Seafloor Spreading Flashcards
How did evidence mount toward a mechanism for continental drift?
Evidence from continents?
Evidence from oceans? - World War II -> systematic mapping of the seafloor -
(1) Echo sounding -> depths -> seafloor topography
(2) Magnetometers -> magnetic signature of ocean floor
What is another word for seafloor topography?
bathymetry
When were mid-ocean ridges (MORs) discovered?
1950s-60s
What are the characteristics of Mid-Ocean Ridges?
long, continuous, notched submarine mountain chains 3 km above surrounding ocean floor 65,000 km long 1-3 km deep rift valleys at ridge crests
When (and by who) was the hypothesis that sea floor spreading was a mechanism for continental drift put forward?
1960
Harry Hess
What did Hess propose?
that the ocean floor spreads apart at mid-ocean ridges, leaving a rift valley
provided a mechanism for Continental Drift
still unproven – what evidence?
How were seafloor magnetic stripes discovered?
Vine and Matthews, 1962[& Morley]
Magnetic survey (towed magnetometers) revealed:
pattern of parallel magnetic anomalies
alternation between normal and reversed polarities
What is the origin of earths magnetic field?
Earth has an internally generated magnetic field
Generated by convection of electrically conductive liquid iron in outer core
How are rocks magnetised?
Lava is hot, molten -> cools to crystallize -> rock
Magnetite (Fe₃O₄) minerals in cooling lava line up to magnetic field lines
Magnetic ‘record’ becomes frozen into basalt rock
How does the latitude of formation affect alignment of magnetic minerals?
rocks formed at poles have vertical alignments
rocks formed at the equator have horizontal alignments
What are the normal and reverse polarities of the magnetic field?
Normal: near geographic south is magnetic north near geographic north is magnetic south Reverse: near geographic south is magnetic south near geographic north is magnetic north
Magnetic pole also … with time relative to geographic pole
‘drifts’
Magnetic field reversals can be recorded in…
successive lava flows of a young volcano
What do magnetic ‘stripes’ on the seafloor record?
One of first marine magnetic surveys in world: offshore Vancouver Island
Magnetic patterns -> ocean floor recorded magnetic reversals in symmetric pattern on both sides of the Juan de Fuca mid-ocean ridge
How do magnetic stripes form?
Lava erupts on seafloor at MOR: magnetic minerals align with Earth’s magnetic field
