Geology 101 scott white usc exam 2
Geology 101 scott white usc exam 2 Geo 101
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]00 Chapter seven Consequences and Causes of Metamorphism ● Metamorphism the transition of one rock into another by temperatures and/or pressures unlike those in which it formed ○ during metamorphism rock must remain solid(no melt) ○ recrystallization as new minerals form and replace original minerals ○ metamorphic rocks are those that have undergone solidstate alteration of preexisting rocks ■ Assemblage and texture characterize most metamorphic rock ○ the pre existing rocks that are altered during metamorphism are called protol ths. ■ metamorphism can alter any protolith(igneous, sedimentary, or metamorphic) ■ protoliths undergo changes in texture and mineralogy. These changes are due to variations in temperature, pressure, tectonic stress, and the amount of reactive water ○ metamorphic changes slowly in the solid state ○ Metamorphism causes change in mineral makeup and texture ● What is a metamorphic rock? ○ metamorphic minerals A mineral formed by solidstate transitions under metamorphic conditions ○ metamorphic texture defined by distinctive arrangements of mineral grains not found in other rock types ■ texture changes during metamorphism create intergrown and interlocking grains ■ often imparts a foliation upon the new rock. ○ metamorphic foliation Foliation is defined by alignment of platy minerals(i.e micas), or creation of altering light/dark bands ■ Foliation is a planar fabric that cuts through the rock ■ foliation develops because the rocks have been subjected to compression and or shear and they have a significant component of platy minerals ○ The process of forming metamorphic minerals and textures takes place very slowly and it involves several processes, which sometimes occur alone and sometimes occur together. The most common processes are: ■ Recrystallizationchanges the shape and size of grains without changing the identity of the mineral making up the grains ● ex limestone to marble ■ Phase Change transforms one mineral into another mineral with the same composition but a different crystal structure. On an atomic scale, phase change involves the rearrangement of atoms ● kyanite ■ Metamorphic reaction or neocrystallization new crystals form from old, Protolith minerals become unstable and undergo chemical reactions that recycle elements to form a new mineral assemblage ● ex. shale> garnet mica schist ■ Pressure solution when a wet rock is squeezed more strongly in one direction than in others. Mineral grains dissolve when their surfaces are pressed against other grains, producing ions that migrate through the water to precipitate elsewhere ● i.e dissolved ions migrate in a thin water film and reprecipitate. this process requires small amounts of water ■ Plastic deformation mineral grains soften and deform to when rock is squeezed or sheared at elevated temperature and pressure. The minerals change shape without breaking, like a plastic ● The agents of metamorphism are heat(t), pressure(P), compression and shear, and hydrothermal(hot water) fluids ○ not all agents are required to alter a rock mass, although they often cooccur. Rocks may be overprinted by multiple events ● Metamorphism occurs between 250 and 850 degrees celsius and the depth to this temperature varies with tectonic setting ● Metamorphism due to heating ○ when you heat a rock, its ingredients transform into new material metamorphic rock ○ solid state diffusion rearrangement of atoms within grains, or to migration of atoms into and out of grains ○ heat ■ most important agent ■ recrystallization results in new, stable minerals ■ two sources of heat: ● contact metamorphism due to heat from magma ● an increase in temp with depth due to the geothermal gradient ● Metamorphism due to pressure ○ if you subject minerals to extreme pressure, the atoms pack more closely together and denser minerals tend to form ○ Pressure ■ increases with depth ■ confining pressure applies forces equally in all directions ■ rocks may also be subjected to differential stress where the pressure is stronger in one direction ● Compression, shear, and development of preferred orientation ○ compressionis a stress that is greater in one direction, pressure is of equal magnitude in all directions ■ compression is a common result of tectonic forces. mountain building creates horizontal compression ○ shearacts parallel to a surface. Shear stress moves part of a material sideways causing it to be smeared out. It is like the sliding of deck cards ○ preferred orientatioThe metamorphic texture that exists where platy grains lie parallel to one another and/or elongate grains align in the same direction ● The role of hydrothermal fluids ○ hydrothermal fluidsspeed up chemical reactions and add or subtract elements. ○ metasomatismHydrothermal alteration is called metasomatism Types of metamorphic rocks ● Foliated metamorphic rocks have a throughgoing planar fabric ○ foliatioLayering formed as a consequence of the alignment