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Physical Geology

by: Ms. Sherman Sipes

Physical Geology GEOSC 001

Ms. Sherman Sipes
Penn State
GPA 3.73


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This 0 page Class Notes was uploaded by Ms. Sherman Sipes on Sunday November 1, 2015. The Class Notes belongs to GEOSC 001 at Pennsylvania State University taught by Staff in Fall. Since its upload, it has received 25 views. For similar materials see /class/233170/geosc-001-pennsylvania-state-university in Geoscience at Pennsylvania State University.

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Date Created: 11/01/15
SHORELINE PROCESSES The Motion of Water Water within rivers lakes seas and oceans moves in response to forces acting on it Currents in rivers due to the force of gravity and in the oceans due to large scale circulation patterns the relentless tidal motion of all water bodies in response to the gravitational attraction of the moon waves formed by the interaction of the surface of bodies of water with the wind are all manifestations of the continuous motion of water in the Earth system This motion of water is the single most natural erosive process on the planet Currents in rivers erode banks especially in times of oods the continuous pounding of waves on a shoreline moves sand along the shore and erodes the shoreline Along parts of Cape Cod wave erosion is cutting back the coastline poorly consolidated glacial deposits by as much as 1 meter per year The migration of sand with long shore currents deposits sand bars at the mouths of harbors blocking shipping routes The motion of ocean currents also alters climate The Gulf Stream a warm ocean current with its source in the Gulf of Mexico moves up the East Coast of the US and then across the Atlantic past the coast of Ireland In winter these warm waters moderate the climate in the United Kingdom On the East Coast of the US the ocean currents move northward and so you can swim raw sewage aside in the waters off Maine throughout the latter part of the Summer On the West Coast the currents move southward from Alaska so that even in the warmth of Summer the waters in San Francisco Bay are frigid Waves The undulations of the water surface called waves derive their energy and motion from the wind If a breeze of less than 3 kmhr s s to blow across still water small wavelets appear When the breeze dies the ripples disappear as suddenly as they formed However when the wind exceeds 3 kmhr more stable waves gradually form and progress with the wind The tops of the waves are the crests which are separated by troughs The vertical distance between the trough and crest is the wave height and the horizontal distance between successive crests is the wave length The wave period is the time it takes for the passage of successive crests past a stationary point r MIAVE LENEx n J T WAVE Haleu r mustl Height length and period depend on 3 things 1 wind speed 2 the length of time the wind has blown 3 the fetch or distance the wind has travelled across open water Needless to say the largest longest waves occur in the Roaring Forties south of New Zealand Australia and South America where the ocean meets no land in the entire girdle of the Earth As the quantity of energy transferred from the wind to the water increases the height an and steepness of the waves increase as well Eventually a critical point is reached and ocean breakers called whitecaps form Above a certain fetch and duration the waves no longer increase in size and the waves are called fully developed This occurs because the breaking of the waves dissipates energy as fast as it is added When the waves move away from the area where the wind that forms them eXists or the wind dies you get a swell which are generally lower and longer as they get further from the source Waves move independently of the water particles within them It is the wave that moves forward not the water The water particles follow circular motion up and forward as the wave crest passes then down and back into the trough Water particles only down to a certain depth feel the movement of the waves mavennorquotr M A Jr l 739 I mWEEquot IT Eta u I z woulelaw SHn J J I When the water particles on the sea oor feel the motion of the waves the friction slows the wave and shortens its wavelength As the speed and length of the wave diminish its height increases until it collapses and breaks Ways wh k asshm m Vela l3 decrmsa 39l we he39shf 1mm C DayHI A wave 3115 qu Wave Erosion and Transportation Although beaches do not appear to change to the idle observer the sand is always on the move often very rapidly On some beaches sand moves faster than it is replenished resulting in erosion Sometimes the net result is homes in the sea Frequently sand is deposited in very inconvenient places often blocking the outlets of navigable rivers with sand bars The ocean is unrelenting in its gradual but persistent attack on land the irresistible force meets the almost immovable object at the shore With rivers most erosion occurs when the river is in ood with little erosion occurring during quiescent times The same is true of the ocean most shoreline erosion occurs during times of storms when the shoreline is battered by large waves Each wave may beat thousands of tons of water against the land sometimes causing the ground to shake In addition to the force of water thrown against the shore erosion is frequently assisted by the abrasive effects of the sand particles suspended in the waves Even along straight segments of shoreline waves generally come to the shore at an oblique angle The retreating wave however always moves directly down the slope of the beach The result is a zigzag path of sand swept along the beach referred to as beach drift Further back from the beach the motion of the waves in this way produces a longshore current which carries sand rolls gravel and moves suspended particles along the shoreline The net effect of beach drift and longshore currents is the movement ofa hell ofa lot of sand 750000 tons per year past Sandy Hook in New Jersey 15 million tons per year past Oxnard in California Without the constant supply of sand from rivers which produce most of the sand on beaches few beaches would have any sand at all 1 AND Pasl39holwgiwns BEACHFACE ananmamnamnnn When a spur of land juts out into the ocean the waves bend when they get closer to the shore This bending called refraction causes most of the energy of the waves to be concentrated against the walls of the spur of land accelerating erosion there Thus refraction acts to smooth coastlines Shoreline Features Along many coasts we nd wavecut cliffs where the action of the surf against the base of the coastal land smooths the coastline leaving coastal cliffs A number of beach features are formed by the motion of sand along the beach front Spits are elongated ridges of sand that project from the land into the mouth of an adjacent bay O en the end in the water hooks landward in response to wavegenerated currents The term baymouth bar is applied to a sand bar which completely crosses a bay sealing it from the open ocean Such a feature tends to form across bays where currents are weak allowing a spit to extend to the other side A tombolo a ridge of sand that connects an island to the mainland or to another island forms in much the same way as a spit Along the Atlantic and Gulf coasts the coastal plains are