field The magnetic north pole does not exactly coincide with the geographic north pole Certain rocks that contain iron or magnetite will preserve the Earth’s past magnetic field or dipole At temperatures greater than 350550 Celsius, there is no net magnetism As rocks cool, the dipoles of the magnetite grains in the rock lock into parallelism of the earth’s dipole Sediments settle in the earth’s magnetic field. The magnetic grains in the sediments retain the paleomagnetic field even after the sediment turns to rock Geologists discovered that in successive layers of rock in one location Either the pole itself is moving (wandering pole) or the continent has drifted over time (fixed pole) By the late 1950s, scientists had reconstructed the apparent polar wander paths for several different continents, and found that these paths did not align. This meant that the continents must have moved Ocean floor is covered by a thin veneer of sediment o Sediment is thinnest near the midocean ridges o Sediment is thickest near the continents o This thin veneer of sediment on the ocean bottom is too thin to be accumulating over earth’s entire existence Dredged rock samples from the bottom are basalt—very dense, Fe and Mgrich rocks. There are no “Continental” rocks like granite Bathymetry 1. A deep trench 2. a. 4,000 km b. 3,560 km c. 10,000 km d. 350 m 3. a. midocean ridge b. trench c. midocean ridge d. mountain Heat flow 1. The middle of the band is deeper and towards both ends of the band, it is more elevated Earthquakes 1. They occur where the tectonic plates collide 2. They occur more where there are trenches Hannah Millirons GEO 101 March 31, 2016 Crustal Deformation Where do we find mountains? o Convergent boundaries at the edge of continents Building continents North America was assembled like a puzzle from many pieces o Craton: a region of continental crust that has remained tectonically stable for 100s of million years o Orogen: an elongated region of crust that has been deformed and metamorphosed, often by a continental collision Processes involved in mountain building o Deformation Faulting and folding Uplift Changing location, orientation, or shape of rocks Strain: a change in shape of volume of a rock (deformation) in response to stress Elastic limit: after exceeding elastic limit, rocks are deformed permanently Brittle deformation: joints and faults Joints are fractures in rock that show no slippage or offset along the fracture Sets of parallel planar fractures with regular spacing Form as a result of contraction due to cooling or as a result of removal of the weight of overlying rock o Metamorphism o Igneous processes o Erosion By rivers By glaciers o Sedimentation Large volume of sediment eroded from mountains Deformation also produces basins (low areas) o Mountains reflect a balance between uplift and erosion o Mountains are steep and jagged due to erosion o Types of stress Stress is the same as pressure (force/area) When you are under pressure, you are stressed! Three major types Compression o Crust is compressed and becomes shorter and thicker Tension o Pulling apart of crust, usually at continental rifts Shear o Two plates are moving past each other, usually at transform boundaries o Types of faults Normal fault Due to pulling apart=extensional Hanging wall moves down relative to the footwall Reverse (thrust) fault Due to pushing together=compressional Hanging wall moves up relative to the footwall Strike slip fault Plates are moving past each other Left lateral slip o Fault moves left Right lateral slip o Fault moves right Dip slip faults (fault scarps) Faults that are not vertical and are dominated by vertical movement Footwall: block beneath the fault (bc miners put their foot on it) Hanging wall: block above the fault (bc that’s where miners hang their lantern) Blind thrust At depth, the fault is displacing the layers but it isn’t displayed at the surface, so it may only cause some folding People may not know about them until there is an earthquake Recognizing faults in the field Often linear features Fault scarps Triangular faces o Edges of mountains are triangular Evidence of movement on fault plain Fault breccia: angular fragments of rock ground up and crushed by movement on a fault When the rock is broken into clay or silt size particles as a result of slippage on the fault, it is referred to as fault gouge Slickensides: scratch marks left on the fault plane as one block moves relative to the other Mylonite: a metamorphic rock showing evidence of shear; forms as a result of dynamic metamorphism along the fault zone Recognizing faults in the geologic record Offset strata or juxtaposition of different rock types Planar surface Fault gouge (ground up rocks) Sometimes striations Ductile deformations: folds and foliations Folds o Folds arise from compressional stress o Geometry of folding Anticline: looks like an arch; the beds dip away from the hinge; has economic value as well as scientific value bc they usually have oil beneath their