of mineral grains, or of compositional banding in a metamorphic rock ■ slate is a fine grained, low grade metamorphic shale. It has distinct foliation called slaty cleavage ● slaty cleavage develops perpendicular to compression by parallel alignment of clays ■ phyllite is a fine grained micarich rock that forms by the metamorphic alteration of slate. With further alteration, phyllite turns into schist ● clay minerals in the slate protolith neo crystallize into tiny micas. Phyllite has a silky sheen called phyllitic luster ■ schist is a fine to coarsely crystalline rock with larger micas indicating medium to high grade metamorphism. It has a distinct foliation from large micas called schistosity ■ gneiss has distinct compositional bands, composed of light bands of felsic minerals(quartz and feldspars) alternating with dark bands of mafic minerals(biotite or amphibole) ● compositional banding can develop by metamorphic differentiation ● non foliated metamorphic rocks like marble, have no planar fabric evident because they lack inequant minerals and they recrystallized without differential stress ○ hornfels ○ quartzite is a metamorphic quartz sandstone. The sand grains in the protolith recrystallize and fuse to form a rock that is hard, glassy, and resistant ○ marble is a coarsely crystalline calcite or dolomite from a limestone or dolostone protolith ■ extensive recrystallization completely obliterates original textures and fossils. It is a favorite stone for sculptures ● Defining Metamorphic intensity ○ metamorphic grade is a measure of the intensity of T and P conditions that lead to alteration. Different T and P conditions occur in different geologic settings ■ the metamorphic grade of a rock is determined by observing the metamorphic mineral assemblage ○ A metamorphic facies is a set of mineral assemblages that indicate a certain range of P and T conditions ○ Index minerals indicate the metamorphic grade of a rich and make useful maps that define metamorphic zones ○ metamorphic zones The region between two metamorphic isograds, typically named after an index mineral found within the region ■ metamorphic zones are divided by isograds Where does metamorphism occur? ● Thermal or contact metamorphism heating by a plutonic intrusion ○ metamorphism due to heat from a body of magma invading host rock, It creates zoned bands of alteration called a metamorphic or contact aureole, zoned from high grade to low grade(cool) ○ metamorphic aureoleThe region around a pluton, stretching tens to hundreds of meters out, in which heat transferred into the country rock and metamorphosed the country rock ● Burial metamorphism deep burial in a basin ○ P and T increase as sediments are buried in a basin due to the weight of overburden and the geothermal gradient. Below depths of around 815 km, metamorphic changes replace diagenetic effects ● Dynamic metamorphism P and T change due to orogenesis ○ dynamic metamorphism involves breakage of rock by shearing within a fault zone, In the shallow crust( 0 15 kn) rocks are brittle, crushing to form fault breccia. In the deeper crust (15 km and deeper) the rocks are ductile. Rocks in Fault zones smear like taffy to form mylonite. Mylonite has very tiny grains ○ myloniteThe resulting rock from dynamic metamorphism; has a foliation that parallels the fault. ● Dynamothermal(regional) metamorphism Tectonic collisions deform huge “mobile belts” hundreds to thousands of km long. Directed compression smashes preexisting rocks and buries them deeply, where they are heated by the geothermal gradient and plutonic intrusion. The heat and pressure of orogenesis creates huge volumes of metamorphic rock, more than any other mechanism ○ Regional metamorphism results from continental collision ● Hydrothermal metamorphism at MidOcean Ridges alteration by hot water ○ At mid ocean ridges, hot, chemically aggressive water chemically alters the basalt. ○ The process starts when cold ocean water seeps into fractured crust. Heated by magma, this water then reacts with the mafic rock and is ejected via black smokers. The mafic rock is metaphorically altered ● Metamorphism in subduction zones high P and low T alteration ○ Trenches and accretionary prisms have a low geothermal gradient. These conditions produce an unique lowT, high P mineral assemblage called blueschist, after glaucophane, a blue amphibole mineral ● Shock Metamorphism extreme high P from a bolide impact ○ when earth is struck by a comet or asteroid, the impact generates high pressure minerals found nowhere else ○ Pressure generated by the impact condenses rock, creating a new rock ● Where do you find metamorphic rocks ○ exhumationis due to uplift induced softening of the tectonically thickened crust, which leads to eventual collapse and thinning. Erosion takes over and removes the upper material exposing deeper rock below. Many metamorphic rocks are dry, which prevents retrograde reactions ○ Large regions of ancient high grade metamorphic rocks are exposed in continental interiors ■ Called shields, these are the eroded remnants