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

• Sima: oceanic crust. More dense the continental crust

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 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.

Isostasy

• 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

• 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

• 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)

Divergence

• 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

Divergence

• 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

Divergence

• 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

• 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

Convergence

• 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

Convergence

• 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

Convergence

• 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

• 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 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

• 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 Deformation

• Upfolds are called anticlines, downfolds are synclines

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

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

• 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

• 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

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

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 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

Earthquakes

• 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

Earthquakes

• 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

• 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

Earthquakes

• 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

Earthquakes

• Common earthquake hazards:• Fire• Landslides• Liquefaction• Tsunamis (eg. Indonesia 2004, Japan 2010)• What causes a tsunami??

Volcanism

• 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

Volcanism

• 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

Volcanism

• 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

Volcanism

• 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

Volcanism

• 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

Volcanism

• 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

Volcanism

• 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• Nuees Ardentes: superheated pyroclastics, denser

than air, 1000+ degrees- destroy everything in its path