Plate Tectonics
Theory of Plate Tectonics
• Explains the formation and
movement of Earth’s plates
(pieces of lithosphere)
Wegener’s Hypothesis
• Continents were once joined in a
single supercontinent called
Pangaea, which then broke apart to
become our current continents
• Move in process called “continental
drift”
Proof of Wegener’s hypothesis
• Same fossils found on east coast of S.
America and west coast of Africa
• Continents look like they fit together
like a puzzle.
Problems with hypothesis
• Unable to explain how continents
could go through rock of sea floor
• What force moved them?
Sea Floor Spreading
• Mapped ocean floor
• Found underwater mountains called
mid-ocean ridge
Mid-ocean ridge
• Youngest rock is in the middle of the
ridge so must be coming up from
underneath
• 1960 Hess proposed sea floor
spreading – process by which new
oceanic crust is created, older crust
moves away: magma is pulled up
Sea Floor Spreading
Subduction of Oceanic Plates
• When spreading occurs, old oceanic
plates sink into the mantle due to
density
• Occurs near edges of oceanic plates
• When subducts, forms a depression
in ocean floor called a trench
Evidence of sea floor spreading
• Sampled rocks on both sides of spreading
• Found magnetic stripes based on magnetic
fields that were identical
Cause of plate movements
• Convection currents from in mantle
• Hot rock rises at mid-ocean ridges,
cools and spreads out horizontally,
then sinks back down.
Where does the heat come from?
• Gradual cooling of Earth’s interior
• Decay of radioactive isotopes
Plate Boundaries
• A dozen or so major plates
• Contain both continental and oceanic
crust
• Where the plates meet are called
boundaries
Three types of Boundaries
• Divergent – move away from each other
• Convergent – move towards each other
• Transform – slide past each other
• Plates move about 0.1-10
centimeters/year
Divergent
• cotf.edu
Convergent Boundaries
•
earth.usc.edu
Transform Boundaries
•
gweaver.net
Mountain Building
• Form along plate boundaries
• Two plates collide, but neither is
subducted (Ex. Himalayas)
Or
• When oceanic plate collides with
continental plate (Ex. Andes)
Mountain building
Or
• Two diverging plates (Ex. Mid ocean
ridge)
– can rise above sea level to produce an
island (Ex. Iceland)
Earthquakes
• Movement of Earth’s lithosphere
when rock in the lithosphere
suddenly shifts, releasing stored
energy
• energy is carried by seismic waves
• Where it begins is the focus
• Epicenter is the land above the focus
Tsunami
• A large sea wave generated by an underwater
earthquake, volcano, or landslide
• Reach speeds of 500 mph, heights of 30-70 ft.
Types of Seismic waves
• P wave – longitudinal waves
–Fast
–First to be detected at a distance
–Travel through solid and liquids
Types of Seismic waves
• S wave – transverse waves
– Can’t travel through liquids
• Surface wave – develop when seismic waves
reach the Earth’s surface
– Move slower than P and S waves
– Cause more damage
Measuring Earthquakes
• Use seismographs to pinpoint epicenters
• Richter scale – rates earthquakes on times
and amplitudes of seismic waves
• Moment Magnitude scale – measures amount
of energy released
– Each number represents 32 x increase in energy
Modified Mercalli scale – scale of 1-12
based on intensity of ground shaking
and damage
rating of 12 causes total
destruction of towns and cities
Where do earthquakes occur?
• Along boundary plates
• San Andreas Fault in California
Stress in the Earth’s Crust
• Faults – a break in mass of rock along which
movement occurs
• Fold – bend in layers of rock
Volcanoes
• A mountain that forms when magma reaches
the surface
How a volcano erupts
• Magma under pressure rises to the
surface
• When it gets closer to surface,
pressure is lower so gases (carbon
dioxide and water vapor) expand
rapidly
Structure of a volcano
mail.colonial.net
Volcano terms
• Magma chamber – place where magma
gathers before blowing
• Vent – opening in ground where magma
escapes
• Crater – top of central vent, bowl shaped
• Caldera – depression at top of volcano when
central vent and magma chamber collapse
inward
Magma
• Three factors determine viscosity
–Temperature – higher = low
viscosity
–Water content – higher = low
viscosity
–Silica content – high silica = high
viscosity
Quiet Eruptions
• Magma is very hot and low silica
• Get lava flows
–Two types of lava
• Pahoehoe – hot, fast moving
• Aa – cooler, slow-moving
Explosive Eruptions
• High silica – thick magma clogs vent pipe and
pressure builds until it blows
• Lava and gases are thrown outward
• Lava solidifies quickly and shatters into pieces
• Fine dust, ash, pebble-sized to car-sized
Mt. Krakatau
Location of volcanoes
europlanet.dlr.de
Location of volcanoes
• Along plate boundaries or hot spots
• Hot spot – region where hot rock extends
from deep within the mantle to the surface
• Three types of volcanoes:
– Shield - wide, flat volcano formed from lava
– Cinder - small, steep sided formed from
ash/cinders
– Composite – forms from explosive eruptions that
produce lava and ash
Other igneous rock features
• Batholiths – from core of mountain range,
large intrusive rock mass
• Sill – formed when magma hardens in a crack
parallel to existing rock layers
• Dike – formed when magma hardens in a
crack that cuts across rock layers
• Volcanic neck – formed when magma hardens
in pipe