geography

• Rocks are aggregates of crystals or grains of one or more minerals, with individual mineral particles generally small but identifiable
• Minerals must satisfy 5 conditions:

  • Naturally-occurring
  • Formed by inorganic processes
  • Solid element or compound
  • Definite/consistent composition
  • Regular internal crystal structure
• Igneous: Formed by solidification of molten rock (magma)

  • Extrusive: above the surface with fine grains (e.g., basalt)
  • Intrusive: under the surface with detailed crystals (e.g., granite)
• Layers of the Earth:

  • Crust: oxygen, silicon, aluminum, iron, calcium, sodium, potassium, magnesium
  • Upper Mantle (700-1300°C): magnesium, iron, aluminum, silicon, oxygen
  • Lower Mantle (1800-2800°C): olivine, pyroxene, feldspar
  • Outer Core (3200°C): liquid iron, sulphur, nickel, oxygen
  • Inner Core (4500 °C): solid iron and nickel
• Continental Drift Theory:

  • 225 million years ago: Pangea (supercontinent) existed, made up of 7 tectonic plates
  • Evidence: continental shape and “fit,” structural evidence, lithological evidence, palaeoclimatic evidence
  • Objections: difficulty in conceiving large continents moving, lack of mechanism explanation
• Plate Tectonic Theory:

  • Earth's surface composed of plates (lithosphere) moving on a “conveyor belt” (asthenosphere) due to convection currents in the mantle
  • Plate boundaries driven by convection currents, aided by Ridge-push mechanism at divergent boundaries, Slab-pull mechanism at convergent boundaries
• Three types of Plate Boundaries:

  • Divergent (Constructive): Ridge push mechanism
  • Convergent (Destructive): Slab-pull mechanism
  • Transform (Conservative): lateral displacement of rock along faults, intense seismic activity
• • Sedimentary rocks - compaction and cementation of sediment 
  • Contains multiple layers of rocks due to the deposition of sediment on top of each other
  • Eg. limestone, sandstone
• • Metamorphic - realignment of sheets causes melting/deformation/recrystallization due to high heat and pressure 
  • Contains rounded grains/fossils or trace fossils 
  • Eg. gneiss, quartzite (from sandstone) and marble
• Layers of the Earth:

  • Crust (what you stand on)
  • Oxygen, silicon, aluminum, iron, calcium, sodium, potassium, magnesium
  • Upper Mantle (700-1300°C)
  • Magnesium, iron, aluminum, silicon, oxygen
  • Lower Mantle (1800-2800°C)
  • Olivine, pyroxene, feldspar
  • Outer Core (3200°C)
  • Liquid iron, sulphur, nickel and oxygen
  • Inner Core (4500 °C)
  • Solid iron and nickel
• Evidence for continental drift includes:

  • Continental shape and “fit”: S America and Africa appear to fit together neatly
  • Structural evidence: matching of ancient shields across S America and Africa, and fold mountains across Scandinavia and N America
  • Lithological evidence: matching rock-types like India and Australia having identical rocks of the same type and age found on both sides of the Atlantic Ocean
  • Palaeoclimatic evidence: ancient rock-types reflecting ancient climates
  • Tillites: glacial deposits in S. America and glacial erosion in Africa
  • Limestones and coral discovered beyond Arctic Circle
  • Dune-bedded sandstones found in temperate latitudes today
• Objections to Continental Drift theory:

  • Difficult to conceive of large continents plowing through the seafloor to move to new locations
  • No mechanism to explain the drift of continents
  • Questioned what kind of forces could be strong enough to move such large masses of solid rock over great distances
• Holmes suggested convection currents in the mantle, generated by radioactivity, as a possible mechanism for continental drift
• In the 1950s, new technology used in studying the ocean floor discovered:

  • Oceanic crusts are thinner than continental crust; long under-sea mountain systems or mid-oceanic ridges running through all oceans
• Convection currents (1960 - Henry Hess and idea of Sea-floor Spreading)

  • Convection currents idea revived - conveyor belt concept
  • Ocean floor depths increase systematically with seafloor age, moving
  • away from mid-ocean ridges - As oceanic crust gets older, it cools and becomes denser, therefore sinking a little lower into mantle
  • Weight of sediments on plate also cause it to sink a little into mantle
• Evidence for Convection Current:

  • Shallow earthquakes occur at mid-oceanic ridges
  • Medium-depth to deep earthquakes happen at trenches
  • High heat flow is observed at mid-oceanic ridges
  • A thicker layer of sediments is found away from ridge crests
  • Layers of sediment on the sea-floor increase in thickness and age away from the ridge crest
  • Paleomagnetism studies show a pattern of paleomagnetic stripes on the ocean floor where magnetic poles have reversed over time
• The magnetic pattern on the ocean floor is due to atoms in iron-bearing minerals lining up in the direction of Earth's magnetic field when lava cools below 550 degrees (Curie point)
• Plate Tectonic Theory 