surface Syncline: looks like a trough; the beds dip toward the hinge Monocline: looks like a stair step; it may drape over a fault block Plunging anticline has a titled hinge Dome has the shape of an overturned bowl Basin has the shape of an upright bowl How does deformation of the crust through faulting and folding in high mountain topography o Isostasy A state of gravitational equilibrium whereby the lithosphere floats on the asthenosphere at an elevation dependent upon the former’s thickness and density Thickening of the continental crust results in surface uplift Can also cause uplift when lithospheric material is replaced with less dense asthenosphere Lithospheric delamination o Thicken up the crust and deepest parts of lithosphere get pushed deeper into the earth and are under a lots of pressure; lower part of lithosphere becomes denser than the asthenosphere and will break away, causing the less dense part of the lithosphere to break away o Tectonic settings Continental collision When two continents collide, the resulting compression shortens and thickens the continental crust forming a large mountain range Characteristics of collisional orogens o The ridges and valleys of the Appalachians represent eroded folds of the foldthrust belt that formed in a late Paleozoic collisional orogeny Convergentmargin collision At some convergent margins, compression between the downgoing and overriding plates uplifts a mountain range in which volcanism occurs Example: the Andes Orogens: accreted terranes o The western portion of North American Cordillera includes many accreted terranes Riftrelated orogens Normal faults Rifting leads to the development of numerous mountain ranges Thinning of the lithosphere causes melting and eruption of volcanoes Example: basin and range Orogens: mountains Orogeny: mountain building o History of the Appalachians Grenville Orogeny ~ 1 Ga Collisional event Supercontinent Rodinia formed Metamorphic rocks under eastern U.S. (Blue Ridge) Exotic terranes & island arcs accreted onto North America during Taconic Orogeny (440 Ma) & Acadian Orogeny (370 Ma) Sediments deposited in Appalachian basin during Acadian orogeny (e.g. Marcellus Shale) Alleghenian orogeny 320290 mya Collision of Africa with North America to form supercontinent of Pangea Himalayalike mountains, foldandthrust belt to west (valley and ridge) 180 mya Pangea starts to break apart Rifting forms Atlantic ocean basin Passive margin develops on coastal plain Neogene rejuvenation of Appalachian topography? Hannah Millirons GEO 101 April 12, 2016 Earthquakes What causes earthquakes? o Earthquake: the sudden slip on a fault (release of elastic energy), and the resulting ground shaking and seismic energy that is radiated in all directions. Case studies o Northridge, California 1994 Magnitude 6.7 (but exceptionally high ground accelerations) Shaking lasted 11 seconds Earthquake occurred on a blind thrust $20 billion in damage 57 dead Buildings and freeways collapsed Since then, building codes have been revised Broken gas and water lines o Tohoku, Japan 2011 Magnitude 9.0 megathrust $235 billion in damage (most expensive natural disaster in history) Over 15,000 dead (vast majority as result of tsunami) Shaking lasted several minutes Large number of foreshocks, hundreds of aftershocks Early warning system issued warning to public ~30 seconds after earthquake occurred, 1 minute before shaking in Tokyo Liquefaction caused damage to several buildings Tsunami damaged nuclear power plants o Haiti 2010 Magnitude 7.1 Shallow depth (8 miles) ▯ strong shaking As many as 300,000 dead, 1.5 million displaced Densely populated area Poor construction ▯ many buildings collapsed Heavy damage to already weak infrastructure Cholera outbreak made worse by conditions resulting from the earthquake (displaced people, poor sanitation, etc.) Tsunami o Sudden displacement of seafloor o Influence entire water depth o Long wavelength o High velocity Earthquaketriggered landslides in Nepal o M 7.8 earthquake on April 25, 2015 o Series of M>6 aftershocks, including M 7.3 on May 12th o Triggered 1000’s of landslides, which killed 100’s of people Liquefaction o Occurs in areas where there is saturated soil o Loosely packed grains o Pore spaces filled with water o When it is shaken, sediment becomes like water and everything sinks into it How can we reduce earthquake risk? o Understand the science of earthquakes Where are earthquakes likely to occur? Oceanocean convergent boundaries Subduction zones big earthquakes Along all different types of plate boundaries How much shaking? (What causes?) How likely? Plate tectonic setting? Any nearby faults? Remember how to recognize faults in the landscape and geologic record o How often have earthquakes occurred on those faults in the past? o How big were they? o Are there segments of the fault that haven’t slipped as recently? o Engineer safer buildings, roads, power plants o Public policy: e.g., building codes, early warning systems o Community preparedness: e.g., earthquake drills, emergency response Seismic waves o Body Waves travel through Earth’s interior Pwave (Primary Wave): Pressure or compressional wave. Like a Slinky being pushed and pulled. Arrive first because it travels faster Swave (Secondary Wave): Shear wave. Particles are moved up and down to propagate the wave. Like flicking a rope. Arrives after the Pwave because it’s a little slower o Surface Waves travel along Earth’s surface Arrives last Seismographs o Measure wave arrivals and magnitude of motion. Straight line = background. 