drifting continents & spreading seas Flashcards
who proposed the idea of continental drift
Alfred Wegener - hypothesized a former supercontinent Pangaea
when did The Origins of Continents and Oceans publish
1915
what initially happened to Wegeners hypothesis
- Wegener’s idea was ridiculed
- Lack of a mechanism for drift - a major criticism
- His idea faded away after his death in 1930 (at the age of 40) - was revived in the 1950s
3 ways we can measure drift
- Sea-floor spreading
- Subduction
- Plate tectonics
what is the plate tectonic theory
theory that the Earth’s surface is covered with a small number of internally rigid lithospheric plates
- The plates move about on the Earth’s surface, and the vast majority of earthquakes, volcanoes and tectonic activity occurs where they interact
what were scientists thinking before continental drift
- The oceans and the continents were permanently fixed
- Evidence of drastic changes interpreted as… =Shrinkage effects
= Rebound from thick sediment loading
= The result of “upheaval.”
5 geological evidence for wegeners continental drift theory
< Fit of the continents
< Locations of past glaciations
< Location of past tropical regions (Paleoclimatic evidence)
< Distribution of fossils
< Matching geology between continents
explain the Locations of past glaciations evidence for wegeners continental drift theory
- Glaciers form mostly at high latitudes
- Past locations of glaciers -> past locations of continents
- At several times during Earth’s history, glaciers covered large areas of continents -> ice ages
what are glacial tills
a mixture of mud, sand, pebble, and larger rocks
- Permian glacial till is found on 5 continents - easily explained when continents where in Pangea
- The tills in Africa and India are now near equator
explain the Paleoclimatic Evidence for wegeners continental drift theory
- Placing Pangea over the Permian South Pole…
- He correctly predicted…
= Tropical coals
= Tropical reefs
= Subtropical deserts
= Subtropical evaporites
explain the Fossil evidence for wegeners continental drift theory
Identical fossils found on widely separated land
= Mesosaurus (A freshwater reptile)
= Glossopteris (Subpolar plant with heavy seeds)
= Lystrosaurus
= Cynognathus
explain the Matching geology for wegeners continental drift theory
Geologic phenomena match across the Atlantic
= Geologic structures
= Rock types
= Rock ages
= mountain belts (Appalachians)
Why wasn’t continental drift theory accepted
Wegener suggested that continents plowed through the ocean crust - wrong
- Cambridge Professor Sir Harold Jeffreys proved that Wegener’s mechanism was impossible
who is Arthur Holmes and his argument
- British geologist
- Did pioneering geochronologic work on the age of the Earth –> billions of years old
- In the 1930’s + 40’s argued for the existence of convection cells in the Earth’s mantle as a mechanism of continental drift
what did scientist find with Paleomagnetism
rocks can become magnetized in the direction of Earth’s magnetic field at the time of their formation - This magnetic signature can be preserved in rocks for millions of years, providing a record of past magnetic field changes
what kind of magnetic field does earth have
Dipole Magnetic Field: behaves like a giant bar magnet with north and south poles
- During periods of normal polarity, a compass needle points north
- angle of Earth’s magnetic field changes based on latitude (inclination):
0° at the equator
90° at the magnetic poles
what is inclination
the angle between the Earth’s surface and the magnetic field line - increases towards the magnetic pole
what is Magnetic declination
the angle between magnetic north and true north
what is curie point
Temperature above which a material loses all magnetic information - important as it forms a strong magnetic field
What is actually measured in paleomagnetism
- Inclination: Angle of the remnant magnetic vector with respect to horizontal (0-90 deg.)
- Declination: Angle of the remnant magnetic vector with respect to geographic - North (0-360 deg.)
- Allows the latitude (N-S) at which a rock formed to be calculated based on inclination
- Allows determination of the location of the magnetic pole
what was discovered in In early 1950s through a collection of rock samples from Britain
that Earth’s magnetic poles appeared to have shifted over geologic time -> polar wander
- Two Possible Explanations:
1. The magnetic poles moved relative to Britain
2. Britain moved (drifted) relative to Earth’s fixed magnetic poles
what was concluded from the discovery from rock samples from Britain
- Earth’s magnetic poles remain relatively fixed over time
- Continents move relative to each other, creating an “apparent polar wander path” in rock records