relatively at and the shore line is characterized by barrier islands such as Cape Hatteras These islands are really low ridges of sand parallel to the coast 330 km from the shore The highest reliefis sand dunes 510 meters high 15 km wide and up to 30 km long These features could have been formed at times of lower sea level when the barrier islands were just beach ridges or could have been caused by storm waves piling up sand from deeper water Various manmade features can drastically change the shoreline and not always in the way that man has expected Jetties are built in pairs to restrict the motion of a stream or river where it meets the ocean to prevent deposition in the stream channel Wave action can result in sand being piled up against the jetty on one side of the channel and eroded away on the other Breakwaters are frequently constructed parallel to the shore to protect boats from the force of large breaking waves However the reduced wave activity along the shore behind a breakwater can cause sand to accumulate so that the marina may eventually fill with sand unless it is continually dredged To maintain and widen beaches that are losing sand groins are often built A groin is a ba1rier that is built at a right angle to the beach for trapping sand that is moving parallel to the beach The result is an irregular but wider beach Often these features work so well that the zone along the shore down the longshore current is denuded of sand Seawalls are often built along beaches to protect onshore structures and roads from the force of the waves These structures de ect the force of the waves backwards focussing their force on the zone seaward of the wall This often results in higher rates of beach erosion and denudation As sand os lost the wall gets beaten up more by the waves and eventually fails and has to be replaced by another larger and more expensive wall L Managing beaches is a huge industry in beachcrazed America Virginia Beach pays 15 million per year on resanding their beaches Replacement of Waikiki s calcite sand with quartz sand acted to protect offshore reefs and limit beach erosion INTRUSIVE ROCKS The most common extrusive rocks are basalts mafic rocks mostly at midocean ridges The most common intrusive rocks are granites felsic rocks Clearly the processes giving rise to intrusive and extrusive rocks are different The bulk of the continents was is formed from intrusive rocks Granite is the igneous rock of the continents Basalt is the igneous rock of the oceans Andesite is the building material for young volcanic mountains Morphology of Intrusive Rocks The magma intrudes into the crust by fracturing the country rock forming vein type structures The country rock around the intruded magma is metamorphosed by the high heat and the magma body itself often shows chill zones near the contact with the initially cool country rock Some of the country rock may break away and form inclusions in the magma body Mdamorf b05944 Coun l tb K0 C m 39 d k 3 quot l zone u 39u cquot39 IIIIICO V Shallow intrusive rocks are mostly the blocked up plumbing of volcanoes or the associated shallow magma chambers They are ner grained as the cooled relatively fast without the insulating blanket of thick crust above The shallow magma intrusions are generally ner grained like their extrusive counterparts and form usually form regular shaped planar intrusive structures Dikes are tabular planar shaped discordant not parallel to the rock layering bedding intrusive structures Sills are tabular concordant parallel to bedding intrusive structures Laccoliths are sills that are blister shaped Deep intrusive rocks Plutons deeper than several km are coarse grained usually irregular in shape and mostly discordant Stocks are small plutons area exposed at the surface is lt 100 square km Batholiths are large plutons area eXposed at the surface is gt 100 square km These mostly extend no more than 20 km below the surface Although batholiths may be ma c intermediate or felsic most are composed of granite felsic Mountain ranges are often built from hard granites eg the Appalachians the Sierra Nevada m k v Mikel k Plutonic Rocks Granite the most common intrusive rock is coarse grained felsic mostly quartz and feldspar both orthoclase Kfeldspar and plagioclase CaNa feldspar with a small amount of ferromagnesian minerals mostly micas The rocks are lighter colored with dark ecks the micas These are the intrusive equivalents of rhyolites Granodiorites are granites that are nearest in composition to diorites having more plagioclase than orthoclase feldspar Diorite intermediate medium gray to medium green no quartz just feldspars with 3050 ferromagnesians Plutonic equivalent of andesite Gabbro ma c dark gray to black mostly ferromagnesians with 050 feldspar Plutonic equivalent of basalt Ultramu c Peridotite all ferromagnesian minerals olivine and pyroxene mostly with some plagioclase feldspar very dark green to black No ner grained counterpart Temperatures greater than 2000 C needed to stay molten so only form near the base of the crust Pegmatite extremely coarse grained up to 10 m plutonic rock usually granitic in composition Coarse grains indicate slow cooling and low viscosity of the magma due to lots of water at high pressure as a pluton cools the water is le in the residue of silica rich material which crystallizes slowly O en economically important for minerals but only usually in small pockets Formation of Magma A magma is a molten rock A rock will only melt llly if the temperature T is higher than the melting temperature Tm of all the minerals in the rock If Tm highest melting point mineral gt T gt Tm lowest melting point mineral then we get a partial melt Tm of a mineral depends on pressure Tm increases with P the presence of gases particularly water give signi cant decreases in Tm and the nature of the surrounding minerals eutectic melting Igneous Rocks Fine grained or Rhyome Dacite Andesile Basalt Porphynuc Coarse grained Grani e Granodiorile Diorite Gabbm Peridome 100 I lagittlase D Aquot A g 30 1 Sodiumrich I CaICIumrlch 5 1 I E 60 Potassium Iquot feldspar I I V I g 40 2quot Biotite a 39E JJIMUSCOVHB yroxen o 20 quot 2 s Jr o 0 T I m V Rocks become increasingly darker in color Rocks have increasing silica content The sources of heat for melting may be Geothermal gradient the normal temperature distribution as a function of depth about 25 Ckm Radioactivity gives off localized heating There are lots more radioactive elements in felsic than ma c rocks Friction during earthquakes or along fault zones during mountain building Localized Hot mantle plumes m M F39 I Origin of Magmas Di 39erentiation is the process by which dilTerent minerals separate out from an originally homogeneous mixture Bowen39s Reaction Series ka 59h zc 4055 h Restrunlg lm Mingale Cr nealh c g 41 33 Adg q L in 3 3 U E 5 Z RMISOUH 0 Ladc Fwd Men Gradalum Loprem lure 0 39P hm Fe dirt Crgehll nzm on Gum1 Musmw39l39e Discontinuous branch 1 OliVine crystallizes out of the magma as it cools leaVing the magma more silica rich 2 As the magma cools pyroxene starts to crystallize out At the same time the oliVine reacts with the silicarich magma to form pyroxene so we have all pyroxene crystals in a silica rich magma 3 As the magma cools further amphibole starts to crystallize out and the pyroxene reacts to form amphibole 4 Next biotite comes out and the amphibole reacts to form biotite All this time the