1st wave causes frame to sink (pen goes up). Next vibration causes opposite motion. Locating the source of an earthquake o P waves travel about 1.7 times faster than S waves o Farther from hypocenter, greater lag time of S wave behind P wave (SP) o In order to calculate the distance from the focus of an earthquake, you need the difference in arrival time of the Pwave and Swave on a seismogram o Need distance of earthquake from three stations to pinpoint location of earthquake: Visualize circles drawn around each station for appropriate distance from each station (time difference between P and S wave arrivals) Three circles intersect at one point that’s the epicenter. Measuring earthquakes o Richter Magnitude: Relative Size of an Earthquake (based on seismograph shaking, normalized for distance from epicenter) o Seismic Moment/Moment Magnitude: Absolute Size of an Earthquake (based on energy released) o Modified Mercalli Intensity: Shaking Damage (based on talking to people) Intensity of shaking depends on… o Earthquake magnitude o Distance from hypocenter o Type of rock or sediment making up ground surface (amplification) Amplification In soft sediments, seismic waves travel more slowly and amplitude increases Longterm earthquake prediction o Probability of a certain magnitude earthquake occurring on a timescale of 30 to 100 years or more o Based on the premise that earthquakes are repetitive o Recurrence interval: average time between earthquake events o Slip rate: average rate at which one side of the fault moves with respect to the other side Slip rate and recurrence interval (problem set 3) Offset: distance one side of the fault has moved relative to the other Smallest offset: slip from one event Larger offsets: cumulative slip from multiple events Average slip rate: offset / (age of offset feature) Recurrence interval: (slip per event ) / (longterm slip rate) Difference between earthquake and aftershock o Aftershock is a type of earthquake o Biggest earthquake is the actual earthquake o Any small shakes before is a foreshock and any small shakes after is an aftershock Induce seismicity o Commonly caused by wastewater injection (not “fracking” itself) o Last year, there were 3x as many magnitude 3+ earthquakes in OK as in CA Metamorphism Three types of rock: o Igneous o Metamorphic o Sedimentary Rocks tell us… o The conditions under which they formed o Tectonic history o Processes occurring within the Earth Metamorphic rocks tell us about ancient plate boundaries and history of mountain building Metamorphism: the mineralogical, textural, chemical, and structural changes occurring in response to elevated temperatures and/or pressures Pressure: o What happens? Minerals collapse inwards Minerals on the earth’s surface have large spaces between atoms (are less dense) Apply pressure and denser minerals tend to form through phase changes and/or neocrystallization Heating: o What happens? Chemical bonds bend and stretch If they break, they can move slightly and reattach to other atoms Occurs between 200C and 850C in the solid state Question: magma exists at temps of about 600C. How can metamorphism occur at higher temps? Recrystallization o Minerals change size and shape, but not composition o Dissolution and growth of mineral grains o Example: limestone >> marble Phase change o Same chemical formula, new crystal structure (polymorph) o Examples: kyanite/adalusite/sillimanite quartz/coesite Neocrystallization o Initial minerals become unstable and morph into new ones Original protolith minerals are digested in reactions Elements recombine into new minerals Plastic deformation o Change shape (example: stretching or flattening), but not composition of grains What are some things a metamorphic rock can tell us o Bulk composition of the rock, preserved features >> protolith o Foliation >> differential stress Differential stress: pressure that is greater in one direction A commonplace result of tectonic forces Opposite: confining (lithostatic) pressure = same pressure in all directions Foliated – have throughgoing planar fabric. Subjected to differential stress. Significant component of platy minerals. Slaty cleavage: produced by mild differential stress Gneissic banding = compositional banding Nonfoliated – No planar fabric evident. Crystallized without differential stress, and/or Composed of equant minerals only (e.g. marble & quartzite) o Metamorphic mineral assemblage >> temp and pressure Metamorphic grade o