of orogenic belts. Shield rocks form the basement under sedimentary cover over much of the world Chapter 8 What causes earthquakes? ● most earthquakes are due to slip on faults ● The place within the earth where rock ruptures and slips, or the place where the explosion occurs, is the hypocenter or focus of the earthquake ● energy radiates from the focus ● Epicenter the point on the surface of the Earth that lies directly above the focus ● Faults in the Crust ○ displacement the distance between two ends of the marker, as measured along the fault surface in the direction of slip ○ Fault scarp step produced when faults intersect and offset the ground surface ○ fault trace the ground surface exposure of a fault ● Generating earthquake energy:stickslip ○ Earthquakes due to fault formation ○ Earthquakes due to slip on a preexisting fault Most earthquakes happen when stress overcomes friction on a preexisting fault, and the fault slips again ■ the alternation between stress buildup and slip events is referred to as stickslip behavior ■ The breaking of a rock that occurs when a fault slips generates earthquake energy ■ The concept that earthquakes happen because stresses build up, causing rock adjacent to the fault to bend elastically until slip on the fault occurs is called the elastic rebound theory ■ the major earthquake along a fault may be preceded by smaller ones called foreshocks ■ smaller earthquakes, called aftershocks, occur in the days to months following a large earthquake ● The amount of slip during an earthquake ○ the larger the earthquake, the larger the slipped area and the greater the displacement ○ The amount of slip varies along the length of the fault line Seismic Waves ● Earthquake energy travels through rock and sediment in the form of waves. We call these waves seismic waves ○ Where the wave moves: ■ Body waves pass through the interior of the earth ■ Surface waves travel along the earth’s surface ○ How the wave moves: ■ compressional waves waves that cause particles of material to move back and forth parallel to the direction in which the wave itself moves ● as a compressional wave passes, the material first compresses, the dilates ■ shear waves waves that cause particles of material to move back and forth perpendicular to the direction in which the wave itself moves ○ Four categories of seismic waves ■ Pwaves Primary compressional body waves ● travel the fastest and arrive first ■ Swaves Secondary shear body waves ● Travel only through solids. Greater amplitude than P waves ■ L waves Love surface waves that cause the ground to ripple back and forth ■ R Waves surface waves that cause the ground to ripple up and down ● surface waves are the slowest waves. complex motion. How do we measure earthquakes ● Seismometer instrument used to systematically measure the ground motion from an earthquake. ● Seismologists use two basic configurations of seismometers, one for measuring vertical and the other for measuring horizontal(back and forth) ground motion ● An earthquake record produced by a seismometer is called a seismogram ○ the horizontal axis represents time, and the vertical axis represents the amplitude(the size) of seismic waves ○ The first squiggles on the line represent Pwaves because they travel the fastest. Next come the Swaves, and finally the surface waves (R and L waves) ○ The surface waves have the largest amplitude and arrive over a relatively long interval of time ● Finding the epicenter ○ measuring the difference between the time that the Pwave arrives and the time that the Swave arrives at a seismometer station ○ The delay between Pwave and Swave arrival times increase as the distance from the epicenter increases ○ time travel curve shows how the time for an earthquake wave to move from its origin to a seismometer station increases as the distance between the epicenter and the seismometer station increases Defining the size of earthquakes ● The first scale focuses on the severity of damage at a locality and is called the mercalli intensity scale. The second focuses on the amount of ground motion at a specific distance from the epicenter, as measured by a seismometer, is called the magnitude scale ● Mercalli intensity scale ○ the intensity of the earthquake refers to the effect or consequence of an earthquake’s ground shaking at a locality of the earth's surface ○ The mercalli intensity scale varies with location for a given seismic event ● Earthquake magnitude scale ○ the magnitude of an earthquake is a number that represents the maximum amplitude of ground motion that would be measured by a seismometer placed at a specified, standard distance from the epicenter ○ The american scientist Charles Richter developed a method for defining and measuring earthquake magnitude in 1935 ■ Richter scale is based on the maximum amplitude of motion that would be recorded at a station about 100 km from the epicenter ○ other magnitude scales are used because the Richter scale works well only for shallow earthquakes that are close to the seismometer station ○ The moment magnitude