magma is getting more and more silica rich 5 Ifthere is enough water in the magma muscovite may form from the biotite Continuous branch At the same time as the discontinuous branch is progressing the minerals in the continuous branch are crystallizing 1 calcium rich feldspars are the rst to crystallize highest Tm 2 as the amount of calcium in the melt decreases and the temperature drops more and more the feldspar that crystallizes gets richer and richer in sodium giVing rimmed feldspar crystals 3 when the temperature gets low enough the potassium feldspars lowest Tm crystallize 4 nally the quartz residue crystallizes For mafic intrusions the progression down the reaction series stops long before reaching the bottom with all of the melt crystallizing out as the more ma c minerals forming a gabbro Consequently basalt intrusions from the mantle will form gabbros making it difficult to explain the predominance of granite felsic intrusions So Question Why Granites why not gabbros as the most common plutonic rocks 1 Crystal settling would result in the more ma c minerals settling to the bottom leaving a zoned batholith with ma cs at the bottom and felsics at the top What you would really get is a large ma c body with a little bit of granite on top and a lot of ma cs underneath a structure that is not often seen Remember too that crystal settling requires that the crystals sink down through the tacky melt to the bottom of the chamber which is rather unlikely to occur So you cannot get large granitic bodies by crystal settling 2 Assimilation if felsic chunks from the country rock fall into the magma chamber as inclusions these may be assimilated into the magma by melting Some wall rock may also be melted into the magma increasing the silica content Unfortunately this process cannot generate granitic magmas from a basaltic magma because there is just not enough heat stored in the magma chamber to melt suf cient country rock you would have to melt a volume of country rock at least 3 times the volume of the original magma chamber to turn a basaltic magma into a granitic one 3 Crustal melting in the lower crust due to contact with hot mantle material basalts provides the only realistic answer If a ma c intrusion occurs in the lower crust and the high temperatures of the intrusion cause the local felsic country rocks to partially melt in the reverse sense of Bowen39s reaction series the quartz and orthoclase melt rst leaving the ma c minerals unmelted The felsic melt can then segregate move upward in the crust and accumulate as granitic magma The heat source for this melting of crustal rock could be due to mantle plumes or magma from subduction zones underplating or intruding the lower crust Obviously several things will affect the composition of the magma The degree of partial melting the higher the temperature the more ma c minerals will be melted and assimilated into the magma making the magma more ma c dioritic The initial composition of the country rock that is melted will also strongly affect the composition of the magma A remelted granite could hardly be able to generate a gabbro without mixing with a more ma c magma WIND AND DESERTS Deserts What Deserts are arid regions de ned as regions that receive less than 25 cm of rain per year They are generally bordered by semiarid regions that receive less than 50 cm of rain per year Deserts are really only regions where essentially nobody lives due to the fact that the rainfall is very low Few deserts are composed predominantly of sand dunes Even in the Sahara the world s largest desert sand accumulations only account for 10 of the surface The rest is predominantly barren rock or expanses of stony ground Even the sandiest desert the Arabian desert contains only 30 sandy cover Another main misconception about deserts is that they are essentially lifeless Although reduced in amount and different in character plant life is generally present usually low scrubby plant or occasionally cactus Such plants are speci cally designed to survive in dry regions large root systems small evaporation resistant leaves etc Deserts are not always hot regions either Polar deserts also exist such as on Antarctica Deserts Where and Why 1 Most deserts occur along two bands which circle the globe at about North 30 and South 30 latitude The predominance of deserts at these latitudes re ects the motion of air within the atmosphere At the equatorial regions warm moist air rises producing much of the high rainfall that occurs in these areas Much of the moisture in this air is left behind at the equator This warm air cools and expands as it rises and moves north and south away from the equator As it descends back towards the ground it heats up permitting more moisture to be absorbed This warm dry air sinks back to the surface at about 30 North and South and is so dry it soaks moisture from the ground Thus the regions at 30 North and South generally have clear skies low rainfall and high evaporation prime candidates for deserts 2 Another cause of deserts results from rainshadows In this case moist air is forced to rise above mountain ranges As the air rises the moisture condenses as cloud and precipitation occurs on the mountain slopes The dryer cooler air descends on the other side of the mountains where it soaks water from the land producing an arid or semiarid environment An example of this effect is the Basin and Range province of the US Winw m1 Wes Elsi nirmss 56 Am inle a 531mm 39 in mard 2mg 3 Deserts can also form along coasts where the ocean currents are cool The cool air over the ocean warms as it moves over the land and rises drawing moisture from the surface The deserts along the coast of South America result from this effect 4 Large distance from the oceans can also cause deserts due to the lack of moisture for rain The Gobi desert in central Asia is an example of this sort of desert Geologic Processes in Deserts Temperature At night the lack of vegetation and the clear night skies permit much of the warmth of the ground to be radiated upwards Thus the nights can be very cold often below freezing Chemical Weathering Chemical weathering in deserts is relatively minor Some occurs due to the light dew that settles at night due to the cool temperatures or due to the moisture that is temporarily retained in the soil after a rain Some of this moisture can cause frost wedging Stream Action The main erosion in deserts is caused by running water Although rainfall is low it generally occurs as brief heavy falls As the desert has little vegetation and low infiltration rates the water runs off initially as sheet ow and then forms streams Flash oods from heavy rains cause massive erosion resulting in deeply incised stream beds or arroyos wadis dry most of the year but deadly to sleep in when there are thunderstorms around Sometimes the water is so laden with sediment that you have mud ows The drainage patterns of deserts are frequently internal there is no outlet for the surface drainage and landlocked lakes with high evaporation rates playa lakes may form These lakes may survive for days or weeks after stream activity When they dry up the dissolved load from the stream forms a bed of evaporites Some deserts do however have external drainage or have through owing streams e g the Nile or the Colorado rivers Another common desert feature is the pediment bedrock surfaces that slope very gently less than 5 away from the