Minerals present change as the intensity of metamorphism increases from low grade to high grade Low grade slight High grade intense o Shale >> gneiss = increasing metamorphism o Shale: protolith o Slate: Clay minerals recrystallize and align to form slaty cleavage o Phyllite: clay minerals neocrystallize into tiny micas; micas reflect a satiny luster o Schist: Abundant micas grow larger; Foliation due to subparallel alignment of large mica crystals; New minerals (e.g., garnet) form o Gneiss: Compositional banding: Dark – biotite, amphibole Light – quartz, feldspar Metamorphic facies o Mineral assemblage that results from specific PT conditions o Which minerals form also depends on protolith composition o Metamorphic facies tell us about the environment of metamorphism Metamorphic environments o The types (and settings) of metamorphism are... Thermal/Contact – Heating by a plutonic intrusion. Hydrothermal – Alteration by hotwater leaching. Subduction – Various bands of alteration. Regional – Range of alteration due to continental collision (orogenesis). Burial – Increases in P and T by deep burial in a basin. Dynamic – Shearing in a fault zone. Shock – Extremely high P due to bolide impact. o Three major pathways to metamorphism in PressureTemperature space : High T at low P “contact” metamorphism (intrusion of plutons) Produces “hornfels” Basically cooks the surrounding rocks Normal geothermal gradient “regional” metamorphism (mountain building) Convergent margins/ancient orogens Generally strong foliation Low T at High P “blueschist” metamorphism (subduction zones) Hannah Millirons GEO 101 April 19, 2016 Geologic Time Relative Age o Determining relative ages empowers geologists to easily unravel complicated geologic histories based on simple principles Uniformitarianism o The present is the key to the past Principle of original horizontality and lateral continuity o Deposited horizontally in layers Principle of superposition o Everything deposited goes on top of the previous layer, making the oldest on the bottom and the youngest on the top Principle of cross cutting relationships o If one feature cuts across another feature, it means the one being cut is older and the one cutting is younger The great unconformity of the grand canyon o Rock record is incomplete o Unconformity = missing time (due to erosion) Contact between underlying metamorphic schist and the overlying sedimentary rocks Angular unconformity Enough time missing where rocks that are deposited are deformed and then eroded before younger rocks are deposited Principle of inclusions o A piece of one type of rock in another type o The rock that is included in the other rock is older Numerical age o Many relative ages can now be assigned actual dates o Radiometric dating is based on radioactive decay of atoms in minerals Radioactive decay proceeds at a known, fixed rate Radioactive elements act as internal clocks o Numerical dating is also called geochronology Radioactive decay o Isotopes – Elements that have varying #s of neutrons. Stable – Isotopes that never change (e.g. 13C). Radioactive – Isotopes that spontaneously decay (i.e. 14C). o Parent isotope – The isotope that undergoes decay. o Daughter isotope – The product of this decay. o Radioactive decay of a particular nucleus is spontaneous, but has a certain probability of occurring o As a result, the number of parent isotopes decreases with a certain halflife Halflife = time for ½ of the remaining unstable nuclei to decay Exponential decay o Halflife (t½) – Time for ½ of the remaining unstable nuclei to decay. T½ is a characteristic of each isotope. o As the parent disappears, the daughter “grows in”. o In order to calculate the age, we need to know the halflife and measure the ratio of parent to daughter isotopes What is a radiometric date? o Radiometric dates give the time a mineral began to preserve all atoms of parent and daughter isotopes. Requires cooling below a “closure temperature.” If rock is reheated, the radiometric clock can be reset. o Igneous & metamorphic rocks are best for geochronologic work. o Sedimentary rocks cannot be directly dated. The oldest rocks o Oldest whole rocks: 4 billion years old, gneiss from Canada o Oldest fragment of rock: 4.4 billion years, zircons from Australia o Age of Earth: 4.56 billion years, meteorites Dating the geologic column o Geochronology is less useful for sedimentary deposits. o Sediments can be bracketed by numerical dates. o Fossil assemblages also constrain age of sedimentary rocks. o Stratigraphic sections can also be dated with magnetostratigraphy Can you answer the following questions? o Given, a cartoon of an outcrop, determine the relative ages of events based on the principles of original horizontality, superposition, crosscutting relationships o Define the following terms: uniformitarianism, unconformity, and radiometric dating, halflife. o How is radioactive decay used to date rocks? o What are 2 reasons why carbon14 can’t be used to date most rocks? o Explain why