Week 3 Notes
Week 3 Notes MAST200010
Popular in The Oceans
Popular in Journalism and Mass Communications
This 13 page Class Notes was uploaded by Ashley Thompson on Sunday September 20, 2015. The Class Notes belongs to MAST200010 at University of Delaware taught by Thoroughgood,C A in Spring 2015. Since its upload, it has received 21 views. For similar materials see The Oceans in Journalism and Mass Communications at University of Delaware.
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Date Created: 09/20/15
Plate Tectonics II 92015 639 PM Outline for Today s Lecture 0 Plate tectonic model 0 Characteristics of plate boundaries 0 Plate mechanics 0 Dynamics of mantle plumes 0 Seismic activity away from plate boundaries Plate Tectonics 0 Basic idea depends on structure of upper layers of Planet Earth 0 Continental crust o Oceanic crust o Mantle Lithosphere cold amp brittle Asthenosphere hot amp molten Lower mantle 0 General View 0 Heat source comes from deep in the planet Drives convective flow in asthenosphere Molten basalt upwells at divergent points in flow Upwelling flow ruptures thin oceanic crust along axis of mid ocean ridges Cooling by ocean water forms spanking new oceanic crust 0 New crust carried away from axis by frictional drag associated with convective flow in asthenosphere Plate tectonics o Plates are driven by cooling of earth convenction Heat within Earth comes from residual heat and radioactive decay of naturally occurring elements 0 Gravity provides additional force to move plates 0 What are the Tectonic Plates 0 Lithospheric plate There are a dozen large plates some of continents some don t All are in motion a The 100km thick surface of the Earth a Contains crust and part of the upper mantle n It is rigid and brittle n Fractures to produce earthquakes 000 O o What is the Asthenosphere o The hotter upper mantle below the lithospheric plate 0 Can flow like silly putty o It is viscoelastic solid NOT LIQUID Subduction and Seismicity 0 Creation and destruction of oceanic crust are cataclysmic processes 0 Occur in conjunction with high seismic activity 0 Earthquakes o Volcanoes 0 Heat flow from interior 0 Activity concentrated along plate boundaries 0 Seafloor spreading zones 0 Subduction zones Plate Tectonic Model Assumptions 0 Continental crust is thick and cool with low density rock 0 Oceanic crust is thin and cool with mgh density rock 0 Lithospheric plates are rigid and brittle and are spatially discrete o Asthenosphere is viscous hot and dynamic o Lithospheric plates above it move along with movement of asthenosphere because of frictional drag 0 General Features of Model 0 New crust is formed at spreading zones 0 Crust becomes part of lithospheric plates 0 Crust and plates move away from point of origin in response to convection in asthenosphere 0 Therefore lithospheric plates must be moving around 0 So what happens at the plate boundaries Three basic types of plate boundaries In Divergent Stages of Development 0 A Upwarping of lithosphere and continental crust B Upwelling of magma from asthenosphere and formation of new oceanic crust in rift valley o C Formation of more oceanic crust and Hnearsea 0 Red Sea is a geologically new spreading zone 0 D Formation of midocean ridge and extensive new oceanic crust n Convergent9Plate Boundaries Collision of two plate boundaries 0 One plate slides under the other subduc on Crustal material in subducting plate is destroyed melted and becomes part of asthenosphere Types 0 I Oceanic plate collides with oceanic plate Oceanic plate subducted and destroyed Trenches formed II Oceanic plate collides with continental plate Oceanic subducted and destroyed Continental crust is uplifted III Continental plate collides with continental plate Uplifting and mountain formation 0 Characteristics of convergent zones 0 O O O O Crust lost or deformed Deep earthquakes Deep trenches with island arcs or continental mountain ranges Rock silicarich after remelt Major source of oil a Transform Plate boundaries 0 Differential movement of two adjacent plate boundaries 0 Crust and lithosphere are neither created nor destroyed o Crust is conserved EX San Andreas Fault 0 Implications of Plate Tectonics While ocean basins are created and destroyed continents are too light to be subductedso simply break apart and collide Earthquakes coincide with plate boundaries Volcanoes tend to occur along plate boundaries 0 How Fast are the Plates Moving o Plates move 110cm per year rate of fingernail growth Seismic Activity Away from Plate