Composition of the Earth

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Geographical Biogeosciences
GEOB 103
Brett Eaton

Geob103 notes COMPOSITION OF THE EARTH THE ROCK CYCLE CORE - Inner core is solid iron & some nickel - Outer core is liquid iron MANTLE - A solid shell of ULTRAMAFIC materials surrounding the core - THICKNESS – 2900km - TEMP – 2800 – 1500 degrees C CRUST - Thin outer layer composed of mafic (Mg, Fe) and felsic (Ca, Al, Si) rocks – mostly igneous - THICKNESS – 8-50km - TEMP – 0-1000degrees - Boundary between crust and mantle is sharp; known as the Mohorovicic Discontinuity (MOHO) CONTINENTAL CRUST - ~35 km thick - 2 layers – upper felsic and lower mafic OCEANIC CRUST - ~7km thick - Composed of mafic rocks THE CRUST & ITS COMPOSITION - Crust: thin outer layer of the earth - 8.5km thick - All the relief on the Earth’s surface (from high mountains to low ocean floors) is contained in the top ~10km (0.2% of total thickness) MATERIALS AT THE EARTH’S SURFACE - Rock: a solid, cohesive aggregate of grains composed of one or more minerals. Grains are essentially all inorganic. - Sediment: Generally unconsolidated inorganic materials originally derived from weathering and erosion (fractionation) of rocks. Deposited after transport by water, ice, air, or gravity. - Organic matter: Deposits composed of mainly dead and decaying matter from organisms (plans & animals). Includes soil (litter), peat, shell beds. - WATER ***The circulation of rocks = a slow process powered by the heat of earth’s interior ROCKS - Rocks constitute earth’s crust IGNEOUS - Formed by crystallization of (cooling) of molten material (magma) o INTRUSIVE (plutonic)  Magma cools slowly below the surface  Larger crystals formed o EXTRUSIVE (volcanic)  Magma cools rapidly at the surface  Small crystals formed  Can also be classified as mafic - Intrusive rocks – granite – felsic - Extrusive – basalt columns – mafic ALTERATIN OF IGNEOUS ROCKS - Chemical weathering can lead to formation of clay minerals (secondary minerals = sediments) - Oxidation: addition of oxygen to minerals (rust) - Hydrolosis: addition of water to minerals  Physical weathering breaks off individual mineral grains (fragmentation, erosion = sediments), facilitates chemical weathering SEDIMENTARY - Deposition compaction and cementation (lithification) of eroded rock/mineral particles - Derived from sediment and chemical precipitates that result from weathering and erosion processes. Sediment and chemicals precipitates accumulate in sedimentary basins (lakes & oceans) and are converted to sedimentary rock over time - Salt flats form when lakes evaporate HOW ARE THEY FOUND? - Weathering of rocks > small mineral particles - Particle transported by air, water or gravity > sediment - Sediment deposited in various environments > strata - Over geological time, sedimentary strata become buried - Physical and chemical changes > strata hardened, compacted and cemented to form sedimentary rocks > lithifaction - Radioactive decay, the sun (affecting the hydrological cycle) drive the rock cycle - Sediments are small particles that haven’t been changed into a rock (lithification) ROCKS - Metamorphic: physical or chemical change of existing igneous or sedimentary rock by application of heat and/or pressure o Limestone > marble - Igneous or sedimentary rock altered by intense heat and/or pressure (plate tectonics) - Can classify metamorphic rocks by degree of metamorphism (grade): o (LOW) Basalt > Greenstone > Amphibolite (HIGH) Where does the heat come from? Radioactive decay in the earth’s core - Uranium (U) and Thorium (Th) o Other elements + lead + radiation (about 90%) 40 - Potassium ( K) o Calcium-40 (or Argon- 40) + radiation (10%) Where does the pressure come from? - Air pressure = 1 atmosphere (Atm) or 1 bar - Pressure beneath 10 meters of water o = 1 Atm. = bar o = 3.5 m of overlying rock - Pressure in the deepest part of the ocean = 1000 bars = 3 km of rock - Pressure under 1 km of rock = 294 bars The Tectonic Cycle Upwelling, sea-floor spreading, subduction, crust formation - There are 2 major layers in the outer part of earth o Lithosphere and asthenosphere (not the same as the crust/mantle boundary) Lithosphere - Geological term describing the outer, rigid rocky shell of the crust and uppermost mantle (60- 150km thick) - Includes felsic, mafic, and ultramafic rocks  (2.7kg/m^3, 3.3 kg/m^3, >3.3kg/m^3) Asthenosphere - Heating of rocks below lithosphere results in “plastic” or soft rocks. - Rocks are dense (ultramafic) - SCALE NOTE: Together, the lithosphere and asthenosphere form a thin (300km) skin over the earth (about 5% of its total thickness). - Temperatures in the asthenosphere is higher (uniform temp) Lithospheric plates - Fit like jigsaw