base of mountains The formation of the pediment is associated with the erosional retreat of the mountain front A mesa is a broad attopped erosional remnant bounded on all sides by steep slopes and cliffs The mesa forms as the weaker underlying rocks are weathered and eroded by mass movements and streams The resistant rock on top forms a cap that prevents erosion A butte is similar to a mesa but is smaller Stream Deposition The bases of mountains in the desert are mantled with sediment supplied by the ephemeral streams Alluvial fans are fanlike stream and mud ow deposits that extend basinward from the mouths of many canyons They vary in size from hundreds of meters to tens of kilometers and generally have a surface with slopes from 5 10 near the apex to less than 1 at the basin end The merging of adjacent alluvial fans forms a sloping depositional surface called a bajada Wind Activity Although wind is not the major factor in forming desert landforms it is certainly an important one Erosion and transport by Wind Wind readily erodes sand silt and clay particles Sandsized particles can be moved quite rapidly at a meter or so above the ground surface Friction causes the velocity to be slower nearer the ground Sand grains may move by saltation moving by leaps across the ground Clay and siltsized particles are more difficult to erode as they are more cohesive but can be broken free by plows trucks ATVs the steps of a large animal saltation of sand grains or rolling gravel The low mass of these grains allows them to be lifted high into the atmosphere and can be carried for hundreds or thousands of kilometers Wind erosional features The erosion of loose particles by wind is called de ation As air has a very low density it can only transport fine particles Smaller particles can be carried far further than coarser ones so wind erosion is an excellent sorter of sediments Sand particles eroded from the surface may travel downwind by saltation to form dunes Finer particles are lifted higher into the atmosphere and may travel great distances before being deposited beyond the edges of the desert as loess As the original deposit has sand silt and clay particles eroded away only the coarser particles remain forming desert pavement which armors the surface from further erosion Sand grains impacting rocks will eventually erode the rock surface polishing the exposed surface to form a at face Such rocks are called ventifacts The erosive action also necessitates metal rings around wooden telephone poles But the rings only need come up about 1 2 meters from the ground as the saltation does not carry much sand higher than that Wind depositional features Dunes Dune formation all you need to form a dune is dry granular material and wind Dunes begin to form when moving sand grains encounter an obstacle such as a rock The dead air behind on the leeward side of the obstacle forms a settling point for the sand Once started this sand pile will grow The sand moves up the windward side and is deposited on the leeward side of the dune The angle of the leeward side of a dune is about 34 the angle of repose of dry sand As growth of the dune occurs by deposition of sand on the leeward side of the dune cross beds may form Dune migration as the dune grows it may start to move downwind Saltating sand grains landing on the windward side of the dune may kick up grains which move up the slope and get deposited on the leeward side Dune types several distinct shapes of dunes may form depending on sand supply wind speed constancy of wind direction type of surface over which the dune moves and the presence of vegetation Barchan Dunes are crescent shaped dunes with the tips pointing downwind They move as isolated dunes on bare rock or gravel surfaces The sand moves up the windward slope of the dunes and cascades down the leeward side or out along the tips moving the dune downwind They form best where there is a limited supply of sand limited vegetation and a constant wind direction Longitudinal Dunes are elongate dunes with the long aXis parallel to the wind They form in areas with a large supply of sand and high wind velocity from one direction and are common in the central areas of large sandy deserts such as the Sahara The dunes can reach heights of 200 m and may be as long as 100 km Transverse Dunes form at right angles to the wind in areas of large sand supply and moderate winds They may grade into barchans at the edges of the dune eld where there is less sand They are frequently found in coastal regions often in association with stable vegetation Parabolic Dunes are similar in shape to barchans but have the dune tips upwind They may form from transverse dunes where the vegetation has been stripped from the top On the upwind side of the dune a de ation basin forms and the wind moves the sand in the middle of the dune preferentially Such dunes may invade coastal regions lntrnll Strumquot fths Earth The Earth 15 a complai demmsxonal layaed strucmre The dommant forces whxch gwe nse to the behavxor othe rust earthquakes volcanoes udal waves emc are genaated from thhm the Earth De nitions hurt The outer layer of rock forming a thin skin over the Earth39s surface Makes up less than 2 of the Earth39s volume and less than 1 of the Earth39s mass The rocks that make up the Earth s crust are less dense than those in the mantle and core and are richer in silicon and aluminum and poorer in iron and magnesium The crust varies from about 6 km thick for oceanic crust to 3545 km thick for continental crust Except for localized melting the crust is solid Mzm e A thick shell of rock that separates the Earth39s crust above from the core below The mantle makes up most of the Earth s volume The rocks are still silicates and oxides as in the crust but are much richer in iron and magnesium and so have higher densities Except for localized melting the mantle is solid Core The central zone of the Earth Although much smaller in volume than the mantle the core comprises a signi cant proportion of the Earth39s mass This fact is due to the much higher density of the core being composed primarily of iron metal with some minor component of nickel and sulphur or oxygen The core is composed of two distinct regions the liquid outer core and the solid inner core L1H105p113139e The rigid outer shell of the Earth approximately 100 km thick The lithosphere contains the crust and a part of the uppermost mantle It is the zone where temperatures are low enough so that the rocks are strong Astbenospbere A region in the Earth s outer shell beneath the lithosphere The asthenosphere is of indeterminate thickness and behaves plastically In the asthenosphere the rocks are quite weak allowing the slow circulation called mantle convection which is the dominant force in shaping the Earth39s surface structure and dynamics NOTE the rocks in the asthenosphere are solid not liquid Convection occurs by the deformation of the rock not by uid motion as in a pot of water Mesosplzere The region in the mantle of the Earth below the asthenosphere Like the asthenosphere this region is characterized by rock that is weak A slow clrculatlon of a substance drle by dlffa mces In tenpecatme and denslty mvedim vvlthln that substance Convectlon IS the Earth s way to dtsslpate the hush Lanpemtures deep mmeh atthmu h b A and a mlu owave Sur ce Futures oflhe EIth The lithosphee IS constructed of a senes ofngld plates whlch lntecact and move