Geol 201 Weekly Notes
Geol 201 Weekly Notes geol 201
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Ge01201 notes for week 1 Introduction and Overview Geology 0 Study of the Earth observational science integrated science 0 Present is the Key to the Past Uniformitarianism 0 Earth is a product of 456 billion years Ga of thermal evolution 0 Much has changed and evolved on the Earth during this time very dynamic Earth Formation of the EarthMoon System Nebular Hypothesis Processes Active during Early Planetary History 1 Impact Collision accretion of asteroids comets etc most collisions have occurred by 39 Ga kinetic impact imparts heat to warm early Earth 2 Gravity Masses attract the Earth condenses condensing imparts heat to warm early Earth 3 Radioactive Decay of Short Halflife Radioactive Elements Rapid decay imparts heat to warm Early Earth 4 Differentiation of the Planet Accumulated heat causes iron dispersed in Earth to melt iron sinks to form core of Earth Iron Catastrophe adds more heat planet segregates into coremantlecrust 5 Glancing blow collision with large MercuryMarssize Planetesimal Most of its core is captured by Earth The rest of the impact material is jettisoned into a disc around the Earth that rapidly coalesces into the Moon 6 Complete to Nearly Complete Melting of the Outer Earth Accumulated heat leads to melting of upper Earth magma ocean extensive volcanism 7 Volcanic Outgassing and Formation of the Atmosphere and Ocean By approximately 42 Ga volcanic gasses trapped by Earth s gravity additional H20 from comet impact continues Evidence of the Earth s Early Planetary Evolution 1 Astronomical Studies 2 Meteorites extraterrestrial rocky frags that survive the passage through atmosphere Stony Meteorites fragments of mantle and crusts of early differentiated planetsplanetesimals most dated to 454 Ga Iron Meteorite fragments of cores of early differentiated planetsplanetesimals most dated to 454 Ga Carbonaceous Chondrites fragments of nondifferentiated rocky materials dated to 457 Ga came out of dust clowds 3 Age of the Moon rocks Apollo missions retrieved rocks that date to 453 Ga 4 Physical and Chemical Modeling Differentiated Planet The Earth has evolved into a layered planet both compositionally and physically Chemical Compositional Zonation product of early differentiation and continued partial melting Core Iron nickel Mantle Ultamafic peridatite Crust oceanic more dense basalt mafic high in iron and magnesium continental granite felsic high is silicon low in iron and magnesium Physical Zonation product of thermal cooling of the Earth over 454 Ga present zonation is produced by the pressure and temperature conditions within the Earth today Inner core solid generates magnetic field Outer core liquid Lower Mantle Transitional Upper Mantle Asthenosphere plastic layer where materials start to melt Rigid Upper Mantle right below the last layer of crust also known as mantle lithosphere Continental crust rigid solids Movement of Heat Through the Earth Convection movement of heat by the transfer of material that is plastic or liquid occurs in the outer core lithosphere Conduction movement of heat by touch thermal Vibrations in material that is rigid occurs in the inner core and lower mantle Major Cycles of the Earth Plate Tectonic Cycle powered by the Earth s internal geothermal energy Movement of the Earth s outer rock layers lithosphere upon the plastic asthenosphere with the accompanying formation of new lithosphere and recycling of old lithosphere Generalities Plates meet at one of three types of boundaries divergent convergent or transform 0 Plates may be all oceanic lithosphere or a combination of oceanic and continental lithosphere 0 Creates most of the physiography topographic highs and lows of the Earth s surface 0 Cause of most of the earthquakes and partial melting pm that produces magmas and volcanoes Parts of the Continental Crust Continental Basement ancient eroded roots of mountain ranges generally gt 10 Ga mainly igneous intrusions and metamorphic rocks Stable Platform thin veneer of young undeformed atlying sedimentary rocks within the continental interior generally lt 600 million years ma Physiographic Landforms of the Continental Crust Exposed Continental Basement shield Exposed Stable Platform great plains Young Mountain Range Old eroded Mountain Range Lithospheric Plates are in Isostatic Balance with the Asthenosphere 0 Isostasy is the gravitational balance between the rigid lithosphere and the plastic asthenosphere that is determined by the otation buoyancy properties of the lithosphere upon the plastic asthenosphere that is capable of ow Analogous to wood block oating in water 0 The otational or buoyancy properties of the material can be because of 1 the density of the composition of the material 2 thickness of the material andor 3the temperature of the material Types of Relative Plate Boundaries 0 Plates are moving relative to each other 0 The three types divergent convergent amp transform are endmembers relative plate boundaries can also be combinations or have oblique properties Divergent Plate Boundaries 0 2 plates are spreading apart 0 Area of extensional or tensional stress 0 Area of hot rising mantle peridotite 0 Area of partial melting pm 0 Area of volcanism extrusion and plutonism intrusion of melts magmas Area of thermal doming isostatic response to heating Evolution of a Divergent Plate Boundary Passive Continental Margin margin or edge of the continent that is not a plate boundary but rather is an oceaniccontinental weld produced during the rifting of a large supercontinent The area is subject to deposition of sediment and subsidence Convergent Plate Boundaries Area of two plates converging or colliding Area of compressional stresses Area of subduction of old cold lithospheric oceanic plate and recycling Area of accretion of sedimentary material andor buoyant landmasses Area of partial melting Area of volcanism extrusion and plutonism intrusion of melts magmas Accretion of a Buoyant Landmass Terrane Buoyant Landmass any thick light crust embedded in a subducting lithosphere Buoyant Landmass is scraped off the subducting oceanic plate and sutured to the overriding plate Connects to the continent and adds mass to the size of it A new subduction zone is