Plate Tectonics Review

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

Plate Tectonics Review