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Plate Tectonics Review. Earth’s Structure. Lithosphere : rigid, outer crust of the earth Rocks are less dense, more rigid than those in asthenosphere Sial : continental crust. Less dense than oceanic crust. Granite most common type of rock

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Earth s structure
Earth’s Structure

  • Lithosphere: rigid, outer crust of the earth

  • Rocks are less dense, more rigid than those in asthenosphere

  • Sial: continental crust. Less dense than oceanic crust. Granite most common type ofrock

  • Sima: oceanic crust. More dense the continental crust

Earth s structure1
Earth’s Structure

  • Mesosphere: the mantle. Extends from bottom of lithsphere to about 2900km below. 83% of earth’s volme Mostly made of periotite.

  • Mohorovicic Discontinuity (moho): boundary between lithsphere and mantle

  • Asthenosphere: weak, partly molten layer in upper mantle. Where convection currents are present

Earth s structure2
Earth’s Structure

  • Centrosphere (core):

  • Outer core: made of iron & nickel, permanently molten

  • Inner core: solid ball of iron and nickel. Pressure is so great it cannot melt.


  • The idea that a solid shell of basaltic sima (ocean crust) surrounds the earth and the sial (continental crust) floats

  • The sial is therefore always adjusting position

  • Isostasy= continental movement

Continental drift
Continental Drift

  • Pangaea, proposed by German Alfred Wegener, was original super continent – broke up into current continents

  • Evidence to support continental drift:

  • 1. coastline fit (continents fit like puzzle)

  • 2. geologic fit (similar age landforms and rocks on opposite continents)

  • 3. paleoclimatology (evidence of past similar climates)

  • 4. fossil correlation ( matching fossils found on opposite continents)

Plate tectonics
Plate Tectonics

  • The idea that rigid plates are traveling on a zone in the upper mantle, pushed by convection currents in the asthenosphere

  • Three types of movement:

  • 1. Divergence (plates move apart)

  • 2. convergence (plates come together)

  • 3. transform (plates slide past each other)


  • Between the two plates, new land forms – this is the youngest land

  • Found almost always on ocean floor – boundaries are called mid-ocean ridges or rift zones

  • The new land formation is also called sea floor spreading- as plates move, magma pushes up from asthenosphere & solidifies


  • A mid-ocean ridge does not always run in a straight line, but can be in segments

  • A crack can appear between the ocean floor of one section of the ridge

  • At this crack, plates move horizontally- these become transform faults


  • African Rift Valley- the greatest continental crack in the Earth’s crust. Runs from Turkey through eastern Africa

  • New land is created at rate of a few centimetres a year


  • convergence- when 2 plates of different density move toward each other

  • Eventually, one will be forced down under the other (the most dense plate)

  • Because ocean plates are more dense, they dive down into the mantle

  • Where this land is “reclaimed” is known as the subduction zone


  • The area of collision between two plates is often marked by an ocean trench, as the diving plate pulls down the edge of the plate is it diving under

  • This type of boundary is sometimes called a destructive plate boundary


  • If both colliding plates are oceanic, they are of the same density and neither wants to dive down

  • Eventually, one will be forced down but some material makes its way back to the surface and forms volcanic islands called island arcs


  • When both plates are light continental plates, they can buckle upwards to form fold mountains

  • Eg. Himalyan and Rocky Mountains

  • Fossils of ocean creatures can be found in these mountains because they used to be ocean floor

Subduction zone
Subduction Zone

  • Ocean trench: when 2 plates of different density collide, the denser plate pulls the leading edge of the less dense plate

  • Continental shelf: flat areas that extend from the shoreline and drop off at the trench

  • Continental slope: the steep drop off from the shelf into the trench

  • Benioff zone: the point where a subducting plate descends into the mantle

  • Convection currents: circular movement of material caused by heating and cooling

Transform boundaries
Transform Boundaries

  • Transform boundary- when two plates slide horizontally past each others

  • There’s great friction- sometimes the plates get stuck, then suddenly slide, resulting in a major earthquake

  • Eg. San Andreas Fault

Surface deformation
Surface Deformation

  • Compression (pushing together) and tension (pulling apart) important in folding and faulting of rock

  • Tension stretches and thins rock- creates normal faults

  • Compression pushes rock layers together and causes folding

Surface deformation1
Surface Deformation

  • Upfolds are called anticlines, downfolds are synclines

  • Folding is the dominant process in forming mountain ranges – known as fold mountains

Surface deformation2
Surface Deformation

  • Rocks respond to stress in 3 ways:

  • 1. brittle fracture (rock breaks)

  • 2. Elastic deformation (slow steady stress causes bending or folding, but when stress is released, rock returns to original form)

  • 3. ductile deformation (slow steady stress over a long period- when stress released, rock is permanently deformed)

  • Fractures more common at the surface, folds more common deep in the earth- more pressure and higher temps.