radioactive decay can’t be used to date sedimentary rocks directly, and how we can date sedimentary rocks (bracketing, fossils, magnetostratigraphy). o How old is the Earth? How do we know this? Eons: o Phanerozoic “Visible life” (542 Ma to the present). Started 542 Ma at the Precambrian / Cambrian boundary. Marks the 1st appearance of hard shells. Life diversified rapidly afterwards. The Phanerozoic is divided into three eras: Paleozoic – Ancient life. Mesozoic – Middle life. Cenozoic – Recent life. These eras are further divided into periods & epochs. o Proterozoic – “Earlier life” (2.5 to 0.542 Ga). Development of tectonic plates like those of today. Buildup of atmospheric O2; multicellular life appears. “Snowball Earth” glaciations between 720 – 635 Ma Ediacaran fauna appears ~620 Ma Multicellular invertebrates o Archean – “Ancient” (3.8 to 2.5 Ga). Birth of continents. Appearance of the earliest life forms. o Hadean – “Hell” (4.6 to 3.8 Ga). Differentiation (core & mantle). Formation of the oceans and secondary atmosphere. o Formation of the earth: 4.564.54 GA o The Paleozoic Era (542251 MA) History of Appalachians Taconic Orogeny o During Ordovician (ended 440 Ma) o Convergence with exotic terrane, volcanic arc o Continent grows from accreted terranes Acadian Orogeny o 370 Ma (Devonian) o More accretions o Collision with Avalon Terrane (volcanic arc) o Sediments deposited in Appalachian Basin Alleghanian Orogeny o 320290 Ma (Carboniferous) o Collision of Africa with N. America o Formation of supercontinent Pangaea o Himalayalike mountains, fold and thrust belt to west (Valley and Ridge) o West of mountains is a shallow sea & coal swamps The biggest extinction in Earth’s history (251 MA) o Carboniferous and Permian life evolution: o The Paleozoic ended with the Permian extinction. o 90% of all marine species disappeared! o Some evidence links the extinction to a bolide impact or flood basalts. o The Mesozoic Era (25165.5 MA) Break up of Pangea (late Triassic – Jurassic) The KT (Cretaceous–Tertiary) boundary event (65 Ma) Sudden mass extinction of most species on earth. o The dinosaurs that had ruled the planet for 150 Ma vanished. o 90% of plankton disappeared. o 75% of plant species vanished. Catastrophic impact by a 10 km comet or meteorite. The Chicxulub crater lies beneath the northern Yucatan. The Late Mesozoic The KT (Cretaceous–Tertiary) boundary event. o Evidence for an impact end to the Mesozoic? Thin clay interrupts deepsea chalk at the KT boundary. This suggests that, for a short time, all plankton died. Iridium in the clay is rare on Earth; common in meteorites. Iridiumenriched clay found at the KT boundary worldwide. The clay contains shocked quartz and tiny glass spheres. An immense impact best explains these features. o The Cenozoic Era (65.50 MA) IndiaAsia Collison (50 MA) Pleistocene Ice Ages The Quaternary Period (~2.65 Ma to present): Pleistocene is defined by continental scale glaciation. o Glaciers have advanced and retreated at least 20 times. o Last Glacial Maximum ~20 ka HOLOCENE is defined as the interglacial period we’re in now: last 11Ka Human Evolution Mostly bipedal genus appeared about 4 Ma (Australopithecus). Earliest tools by 2.6 Ma 3.3 Ma modern Homo sapiens (~200 ka) Start of recorded history 5 ka The Anthropocene Could be added as a new epoch on top of the Holocene or the timescale could remain unchanged, in which case the Anthropocene would function as an informal time unit. Hannah Millirons GEO 101 April 26, 2016 Global Climate Change Paleoclimate o How do we know what the climate was like in the distant past? o Sedimentary record of paleoenvironment o Fossils (esp. pollen) o Isotope geochemistry Ocean sediment cores Ice cores o How do we know? Rock record of paleoenvironment Warm climates indicated by: Fossil reefs Limestones Bauxite (tropical soils) Evaporite minerals (like halite and gypsum) Frostintolerant flora and fauna at high latitudes Cold climates indicated by: Glacial till Glacial striations, Glacial landforms Coldweather flora and fauna at low latitudes. Oxygen isotopes 1O and O = stable isotopes of oxygen We are interested in the ratio of these two isotopes in H2O During glacial periods, more water with low O/ O is trapped in glacial ice, so the water remaining in the oceans has even higher 1O/ O Oxygen isotope ratios are preserved in carbonate shells of foraminifera. o The oxygen in CaCO3 shells mirrors oceanic O/ O.18 16 o Higher O / O in the shells means colder climate, and lower ratio means warmer climate o Thus O/ O of foraminifera in sea floor sediments provide a record of climate change. o This record goes back ~65Ma The colder it is, the lower the O / O in glacial ice (opposite of what we see in ocean sediment cores) Colder air can hold less moisture. When it’s colder, the air masses arriving over ice sheets have formed more precipitation (which removes the heavy isotopes), leaving them with lower O/ O than when it’s warmer Ice traps air bubbles that tell us about the composition of