of the orderofl e 10cmpecyeargt PhD haunt Whut we dune Sprading hmndlries as at mldroceanlc n zones also In conttnents such as the East Afncan ntt Hae molten mantledenved rock breaches the surface llke a long llnear volcano or a smng of small ones the convectlon In the mantle undemeath Causlng the ntt The polnt under the ntt IS the zone dlrecLly above the upwelllng mantle whee the tenpeatures are lghe Collisionnl boundaries as at mbduction zmel eg the collision between the Nazca plate and the South American plate where oceanic crust is pushed down into the mantle Or as at continental collision zones eg the Himalayas where the IndianAustralian plate and the one attuthet Siding boundariu such as the San Andreas fault a strikeslip fault where two plates slide past each other Hsknoz here also at mid ocean ridges you can see faults called transform faults which link different ridge segments JI ltgt A e 9 7 Where any 3 plates meet you have a trigle 39unction which we will come back to later Lithagheric Plltes OW nm m l Ali 5 l 9 Cgmrd 06m Basin 56 Notice that the plates thicken away from the ridge Thevery m 39 quot 1 um39 I u ll 39 Volcanic islands that are not due to the normal plate motions hut occur in the middle of plates eg Hawaii are called mmounts or guyota The continent ocean boundary on the le is called a passive mugjn as there is not major activity there You can clearly see the coastal plain and the continental mugin 25 By contrast the boundary below is an active margin with subduction of oceanic plate beneath the continent This process causes deep troughs or tranchcs in the ocean oor at the margin In addition the heating of the subducting plate causes melting which works is way to the surface to for volcanic arcs at the continent edge Iahml an rv bh I 39bnukprc baan I Cm39linenl ul Crud39 Sellamoth wedch Haitians Continents he continents are made up of distinct and different zones also The older continental landmasses such as the Canadian shield are basically at old thick continental crustal masses Newer regions are formed at continental margins which collision of plates causes mountain mnrh nlln er Major Lithospheric Plates of the World ORIGIN AND GENETIC CLASSIFICATION OF ROCKS De nitions Rocks 39 naturally formed consolidated material composed of grains of one or more minerals Rocks can be subdivided into 3 general categories Igneous rocks a rock formed or apparently formed from the solidi cation of a magma Includes intrusive 0r platonic rocks such as granite and cXtrusivc 0r volcanic rocks such as basalt Sedimentary rocks rock that formed from 1 lithi cation of any type of sediment 2 chemical precipitation from solution 3 consolidation of the remains of plants or animals Metamorphic rocks a rock formed through the transformation of preexisting rock into texturally or mineralogically distinct new rock as a result of high temperature high pressure or both but without the rock melting in the process The depth and temperature of the metamorphism de ne the grade of metamorphism The Rock Cycle The way in which the various rocks are interrelated and form from each other can be Visualized using the rock cycle Li H iF u h39nn QEIMEHTF K3 ROCK MetamorrLism I METHMDKYHIC DCK Coolins Cragha an M6 quotquotJ Um crina SEDIMENTS AND SEDIMENTARY ROCKS I Formation of Sediments Sediments are formed from the breakdown of rock Weathering breaks down rocks erosion li s the products away from the bedrock transportation moves them away to the zone of deposition L iliicah39on emmeu39rh K3 ROCK Nu l nt nj Meimnorrkism IGN ED U5 ROCK METBMD uHHC ROCK Cooling Cr34m Wildinn Meli mi M HGMA 1 Weathering the generally slow process by which a surface rock is broken down into smaller fragments Occurs as mechanical weathering where a rock is broken up without altering the atomic structure of the fragments frost wedging plant growth frost heaving abrasion pressure release The common results of mechanical weathering are boulders gravels and quartz sand and silt Also occurs as chemical weathering where the rock composition is changed eg iron becomes rust or the rock is dissolved Changing the rocks composition by exposure to atmospheric water or carbon dioxide can cause the grain contacts to weaken so that the rock crumbles e g feldspar plus water gives clay which is more easily washed away by water Salt or limestone is readily dissolved Chemical weathering is exacerbated by the presence of carbon dioxide which causes water to become more acidic Chemical weathering at the interface between the soil and underlying rock continues the breakdown of the rock in fact the presence of the soil holding water can accelerate the weathering Chemical weathering is more effective than mechanical weathering Di erential weathering occurs when rock which is less easily weathered remains intact although the rock around it is worn away Soil is the weathered unconsolidated material that sits above bedrock made up of clays and decomposed organic matter A J g 3 at W y 1 114quot l H39 2 Erosion Transportation Erosion removes the weathered particles from their original rock When we talk about erosion as bad we refer to the problem where the soil is washed away exposing the unweathered rock upon which no plants will grow Transportation carries the eroded material to the zone of deposition Several important things happen to the sediment during transport Rounding of sediment occurs as the sharp edges and corners of the particles are ground away during transport in rivers in glaciers or in the ocean wave motion Sorting is the process where sediments are separated according to size and results from the ability of a stream to carry coarse particles only at its higher reaches and only the nest across a low at plain 55h mewsquot9 1quotquot RHTE 395 lower slow 31 E 9 s sq d alli39iLIQ Em church 55 awam M quotai 5 Elaihs Lea Glaciers move too slow to sort sediments so glacial moraine is a mixture of particles of all sizes Airborne transport is also important but only in waterless environments deserts Venus Mars and only the nest particles sand or smaller can be carried However such airbome particles can cause weathering by abrasion sand blasting Sediment includes sand silt mud boulders pebbles dust carried by the wind clam shells or coral fragments deposited on the sea oor and also precipitated minerals from chemical weathering Sediments are unconsolidated grains are separate and unattached and are generally classi ed according to size Gravel gt 2 mm includes boulders cobbles pebbles granules Sand 116 to 2 mm gritty to the touch Silt 1256 to 116 mm too small to see individual grains not gritty to the touch but gritty on the teeth Clay lt 1256 mm not gritty to touch or teeth Mud includes both silt and clay 3 Deposition Deposition occurs on the sea oor desert valleys river channels behind coral reefs in lakes behind dams on beaches on sand dunes etc etc Mechanical Deposition occurs when sediment comes to rest when the running water glacial ice waves or wind lacks suf cient energy to carry that size sediment Chemical Deposition occurs when minerals precipitate as NaCl or gypsum as sea water evaporates or as calcite or silica precipitates as hot spring water cools etc or as organisms die and their remains accumulate within an area such as in a shallow sea or behind a coral reef SEDIMENTARY ROCKS Lithiflcation the processes that converts deposited sediment into sedimentary rock Mostly the