Composition of lithosphere
Composition of Lithosphere

  • Three main types of rock: igneous, sedimentary, metamorphic

  • Igneous: when molten lava hardens to form rock. Extrusive if it makes it to the surface, intrusive if it does not reach the surface

    • Most igneous rocks found in the ocean are basalts and gabbros- heavy & dark

    • Most igneous rocks found on the continents are andesite and granite- less dense and light colour

Composition of the lithosphere
Composition of the Lithosphere

  • Sedimentary rocks-when igneous rock is eroded by water, wind, and ice it becomes small particles called sediment

  • Sediment is transported to low lying areas and over time, layers form and compact to form sedimentary rock

  • Metamorphic rock- created through heat, pressure, and chemicals on other rock types (marble, slate, quartzite). Often in mountains.

The rock cycle
The Rock Cycle

  • All rocks are made by either melting, weathering, or compaction of particles.

Hot springs and geysers
Hot Springs and Geysers

  • Commonly seen in volcanic regions where magma is close to the surface

  • Hot springs: Surface water that infiltrated the ground comes in contact with the magma and heats - If it can find a route to the surface, it can emerge and still be hot

Hot springs and geysers1
Hot Springs and Geysers

  • Geysers are similar, but the fault line/channel that carries water to the surface has an obstruction

  • The obstruction allows water to pool and become VERY hot, creating steam, which creates enough pressure to forcefully expel the water

  • Hot water has many dissolved minerals, and areas around geysers often have layers of mineral deposits

  • How a Geyser Works


  • When stress-deformed rocks break or shift, earthquakes happen

  • The shaking is caused by seismic waves, which originate where the fracture or shift occurred

  • The exact point where the fracture/shift happened is the focus

  • The focus can vary in depth- the closer to the surface, the stronger it is

  • The point on the surface above the focus is the epicentre

  • The distance between the focus and epicentre is the focal depth


  • As plates move, pressure is put on them and cracks or faults form

  • These faults can become stuck for a long time, and as rock deforms, the pressure becomes too great and the rock suddenly moves, releasing the energy in waves- known as isostatic rebound

  • Earthquakes can also be caused by underground movement of magma (eg. volcano)

  • When land masses sink or rise due to weight adjustments, earthquakes can occur (isostatic readjustment)

  • California Earthquake


  • Earthquakes usually occur along fault lines

  • Instruments that record earthquakes are called seismographs

  • Earthquakes are measured using the Richter Magnitude Scale- each number on the Richter Scale represents a magnitude of 10x larger than the previous number (eg. 7 earthquake is 10x larger than 6, and 100x larger than a 5)

  • Earthquake Footage


  • Three kinds of waves caused by earthquakes:

  • Primary wave- compression wave- travels by compressing and expanding the ground. Moves the fastest

  • Secondary/shear wave-slower, moves in side to side motion

  • Surface wave- when primary and secondary waves reach the surface- like ripples on water- responsible for ground shaking

  • P & S Waves


  • Common earthquake hazards:

  • Fire

  • Landslides

  • Liquefaction

  • Tsunamis (eg. Indonesia 2004, Japan 2010)

  • What causes a tsunami??


  • Types:

  • Fissure eruption- fissure= crack in the lithosphere, from which molten volcanic rock can spew

  • Fissure Eruption

  • When basaltic lava cools it tends to fracture in cylindrical columns creating what is known as columnar joining


  • Shield Volcanoes: occur predominantly on ocean floors- gentle slopes & can be kilometres in diameter. Made of fluid basaltic lava (violent eruptions do not occur- fluid lava like wet concrete)

  • How Shield Volcanoes Form


  • Cinder Cone Volcanoes: found on continents, steep sides, violent eruptions. Trapped gases cause explosions. The violently erupted material is called pyroclastics.

  • How Cinder Cone Volcanoes Form


  • Composite volcanoes (andesitic volcanoes): erupt different materials at different times. Built up in layers of lava and ash. Symmetrical, often snow and ice capped (like those around the ring of fire). Violent, sudden eruptions.

  • The heat from these can cause lahars (mudflows) from melted snow/ice

  • How Composite Volcanoes Form


  • Hot spots: volcanic activity that takes place in the middle of a tectonic plate. Super heated plume in the asthenosphere

  • Pressure keeps the area from melting completely, but it is ten percent molten- called the plastic zone

  • Hotspots tend to create shield cones

  • Hawaii Hot Spot


  • Volcanic features:

  • Dike- the rock layer through which magma moves fractures and creates a dike

  • Sill- when rock intrudes between rock layers but cannot penetrate them

  • Laccolith- pooling of magma between rock layers

  • Volcanic neck- when layers of soil are eroded away from an ancient volcano

  • batholith- when magma pushes up from the mantle- can form great mountain ranges


  • Lava: igneous rock that has reached the surface

  • Pyroclastics: any material blasted out of a volcano (cinder, ash, gases, rocks...)

  • Caldera: volcanic crater formed when magma empties from chamber OR when the top of the volcano blows off

  • Lahar: volcanic mudflows

  • NueesArdentes: superheated pyroclastics, denser than air, 1000+ degrees- destroy everything in its path