The Structure Of The Earth, Plate Tectonics And Landforms

The Structure of the Earth,Plate Tectonics and Landforms

Rock CycleIgneous: formed through cooling and solidification of magma or lavaMetamorphic: transformation of an existing rock through heat and pressureSedimentary: formed through deposition and cementation of weatheredrock products

Structure of the EarthMantle The Earth ismade up of 3main layers:Outer coreInner core– Core– Mantle– CrustCrust

The Crust Upper thermal boundary layerassociated with mantleconvection The Earth’s crust is made of:Continental CrustOceanic Crust- thick (10-70km)- buoyant (less dense thanoceanic crust)- mostly old- thin ( 7 km)- dense (sinks/subductsunder continental crust)- young (Atlantic 200 my)

Earth’s Crust: cold, brittleThin layer, 0.4% of Earth’s mass and 1% of its volumeContinental Crust Primarily granitic type rock (Na, K, Al, SiO2) 40 km thick on average Relatively light, 2.7 g/cm3Oceanic Crust Primarily basaltic (Fe, Mg, Ca, low SiO2) 7 km thick Relatively dense, 2.9 g/cm3cool, solid crust and upper (rigid) mantle “float” andmove over hotter, deformable lower mantle

Knowledge of the Earth’sStructure Geophysical surveys: seismic, gravity, magnetics,electrical, geodesy (geodetics)– Acquisition: land, air, sea and satellite– Geological surveys: fieldwork, boreholes, mines

Plate Tectonics The Earth’s crust is divided into plates whichare moved in various directions. This plate motion causes them to collide, pullapart, or scrape against each other. Each type of interaction causes acharacteristic set of Earth structures or“tectonic” features. The word, tectonic, refers to the deformation ofthe crust as a consequence of plateinteraction. The surface expression of mantle convection

World PlatesMajor plates – Pacific, African, Eurasian, North American, Antarctic, SouthAmerican, AustralianMinor plates – Nazca, Indian, Arabian, Philippine, Caribbean, Cocos,Scotia, Juan de Fuca

Plate Composition Plates aremade of rigidlithosphere.The lithosphere ismade up of thecrust and the upperpart of the mantle.

Lithosphere & Asthenosphere: More detailed description ofEarth’s layered structure according to mechanical behaviorof rocks, which ranges from very rigid to deformable1. lithosphere: rigid surfaceshell that includes uppermantle and crust (here iswhere ‘plate tectonics’work), cool layer2. asthenosphere: layerbelow lithosphere, part ofthe mantle, weak anddeformable (ductile,deforms as plates move),partial melting of materialhappens here, hotter layer(100 – 200 km)(200 – 400 km)

Craton: an old and stable part ofthe continental lithosphere.Having often survived cycles ofmerging and rifting of continents,cratons are generally found in theinteriors of tectonic plates.They have a thick crust and deeplithospheric roots that extend asmuch as several hundred km intothe mantle.The term craton is used todistinguish the stable portion ofthe continental crust from regionsthat are more geologically activeand unstable.

Cratons can be described as Shields, Precambrian crystalline rock thatcrops out at the surface and Platforms, in which the basement rock isoverlaid by younger sediments and sedimentary rock. The age of theserocks is in all cases greater than 540 million years, and radiometric agedating has revealed some that are as old as 2 to 3 billion years

Plate Movement “Plates” of lithosphere are moved around bythe underlying hot mantle convection cells

Convergent plateboundary marked bytrenchAsthenosphereAfricaDivergent plate boundarymarked by mid-ocean ridge Transform fault(spreading center)Oceanic lithosphereSubduction fuelingvolcanoesAsiaDescending platepulled down bygravityPhilippine TrenchMantleupwellingSuperplumeOuter coreMariana TrenchMantleMid-AtlanticRidgeInnercoreHotSouth AmericaColdPossibleconvection cellsRapidconvectionat hotspotsPeru–ChileTrenchHawaiiEastPacific Rise16

Plate BoundariesSites of significant geologic activityearthquakes, volcanism, orogenesis

Three types of plate boundary Divergent Convergent Transform

Divergent Boundaries Spreading ridges– As plates move apart new material is erupted tofill the gap

Age of Oceanic CrustCourtesy of www.ngdc.noaa.gov

Iceland: An example of continental rifting Iceland has a divergent plate boundary runningthrough its middle

Modern divergenceEast African Rift System22

East African Rift System24

Convergent Boundaries There are three styles of convergentplate boundaries– Continent-continent collision– Continent-oceanic crust collision– Ocean-ocean collision

Continent-Continent Collision Forms mountains, e.g. European Alps, Himalayas

Himalayas

Continent-Oceanic Crust Collision Called SUBDUCTION

Subduction Oceanic lithospheresubducts underneath thecontinental lithosphere Oceanic lithosphere heatsand dehydrates as itsubsides The melt rises resulting involcanism E.g. The Andes

Continent – OceanWest Coast of South America

Continent – Ocean Mount St. Helens31

Ocean-Ocean Plate Collision When two oceanic plates collide, the older moredense slab will sink back into the mantle forming asubduction zone. The subducting plate is bent downward to form a verydeep depression in the ocean floor called a trench. Trench systems occur for both continent-ocean andocean-ocean boundaries The worlds deepest parts of the ocean are foundalong trenches.– E.g. The Mariana Trench is 11 km deep!

Island Arcs Form, Continents Collide, and Crust Recycles atConvergent Plate BoundariesThe formation of anisland arc along atrench as two oceanicplates converge. Thevolcanic islands form asmasses of magmareach the seafloor. TheJapanese islands wereformed in this way.Motion of the plates: Rates: average 5 cm/year Mid-Atlantic Ridge 2.5 – 3.0 cm/yr East-Pacific Rise 8.0 – 13.0 cm/yr34

Convergent Plate BoundariesOcean-OceanAleutian Islands, Alaska

Ocean – OceanCaribbean Islands

Transform Boundaries Where plates slide past each otherAbove: View of the San Andreastransform fault

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Wilson Cycles

unaFormation (Ma)3506509001800Breakup (Ma)2505507601500Mode Ref.Atlantic (4)Pacific (4, 5)Pacific (4, 6, 39)Atlantic? (4)Craton/Shields

Volcanoes & Plate TectonicsPacific Ring of FireVolcanism ismostlyfocused atplatemargins

Volcanoes are formed by:- Subduction - Rifting - Hotspots

Pacific Ring of FireHotspotvolcanoes

What are Hotspot Volcanoes? Hot mantle plumes breaching thesurface in the middle of a tectonic plateThe Hawaiian island chain areexamples of hotspot volcanoes.Photo: Tom Pfeiffer / www.volcanodiscovery.com

The tectonic plate moves over a fixed hotspotforming a chain of volcanoes.The volcanoes get younger from one end to the other.

Plate Tectonics Summary The Earth is made up of 3 main layers (core,mantle, crust) On the surface of the Earth are tectonicplates that slowly move around the globe Plates are made of crust and upper mantle(lithosphere) There are 2 types of plate There are 3 types of plate boundaries Volcanoes and Earthquakes are closelylinked to the margins of the tectonic plates

Plate Tectonics and Landforms . Rock Cycle Igneous: formed through cooling and solidification of magma or lava Metamorphic: transformation of an existing rock through heat and pressure Sedimentary: formed through deposition and cementation of weathered rock pro