Boundaries Hotspot Volcanoes o Are produced by movement of plate over stationary magma plume from a point source of heat in the mantle o Formed independently from volcanoes at divergent and convergent boundaries and can actually occur in the stable interior of a plate 0 Heights of hotspot volcanoes decrease with increasing distance from point of active volcanism Mantle Plumes and Hotspots o Plumes and lithospherecrust move relative to each other 0 Hotspot the linear volcanic chains trace the drift path of a plate over a mantle plume Passive Continental Margins EX Atlantic Seaboard Margin Characteristics 0 Few earthquakes Few faults Wide continental shelf 20200km wide Deltas Amazon Coral reefs Australia Economically important oil gas fisheries OOOOO Active Continental Margins EX West coasts of North and South America Margin Characteristics 0 00000 Frequent earthquakes Active volcanoes faults Narrow steep shelf Troughs Faults Trenches Island arcs Marine Sedimentation 92015 639 PM Today s Outline 0 Marine Sedimentation o Sediment in the sea Probing the sea floor Classification of marine sediment Factors that control sedimentation o Sedimentation in the ocean Shelf sedimentation Deepsea sedimentation Marine Sediments Unconsolidated organic and inorganic particles that accumulate on the ocean floor Originate from numerous sources 0 weathering and erosion of the continents o volcanic eruptions o biological activity 0 chemical processes within the oceanic crust and seawater o impacts of extraterrestrial objects 0 Classified by size 0 Transport to ocean 0 Deposit by settling through water column Paleoceanography and Marine Sediments Paleoceanography study of how ocean atmosphere and land interactions have produced changes in ocean chemistry circulation biology and climate 0 Marine sediments provide clues to past changes 0 A vast library recording geologic oceanographic and climatic conditions 0 Remarkably complete compared to land Sampling the Sea Floor 0 Hard rock can be sampled with the durable bottom dredge 0 Surface sediments are collected by grab samplers that take a bite out of the sea bottom 0 A heavy weight drives the core barrel into the sediment o A piston corer can take a much longer core than can a gravity corer because of a piston in the core barrel Probing the Sea Floor 0 Acoustic Techniques 0 Echo sounding 0 Seismic reflection 0 Seismic refraction Classification of Marine Sediment 0 Classification by Grain Size 0 Sediment Texture Grain size sorting grain size indicates condition under which sediment is deposited a High energy environments characteristically yield sediments larger in size a Small particles silts clays indicate low energy environments Indication of selectivity of transportation and deposition processes a Considered wellsorted if most particles appear in the same size classification a Poorly sorted sediments comprised of multiple sizes Textural maturity n Increasing maturity if 0 Clay content decreases Sorting increases Nonquartz minerals decrease Grains are more rounded abraded 0 Classification by Origin 0 Terrigenous Continental origin Transport mechanism a Rivers n Wind 0 Wind Driven 0 Sediment delivered to the openocean by wind activity as particulate matter dust primary dust source is deserts in Asia and North Africa 0 Comprise much of the finegrained deposits in remote openocean areas red clays o Volcanic eruptions contribute ash to the atmosphere which settles within the oceans n Glaciers n Turbidites Rapidlyaccumulated terrestrial sediments Earthquakestriggered submarine avalanches High velocity 50mph erosive events a Sealevelchange Derived from weathering of rocks at or above sea level eg continents islands 2 distinct chemical compositions n ferromagnesian or ironmagnesium bearing minerals n nonferromagnesian minerals eg quartz feldspar micas Largest deposits on continental margins less than 40 reach abyssal plains Transported by water wind gravity and ice Transported as dissolved and suspended loads in rivers waves longshore currents o Biogenous Composed primarily of marine microfossil remains Shells of onecelled plants and animals skeletal fragments median grain size typically less than 0005 mm ie silt or clay size particles Characterized as CaCO3 calcium carbonate or SiOZ silica dominated systems Tests shells skeletal structures produced by marine organisms In Siliceous Oozes Primarily diatom oozes 0 Cover 15 of the ocean floor Distribution mirrors regions of high productivity Common at high latitudes and zones of upwelling