scale provides the most accurate representation of an earthquake's size ○ all magnitudes are logarithmic, meaning that an increase of one unit of magnitude represents a tenfold increase in the maximum amplitude of ground motion Where and why do earthquakes occur ● most earthquakes occur in fairly narrow seismic belts, or seismic zones ● Earthquakes that occur away from plate boundaries are called intraplate earthquakes ● shallow focus earthquakes occur in the top 60 km of the earth, intermediate focus earthquakes take place between 60 km and 300 km, and deepfocus earthquakes occur down to a depth of about 660 km ● Plate boundary earthquakes ○ divergent plate boundary seismicity ■ Along spreading segments, stretching generates normal faults, whereas along transform faults strikeslip displacement occurs ■ Earthquakes along midocean ridges take place at depths of less than 10 km, and thus are shallowfocus earthquakes ○ Transform plate boundaries seismicity ■ most faulting results in strikeslip motion ■ The majority of transform faults in the world link segments of ocean ridges, but a few such as the San Andreas fault of California, the Alpine fault of New Zealand, and the Anatolian faults in turkey, cut through continental lithosphere or volcanic arcs ■ All transform fault earthquakes have a shallow focus, so the larger ones on land can cause disaster ● ex 2010 earthquake in Haiti ● slip of the San Andreas fault near San Francisco in 1906 ○ Convergent plate boundary seismicity ■ shallow focus earthquakes occur in both the subducting plate and the overriding plate. ■ also host intermediate focus and deep focus earthquakes. These occur in the downgoing slab as it sinks into the mantle, defining the sloping band of seismicity called a WadatiBenioff zone ■ Earthquakes in southern alaska, eastern Japan, the western coast of South America, the coast of Oregon and Washington, and along island arcs in the western Pacific serve as examples of Convergent boundary earthquakes ■ 1906 earthquake in Chili the largest earthquake on record, the 1964 good Friday earthquake near alaska, and the 2011 Tohoku earthquake which also triggered a tsunami ○ Earthquakes due to Rifting and collision ■ Continental Rifts the stretching of crust at continental rifts generates normal faults ● active rifts today include the East African Rift, the Basin and range province, and the Rio grande rift ● in all these places shallow earthquakes occur ■ Collision zonestwo continents collide when the oceanic lithosphere that once seperated them has been completely subducted ● such collisions produce great mountain ranges such as the Alpine Himalayan chain and caused the catastrophic 2005 earthquake in Pakistan ● though a variety of earthquakes happen in collision zones, the most common result from movement on thrust faults Chapter 10 The concept of geologic time ● Setting the stage for Studying the past ○ James Hutton was first to articulate the Principle of uniformitarianism ○ He realized that vast amounts of time were necessary for earth processes to create rocks. “father of modern geology” ○ according to this principle, physical processes that operate in the modern world also operated in the past, at roughly the same rates, and these processes were responsible for forming geologic features preserved in the outcrops. ○ the present is key to the past ● Relative vs Numerical age ○ We specify the age of one feature with respect to another in sequence as its relative age ○ the age of a feature in years as its numerical age ● Geological principles for defining relative age ○ Uniformitarianism the processes observed today were the same in the past ■ mud cracks found in ancient sediment formed in just the same way as the mudcracks we see forming today ○ Original horizontality states that because sediments settle out of a fluid by gravity, they tend to accumulate horizontally. Sediment accumulation is not favored on a slope. Hence, tilted sedimentary rocks must be deformed ○ Superposition in a undeformed sequence of layered rocks, each bed is older than the one above and younger than the one below ○ lateral continuity sediments generally accumulate in continuous sheets within a given region. Subsequent erosion dissects oncecontinuous layers ○ cross cutting relations younger features truncate(cut across) older features. ■ Faults, dikes, erosion etc must be younger than the material that is faulted, intruded, or eroded (A volcano cannot intrude rocks that aren't there yet) ○ baked contacts observes that an igneous intrusion cooks the invaded country rock. The baked rock most have been there first. A chill margin is formed within the igneous intrusion at the contact from rapid cooling ○ inclusions explains the occurrence of one rock fragment within another. Inclusions are always older than the enclosing material ● Fossil succession ○ The principle of fossil succession describes the predictability of fossil distribution through time. Specific fossils are only found within a limited, often narrow, time range ○ The first appearance, range of