Composition of the Earth

13 Pages
153 Views
Unlock Document

Department
Geographical Biogeosciences
Course
GEOB 103
Professor
Brett Eaton
Semester
Fall

Description
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
More Less

Related notes for GEOB 103

Log In


OR

Join OneClass

Access over 10 million pages of study
documents for 1.3 million courses.

Sign up

Join to view


OR

By registering, I agree to the Terms and Privacy Policies
Already have an account?
Just a few more details

So we can recommend you notes for your school.

Reset Password

Please enter below the email address you registered with and we will send you a link to reset your password.

Add your courses

Get notes from the top students in your class.


Submit