Convergent_Plate_Boundaries

A.M.D.G                                              Geography                                            08/07/13                 CONVERGENT PLATE BOUNDARIES ·        Planet is not getting any bigger so if new land is created other land has to be destroyed. ·        These boundaries are also called destructive plate boundaries. ·        This happens where plates dive into mantle ·        Solid rock of crust melts ·        This is part of tectonic cycle.   3 Convergent Boundaries ·        Continental -> Oceanic ·        Oceanic ->Oceanic ·        Continental-> Continental   ·        Oceanic meets continental   ·        Oceanic material (sima) is heavier than continental (sial) ·        Oceanic dives beneath continental and about 70km melts. ·        The plate (lithosphere) becomes part of the mantle (asthenosphere). ·        Hence it is called a destructive boundary ·        Stress of this collision causes the continental plate to buckle to form mountains. ·        The melting plate mixed with gases from the sea water may also cause violent volcanic activity Pacific Plate North America -> Mt St Helen’s and Rockie Mts.     ·        Oceanic meets Oceanic ·        Older heavier plate dives under. ·        This is subduction long, deep, narrow trench developed. ·        Marianas trench (Pacific and Philippines) about 11km at deepest. ·        Crystal rock destroyed. ·        Magma formed volcano activity occurs. A line of volcanic islands appear along boundary. ·        These are Island Arcs. ·        Philippines and Japanese Islands ·        Part of Pacific Rim   ·        Continental meets Continental ·        Two continental plates collide head on. ·        Light but very thick. ·        The lithosphere subducts but plates collide and all sediments at edges of plates are swept up and deformed causing mountain ranges. ·        Indian sub plate collided with Eurasian 50-20 million years ago resulting in Himalayas. ·        Ireland: Cadedonian mts Donegal ·        Faulting, thrusting and folding occur. ·        Huge amounts of deposited material from rivers. ·        They collided recently therefore mountains are still high i.e. ->not worn down by weathering and erosion. ·        Very destructive earthquakes occur along fault lines within Himalayas.   Conservative Plate Boundaries    ·        Plates slide past ·        Along fault ·        Transform faults ·        Crust neither created nor destroyed ·        Therefore a passive or conservative boundary ·        Intense earthquake activity ·        San Andreas Fault, California 1200km long move about 5cm a year   Plate Boundary Summary Direction Name Created Moves apart Divergent or Constructive Crust created Continents split Move together/Collide Convergent or Destructive Fold Mts., Earthquakes, Volcanoes, Island Arcs and crust destroyed Moves alongside/past Transform, neutral, passive Earthquakes Move over hot spot Mid plate Volcanic Island        