existence, and final extinction are used for relative dating, Fossils succeed one another in a known order. Time periods are recognized by their fossil content ○ once a species disappears from the fossil record it never reappears ○ Index fossils are a diagnostic of a particular geologic time ○ Extinct fossil organisms are found in the same order everywhere Unconformities ● an unconformity is a time gap on the rock record ○ Angular unconformity cuts across the underlying layers, and the orientation of layers below an unconformity differs from that of the layers above ■ the outcrop at Siccar point ○ A nonconformity is an unconformity where igneous or metamorphic rocks are capped by sedimentary rocks. Crystalline igneous or metamorphic rocks were exposed by erosion. After a renewed marine invasion, sediment was deposited on this eroded surface ○ A disconformity is an unconformity within sedimentary layers due to an interruption in sedimentation. often subtle Stratigraphic formations and their correlations ● The succession of rocks in the grand canyon can be divided into formations, based on noteable changes in rock type and included fossil ● lithologic correlation geologists correlate formations between nearby regions based on similarities in rock type ○ based on rock type in a particular region. Several rock columns kilometers apart look slightly different but can be correlated by matching rock types The geologic column ● by correlating rocks from locality to locality at millions of places around the world, geologists have pieced together a composite stratigraphic column, that represents the entirety of earth's history ● The column is divided into segments, each of which represents a specific interval of time ● The largest subdivisions break earth history into the hadean, archean, proterozoic and phanerozoic eons (the first three together constitute the precambrian ● The Phanerozoic is subdivided into eras. In order from oldest to youngest, they are the paleozoic, Mesozoic, and cenozoic eras. ● We further divide each era into periods and each period in epochs ● Correlation of stratigraphic sequences from around the world led to the production of a chart, the geologic column, that represents the entirety of Earth history. The column, developed only using relative age relations, is subdivided into eons, eras, periods, and epochs. ● the oldest sedimentary rocks of the region crop out near the base of the grand canyon, whereas the youngest form the cliffs of cedar breaks and bryce canyon ● correlation from cedar breaks to the grand canyon encompasses rock representing almost half of earth's entire age How do we determine numerical age ● radioactive elements decay at a constant rate that can be measured in a lab and can be specified in years. ● many relative ages can be assigned actual numerical dates because of radiometric dating or geochronology ● this technique measures certain radioactive isotopes in minerals that decay at a known, fixed rate ● radioactive isotopes act as internal clocks ● radiometric dating is determined by measuring the ratio of parent to daughter isotopes. The age can be calculated from the parent half life ● three types of radioactive decay: Alpha beta and gamma ● parent an unstable radioactive isotope ● daughter product the isotopes resulting from the decay of a parent ● half life the time required for one half of the radioactive nuclei in a sample to decay ● after one half life, one half of the original parent remains. After three half lives one eighth of the original parent remains ● sources of error ○ A closed system is required ○ 100 percent parent isotope at “time zero” ○ no escape of isotopes over time ○ enough isotope must be present to measure accurately Isotope date ● isotope dating give the time a mineral began to preserve all atoms of parent and daughter isotopes, which require cooling below a “closure temp” ● If rock is reheated, the radiometric clock could be reset ● igneous and metamorphic rocks are best for geochronologic study sedimentary rocks cannot be directly dated ● other numerical ages ○ annual growth rings from trees or shells are able to be counted to establish dates ○ rhythmic layering annual layers in sediment or ice can be counted to establish numerical dates Dating the geologic column ● geochronology is less useful for sedimentary deposits, although it is able to constrain these deposits ● sediments can be bracketed by numerical ages derived from datable materials that cross cut them. This yields age ranges that narrow as data accumulates Chapter 11 ● The earth formed about 4.57 billion years ago. For part of the first 600 years, the hadean eon, the planet was so hot that its surface was a magma ocean ● The Archaean Eon began about 3.5 ga, when the first continental crust that still remains formed. This crust assembled out of volcanic arcs and hot spot volcanoes that were too buoyant to subduct ○ the atmosphere contained very little oxygen and the first life forms bacteria and archaea appeared ○ archean strata at some localities contain