development of the theory of plate tectonics
plate tectonics
earthquakes
volcanoes
Plate Tectonics
Theory of plate tectonics
- lines of evidence leading to development of current theory
- plate movements
- faulting and volcanism at plate boundaries
- earthquakes and volcanoes
- Based on 6 lines of evidence
- shapes of continents
- similar distribution of geologic features, fossils, some living species
- non-uniform distribution of earthquakes and volcanoes
- sea floor topography, especially the Mid-Atlantic Ridge
- age of volcanic islands in the Atlantic
- sea floor magnetism & sea floor spreading
Shapes of continents
- Historical perspective
- throughout most of history
- physical features of earth believed to be were fixed
- beginning centuries ago
- noticed outlines of western Africa and eastern South America matched
- in 1915
- concept of continental drift was proposed
- by Alfred Wegener
- Alfred Wegener (German,1880-1930)
- meterologist
- wrote The Origin of Continents and Oceans in 1915
- Wegener’s continental drift proposed
- all large continents of current world were joined together in late Paleozoic Era
- as single supercontinent, called Pangaea
- broke apart
- fragments had drifted about
Figure: history of continental drift
Shapes of continents
- Fit of continental shelves
- lended further support
- more striking than fit of continents
- rejection of continental drift
- despite mounting evidence
- Wegener’s theory largely rejected
- due to lack of mechanism for moving the continents
- Animation of Pangea, continental drift
Similar distribution of geologic features, fossils, some living species
- Geologic features
- cited by Wegener in support of continental drift
- marine and non-marine rock sequences of same age
- mountain ranges and glacial deposits match
- diamond fields of Africa and South America
- Fossils
- cited by Wegener in support of continental drift
- many of same extinct plants and animals in matching locations
- Glossopteris
fossils
- plant found on five continents thought to have been joined to form Gondwana
- former supercontinent formed by southern continents
- proposed in 1885 by Austrian geologist Edward Suess
- Figure: Fossil plant evidence - Glossopteris
- extinct group of seed plants that arose during Permian on great southern continent of Gondwana
- evidence provided in 1937 by Alexander Du Toit, South Africa geologist
- found Mesosaurus fossils in same sediment layer in Brazil and in South Africa
- fossils of a unique trilobite species
- found only in Boston, Massachusetts and Scotland
- Living species
- earthworm genera
- one genus found only at southern tips of South America and Africa
- another genus found only in southern India and southern Australia
- Non-uniform distribution of earthquakes and volcanoes
- earthquakes and volcanoes
- do not occur uniformly over surface of earth
- unheard of in some areas
- routine in other areas
- earthquakes and volcanoes
- ring of fire
Figure: Crustal plate boundaries
Sea floor topography
- historical perspective
- prior to advent of technology that enabled its exploration
- ocean floor was thought to be
- feature-less plain
- declassification of sonar
- helped document presence
- mid-ocean ridges
- Mid-Atlantic Ridge
- under water mountain range
- longest mountain range on earth
- runs down middle of Atlantic Ocean
- roughly equidistant between continents
Age of volcanic islands in Atlantic
- age of islands in Atlantic Ocean
- increase the farther they are to either side of Mid-Atlantic Ridge
Sea floor magnetism & spreading
- Earth’s magnetic field
- strong
- dipolar
- Earth’s magnetic polarity
- present north magnetic pole is located near the north geographic pole
- south magnetic pole is located near the south geographic pole
- Interest in continental drift
- revived during 1950s
- result of paleomegnetism studies
- paleomagnetism
- remnant magnetism in ancient rocks
- records direction of Earth’s magnetic poles at time of rock’s formation
- magnetic iron-bearing minerals (usually magnetite) align themselves with Earth’s magnetic field, "freeze" in place
- recording direction and strength
- Magnetic reversals
- in geologic past
- earth’s magnetic field has reversed, 180 degrees opposite of present
- reversals are recorded in rock
- magnetic anomalies
- average regional magnetic field of earth
- magnetic field at a point
- can be measured, values displayed graphically
- magnetic stripes
- discovered during oceanographic research in 1960s
- resulted from plotting magnetic anomalies measured on ocean floor
- produced pattern of "stripes"
- first seen in Atlantic, later in Pacific
- striping patterns were symmetrical about mid ocean ridges
- magnetic stiping
- led to development of
- theory of sea floor spreading
- proposed in 1962 by Harry Hess, Princeton University
- helps accounts for continental movement
- sea floor spreading
- sea floor separates at mid-oceanic ridges
- new crust forms by upwelling magma
- as it cools
- newly formed crust moves laterally away from ridge over time
- during sea-floor spreading
- magnetic field of the rock is fixed, in alignment with the earth’s field, at time rock cools
- after a polarity reversal
- it will be aligned against (opposite) earth’s field
- mechanism to drive this system
- thermal convection cells in mantle
- hot magma rises from mantle
- intrudes into fractures along mid ocean ridges, forming new crust
- cold crust is subducted back into mantle at deep-sea trenches
- where it is heated and recycled, completing cell
- convection cells in mantle
- example
- heat beaker
- water expands and rises
- it spreads and cools at the top
- cool water sinks
- consequence
- ocean basins are geologically young features
- continental fossils are at least 3.5 billion years old
- oldest marine fossils are about 180 million years
- there is relatively little sediment
Plate tectonics
- Plates
- types of plate boundaries
- Plates
- consist of lithosphere (oceanic and continental crust) and underlying mantle