processes involve a combination of compaction and cementation Crystallization occurs when minerals precipitate from solution as in NaCl Rocks formed purely by crystallization have no pore space and no cement Precipitation of minerals within the pore space of clastic rocks forms cements as in CaC03 or SiOz cements 1 Clastic Detrital Sedimentary Rocks are formed from sediment grains that are mechanically weathered fragments of preexisting rocks Transported clay minerals also form clastic sedimentary rocks Sedimentary breccia is a coarsegrained rock made up of angular rock fragments suggesting that the sediment did not travel far from its source as that would have rounded the fragments Most likely sources for this rock are landslides or gravel fans Conglomerates are coarsegrained rocks formed by the cementation of rounded gravel Intermediate travel distances of a few kilometers enough to round but not enough to break the sediment into smaller pieces Sandstones are rocks made up predominantly of cemented sand usually quartz sand The sand grains are o en surrounded by ne clay or silt particles Sometimes there can be some relict feldspar that has not weathered to clay in the rock Shale is a negrained rock formed mostly from silt and clay As the rock lithi es and eXpels water the platey clay minerals become preferentially oriented horizontally like a collapsing house of cards As these minerals can easily be cleaved parallel to the plates shales are generally ssile parallel to bedding the horizontal 2 Chemical Sedimentary Rocks are rocks formed by precipitation of minerals from solution eg NaCl These rocks may also be formed by organisms eg limestones formed by precipitation due to algae or corals Evaporites including salt NaCl and gypsum CaSO42HzO are formed when water laden with minerals evaporates leaving a dense interlocking crystal structure 3 Organic Sedimentary Rocks form from the remains of organic species coal from plants and trees limestones from shell sh Limestone is composed predominantly of calcite CaC03 Although limestones may be formed as chemical sediments they form mostly from the wavebroken fragments of shell coral and algae These fragments can vary in size from gravel to clay and silt and are often rounded Chalk is ne grained limestone formed from the ner particle sizes Once buried limestones readily recrystallize obliterating the original texture of the rock Fossils are frequently found in limestone Dolomite is a rock where approximately half of the calcium has been replaced by magnesium Coal is a sedimentary rock which forms when plant matter is consolidated by burial Chert a negrained rock composed dominantly of silica is mostly formed from the accumulation of hard shelllike parts of small marine organisms Sedimentary Structures The law of original horizontality states that sediment deposited from sedimentladen water will be deposited in horizontal or near horizontal layers that are essentially parallel to the Earth39s surface These horizontal layers are called bedding Bedding rarely stays horizontal very long into the life of a rock sequence When sediment is deposited by the wind or by the motion of currents in the ocean or in a river the sediment layers may form at angles to the bedding Such a process produces crossbedding A gazes bedding Ma ins When the sediment on the sea oor or river bed is stirred up by currents the heavier larger particles will settle to the bottom with progressively ner particles above This process causes graded beds with a gradation from large to small particles from bottom to top Graded beds may allow us to determine which direction in a deformed rock was originally up J M METAMORPHIC ROCKS What Metamorphism is derived from the Latin meaning change of form With respect to rocks it refers to the process of alteration of rocks as they undergo a change in temperature andor pressure The changes may be in the fundamental crystal structure of the individual minerals they may be in the density and appearance of the rocks Hence metamorphism is the solidstate transformation of preexisting rock into texturally or mineralogically distinct new rock as a result of high temperature high pressure or both Single minerals may change their form to higher temperature andor pressure forms e g quartz goes to cristobalite at high temperatures and coesite at high pressures and minerals may interact chemically to form new minerals eg quartz plus calcite gives wollastonite Factors affecting metamorphism 1 Temperature Temperature affects metamorphism in 2 ways as temperatures increase a mineral may no longer be stable and may evolve into a new form which is stable at these elevated temperatures temperature also speeds chemical reactions so that mineralogical changes will occur more rapidly 2 Pressure Pressure affects metamorphism in 2 ways also Con ning pressure is the pressure that a rock feels from all sides due to the overburden of all the rock mass above like a deep sea diver Minerals that are stable at low pressures become unstable at high pressures and metamorphose to forms that are only stable at the higher pressures e g coesite is the higher pressure form of quartz Higher pressure forms of minerals are invariably higher density Directed pressure stress is pressure applied unequally on dilTerent surfaces of a body Directed pressure causes compaction or shearing of a rock o en resulting in attening of the grains in a rock This compaction or shearing motion may cause a foliation in the rock A foliation is usually manifest as the preferred alignment of minerals such as micas with the direction of alignment perpendicular to the direction of the compaction or parallel to the shear direction Pressure MP3 200 400 600 800 1000 1200 0 100 200 300 Temperature C 400 500 Zeolite Greenschisl Eclogite Granulite 20 30 Depth km Increasing temoerature and pressure L Diagenesis Low grade Intermediate grade L High grade Quartz Quartz I L Mixed clay minerals J r Plagioclase Chlorite Muscnvite J KFeidspa Garnet j I7 Biotite I 7 Kyanite 4 I Sillimanite J jkgfrcss COMPQCHM 5amp3 Sin 65 39 1395 s 392 I Lg Z 39 2 5 quot quot quot39 quot quotquot39 F There are 3 different types of foliation in metamorphic rocks 1 If the rock splits very easily along nearly at and parallel planes indicating that preexisting microscopic platy minerals were pushed into alignment during metamorphism we say the rock is slatyor that it possesses slatycea vage 2 If the visible platy or needleshaped minerals have grown essentially parallel to one another to one another while under the in uence of directed pressure the rock is sclu39stose 3 If the rock became very plastic and the new minerals separated into distinct light and dark layers of lenses the rock has a layered or gna39ssw texture Metamorphic Rocks Marble is formed from the metamorphism of limestones It is made up of coarse interlocking grains of calcite or dolomite The grains are much larger than in the original parent rock limestone as they have coarsened at the higher temperatures of metamorphism Quartzite is formed from the metamorphism of sandstone It has a sugary appearance and is made up of interlocking small grains of quartz The grains have been welded together during the high temperatures of metamorphism so that quartzites are much less friable than the parent sandstones Hornfels are the rocks formed from the metamorphism of either shale or basalt These rocks are made up of negrained