A.M.D.G                             Geography                                         12/07/13                                     VOLCANICITYIntrusive activity: Magma forces itself into crust and cools. These form batholiths, sills and Dykes. Often called plutons. This activity is Plutonic activity. It is all underground. Extrusive activity: Lava comes out at the surface. This forms Lava plateaus and volcanoes of different types. Called Volcanic Activity. Volcano·        A volcano is a vent through which molten rock escapes to the Earth’s surface. Unlike other mountains which are pushed from below, volcanoes are built by surface accumulation of their eruptive products layers of lava, ashflows and ash.·        When pressure from gases within the molten rock becomes too great, an eruption occurs. Volcanic hazards include gases, lava an pyroclastic flows, landslides, earthquakes and explosive eruptions. Locations for Volcanic Activity 1)    Subduction zones at convergent plates (Mt. St. Helens)2)    Spreading centres Rifting/Doming (East African Rift Valley +Mt. Kilimanjaro +Giant’s Causeway)3)    Hot spots (Hawaiian Islands) Hot Spots·        Volcanoes can be caused by mantle plumes, these so called hotspots for example Hawaii can occur far from a plate Boundaries.·        A plume of magma a volcano to occur on the surface, directly above it.Over time lithosphere moves and a new volcano occurs over the hot spot or plume. This volcano will in time move away from the plume and another occur over the active plume. What is erupted?Basic lava: Low in silica, hot, runny gentle eruptionsAcid Lava: High in silica , less hot, plastic/ sticky , violent.Gases: Carbon monoxide , sulphur dioxide , hydrogen.Pyroclastic materials : Name for broken rock/ash/cinder bombsPumice: Rock with so many air spaces it floatsLahar: A mudflowNew Ardente: Gases/ superheated steam/ dust moves very fastTypes of VolcanoClassification of Volcanoes1.     Frequency of eruptionExtinct – erupted in geological timeDormant – erupted in historic time, not recentActive – erupting continuously Mt Ethen2.     Shape of volcano Types of Volcano:·        Shapes are based on type of lava/frequency and violence of eruption·        Type of lava is based on silica·        Silica is a mixture of silicon and oxygen·        More than 45%-60% silica = very acidic·        Less than 45% silica=basic ·        Violent eruptions are typically acidic·        Silica traps gases·        Infrequent eruptions (dormant) tend to be very violent.·        Subduction also converts sea floor organic remains into CO2. Types of volcanoes or Central Vent Eruptions1.     The shield volcano -> This is a broad, shallow volcanic cone, which arises because the running lava, which is fluid and hot cools slowly.2.     This Dome Volcano -> This one has a steep, convex slope formed from thick, fast cooling lava.3.     The ash-cinder Volcano -> Throws out besides lava- much ash into air. Though this the volcanic cone is built up from alternate layers of ash cinder.4.     The composite volcano5.     The caldera volcano 1.      Shield VolcanoBasic lavaLow viscosity (flows)Runny lavaFlows great distanceGentle slopesWide baseMauna Loa Hawaii really wide base of 120km 600m below sea level and 4000m above sea level. 2.     Dome ConeAcidic lavaViscousBulbous shape Like tooth paste being squeezed out of tube.Vents block easilyViolent eruptions can therefore followMt. St Helens 3.     Cinder ConeViolent EruptionCinder Rocks and volcanic bombsMore gentle at baseE.g. -> Parcutin, MexicoBig Crater 4.     Composite VolcanoStrato VolcanoMade up of lava flow and pyroclastic materialLava acts as a cap so volcano does not erode and can grow to a great height.E.g. -> Mt Etna , Fujiyama,  Vesavius 5.     CalderCalderas, which are circular depressions are found on the summits of many volcanoes “Giant” calderas are the largest of these: huge craters up to many tens of miles across.Giant calderas form by collapse in gigantic eruption that spew volcanic rocks out hundreds or even thousands of miles in all directions. Lahar or mudflow: Mudflows set off by Volcanic activity                                   Examples Nevada Del Ruiz, Colombia (1985)                                    Destroyed city of Armero                                    River of mud flowed over the town caused by the                                     heat from the volcano melting a glacier                                     23,000 died Hot springs and geysersHot springs and geysers are also central vent emissionsGroundwater gets heated in rocks in regions of recent volcanic activity. It may return to surface as spring/ geysers.If spring water is warmer than human body then it is classified as a hot spring.May also contain dissolved materialsVichy France or Blue Lagoon Iceland spas Benefits of Volcanic Activity  Geothermal energy -> electricity is produced from water heated in pipes deep in the earth’s crust and returned to the surface to be used in houses and greenhouses. Mineral rich soil can support more than one crop. Tourism e.g. -> Mt Vesuvius, Canaries , Yellow stone park Iceland Fissure Eruptions:In this type, there is no central crater at all. Instead giant cracks open in the ground and expel vast quantities of lava that spread far and wide to form huge rocks that can cover almost everything around. When those rocks of lava cool and solidify the surface remains mostly flat. Since the source cracks are usually buried, there is often nothing “volcano-like” to see-only a flat plane or plateau. 1.     Antrim PlateauEnormous lava PlateauCovering 4000 square km’sExtending to a depth of 800mPossibly 400 metres erodedFormed during tertiary period 65million years ago. American and European plate pulling apart.Crust stretching , cracks appear and lava pours out.Spreads over surface and cools quicklyOver 2 million years of this, layers build up to form a flat topped mountain called a plateau.Layers vary in size from 5m to 40mEdges have been eroded and undercut. Intrusive featuresKnown as plutonic structuresMagma goes into the crust and eventually cools and hardensGeneral name for structures is Plutons.Only revealed as rock above them is weathered and eroded.   Dykes:Magma forces into vertical fractures.Magma widens them furtherMay be a few cm’s up to several metresOther rocks of crust get worn away over time but rocks of dyke are more resistant and weather slowerThey are left as a ridge Sills:Magma forces it’s way into tiny gaps between sedimentary layers along the bedding planes.It happens close to the crust where there is less pressure of over lying rockOver lying rock may be  pushed up to form laccoliths and loppoliths.(Laccolith pushes up a dome shape loppolith) Intrusive Features: Dublin/Wicklow BatholithsStretches from Dublin to KilkennyComes over 1500 square kms.It is the largest of its kind in Ireland and Britain. Wicklow Mts first formed about 400mya when American Plate collided with European plate  (Iapetus sea began to close)Magma masses rise into the mountainsShale rock of mountains touches off hot magma and gets metamorphosed into quartzite .The magma cools very slowly to form granite. Through weathering the granite can now be seen out the surface.Granite weathers to form rolling hills.But in some places metamorphic rocks remain.Lugnaquillic Mts is still capped with schist and is steep and rugged.Sugar loaf is made up of quartzite and is also steep and sharper in outline.   

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