created behind the buoyant landmass Subduction Zone Jump Upon suturing of the lighter buoyant landmass to the continental margin the front of the dense subducting oceanic lithosphere breaks and continues to subduct into the mantle The sutured buoyant landmass now an accreted terrane is only a portion of the overall convergent plate boundary The remaining subducting oceanic lithosphere on either side of the accreted terrane continues to a subduct applying a compressional stress on the oceanward side of the accreted terrane Eventually the stress causes the plate to break on the oceanward side of the accreted terrane and instigate a new subduction zone subduction zone jump This new subduction zone along the oceanward margin of the accreted terrane connects to the larger subduction on both sides of the accreted terrane ContinentalContinental Convergent Plate Boundary A similar event to the accretion of a buoyant landmass except this buoyant landmass is on a continental scale Transform Plate Boundaries 0 Area of two plates sliding horizontally passed each other 0 Area of horizontal shear stress 0 Boundary is marked by a major strikeslip fault system 0 No rising or sinking of material 0 No partial melting volcanism or plutonism Oceanic Fracture Zone landform area of fractured rock Within an oceanic plate that has no motion or very limited motion along the fracture zone a fossilized or nonmoving fault the evolutionary result of the formation of an oceanicoceanic transform plate boundary Driving Forces of Plate Tectonics The mechanism to drive these changes 0 Convection Trench pull 0 Gravitational sliding Ridge push Trench Pull gravitational pull of a dense oceanic lithospheric plate into the asthenospheat the subduction zone The colder deeper segments are getting more dense and compact Which pulls the material down Gravitational Sliding gravitational sliding of the oceanic lithosphere away from the topographic high of the oceanic ridge Ridge Push forceful injection of magmas into oceanic ridge not a driving force but term ridge push is sometimes used for gravitational sliding magmas coming in and pushing fragments apart Pangeas or Supercontinental Cycle Pangeas are formed by the accumulation or accretion of most of the continental landmasses into a single large landmass during a period of time The attachments do not last long geologically speaking and the continental landmass eventually breaks apart Hot Spot Cycles 0 Semi stationary or fixed mantle plume of rising hot peridotite from deeper than the asthenosphere 0 Area of partial melting and volcanism 0 Area of high heat ow and thermal doming 0 Hot spots rise from deeper than the asthenosphere most from coremantle boundary too concentrated with heat and become buoyant 0 Change from convection to conduction slow the pm from rising to the surface Linear Volcanic Chains physiographic landform that can be used to trace the track the direction of a plate over the stationary hot spot provide a close approximation of the direction of absolute plate motions over geologic time Normal Plume Activity small plumes rising as in today s setting Superplume Activity short periods in the geologic past when large megaplumes rise to produce extensive volcanism and to bring up added heat into the asthenosphere speeds up plate tectonics increase in speed of plate tectonics changes spreading centers from slow to fast changing oceanic ridges to oceanic rises oceanic rises and young oceanic crust is warmer and more buoyant than oceanic ridges and older oceanic lithosphere and so it displaces more water from ocean basins this causes sea levels to rise and oceans to cover the continents in a shallow sea epieric sea When the ocean rises over these ridges the water that was once there is now displaced and this is where we get sea level rise and sometimes continents can begin to be covered Hydrologic Cycle powered by solar radiation with minor geothermal energy Sculpts the continental land surface Plate tectonic cycle vs the hydrologic cycle plates form the mountains and hydrologic sculpts its shape Weathering disintegration and decomposition of minerals and rocks by interaction of water and gases converts solid rock into fragments clay aluminaoxide and ironoxide and dissolved ions Mass Wasting downslope motion of weathered material and rock under the in uence of gravity aided by the presence of water but water is not necessary Dropping down by gravity River Systems main sculpting agent of the continental crust transports sediment erodes land and deposits sediment in adjacent basins rapid residence time of water Biggest erosion facture of the earth Glacial Systems movement of ice under the in uence of gravity important sculpting agent of the continent when they are present glacial periods are rare events over geologic time the present time last 25 million years just happens to be one of the them very long residence time of water Groundwater Systems movement of water through the regolith and bedrock of the lithosphere minor sculptor of the Earth s surface generally slow moving general long residence time of water Ocean System largest water reservoir very important in climate control main sediment deposition site of the Earth Eolian Wind System operates worldwide moving air masses and water mainly capable of moving sand silt and claysize sediment modifies the Earth s surface mainly in arid regions w little vegetation Rock Cycle the interaction between the three major cycles of the earth hydrologic plate tectonic and hot spot recycle the earth materials rocks and minerals creating the rock cycle Igneous Rocks Rocks cooled from a molten mass magma Record the melting and cooling history of the Earth Fire formed rocks Sedimentary Rocks Rocks formed at the earth s surface by the accumulation of fragmental sediment organic remains andor chemical precipitates from water Record the surface history of the Earth Made by surface processes Metamorphic Rocks Rocks generally formed in the roots of mountain ranges through recrystallization in the solid state due to changes in temperature and pressure Record the history of mountain building on the Earth Forcing rocks to bend and fold and recreate Tell you about mountain building Geol 201 notes for week 2 Uses of Minerals Economic Metalbearing minerals used