stromatolites, distinctive layered mounds of sediment. Stromatolites that developed after about 3.2 ga form because cyanobacteria secrete a mucus like substance to which sediment from water sticks. As the new mat gets buried, new cyanobacteria colonize the top of the sediment, building the mound upward ● In the proterozoic eon, which began at 2.5 ga, archean cratons sutured together to form large cratons ○ photosynthesis added oxygen to the atmosphere ○ between 2.4 ga and 1.8 ga huge amount of iron settled out of the ocean to form colorful sedimentary beds known as banded iron formation ■ bif consisted of alternating layers of iron oxide minerals(hematite or magnetite) and jasper(red chert) ○ by the end of the proterozoic, soft bodied marine invertebrates populated the planet, and continental crust had accumulated to form supercontinent ● As the paleozoic era began, rifting yielded several separate continents. Sea level rose and fell, depositing sequences of strata in continental interiors. continents coalesced again to form another supercontinent, pangea ○ early paleozoic evolution produced many invertebrates with shells, and jawless fish ○ Land plants and insects appeared in the middle paleozoic, and by the end of the eon, there were land reptiles and gymnosperm trees ● In the Mesozoic era, pangaea broke apart and the atlantic ocean formed. Convergent boundary tectonics dominated along the western margin of North america. Dinosaurs became prominent land animals through the mesozoic era ○ during the cretaceous period, the continents flooded. Angiosperm appeared along with modern fish. A huge mass extinction event wiped out the dinosaurs at the end of the cretaceous period, probably due to a meteorite ● In the cenozoic era, the collision of africa and india with asia and europe formed the alpinehimalayan orogen ○ convergent tectonics persisted along the margin of south america, creating the andes, but ceased in North America when the San Andreas fault formed. ○ rifting in the western United states produced the basin and Range province. Various kinds of mammals filled niches left vacant by the dinosaurs, and the human genus, Homo, appeared and evolved through the pleistocene ice age Chapter 14 ● streams are movements of water in channels ○ most important cause of erosion ○ transport agent for sediment ● stream runoff is an important geologic agent ○ streamflow is used for drinking water, transportation, waste disposal, commerce, irrigation, and energy generation ○ streams are channels of water that drain the landscape. ○ flowing water erodes, transports, and deposits sediment and sculpts landscapes ○ earth is the only planet in the solar system with flowing water ● Streams carry water out of a drainage basin a drainage basin divide separates two adjacent catchments ● Permanent streams exist where the water table lies above the bed of the channel or when large amounts of water enter the channel from upstream ○ these streams are common where there is abundant rainfall, groundwater discharge, and low rates of evaporation ● where the water table lies below the channel bed, streams are ephemeral ○ do not flow all year ○ they are common in places with low annual rainfall, a low water table, and high rates of evaporation ● the discharge of a stream is the total volume of water passing a point along the bank in a second. Most streams are turbulent, meaning that their water swirls in complex patterns ○ discharge is determined by measuring the cross sectional area of the channel multiplied by the flow velocity ○ discharge varies seasonally due to changes in precipitation and runoff ○ a river with lower cross sectional area and velocity has a lower discharge ● velocity is not uniform in a channel. friction slows water along channel edges; water flows faster in the deeper center ○ in curved channels, the maximum velocity is swept to the outside of the curve which is preferentially scoured and deepened ● The total quantity of sediment carried by a stream is its capacity. Capacity differs from competence, the maximum particle size a stream can carry. When stream water slows, it deposits alluvium ○ dissolved load consists of ions from mineral weathering ○ suspended load is made of fine particles(silt and clay) entrained in the flow ○ bed load is composed of the larger particles that roll, slide, and bounce along the bed of the channel. Movement is called saltation ○ sediment transport changes with discharge, during high discharge, cobbles and boulders that are stranded and low discharge may be mobile ● A meandering streams wanders back and forth across a floodplain. It erodes its outer bank and deposits a point bar on the inner bank. Eventually, a meander may be cut off and turn into an oxbow lake. Natural leeves form on either side of the river channel ○ single main channel ○ sinuous(wiggly) ○ low competence, steady discharge ○ congaree river ● Braided streams consist of many entwined channels ○ straight ○ high competence, flashy discharge ○ chitina river
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