Radiolarian oozes more common in equatorial regions a Calcareous Oozes 0 Cover 50 of the ocean floor 0 distribution controlled largely by dissolution processes cold deep waters are undersaturated with respect to CaCO3 deep water is slightly acidic as a result of elevated C02 concentrations solubility of CaCO3 increases in colder water at greater pressures CaCO3 therefore readily dissolves at depth 0 Level below which no CaCO3 is preserved is the 39carbonate compensation depth typically occurs at a depth of 30004000m Transport mechanism a In situ sedimentation o Hydrogenous Particles that are precipitated directly from sea water Transport mechanism a In situ production Produced by chemical processes in seawater n essentially solid chemical precipitates of several common forms Nonbiogenous carbonates form in surface waters supersaturated with calcium carbonate common forms include short aragonite crystals and oolites Phosphortes El El phosphate crusts occurring as nodules formed as large quantities of organic phosphorous settle to the ocean floor some material is transformed to phosphorite deposts found on continental shelf and upper slope in regions of high productivity surficial deposits of manganese iron copper cobalt and nickel accumulate only in areas of low sedimentation rate eg the Pacific develop extremely slowly 1 to 10 mmmillion years 0 Volcanogenous Dust originating in volcanic eruptions Transport mechanism El El Mantle forces Gravity o Cosmogenous Sediment Sediments derived from extraterrestrial materials El El El includes micrometeorites and tektites tektites result from collisions with extraterrestrial materials fragments of Earth39s crust melt and spray outward from impact crater crustal material remelts as it falls back through the atmosphere forms 39glassy39 tektites 0 Productivity Skeletons and Soft Tissue Accumulation depends on production and preservation SiOz is preserved everywhere CaCO3 is variable depending on P T pH n Productivity 0 reproduction of planktonic organisms In Preservation silica dissolves only very slowly 0 calcium carbonate varies with depth a w are variable lt1 to 15mm1000 yr 0 Carbonate Compensation Depth The depth at which carbonate input from the surface waters is balanced by dissolution in corrosive deep waters In today s ocean this depth CCD varies between 3 km polar and 5 km tropical Thus accumulation rates vary a lot Factors Controlling Sedimentation 0 Factors Controlling Sedimentation 0 Average grain size reflects the energy of the depositional environment 0 Hjulstrom s Diagram graphs the relationship between particle size and energy for erosion transportation and deposition Grain Size proportional to energy of transportation and deposition 0 Relationship Between Energy and Grain Size 0 Energy supplied by currents and waves Low energy beach small waves mud Medium energy beach medium waves fine sand High energy beach big waves course sand and cobbles Sedimentation in the Ocean 0 Shelf Sedimentation 0 Continental Shelf Sediments Sources are mostly terrigenous in temperate and polar regions and in tropical regions that drain large mountain ranges a River input a Glacial n Volcanic Sources are mostly biogenic calcium carbonate in tropics n Coral rubble and shell debris n Weathers into carbonate sands Distribution of carbonate sediments follows global distribution of coral reefs Carbonate shelves are confined to tropical and subtropical settings where the water is shallow warm and clear 0 CrossShelf Distribution of Grain Size Regardless of type of sediment terrigenous vs biogenic grain size should decrease in offshore direction a This is true given a constant sea level I But sea level has been rising for the past 15000 years D So we have a mosaic of ancient sediment gradients on modern continental shelves 0 Depth increases in offshore direction 0 Wave energy near bottom decreases with depth 0 Grain size decreases with wave energy Latitudinal distribution a Biogenic at low latitude n Terrigenous at midlatitude Glacial at high latitude n Crossshelfdistribution n In theory grain size decreases with depth a In practice 60 of shelf sediments are relict n Relict sediments not in equilibrium with recent sediments DeepSea Sediments 0 Sources of Sediment Terrigenous sources a Bulk emplacement n Pelagic sedimentation Biogenic n Pelagic sedimentation Hydrogenous n In situ formation Topics not in lecture in book 0 Stratigraphy of deepsea sediments 0 Effects of plate tectonics
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