in various industries steel wiring computers etc 0 Minerals are used in the building or construction industries concrete drywall etc 0 Minerals are used in agriculture fertilizers and other soil supplements 0 Minerals are used in aesthetics jewelry art etc Academic and Applied Academics 0 A11 minerals form under a given set of pressure temperature and uid conditions that is different for each mineral 0 The minerals can be used to help interpret the formation of the minerals in the rock Geothermometers a mineral that is temperature sensitive example FeMg content within garnet Geobarometers a mineral that is pressure sensitive example Na content in glaucophane Qdiometric Age Datin mineral traps and seals in radioactive isotopes at their time of formation Paleomagnetism ironbearing mineral align to the Earth s magnetic field when they form and cool down Types of Bonding and Relationship to Strength of Minerals Ionic Bonds Chemical bond in which the electrical force between the two oppositely charged ions holds the ions together Common bond in minerals fairly weak Covalent Bonds Chemical bond in which the adjacent atoms share electrons within their outer electron shells Common bond in minerals the strongest Metallic Bonds Chemical bond in which the electrons are shared by many adjacent atoms Uncommon Bond in minerals only a few pure metal minerals weaker than ionic Van Der Waals Bonds Very weak electrostatic bond between polarized atoms or molecules Uncommon bond in minerals weakest of all the mineral bonds Hydrogen Bonds Weak electrostatic bond between water molecules due to the dipolar nature of the water molecule Produces many of the unique properties of water De nition of Minerals and Mineraloids Mineral naturally occurring inorganic crystalline solid With a definite chemical composition that varies only Within certain limits Crystalline vs Amorphous Solids Crvstalline Solid a solid With a regular repeating internal structure or arrangement of atoms crystal lattice Amorphous Solid a solid that lacks a crystalline structure atoms are randomly or semi randomly arranged Mineraloid EX glass Composition Since a mineral has a regular repeating network of atoms it must have a definite chemical composition Composition may vary Within certain limits due to l Ionic Substitution 2 or more types of ions can substitute for each other in the same position in a mineral s crystal structure Ability to substitute depends upon a ionic radii of the substituting ions 15 b electrical charge of the ions l 2 Void Fillings filling of small voids in the crystal structure With various ions Adding substances into the empty space changes the overall composition and chemistry Classification Hierarchy Level Basis Example CLASS upon chemistry Silicate Class FAMILY upon overall crystal structure Nesosilicate Family GROUP same structure but W solid solution substitution Olivine Group SERIES a solid solution series Win the group Forsterite Fayalite Mg olivine Fe olivine SPECIES an individual member of a particular series Forsterite VARIETY a minor variety of a species Peridot transparent Forsterite Common Minerals in the Earth s Crust 0 Silicate minerals are the largest by volume class of minerals of the Earth s crust 0 Of the 8 of nonsilicate minerals the most common by volume are the carbonate and sulfate minerals in many sedimentary rocks 0 The sulfide and oxide minerals are the nonsilicate common metallic ore minerals used in industry Silicate Minerals 0 Silica Tetrahedron SiO4 439 is the basic building block of the silicate minerals the tetrahedral shape is determined by ionic radii of the Si and 0 ions and the valence of ions 0 The silica tetrahedra combine by either ionic bonding to cations Fe2 Mg2 Na Ca2 etc or by linking up through covalent bonds With other silica tetrahedra polymerization 0 Commonly both types of attachments are found in silicate minerals The covalent tetrahedral bonds are the strongest bonding and the ionic cationtetrahedral bonding is the weaker bonding 0 The silicate class of minerals is subdivided into families based upon the type and amount of polymerization of the silica tetrahedra Nesosilicate Each tetrahedra is surrounded or attached to cations no polymerization General Formula SiO4 439 example olivine MgFe2SiO4 Sorosilicate Silica tetrahedra are heaped or clustered into pairs each tetrahedron shares one of the oxygen atoms of the tetrahedra and then surrounded by cations General Formula Si207639 example lawsonite CaAlei207OH2tzO Cyclosilicate Each silica tetrahedron shares 2 oxygen and is arranged in polymerized rings or circles each circle of silica tetrahedra is separated and ionically bonded to cations General formula GF nSi03239 n3 4 6 example beryl BC3A12Si6018 Inosilicate Silica tetrahedra are polymerized into continuous chains each chain is then ionically bonded to cations Inosilicates are subdivided into two groups Pyroxene singlechain inosilicates and Amphiboles doublechain inosilicates Pyroxene singlechain inosilicate GF Si03239 example Enstatite MgFeSi03 Amphibole doublechain inosilicate Si4011439 example Actinolite Ca2MgFe5Si8022OH2 Phyllosilicate Silica tetrahedra are polymerized into continuous sheets by sharing 3 oxygen of each tetrahedron Each sheet is separated from adjoining sheet by bonding to cations General Formula GF Si205239 example Muscovite K2Al4Si6A12OzoOH4 Tectosilicate Framework silicates where silica tetrahedra are polymerized by joining to all four oxygen of the silica tetrahedron General Formula SiOz example Quartz SiOz Orthoclase Feldspar KNaAlSi308 Physical Properties of Minerals The crystalline structure or regular repeating arrangement of atoms in the mineral s structure provides each mineral with a set of distinct mineral properties Crystal Form 0 This is the shape of a mineral that has grown freely in an unrestricted environment A wellformed mineral crystal will have smooth crystal faces and a definite crystal form 0 A mineral has cleavage if it breaks along definite planar surfaces within a mineral Cleavage is identifiable as a series of repetitious stairstepped breaks 0 Cleavage in silicate minerals is a property of unequal bonding within the minerals crystal structure The mineral cleaves in a repetitious manner because it breaks along the weaker bonds Crystalline