the atmosphere when the ice formed In polar ice cores, lower O/ O means that temperatures were cooler. This record goes back 800,000 years o What causes these variations in climate? Tectonics Can’t build up thick glacial ice on the ocean >> need a big land mass at one or more pole to have large glaciations Position of the continents also affects ocean currents, which affect global transport of heat and moisture; bring warm water up from equator to the North Atlantic o Ex: closing of the Isthmus of Panama by 3 MA may have contributes to Pleistocene ice age Mountain building leads to increased weathering, which removes CO2 from atmosphere and cooling Uplift of the Tibetan Plateau also altered atmospheric circulation patterns Rates of volcanic activity affect CO2 o Example: Increased rates of volcanism associated with breakup of Pangea ~200 Ma Increased levels of CO2 in atmosphere Warming? Greenhouse gases Carbon cycle Albedo (reflectivity) = positive feedback o Ice has a high albedo (it’s good at reflecting solar radiation) o Cooling >> increasing ice cover >> more cooling o Positive feedback: enhances and amplifies a process Amplify change (ex: ice albedo effect) o Negative feedback: slows a process down or reverses it Orbital variations Milankovitch Cycles o Orbital cycles Changes in Earth’s orbit are linked to glacial oscillations Enhance or diminish seasonality Feedback mechanisms Hannah Millirons GEO 101 March 24, 2016 Rivers & Coasts Rivers and coasts are very dynamic systems involving interactions between o atmosphere o hydrosphere o lithosphere o Biosphere (including humans) The work of rivers: sediment transport and erosion o The energy available to do this work depends on the slope of the channel and the discharge (amount of water flowing past a point [volume/time]) o Whether erosion or deposition occurs in river also depends on erodibility of the rock and the size and amount of sediment being carried by the river o High slope and discharge > erosion o Low slope and discharge > deposition Sediment load: bed load, suspended load, and dissolved load o Bed load: larger sediment rolls along the bed of the river o Suspended load: sediment is small and is suspended in the water; does not settle and flows with water o Dissolved load: ions dissolve in weathering of rocks and is present in water, but not visible; ex: salty ocean water Floodplain flooding leaves mineralrich sediments Nooksack river o Near the headwater source of the stream, the gradient is steep, discharge is low, sediments are coarse, and the channel is straight and rocky. o Toward the mouth of a stream, the gradient flattens, discharge increases, sediment grain sizes are smaller, and channels develop broad meander belts. Braided streams o Needs high sediment bed load and high gradient Meandering stream o Channels form intricately looping meanders along the lower gradient portion of longitudinal profile The longitudinal profile Base level: lowest elevation to which stream can erode globally = sea level o Base level changes can cause stream readjustments o Raising base level results in an increase in deposition o Lowering base level accelerates erosion Parts of the river o Channels o Floodplains Deltas o When streams enter standing water, velocity decreases and sediment drops out o Delta evolution Factors that may control the gain or loss of land along a coast Gain o Build up land by bringing in sediment o Elevation of land changing Loss o Sea level rise Glacial melting o Storms erode coast Estuary o Where sea floods river valley, freshwater and saltwater mix in an estuary Coastal processes o Coasts are shaped by Erosion and deposition by Waves o Along rocky coasts, waves attack and results in cliff retreat and creation of a wavecut bench o Also transport sediment o When waves arrive at the shore obliquely, longshore drift results from zigzag pattern of swash and backwash Longshore drift can create spits and baymouth bars Tides Rivers o Modulated by Relative sealevel changes Climate Biosphere Globally, absolute sea level is rising due to o Melting glaciers o Thermal expansion of seawater Locally, relative sea level is rising even more rapidly in some places and falling in other places Emergent coast o Land has risen relative to sea level o Form cliffs, marine terraces Submergent coast o Land has sunk relative to sea level o Flooded river valleys, estuaries Changes in land surface elevation may depend on o Plate tectonic setting (ex: uplift along faults) o Glacial isostatic adjustment During ice ages, thick sheets of ice sit on top of earth’s crust and pushes it downward & underneath ice sheet, the earth’s crust has been depressed When the glacier melts, the lithosphere flexes back to its initial position meaning that areas that are pushed down will rise upward and areas that are bulging upward will relax and come down again o Compaction of sediments Pore collapse and land subsidence due to groundwater withdrawal Oil and gas drilling can causes compaction and subsidence o Sediment