ferromagnesian minerals mantle derived minerals such as micas etc Phyllite is a rock formed from the metamorphism of shales where clay minerals have recrystallized into microscopic micas giving the rock a silky sheen Schist is a metamorphic rock derived from shale where there are megascopically visible approximately parallel oriented mineral grains Thus we have a mica schist or a gametmica schist Types of Metamorphism 1 Contact Metamorphism Thermal metamorphism Metamorphism caused mostly by a thermal event such as a volcanic intrusion in the vicinity Although pressure may in uence what new minerals are formed the pressure is not directed and the rocks are nonfoliated 2 Regional Metamorphism The majority of metamorphism occurring in the Earth is regional or dynamothermal metamorphism Here all of the rocks in a region are subjected to increased temperature and pressure both directed and con ning usually through burial 3 Shock Metamorphism Metamorphism caused by the impact of an asteroid or meteor The metamorphism is predominantly high pressure but high temperatures also occur even melting of the rock Frequently associated with coesite the high pressure version of quartz j W Hydrothermal Processes and 39 quot Metasomatism is the metamorphism of a rock mass coupled with the introduction of ions from an external source brought in dissolved in water These ions may be deposited in the rock and the uid may remove other ions as it passes through More ma c rocks can be made more felsic by the deposition of quartz and feldspar from a passing silicarich uid a process called granitization a metasomatic process The passing hot water may also deposit economically important ions along fractures and cracks in the rock As the concentration of such ions in normal rocks is very low the water scavenges them over a large area and then deposits concentrates them is a small crack Such a process occurs in many hydrothermal systems r4r 90D 3olitd 3qu mollael39oni 1 5 Casio Cc1g Cocos MRGM A Cz39se39r RLLllES MASS WASTING e earth39s surface is never perfectly at but consists of slopes some of which are steep or precipitous others moderate or gentle Some slopes are covered in vegetation others are barren rock and mbble Although slopes generally look to be stable and unchanging they are all evolving because the force of gravity causes material to move downslope This motion can vary from gradual imperceptible motion to thundering rockfalls or avalanches J 39 J a quot and a quotquott for a signi cant number oflost lives per year For example May 31 1970 a gigantic rock slide buried more than 20000 people in Peru The avalanche started 14 km from one of the buried towns near the top of one of the tallest peaks in Peru It was triggered by an offshore earthquake The material composing the avalanche was initially rock and ice but became pulverized a er travelling downhill about a kilometer into a huge uid mass containing crushed rock and melted ice and trapped air The avalanche owed down a valley burying several towns until it nally stopped several tens of kilometers later Landslides frequently ow down into valleys darnrning rivers Sometimes when the dam breaches some time later many lives are lost Changes in the shape or water content of the soils in a slope can result in slope failure Mass wasting is a type of erosion where whole masses of material rather than discrete particles are eroded 1n mass wasting the transport distance is generally small and the amount of change in the sediment is minor Mass wasting includes landslides mud ows slope failures rock avalanches slumping etc FACTORS 1N MASS WASTING Gravity is the driving force behind all mass wasting obviously the steeper the slope the greater the tendency for particles to move down it Unstable slopes occur when particles on that slope are on the verge of moving A prime example is a scree slope This tendencyis resisted by the strength or friction on the slope The actual movement of slope materials 1 r m 1 cohesiveness of the material and the iscalled 39 r quot A the quot kinetic energy purer rLial energy is turned into The orientation of rock layers can markedly change the stability of a slope When the rock layers are inclined away from the slope the slope is generally more stable When the rock layers are angled toward the slope the slope is less stable Slope stabilities on either side of a road cut can be very different Water is extremely important in slope stability It can actively assist slope failure by increasing the weight of the sediment or rock or passively assist failure by reducing the friction and cohesion between particles Water can dissolve cement materials in rocks frost wedge rocks soften clays Some clays derived from volcanic ash deposits can swell up to 8 times their original size during rain creating real problems for house foundations Given enough water some clays can liquify and ow downslope Water can also ow between a permeable layer and an impermeable rock layer decreasing friction Cohesive forces A small amount of water in sediments can provide cohesion due to surface tension For example wet sand can be built into sand castles Once the amount of water exceeds that level and the rock becomes saturated the sediments lose their cohesion The critical angle of repose is the angle that a pile of material will have usually 25 45 Angular fragments have higher angles as do poorly sorted sediments Partially saturated sediments also have a higher angle of repose TYPES OF MASS WASTING 1 Falls involve sediment and rock that move through the air and land at the base of a slope 2 Slides are movements of rock or sediment as a unit principally along one planar surface 3 Flows are plastic or semiliquid movements of rock or sediment either in air or water 1 Falls can be dry processes triggered by root wedging or wet processes triggered by frost wedging These are usually extremely rapid and result in a talus pile at the base of the slope 2 Slides can be rockslides or slumps Rockslides are movements of rock masses on a planar surface usually a bedding plane or a fracture joint They are common in road cuts Slumps are sliding of a mass of material along a curved surface they are most common in unconsolidated sediments Frequently occur due to the removal of material at the base of a slope such as due to stream erosion or wave erosion or mining of a gravel pit 3 Flows include a wide range of mass movements from the slowest to fastest creep soli uction mud ows debris ows and debris avalanches Creep is an extremely slow downslope movement of regolith soil and rock under the in uence of gravity Although it is so slow it is the most important mass movement in terms of the total volume of material per year Creep shows up as a downslope displacement of trees fences telephone poles over a long period of time Heating and cooling of the soil aids creep Soli uction is the downslope movement of watersaturated regolith May reach a few centimeters per year Most common in cold climates where freezing and thawing occurs periodically often over the top of permafrost Shows up as curved lobate ows on the surface Mud ows contain a significant amount of water up to 30 and a large proportion off1ne grained sediment They are common in semiarid areas with occasional heavy rains They create problems around places like Los Angeles Debris Flows are mass movements in which rock debris and regolith move very rapidly downslope Most debris