Structure and Relationship to Cleavage 0 In silicate minerals where both covalent bonds within the polymerized silicate tetrahedra and ionic bonds joining the polymerized silica tetrahedra exist cleavage planes are developed along the ionic bonds in the crystal structure 0 Not all of the crystal faces of growth are related to the same locations in the crystal structure as the cleavage planes Fracture 0 This is the way a mineral breaks when it does not possess cleavage or breaks across a cleavage plane 0 Fracture in silicate minerals is a property of equal strength bonding between ions in the mineral s crystal structure 0 The strong bond breaks and it is irregular Luster 0 Luster is the surface appearance of the mineral in re ected light 0 Luster is described in terms of quality and intensity and is generally divided into two basic types metallic and nonmetallic Color 0 The color that the mineral displays 0 This is not always a diagnostic property as many minerals can display a wide variety of colors Streak 0 This is the color of the powdered mineral when scratched against a piece of unglazed porcelain streak plate Hardness 0 This refers to the resistance that a mineral has to being scratched 0 The hardness is a result of the bond strength of the ions in the minerals crystal structure A relative hardness scale known as Moh39s Hardness Scale classifies mineral hardness from one to ten Strong bonds vs weak bonds Miscellaneous Properties 0 Effervescence in weak HCl 0 magnetism 0 taste 0 double refraction 0 smell Geol 201 Notes for week 3 Igneous Rocks Fireformed rocks rocks cooled from a molten state magmalava record the melting and cooling history of the Earth Cont hot spot contcont divergent contoc convergent oc hot spot ocpc divergent ococ convergent Relationship to Tectonics and Hot Spots General Types of Igneous Rocks Plutonic Intrusive vs Volcanic Extrusive Plutonic rocks cooled slowly underground where they are insulated by the surrounding rocks Volcanic rocks cool quickly at the Earth s surface where they loose heat rapidly to the air or ground Plutonic slow cooling Volcanic quick cooling Igneous Compositions Compositions are designated by silica content boundaries between categories are gradational Rock names are based upon composition and texture Generation of Magma Partial vs Complete Melting Melting is accomplished by changing the conditions pressure temperature andor composition that a rock is under Under the present conditions in the Earth only 230 of the original rock melts partial melting During the melting process different minerals melt at different rates In general the lighter minerals melt faster and first and the denser more ironmagnesiumbearing minerals melt slower and less Types of Partial Melting Decompression Partial Melting Dch Partial melting is accomplished by decreasing the pressure on mantle peridotite by allowing it rise convect upward Without loosing too much heat The ultramafic composition peridotite then undergoes partial melting to produce a mafic composition basaltic magma Volatile Flux or Dehydration Partial Melting DhPm This type of melting occurs when volatiles H20 C02 and other molecules are released dehydrated from the subducting oceanic lithosphere The volatiles were originally bound in the mineral crystal structures of the minerals within the subducting oceanic plate As the pressure conditions of the subducting plate change the minerals metamorphose and release the volatiles the minerals dehydrate The released volatiles migrate upward into the overlying mantle wedge where they change the chemistry of the mantle peridotite The volatiles act as a ux to lower the melt temperature of the mantle peridotite ultramafic composition causing it to melt to create a mafic composition basaltic magma Advection or Heat Transfer Partial Melting Ade The hot mafic magmas generated by volatile partial melting rise upward to the base and lower continental crust where they pond or underplate the lower crust The rising magmas transfer heat to the rocks of the lower crust This added heat causes the rocks of the lower crust to undergo partial melting Since the rock of the lower continental crust is mafic and intermediate in composition the partial melting of these compositions of rocks produces intermediate and felsic silicic magmas Factors Associated with the Ascent Rise of Primary Magma Differentiation change in composition by Assimilation Rising magma fragments the surrounding country rock That country rock is incorporated into the magma where it is melted and its composition is added to the magma composition Differentiation by Crystal Settling Fractional Crystallization Early formed minerals that crystallize in the magma chamber are generally Fe and Mgrich minerals that are dense and settle to the bottom of the magma chamber They are then removed from the magma as the melt magma moves upward through the crust Differentiation by Magma Mixing Magmas of different compositions can merge and mix to create a new composition Sometimes this occurs and the result is a catastrophic explosive eruption Differentiation by Combinations of the Above Processes Magmatic processes are complicated and more than one differentiation process can occur with a magma body Magmatic Compositions and Relationship to Magmatic Properties 0 Mafic magmas basaltic with their high temperatures low gas content and low silica content are very uid Capable of rapidly rising through the crust and when they erupt to the surface to ow long distances The low amount of gas content in the magma and uidity of the magma allow the gas to escape easily creating quiescent lava ow eruptions Felsic magmas rhyolitic with their lower temperatures high gas content and very high silica contents are very viscous thick and pasty The magma moves slowly through the crust The high gas content and thick viscosity of the magma keeps the gas dissolved in the magma and under pressure When the pressure is finally released the gases exsolve rapidly and explosive eruptions occur Often after the initial explosive eruption the remaining degassed felsic magma will ooze to the surface to slowly ow short distances from the eruptive center Intrusive or Plutonic Igneous Rocks Plutonic Textures Texture is a term that describes the manner in which mineral