  • Surface of earth composed of “plates” (lithosphere - cold and therefore brittle and rigid) that move on a “conveyor belt” (asthenosphere - hot and therefore ductile)
  • Plates move because of convection currents in the mantle
  • Heat from Core of Earth causes magma in mantle to expand and rise
  • Magma rises, spreads out below plates
  • Plates are moved along with the currents
  • The spreading magma cools and sinks. As magma sinks, plates are dragged towards each other. Thus, repeated heating and cooling causes plate movement
• Rocks are aggregates of crystals or grains of one or more minerals, with individual mineral particles generally small but identifiable
• Minerals must satisfy 5 conditions:

  • Must be naturally-occurring
  • Usually formed by inorganic (non-biological) processes
  • Must be a solid element or compound
  • Must have a definite/consistent composition
  • Regular internal crystal structure
• Three main types of rocks on Earth:

  • Igneous: Formed by solidification of molten rock (magma)
  • Extrusive: above the surface; contains fine grains e.g., basalt
  • Intrusive: under the surface; contains detailed crystals e.g., granite
  • Sedimentary rocks: result from compaction and cementation of sediment, with multiple layers due to the deposition of sediment on top of each other e.g., limestone, sandstone
  • Metamorphic rocks: formed by realignment of sheets causing melting/deformation/recrystallization due to high heat and pressure, containing rounded grains/fossils or trace fossils e.g., gneiss, quartzite, marble
• Rock Cycle is powered by the interior heat of the Earth, earth’s momentum, and energy from the sun, where any of the three basic kinds of rocks (igneous, sedimentary, or metamorphic) can be transformed into another rock of the same kind or either of the two other kinds
• Layers of the Earth:

  • Crust: Oxygen, silicon, aluminum, iron, calcium, sodium, potassium, magnesium
  • Upper Mantle (700-1300°C): Magnesium, iron, aluminum, silicon, oxygen
  • Lower Mantle (1800-2800°C): Olivine, pyroxene, feldspar
  • Outer Core (3200°C): Liquid iron, sulphur, nickel, oxygen
  • Inner Core (4500 °C): Solid iron and nickel
• Continental Drift Theory:

  • 225 million years ago, Pangea (supercontinent) split into Laurasia and Gondwana
  • Evidence includes continental shape and “fit,” structural evidence, lithological evidence, and palaeoclimatic evidence
• Plate Tectonic Theory:

  • Surface of Earth composed of plates that move on a “conveyor belt” in the asthenosphere
  • Convection currents in the asthenosphere drive plate movement, aided by Ridge-push mechanism at divergent plate boundaries and Slab-pull mechanism at convergent plate boundaries
• Three types of Plate Boundaries:

  • Divergent (Constructive): Ridge push mechanism
  • Convergent (Destructive): Slab-pull mechanism
  • Transform (Conservative)
• Oceanic-Oceanic Divergence:

  • Characterized by a prominent submarine basaltic mountain range or ridge of length 64,000 km
  • Emerges above sea level as islands
  • Mid-ocean ridge is marked by a central rift valley and magma extrudes along fissures (vulcanicity)
• Divergent boundaries in the middle of the oceans cause seafloor spreading
• Evidence for Seafloor Spreading:

  • Magnetic striping: Earth has a geographic North and South, but also a magnetic North and South which changes over time
  • Strips of ocean-floor basalt record the polarity of Earth's magnetic field at the time the rock formed, forming a zebra pattern
• Continental-Continental Divergence:

  • Results in the formation of triple-armed rift valleys at different scales
  • Features include above-average geothermal heat flow, alkali-rich lava, upwarping or doming of crust, formation of central rift valley, shallow focus earthquakes, and positive gravity anomaly
• Oceanic-Oceanic Convergence:

  • Denser oceanic plate subducts under less dense oceanic plate
  • Main structural landforms: island arcs, stratovolcanoes & ocean trench
  • Earthquakes may occur as the process of subduction may not be smooth
• Continental-Continental Collision:

  • Features include orogenesis, fold mountain ranges, earthquakes, ocean basin between continents, and sediments compressed & deformed
• Transform Plate Boundaries:

  • Two plates slide past each other along faults, causing intense seismic activity
  • Lateral displacement of rock along these faults occurs, known as strike-slip faults
  • Most famous example: San Andreas fault in California, between the Pacific and North American Plates
• When two oceanic plates move apart, cracks are formed in the oceanic plate. The decrease in the overlying pressure causes the underlying mantle to melt, forming magma. Magma rises up from the mantle material, and flows through the cracks on the oceanic plate to form a new crust of igneous rocks. This process is called seafloor spreading. The youngest rocks are nearest to the plate boundaries. Over time the cooling magma piles up to form a raised ridge along the plate boundary. Magma rises up and flows up through the cracks of the oceanic plate to form submarine volcanoes/volcanic islands.