Download 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 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 wavesknown 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 waterresponsible 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