grains are touching or arranged The texture of rock is an indicator of how the rock formed Igneous textures indicate the cooling history of the magmalava to become a solid rock Plutonic rocks are formed from magmas that cool slowly underground because the magma is insulated by the surrounding warm rocks The slow cooling produces few nucleation sites for mineral growth and allows these minerals to crystallize into minerals with sizes easily visible to the naked eye The crystallizing minerals form into a random interlocking arrangement known as a phaneritic or coarsegrained igneous crystalline texture Igneous Rocks Felsic magmas have high water contents When they cool underground the water that does not like to be part of a mineral s crystal structure is left into the very late stages of crystallization These waterrich felsic melts crystallize slowly Due to the high water content the elements in this last uid can migrate long distances and join up to make a few very large crystals These crystals are generally greater than 25 cm but can be up to 10 meters in size Very coarse grained igneous crystalline textures are termed pegmatitic Plutonic Structures Structures are features in rocks that are larger than the grain to grain relationship of texture Structures are often large enough to require a large hand sample or the rock outcrop exposure to see them Structures in igneous rocks indicate events that occurred during the injection and cooling of the magmalava Xenoliths foreign rocks Fragments of the surrounding country rock that have been included into the magma without assimilation or incomplete assimilation Dikes tabular igneous intrusions that crosscut the adjacent rock layers Sills tabular igneous intrusions that are parallel to the rock layers 0 Batholiths a very large igneous intrusion greater than 45 km2 of exposure Extrusive or Volcanic Igneous Rocks The type of eruptive style landforms structures and textures are determined by the composition of the magmalava Mafic Basaltic Volcanism Magmas are high in iron and magnesium low in silica low in gas content uid low viscosity Resultant eruptions are generally quiescent lava ows with gas escaping easily Lavas accumulate to form large shield volcanoes or plateau lavas Small localized vents where gases cause mafic magma to be erupted as fire fountains will create associated cinder cones Plateaus and cinder cones are normally the formations afterwards Central Vent Eruptions w Short Fissure Flank Eruptions Shield Volcano with Associate Landforms 0 Rift or Fissure Zones Fissure Eruptions ex Hawaii normally has 3 centralized 0 Fire Fountaining and Lava Flow gas charged eruption then goes into a smooth ow 0 Cinder Cones Cinder cones develop as pyroclastic material build up along a small vent 0 Crater or depression at the top of the cinder cone is formed at the central vent where pyroclastic debris builds up Mafic Volcanic Textures Aphanitic and AphaniticPorphyritic Textures 0 Aphanitic textures are finegrained igneous crystalline textures where the individual minerals are generally difficult to distinguish with the unaided eye 0 Some minerals may be large enough to observe tiny outlines but easy recognition is not possible This texture indicates a rapid cooling of the lava owmagma at or near the Earth s surface 0 Aphaniticporphyritic are crystalline textures that have two distinct crystal sizes There are large easily visible minerals phenocrysts in a groundmass that is aphanitic This type of texture is produced by two distinct cooling histories 0 The large phenocrysts were formed by slow cooling in the magma chamber 0 Those early formed crystals and the remaining liquid then erupted to the Earth s surface where the liquid cooled rapidly around the phenocrysts to form the aphanitic groundmass Pyroclastic Textures 0 Pyroclastic or fire fragment texture is formed when fragments either liquid or solid are thrown from an explosive volcanic eruption and those fragments accumulate and become bond together by various means 0 The individual fragments may have aphanitic aphaniticporphyritic or glassy textures 0 Blobs of lava that cool rapidly when they are thrown out during the eruption Glassy Textures Glassy textures are glassy in appearance and the rock is composed mainly of the mineraloid glass 0 In mafic volcanic rocks glassy textures mainly form from the rapid quenching of lavas within water or air Basaltic Lava Flow Features Structures Structures are features larger than texture need a large hand sample or outcrop to view Lava Levees 0 Once the low volume of gases is tapped off the pyroclastic eruptions of cinder cone the remaining magma generally erupts as quiescent lava ows either from the crater or punches out the base of the cinder cone 0 The lava ows are channelized downhill from the source and the channel is path is maintained by the development of lava levees Lava Tubes 0 As the lava ows between the levees its surface crusts over and insulates the owing interior of the ow 0 If the crust is a strong enough the roof may support itself if the lava drains out and produce a lava tube or cave 0 The interior of lava drains out if the crust on top is formed and the middle is now empty Aa Lava Flow Surface Flow Breccia 0 More viscous basalt ows produce a surface rubble of broken rock known as ow breccia or aa Pahoehoe Lava Flow Surface 0 More uid mafic lava creates a very thin crust that becomes rippled and ropey as the underlying lava is continually moving 0 This smooth ow top is known as Pahoehoe 0 Thin rinded only a cm thick ripples rope structure Columnar or Cooling Joints 0 If the lava does not drain out of the tube but instead crystallizes and becomes solid hot rock this hot rock will cool down and contract to produces a series of joints or fractures 0 The fractures generally form perpendicular to the upper and lower cooling surface The result is often a set of six to fivesided cooling columns 0 Very thick lava ows often have a set of three cooling joint patterns 0 Upper and lower cooling surfaces middle is still insulated 0 No more lava is added from the vent solidifies into a solid rock but is still hot so it expands then cools and contracts and turns into joints and columns 0 Submarine Pillow Lava 0 Lava ows that erupt under water or slowly ooze into a