Chapter 12 Flashcards
Paleomagnetism (fossil magnetism)
Rocks become magnetized as they solidify.
Magnetite grains will align parallel to Earth’s magnetic field as it cools. Comparing past vs present magnetization directions shows that either the pole has moved or the continent has moved.
Past Glaciations
Motion of ice sheets formed striations (scratches) that showed the direction of ice flow. Rocks called tillites are left by past ice ages and their locations showed that past glaciers were located in now tropical places.
Earth’s magnetic field
Dipole with a small offset between the magnetic and geographic poles. It is vertical at the poles and horizontal at the Equator.
Seafloor Spreading hypothesis
In the 1960s, Harry Hess proposed that plates moved apart at ridges. Upwelling causes an eruption of lava that creates a new seafloor, and trenches were where material sank back down into the interior. This cycle is mantle convection.
Geomagnetic reversals
The polarity of the earth changes from north to south, and back again, with the last one being 780,000 years ago.
Oceania-continent plate boundary
Convergent, denser oceanic plate sinks beneath the continent plate.
Oceanic-oceanic plate boundary
Convergent, one plate subducts and a volcanic island arc forms
Continent-continent plate boundary
Convergent, begins as an oceanic–continental boundary. Both plates are low density and can’t subduct and form a thickened crust and major mountain belt.
Transform plate boundaries in oceans
Join segments of mid ocean ridges
Plate tectonic model
Earthquake distribution outlines plate boundaries and epicenters map down going slabs as Wadati-Benioff zones.
Plate motion method
Driven by mantle convection as
energy comes from heat escaping from the interior of the Earth.
Plate motion mechanisms (3)
- Mantle traction/drag
- Ridge push/sliding
- Slab pull