supply (ex: to build delta, to replenish sand on beach) Solutions to coastal erosion o Minimize activity that contribute to relative sealevel rise Revetments (seawalls, jetties, groins, breakwaters) Groins are structures built perpendicular to the shore that prevent sediment being removed by longshore drift Jetties (similar to groins, but in pairs) project out into the water perpendicular to the shoreline to maintain entrances to lagoons and harbors o Cons of jetties are the beaches that are downstream are not getting their sediment being replaced, so beaches upstream are wide and maintained and beaches downstream are narrower and eroding Breakwaters reduce intensity of wave action o Small structures built in different places and patterns in the middle of the water in order to make the bigger waves break there and the smaller waves break on the beach A concrete or rock seawall can hasten erosion in extreme storms o Wave energy is concentrated and erosion is enhanced at the base of the seawall; seawalls can then fail Beach replenishment Vegetation Relocation o GEO 101 February 11, 2016 Seafloor spreading Earthquakes occur near both trenches and midocean ridges, but the largest ones occur near the trenches Harry Hess in 1960 said that since the sediments are so thin on the ocean floor, the oceans are younger than the continents o Midocean ridges are the youngest o Trenches or deepest parts are older He proposed o New seafloor forms at ridges so the oceans grow wider o Old seafloor sinks into the mantle at a trench o Midocean ridges lie above the rising limbs of mantle convection cells Hess was on the right track, but there needed to be more evidence to support his hypothesis before it could be considered a true theory; this support came from marine magnetic anomalies o Positive anomalies (dark stripes) are strong o Negative anomalies (white stripes) are weak Geologists did not know what the negative and positive anomalies meant until they discovered that the polarity of Earth’s magnetic field reversed over geologic time Normal polarity matches with earth’s dipole today and is a positive anomaly Reversed polarity is opposite from the earth’s dipole today and is a negative anomaly In 1963, Vine, Matthews, and Morley hypothesized that the magnetic stripes on the seafloor were produced by the combination of seafloor spreading and episodic reversals of the earth’s magnetic field The land geologists had already determined that magnetic reversals had occurred over time Land geologists used radiometric dating of lava flows (basalt) to match magnetic reversals with time When the pattern of magnetic stripes on the seafloor was compared with the polarity of vertical sequences of lava flows on the continent, the pattern matched The width of magnetic stripes in seafloor basalt is proportional to the curation of normal or reversed magnetic field conditions Alfred Wegener observes that the edges of the continents fit like puzzle pieces Plate tectonics What observations have been made to support the theory of plate tectonics? o Ridges and trenches in ocean basins o Thickness of sediment o Overall age of ocean basins o Relative age of basalts o Where the active volcanoes are: trenches and midocean ridges o Global seismicity (earthquakes) o Heat flow: highest heat flow is at midocean ridges due to seafloor spreading o Relative plate velocities measured with GPS What is a tectonic plate? o A rigid plate/piece of outer part of earth that moves with others o Lithospheric plate o Includes crust and uppermost part of mantle o NOT the continents The lithosphere is rigid, whereas the asthenosphere can flow The lithosphere is composed of the crust and the uppermost part of the mantle The boundary between the lithosphere and the asthenosphere occurs at a specific temperature The lithosphere does not float on the asthenosphere because it is solid, not liquid Passive margins are places where the edge of the continental crust is thinner and There are 3 major types of plate boundaries o Divergent: plates spreading apart Lithosphere plates move apart, warm asthenosphere rises Riding asthenospheric mantle rock undergoes melting to form new oceanic crust As lithosphere and uppermost asthenosphere move away from ridge, the lithosphere thickens as it cools Continental rift: if rifting continues long enough, it can form a divergent boundary Earthquakes sometimes occur here o Convergent: plates pushing together Subduction oceaniccontinental or oceanicoceanic o oceanicoceanic: trench forms where plate bends down sediment scraped off the subducting slab form an accretionary prism a volcanic arc forms due to partial melting of the mantle above the subducting slab o oceaniccontinental What is the difference between the oceanocean and oceancontinent boundaries? Most features are the same, there is just continental crust instead of oceanic crust During subduction, one oceanic plate bends and sinks down into the asthenosphere underneath the other Shallow earthquakes occur in subducting