water source develop a thin glassy rind or chilled margin along its outer surface The interior remains molten and oozes along like toothpaste squeezed out of a tube After the interior cools it is aphanitic texture 0 Chills immediately when it hits the cold seawater no time for the elements to adjust so they rapidly become glass This only happens to the outer shell because this then insulates the rest on the inside enough for that to become minerals Basaltic Pyroclastic Features Structures 0 Pyroclastic material that builds up as generally loose material to create the cinder cone consists of blocks of scoria a very gasrich mafic rock that is ejected in a somewhat tacky state 0 spindleshaped bombs tapered football shaped ejecta that is expelled in more uid state Intermediate to Felsic Volcanism Magmas are intermediate to low in iron and magnesium intermediate to high in silica high in gas content thick and pasty high viscosity Resultant eruptions are generally a combination of explosive pyroclastic eruptions and quiescent lava ows Lava ows are thick and pasty and do not ow long distance but build up around the central vent to form a steep cone Central Vent Eruption amp Associated Flank Eruption 0 Composite Cones and Associated Domes 0 Composite of both explosive pyroclastic material and quiescent lava ows 0 Also known as a stratovolcano Pyroclastic Eruptions Ash Fall vs Ash Flow Eruptions 0 Ash fall or air fall pyroclastic eruptions are where the pyroclastic material is jettisoned into the atmosphere where it cools and falls back to the ground eventually as cool or cold volcanic fragments tephra 0 Ash ow or nuee ardente pyroclastic eruptions are hot ZOO600 C dense columns of ash that move along the ground surface at speeds of up to 300 kmhr upon a cushion of compressed air When the ash ow comes to rest it is still hot 0 air fall cools become glass or pumice and falls 0 when too dense then it spills out and rides down a cushion of air to the bottom ash ow Formation of Ash and Pumice 0 Dissolved gas in the magma exolves becomes gas bubbles as the pressure is released 0 Magma chills around the gas bubble becoming glass 0 Continued expansion of the gas may shatter the pumice into small fragments of pyroclastic ejecta or tephra Pyroclastic Texture Glass shards pumice fragments crystal fragments and rock fragments accumulate from the erupting cloud as loose material that then becomes bonded together in various methods by welding of hot ash ow deposits or by later cementation amp compaction in air fall deposits 0 Unwelded tuff the uffy particles that are left after an eruption if it is compacted down it will form into a rock Welding in Ash Flows When hot ash and larger tephra fragments come to rest in pyroclastic ows the particles compact and squeeze gases from the mass of tephra 0 The particles become fused or welded together Caldera Formation Calderas are large circular to elliptical collapse structures above a volcano that are produced when the volcano has a catastrophic pyroclastic eruption that partially drains the shallow magma chamber leaving the upper portion of the volcano fractured and unsupported Postcaldera Felsic eruptions 0 Following the cataclysmic eruption de gassed felsic magma may erupt quiescently to the surface as domes and obsidian ows Pressure release of the magma and the rim is unsupported for it will down drop Ex crater lake Debris Avalanches and Debris Flows Debris avalanches are large mass wasting events where undersaturated weathered material on the slopes of composite volcanoes are mobilized and moved down the slope Ex Big Obsidian ow in Newberry volcano and crater lakes wizard island Thick and pasty magma that ows afterward Debris Flows or lahars are fastmoving slurry ows of debris and water that move down preexisting drainage systems Cementlike mix of ash pumice sedimentary rocks glacier water They pick up anything and everything in their way Intermediate and Felsic Composition Volcanic Textures Glassy Texture Glassy textures in intermediate and felsic compositions are formed in ash and pumice fragments during quenching in the air during pyroclastic eruptions The other way glassy textures are formed is during rapid cooling of upper portions of felsic lava ows during postcaldera dome eruptions The rapid cooling is similar in rate to the rapid cooling that produces aphanitic textures in mafic and intermediate composition lavas The greater Viscosity of the felsic composition lava means that the lava does not have enough time even to make a rock with very small mineral crystals an aphanitic texture and instead gets a solid rock of volcanic glass or a mineraloid Ex obsidian Classi cation of Igneous Rocks Simple Classification of Crystalline Igneous Rocks Name of rock is based upon the composition and texture Igneous Resources Resources produced by processes associated with igneous activity Common source of many of the metal ores Hydrothermal Activity Associated with Igneous Processes Magmatic waters and meteoric waters groundwater that are heated up by the hot magma chamber carry dissolved metals that precipitate out in fractures and pores spaces in the rock as the uids cool The hydrothermal activity produces many of the metal ores of the world such as gold silver copper zinc lead manganese etc Hydrothermal Activity at Oceanic Ridges Hydrothermal activity is very common at the oceanic ridges The fractured oceanic crust allows sea water to pass through the oceanic crust and be heated to form hydrothermal circulation cells The hot water alters the rocks of the oceanic crust and places metals into solution When those hydrothermal waters discharge back into the cold seawater the metals in solution precipitate out to form Black Smoker Chimneys and metalsulfide minerals that precipitate onto the sea oor Although not easily mined in the modern oceans these deposits exist upon land when the ocean oor is accreted to the continents The accreted oceanic oor is known as an ophiolite Hydrothermal activity also supports an intricate array of life based upon hydrogen sulfide gas in a process known as bacterial chemosynthesis Geol 201 notes for week 6 Geologic Time 0 The Earth is 456 billion years old Ga or 4560 million years ma 0 The vast age is known as Deep Time 0 Less information is known for the older