- vary in thickness
- up to ~250km thick
- upper mantle + continental crust
- up to ~100km thick
- upper mantle + oceanic crust
- lithosphere overlies hotter and weaker semi-plastic asthenosphere
- heat transfer system in asthenosphere causes overlying plates to move
- as plates move over asthenosphere
- they separate
- mostly at mid ocean ridges
- collide and are subducted back into mantle
- in areas such as ocean trenches
- Plate movements
- plates move slowly (up to 15 cm/yr)
- may collide, move apart, or slide past each other
- friction during plate movement often generates earthquakes
- Plate boundary types
- divergent plate boundaries
- convergent plate boundaries
- transform faults
- divergent plate boundaries
- plates move apart in opposite directions
- crust is extended, thinned, and fractured as magma arises to surface
- two types
- oceanic
- continental
- oceanic
- produce mid-ocean ridges
- continental
- produce rift valleys
- ex., East African Rift Valley
convergent plate boundaries
older crust is destroyed and recycled
two plates collide
leading edge of one plate descends beneath margin of other via
subduction
key is density of rock types involved
- density = mass/unit volume
more dense plate will be subducted downward (sink) under less dense plate
- density differences as small as 1% are enough to cause subduction
- subducting plate moves downward into the asthenosphere
- is heated and is incorporated into mantle
- results in regional seismic and volcanic activity
- trench usually forms at boundary between two converging plates
- convergent plate boundaries
- characterized by
- defromation
- volcanism
- mountain building
- metamorphism
- seismicity
- important mineral deposits
convergent plate boundaries
three types
oceanic-oceanic
oceanic-continental
continental-continental
convergent plate boundaries
two oceanic plates converge
subducted plate bend downward to form outer wall of an oceanic trench
- leads to formation of a volcanic island arc
oceanic trench
subducted plate
- bends downward
- drags part of surface with it
- forms outer wall of trench
marks top of subduction zones
Volcanic island arc
subducted plate descends into mantle
- is heated and partially melted
- produces magma that is less dense than mantle rocks
- less dense magma rises to surface
- forming a curved (plane intersecting sphere) chain of volcanic islands called a volcanic island arc
examples
- Japan, the Philippines, and Indonesia are examples
oceanic-continental
oceanic and continental plate converge (collide)
denser (3.28 g/cm3) oceanic plate is subducted under less dense (3.0 g/cm3) continental plate
- forms outer wall of oceanic trench
- leads to formation of a volcanic mountain chain
- Volcanic mountain chain
- subducted oceanic plate descends into mantle
- is heated and partially melted
- produces magma that is less dense than mantle rocks
- less dense magma rises to surface inland of the coast
- forming a chain of volcanic mountains
- examples
- Andes Mountains (South America)
- Cascade Mountains (North America)
- continental-continental
- two continental plates converge
- both plates are of roughly equal density
- neither plate can be subducted
- one plate mayslide partly under the other
- leads to formation of interior mountain belts and seismic activity
transform plate boundaries
two plates move past one another in opposite directions
along fractures known as transform faults
majority are in ocean crust
- may along extend into continents
associated with seismic activity
example
San Andreas fault
- separates Pacific plate from North American plate
Figure: San Andreas fault
Earthquakes
- definition
- cause
- features
- fault
- focus
- epicenter
- measurement and rating
- primary effects
- secondary effects
Earthquake- definition & cause
- sudden motion or trembling in Earth along an existing fault
- caused by abrupt release of slowly accumulated strain
- strain
- change in shape or volume of a body as a result of stress
Earthquake- features
- fault
- focus
- epicenter
- foreshocks
- aftershocks
- fault
- fracture in rock of earth’s crust
- along which bodies of rock move past each other
- results from stress in earth’s crust
- focus
- initial rupture point of an earthquake
- where strain energy is first converted to elastic wave energy
- point within Earth which is center of an earthquake
- Depth of focus
- earthquakes can be classified by
- depth of focus below the surface
- shallow
- intermediate
- deep
- 300-700 kilometers
epicenter
point on Earth's surface directly above focus of an earthquake
foreshocks
shock waves
- released from seconds to weeks before main shock
aftershocks
shock waves
- follow main shock of an earthquake for up to several months
- often decreasing in frequency and magnitude over time
Earthquake- measurement
- device
- seismograph
- instrument that detects, magnifies, and records vibrations of Earth, especially earthquakes
- resulting record is a seismogram
Seismogram example
shows earthquake
- three different traces represent vibrations in different directions
Scales
attempt to measure severity of earthquake
- do not directly measure amount of energy released
- imply it
examples
- Richter scale
- Mercalli scale
Earthquake- measurement
- Richter scale
- measures vibrational amplitude of earth’s movement in response to seismic waves
- does NOT measure energy released
- produces numerical scale of earthquake magnitude
- as indicated by amplitude (size) of earth’s vibrations
- devised in 1935 by the seismologist C.F. Richter
- local quake magnitude
- logarithm, to base 10, of amplitude in microns of largest trace deflection that would be observed on a standard seismograph at a distance of 100 km from epicenter
Earthquake- measurement
- Richter scale
- generalized severity rating
- insignificant
- < 4.0 on Richter scale
- minor
- 4.0 - 4.9 on Richter scale
- damaging
- 5.0 - 5.9 on Richter scale
- destructive
- 6.0 - 6.9 on Richter scale
- major
- 7.0 - 7.9 on Richter scale
- great