puzzle pieces (or armadillo armor) - Produces large earthquakes, volcanism, mountain range PLATE TECTONICS - The movement, collision, bending and breaking of lithospheric plates due to internal earth forces. LITHOSPHERIC PLATES - Rigid lithospheric plates are “floating” on the denser, plastic asthenosphere - The lithosphere is broken into large plates (tectonic plates o Oceanic plates: consist of relatively thin, dense oceanic lithosphere (~60km, 3.3 kg/m^3) o Continental plates: made of thick, light continental lithosphere (~150km, 2.7 kg/m^3) - Plates are both flexible and brittle > fractures are known as FAULTS. What drives this movement? >>> CONVECTION PLATE TECTONICS AND CONTINENTAL DRIFT - In 1915, Alfred Wegener proposed that landmasses were once united as a single great continent, PANGAEA, surrounded by a single great ocean, PANTHALASSA o PANGAEA broke apart over hundreds of millions of years, first splitting into LAURASIA and GONDAWANALAND, to form the present-day continents, islands, and oceans. - Wegener’s continental drift hypothesis: o Fit of South America and Africa o Fossils, rock types, and ancient climates match across continents - Main objection to Wegener’s proposal was its inability to provide a mechanism. - Inclination of the earth’s magnetic field varies over latitude. It is parallel to the surface over the equator and vertical over the magnetic poles.  DISCOVERY OF SEA-FLOOR SPREADING THE RISE OF PLATE TECTONICS - Invention of SONAR revealed ocean-floor topography, with deep trenches on some continental margins and linear submarine mountain ranges along mid-ocean ridges. PLATE BOUNDARIES 1. Converging boundary a. Collision of plates b. ‘Subduction’: one plate diving beneath another 2. Diverging (spreading) boundary a. Plates moving away from each other, new (oceanic > mafic/ultramafic) crust formed 3. Transform boundary a. Plates sliding past each other TECTONIC ACTIVITY - Two basic types of tectonic activity at the earth surface: 1. Compression causes folding at converging boundaries (alpine mountain chain) 2. Extension causes faults by rifting > occurs where plates are diverging. Convergent boundary: collision of an oceanic plate with continental plate - Subduction of oceanic plate beneath continental plate > intense tectonic and volcanic activity. - Oceanic plate sinks because it is thin, dense, and cooler than asthenosphere below - Deep oceanic trench forms where plates meet. - Accretionary prism of metamorphic rock forms along edge of continental plate. - Oceans and continental sediments carried into zone of heat/pressure as oceanic plate sinks under continent - Descending oceanic plate melts at depth - Rising magma can form a chain of volcanoes. Convergent boundary: collision of two continental plates - Subduction may not occur - Rocks are uplifted, folded, and faulted into mountains. - Ultimately, continents may be joined together by a deformed, metamorphic rock mass > ‘continental suture.’ Convergent Boundaries - Arc – continent collisions - Island arcs may collide with a large continental plate and become welded or joined. - Western North America: at least 50 accreted terranes recognized as products of ancient collisions - Much of British Columbia SPREADING BOUNDARIES - Continental plate ruptures and new ocean basins form o Continental crust is uplifted and cracked by convective movements in asthenosphere. o Long narrow rift valley forms, widens as the two new plates begin to move apart at the new spreading boundary o Blocks of crust slide up or down o Ocean invades the new valley o New oceanic crust formed along the mid ocean rift, and ocean basin widens. East African rift valley system and the Red Sea - The East African rift system is a divergent plate boundary at an early stage of its development. - It has reached locally the stage of continental rift, with thinning of the continental crust and lithospheric mantle. - The Red Sea is a fully developed divergent plate boundary that has reached the stage of oceanisation. SPREADING BOUNDARIES: Ex. Iceland (Mid-Atlantic Ridge) TRANSFORM BOUNDARIES (California/San Andreas Fault Zone) Example: The San Andreas Fault is a right-lateral strike-slip transform fault – the surface expression of the Pacific plate and North American plate moving by one another. Plate tectonic setting of western N. America - The Juan de Fuca plate is currently being subducted beneath the N. American continent - Cascadia subduction zone - The San Andreas and Queen Charlotte faults lies adjacent to the coastline - Seismic activity along this fault ACTIVE REGIONS - Orogeny (mountain building) is ongoing - Formed by volcanism or tectonic processes - ‘Young’ landscapes - Creates mountain bel
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