events Classi cation Categories of Geologic Time Hierarchy of Subdivisions of Time Eon largest time slice Era Period Epoch smallest time slice Major Subdivisions of the Time Scale Cryptozoic or Precambrian Eon 456 Ga to 0542 Ga 542 ma informal time interval meaning hidden life Subdivided into formal eon names Hadean Eon 46 38 Ga Archean Eon 38 25 Ga Ancient Time Proterozoic Eon 25 0542 Ga First Life Time Phanerozoic Eon 542 ma to present Visible Life time subdivided into 3 era Paleozoic Era 542251 ma Ancient Life time dominated by invertebrates and beginnings of vertebrates Mesozoic Era 25165 5 ma Middle Life time age of dinosaurs Cenozoic Era 6550 ma Recent Life time age of mammals Relative Age Dating Process of logically placing a related group of rocks into a progressive sequence of events Principle of Original Horizontality When looking at sedimentary rocks most sedimentary rocks are deposited in approximately horizontal layers When layers are deposited one on top of another Without gaps or breaks in time they are known as comformable sequences or layers Principle of Superposition Sedimentary rock layers are deposited one on top of another such that the oldest rock layers are on the bottom unless they have undergone substantial deformation Principle of Original Continuity Sedimentary rock layers are often deposited over continuous areas of the basin of sedimentation They may often be traced out over long distances even between states Principle of Faunal Succession Different organisms have lived at different times Rock layers with several fossils can be used to determine concurrent time range when all fossils present would overlap in time Principle of CrossCutting Relationships Younger events crosscut older events Common crosscutting events igneous intrusions faults and buried erosional surfaces unconformities Principle of Inclusion or Fragments Fragments found in a rock or rock layers are older than the unit or layers they are included within Nonconformitv erosion surface between sedimentary rocks above and igneous andor metamorphic rocks below Angular Unconformitv erosional surface between sedimentary layers that are tilted to each other Absolute Age Dating 0 Placing a number of years to an event 0 Most common method in geology is radiometric age dating of minerals Method by which you measure the ratio of radioactive parent isotope and the nonradioactive daughter isotope that is sealed within a mineral in the rock 0 Method works best for igneous and metamorphic minerals to give the general crystallization age of the rock Bracketing Events Bracketing of events involves using both absolute and relative ager dating techniques to place absolute constraint numbers on when particular events occurred Geol 201 Notes for week 8 Earthquake Nomenclature Vibration seismic wave of the earth caused by the rupture and sudden movement of rocks that have been strained beyond their elastic limit Elastic Limit The maximum amount of bending a material will accept before rupture will occur Elastic Rebound Theory Theory of the process of storing energy in the elastic bending of rocks and then release of that energy upon rupture as the form of shock waves or vibrations seismic waves Focus vs Epicenter Focus the point within the earth where initial slippage occurs to generate earthquake energy or seismic waves Epicenter the point on the earth s surface directly above the focus Richter Magnitude Scale A scale that expresses the magnitude or amount of energy released by an earthquake ranges from 0 lowest limit of detection to 8 or 9maximum strength of the rocks before breakage Frequency and Magnitude of Earthquakes Function of the strength of the rock and the amount of time it takes to strain the rock beyond the elastic limit Classi cation of Earthquakes bv Depth of Focus Shallow Focus Earthquakes 070 km depth approx 62 of all earthquakes produced by the brittle failure of rock strained beyond the elastic limit Intermediate Focus Earthquakes 70300 km depth approx 30 of all earthquakes produced by uidinduced brittle failure in the subducting plate Deep Focus Earthquakes 300700 km depth approx 8 of earthquakes produced by mineral phase changes in the subducting plate Plate Tectonic Related Earthquakes Divergent Plate Boundaries 0 Area subject to tensional stress 0 Normal faulting 0 Shallow focus earthquakes 0 Richter magnitudes generally lt5 or 6 Transform plate boundaries 0 Area subject to horizontal shear stress Strikeslip faulting Generally shallow focus earthquakes Richter magnitudes up to 8 Convergent plate boundaries Area subject to compressional shear stress Reverse or Thrust faulting Shallow intermediate and deep focus earthquakes Benioff zone earthquakes Richter magnitudes up to 8 Types of Convergent Plate Boundary Earthquakes Crustal Quakes earthquakes that occur along shallow crustal faults Subduction Zone with Stickslip Frictional Properties Earthquakes shallow to intermediate earthquakes that release strain rapidly Episodic Tremor and Slip ETS Earthquakes slow earthquakes lasting over a couple of weeks seismic Vibrations are very low and difficult to detect never felt release as much energy as a magnitude 5 or 6 event Generation Of Shallow Earthquakes is by the standard model of stickslip properties of rocks along fault zones This applies to both the crustal earthquakes and the shallow subduction zone earthquakes Generation of Intermediate Focus Earthquakes 0 Generation of Intermediate Focus Earthquakes is by a modified model of stickslip properties of rocks along fault zones 0 the subduction zone the conversion of serpentine to olivine releases water 0 water causes uidinduced brittle failure 0 Origin of the waterin serpentinite is from hydrothermal metamorphism of oceanic lithosphere at the oceanic ridge Converts anhydrous ocean oor basalt to hydrous greenstone spillite and mantle anhydrous peridotite with olivine to hydrous serpentine Generation of Deep Focus Earthquakes 0 Generation of Deep Focus Earthquakes is by a model of anticrack failure At depth olivine converts to spinel 0 As lenses of spinel form eventually enough are present that the lenses join and slip occurs along the weaknesses in the crystal structure of spinel Mineral stabilities are deeper in the subducted plate than in typical mantle because