ows start as slumps or slides but are transformed downslope into ows as the mass breaks up and mixes with air and water Such ows can move over large distances and fairly low slopes as they are effectively oating on a cushion of air Debris Avalanches are an extreme form of a debris ow Debris avalanches are similar to snow avalanches in that they move as an elongate tongue down the slope SEDIMENT DEPOSITED BY MASS WASTING The collective term for sediments formed from mass wasting is colluvium The short distances of travel result in minimal rounding of particles and little sorting so that the mineralogical composition of the colluvium is very similar to that of the source and the individual ows are mostly unlayered GLACIERS Although glaciers account for much less erosion than running water in themselves they are much better eroders than streams Geological features that are characteristic of glaciation are signi cantly different than those of running water A glacier is a thick ice mass that originates on land from the accumulation of compaction and recrystallization of snow Glaciers ow generally downhill and accumulate carry and deposit sediment The characteristics of the sediment carried and deposited by glaciers are very different from those of streams There are two fundamental types of glaciers Valley or alpine glaciers are the ones we normally think about Each is a stream of ice bounded by precipitous rock walls that ows downvalley from an accumulation center near its head Ice sheets are much larger and ow out in all directions from one or more centers and completely obscure all but the highest areas of underlying terrain For instance Greenland is covered by an ice sheet that covers 17 million square kilometers 80 of the island with an average thickness of 1500 meters The Antarctic ice sheet is 139 million square kilometers 15 times the area of the US and attains a maXimum thickness of 4300 m Continental ice sheets cover about 10 of the earth s land area When an ice sheet ows into a bay and no longer sits on land but oats you get an ice shelf Noun139an rLsir ahaI it Sur oce D vu m o5 in ow An interesting point to note is that Antarctica s ice sheet constitutes 80 of the world s ice and 23 of the fresh water If this ice melted the sea level would rise 6070 meters covering most of the densely populated parts of the globe Formation of Glacial Ice Snow is the raw material from which glacial ice originates Glaciers form where more snow falls in winter than melts in summer When temperatures stay below freezing after a snowfall the uffy accumulation of hexagonal snow crystals the extremities of the crystals evaporate in the air around them while the moisture condenses near the centers of the akes These smaller granular snow particles pack down under the weight of the snow above pushing the air out and recrystallizing into amass of dense grains with the consistency of coarse sand called rn Once the thickness of the overlying ice and snow exceeds 50 meters the m packs into a solid mass of interlocked crystals glacial ice 0 l o O D So a Sagg Shape and Movement of a Glacier The upper part of the glacier the zone of accumulation is the part of the glacier with perennial snow cover The lower part is called the zone of wastage where ice is lost by melting evaporation and calving bits breaking off The boundary between the 2 zones is called the rn limit What Glaciers do to Valleys Rivers cut Vshaped valleys whereas glaciers cut steepsided Ushaped valleys A cirque is a semicircular basin at the head of a glaciated valley formed by frost wedging and plucking A hanging valley is atributary valley that enters a glacial trough at a considerable height above the oor of the trough Rounded peaks How do Glaciers Move Ice behaves like a brittle solid until the pressure or load upon it is equivalent to about 50 meters of ice Above this limit is the zone of fracture where cracks form which are called crevasses Below 50 meters the ice ows plastically with no cracking Most glaciers slide by basal slip as a solid sheet of ice moving over the ground below Because of friction at the base the motion of ice near the base is always slower than higher up Inyhalposi39Iian JPI PC The surfaces of glaciers provide an indication of the shape of the land over which the glacier is moving Crevasses are tension fractures which form in the upper brittle part of a glacier they are no more than 40 meters deep They form when the glacier is stretched as it begins to ride over steeper terrain Once the terrain gets atter again the crevasses close up An icefall forms when a glacier rides over very steep terrain the brittle surface layer becomes highly fractured with large blocks and pinnacles forming a very jumbled surface Glaciers significantly change the terrain over which they have moved A retreating glacier will leave behind moraines poorly sorted material ranging from ne rock ourto large boulders which the glacier eroded and carried with it When the ice melted the moraine was left behind End or terminal moraines are the ridges of a moraine left like a bulldozer pile at the leading edge of the glacier More speci cally the terminal moraine marks the maximum advance of the glacier Lateral moraine is the moraine that forms along the sides of the glacier Medial moraine forms from the lateral moraine of two glaciers that merge Eskers are long sinuous ridges of waterdeposited sediment apparently formed intunnels below the glacier front edge Drumlins are moraines that have been reshaped into hummocks by the overriding glacier L 4 05902 w 531m got U Ice Ages There have many periods in the Earth39s history when the surface temperature of the planet is cooler than it is now causing much of the planet39s surface to be shrouded inice We all know of the lastice age about 18000 years ago Which covered much of North America and much of Northern Europe With ice sheets 39 Ex39lerrl 0F 145 ice 15 Na ice Cave Era means These ice sheets formed from precipitation of snow from water evaporated from the oceans producing a net drop in ocean level the mean sea level during the last ice age was nearly 100 meters lower than it is now A record of sea level versus time since the last ice age shows that the sea level has been fairly static only over the last 6000 years Mean Sea level Lm II I ZDlalbl412lOSb4ZO Wrds 091915 050 This last ice age resulted in a large ice sheet covering most of Scandinavia As we have previously discussed the increased weight on the land mass causes the area to be pushed down into the upper mantle Thus Scandinavia sank somewhat into the mantle during the last ice age The mantle material that was pushed out of the way by the subsidence of Scandinavia pushed the low lying plains that we now call Holland up higher A er the ice melted Isostasy startedto push Scandinavia up again it is still slowly rising today In the process Holland is now slowly sinking Hence the need for expensive dikes to keep the sea out A Zqomm a Sana1 639 Largely 39rOdalJ me re We are currently in a short warm period in one of the Earth s cooler times taking the last 2 million years Prior to that we can see that there have been periodic warm and cold periods that last tens to hundreds of million years It is ineVitable that this warm spell will soon in geologic time be over and the ice sheets may again advance over the continents M39l39 nsmc ll 0 03m ago


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