Download Chapter 17- Plate Tectonics

Ocean Floor - Plate Tectonics
Lecture Notes
• Early mapmakers noticed the
apparent fit of continents on either
side of ocean (matching coastlines)
• Continental drift (Wegener)- Earth’s
continents had once been joined as
Pangaea
• Similar fossils of
plants and animals
found on
separated
continents with
different climates,
proves that these
continents were
once closer to
equator
• Most scientists rejected Wegener's
hypothesis– Could not explain how or why
continents moved
• Studies of ocean floor discovered
vast, underwater mountain chains
and ocean ridges
• Rock samples taken from areas near
ocean ridges were younger than
samples taken from areas near deep
sea trenches
• Oldest seafloor is 180 million years
old
• Magnetic reversal- change in earth’s
magnetic field
– Same as present field- normal polarity
– Opposite to present- reversed
• Magnetic pattern on one side of a
ridge is mirror image of pattern on
other side
• Seafloor spreading- new ocean crust is
formed at ocean ridges and destroyed at
deep sea trenches
– Magma forced toward crust
– Fills gaps and hardens
– Forms new ocean floor
• Theory of plate tectonics- earths
crust and upper mantle are broken
into slabs called plates
–Plates interact at boundaries
• Divergent boundary– Moving apart
– Most are found on seafloor
– Form ocean ridges
– When on continent, form rift valley
• Convergent boundary–
–
–
–
–
Moving toward each other
Classified according to type of crust involved
Oceanic crust mostly basalt
Continental crust granite and sedimentary rock
Oceanic-oceanic- subduction occurs (one plate
descends) creates deep sea trench, forms arc of
volcanic islands: Mariana trench and islands
• Oceanic-continental: oceanic is
subducted, volcanoes erupt- PeruChile trench
• Continental- continental: mountains
form (Himalayas)
• Transform boundary- plates sliding past
each other
– Crust is deformed or fractured
– Long faults
– Shallow earthquakes
Late Paleocene 59 mya
Early Eocene 52 mya
SJVRocks!! CSUBakersfield Department of
Late Miocene 10 myaGeological Sciences
Pliocene 4 mya
Early Miocene 21 mya
Pleistocene 0.6 mya
San Andreas Fault
• Convection- transfer of energy
between Earth’s hot interior and
cooler exterior
– Hot mantle less dense than cooler
mantle
– Hot mantle forced upward to crust
– Cooler parts sink back down
towards the core
• Rising part of convection current
spreads out as it reaches the upper
mantle and causes both upward and
side to side forces
– Lift and split the lithosphere at divergent
boundaries, Material rising from mantle
hardens forms new crust
– Downward part occurs where a sinking
force pulls plates downward at
convergent boundary
Mantle Convection
• Ridge push- forces in mantle cause
asthenoshere to rise
– Pushes oceanic plate toward trench
– Could create drag on lithosphere
• Slab pull- sinking region of mantle
convection sucks oceanic plate downward