the cold subducting oceanic lithosphere is heated slowly by conduction but rapidly descends because of convection This produces a depressed set of isotherms equal temperature lines allowing minerals to be stable to greater depth Intraplate Earthquakes Re activation of old faults by loading and unloading Re activation of old faults by transmission of stress from plate boundaries into plate interiors Magma movement associated with hot spots Large explosions nuclear and impact Magma movement associated with intraplate hot spots Thermal subsidence of a plate as it move away from hot spot QMPPP SBiSIIliC Waves All seismic waves generated along the fault zone as the rocks rupture and slip once the rocks reach their elastic limit at the asperity Causes of different intensities 0 Movement among fault 0 different material composition 0 loose formationsbonds 0 distance 0 size 0 duration 0 structure Body Waves Waves that travel through the body or interior of the earth Primary Waves PWaves Fastest wave approx 55 kmsec through granite rock particles move back and forth compress and dilate in the direction of wave travel waves travel through all materials solids liquids and gases Secondary Waves Second fastest wave 30 kmsec through granite rock particles move at right angles to the direction of wave propagation waves only travel through solids wave energy is absorbed in liquids more destructive than Pwaves causes widespread damage Surface Waves Travel along the surface of the Earth most commonly observed near the source of shallow focus earthquakes may be the most destructive of the earthquakes waves Love waves Slow waves lt1 kmsec in granite rock particles move at right angles to wave of propagation in along the surface sidewinder wave can be very destructive shear buildings off their foundations Rayleigh Waves Slow wave lt 1 kmsec through granite rock particles move in a circular motion that dies out with depth rolling wave can be very destructive Measuring Earthquakes Seismographs 0 Instrument used for magnifying and recording the motions of the earth s surface and the time at which the motions occur due to the seismic events 0 The recording of a seismic event records waves and precise timing 0 records the precise timing along the horizontal axis 0 records the amplitude of the waves along the vertical axis PS Lag Time the time difference between the arrival of the rst Pwave and the time of arrival of the first Swave at a seismic station Lag time is dependant upon the distance the seismic station is to the epicenter Richter Magnitude Measure of the magnitude amount of energy released by an earthquake Developed in 1935 for local earthquakes in California It is defined as the logarithm to base 10 of the maximum seismic S wave amplitude in thousandths of a millimeter recorded on a standard seismograph that is 100 km from the earthquake epicenter Not very accurate especially for very large earthquakes main use today is for local earthquakes of moderate magnitudes Nomogram graph used to determine the Richter magnitude knowing the distance to the epicenter from PS Lag Time as calculated from the PS lag time off the seismogram and using a P S lag time curve and trace amplitude measured from the seismogram Seismic Waves and the Earth39s Interior Generalities 1 Knowledge of the earth s interior comes from seismic refraction data collected on P and Swaves from seismic stations from around the world 2 P waves travel through all states of matter Swaves travel only through solids 3 4 Swave Shadow Zone Swaves are absorbed in the liquid outer core to form the Swave shadow zone P and S waves velocities are controlled by the density of the material they pass though and the state of matter of the material Changes in velocity of P and Swaves indicate changes in the composition or state of matter Pwave Shadow Zone P waves are slowed down and steeply bent or refracted at outer core to form the Pwave shadow zone Seismic Potential of the Paci c Northwest Over the last 30 years if has been realized that the Paci c Northwest subduction zone shares many of the same qualities that have generated Great Earthquakes and tsunami on subduction zones elsewhere in the world The subduction zone has stickslip frictional properties which means the two plates are locked and the upper plate is storing energy in the form of bent rocks That energy will be released when the plate ruptures along the entire length of the subduction zone to generate a 90 quake The rupture will occur underwater and offset the overlying water column producing a large tsunami that will strike the coast within 1030 minutes with waves that may reach 90100 feet in height 1921 events such as this have occurred over the last 10000 years with an average 1500 years The Cascadia Subduction Zone is the last remnant of a convergent plate boundary that existed along the West Coast over the last 200 million years Three sources of major earthquakes 1 Shallow Crustal Faults 2 Deep Subduction Benioff Faults 3 Shallow Subduction Faults Evidence for Great Earthquakes and Tsunamis One is the sediment record left in the estuaries along the Paci c Northwest Coast Estuaries are shallow coastal embayments with small river systems emptying into the ocean They are often partially enclosed with a sand extension or sand spit Deposition of peat from salt marsh plants occurs along the margin of the estuary and red cedar forests grow just landward of the salt marsh Deposition of fine clays and silt occurs within the estuary center Sand is deposited along the ocean side beaches and on the sand spits the salt marsh migrates over the area that once was estuary clay deposition and the cedar forest migrates over the area that had been peat deposition 0 during the Great Earthquake tsunami waves crash into the shore transporting beach sand over the estuary clay peat and cedar forest This layer of sand is known as tsunami sand 0 sediment again begins to accumulate in the estuary and along its margins Other evidence for the earthquakes 0 Japanese tsunami records 0 Native American oral history records 0 Graded bed turbidite sediment records found in submarine fans at the mouths of submarine canyons located along the continental margin of the Pacific Northwest
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