slab, in crust of overriding plate, and at interface between the two plates Deep earthquakes occur in the subducting slab down to 660 km Oceanic lithosphere> 10 million years old is denser then underlying asthenosphere Once the plate bends down into the mantle, it will continue to sink Continentcontinent boundary Produces thick crust, large mountain ranges, and large earthquakes o Example: Himalayas Death of a plate boundary Oceanic plate is consumed by subduction before continent continent collision Collision Oceanic plate detaches and sinks into mantle o Transform: plates sliding past each other Places these can happen Between ridge sediments Earthquakes sometimes occur here February 16, 2016 What is the driving mechanism for plate tectonics? o Ridge push: outward force from ridge, driven by gravity Convection in the mantle creates the driving energy that pushes the plates o Slab pull: sinking slab pulls the rest of the plate behind it, like an anchor How to plate velocities relate to driving mechanisms? o The fastest velocities occur where there is both ridge push and slab pull happening, not just one or the other Igneous Rocks Three types of rock o Igneous: solidified molten rock that freezes at high temperature o Metamorphic: transformation of preexisting rock subjected to heat and pressure o Sedimentary: deposited at earth’s surface Rocks tell us about: o The conditions under which they formed o Tectonic history o Processes occurring within earth Igneous rock o What causes melting? o What processes affect the texture and composition of igneous rocks? o How do igneous rocks form in different tectonic settings? o Solidified molten rock that freezes at high temperatures o Earth is mostly igneous rock o Melted rock can cool above or below ground Intrusive: cool slowly underground Extrusive: cool quickly at the surface What is magma? o Molten rock in the subsurface o Consists of Liquid: the melt itself Solids: the solid mineral crystals Volatiles: dissolved gas o How does is form? Take hot rocks and add Pressure decrease: decompression melting Temperature increase: heat transfer and melting Volatiles: volatile enhancing melting Combination of the above Geothermal gradient o Average crustal geothermal gradient increase is 25 degrees Celsius per km o The deeper you go, the more rocks you have above you, which increases the pressure o Radioactive decay creates heat in the earth’s crust o The deeper you go into the earth, the higher the temperature is o Why aren’t the rocks of the lower lithosphere and asthenosphere all molten? The pressure is higher Conditions for melting depend on temperature and pressure Solidus: conditions at which rock starts to melt Liquidus: conditions at which rock melts completely Ways to melt rocks o Pressure decrease Pressure decreases when hot rock rises to shallower depths o Heat transfer Rising magma carries mantle heat, which raises the temperature in crustal rock o Addition of volatiles Water, carbon dioxide Volatiles cause rocks to melt at much lower temps Break links between silica tetrahedral What can we learn from igneous rocks? o Texture (grain size) Tells us about the rate at which the rock cooled from the magma Fast cooling: small grain size Slow cooling: large grain size Composition Tells us about the magma source from which the rock formed and the processes occurring in the magma chamber o Depends on Source rock composition Partial melting Fractional crystallization Assimilation Magma mixing Coarse grained: mineral grains are large enough to see with the naked eye; cooled slowly at depth Finegrained: minerals can barely be seen without hand lens or microscope; cooled rapidly near surface Porphyritic: large mineral grains surrounded by a finergrained groundmass; cooled slowly at depth, then rapidly Glassy: lava cools too rapidly for crystal growth to occur Vesicular: magma cools too rapidly for all gasses to escape, trapped bubbles form vesicles Pyroclastic: rocks that have been assembled from erupted material; from o Cooling rates Depend on Depth: deep is hot (cools slowly); shallow is cool (cools rapidly) Size and shape (surface area to volume ratio) Ground water: ground water (or ocean water) removes heat o Chemical composition Mafic vs. felsic Mafic rocks/minerals are rich in iron and magnesium o Typically dark colored minerals Felsic rocks/minerals are rich in silica o Typically light colored minerals Magma composition depends on Source rock composition o Melt intermediate continental crust: felsic magma o Melt ultramafic mantle rock: mafic magma Partial melting o Rocks rarely melt completely or all at once o Upon heating, silicarich minerals melt first o Melt is more felsic than the source rock Fractional crystallization o As magma cools, different minerals crystallize at different temperatures and settle out of magma chamber o Mafic minerals crystallize first o Progressive removal of mafic minerals creates more silicic magma Bowen’s Reaction Series
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