Lecture Twelve Flashcards
What do plate tectonics control and effect?
Controls -
Movement of the continents.
Distribution of the continents and oceans.
Where and why earthquakes occur.
Where and why volcanoes occur.
Where some mineral deposits occur.
Where and why mountain ranges are formed.
Affects:
Climate (including ice ages).
Evolution of life.
Sea-level changes.
What are the other theories explaining the effects of plate tectonics?
Hollow Earth Theory:
We live inside the Earth and all light is bent so it looks like we live on its outside.
Thought it explained the Earth magnetism and variation in the magnetic field.
Expanding Earth Theory:
Mid ocean ridges exist, but the subduction zones do not, therefore the Earth has expanded as it has aged.
This requires a mineral in the mantle that expands as it cools.
What evidence is there to support the face that the continents were once joined?
Animals and plants of the same species existing on different continents.
Rocks of same compositions and properties matching up to one another on the edges of continents.
What are mid ocean ridges and seafloor spreading?
Ocean floor -
Only a few hundred million years old.
Much younger than the continents.
Molten magma from volcanically active mid-ocean ridges.
Spreads sideways to create new sea floor.
Seafloor subducted back into the Earth’s mantle at ocean trenches within 200 million years.
Sea floor spreads from vents in rift, where hot magma oozes up.
Cooling magma forces the existing sea floor away from ridges on each side.
Old sea floor recycled at deep-sea trenches.
Explain magnetism in the ocean crust and continental crust.
Magnetism does not drive crust motion, but records the motion.
It can tell us -
The location of magnetic poles with respect to continents.
Location of sea floor over time and its age.
Some Igneous rocks preserve a record of ancient magnetic fields in oriented magnetic crystals.
Show the orientation of the Earth’s magnetic field at the time when the magma crystallises into rock or is deposited as an ash fall or deep sea sediment.
Explain the reversals of the Earth’s magnetic field.
The Earth’s magnetic field changes polarity though time.
Normal polarity = Earth magnetic field is oriented the same as it is today.
Reversed polarity = The North and South magnetic poles change.
There is no pattern or order to the time between changes in the magnetic field (unlike the sun).
What are sea floor stripes?
New sea floor records direction of magnetic poles at the time of formation.
The ages of the sea floor prove that spreading is occurring.
What is the plate tectonic theory?
1) The outer shell of the Earth is rigid = lithosphere.
2) The mantle behaves in a plastic manner and convects.
3) Lithosphere is broken into ~20 plates that move relative to one another. Rates around 1-18cm/year.
4) Interior of plate remain intact whereas the boundaries are strongly modified.
As the plates move they carry the continents with them.
5) As the plates move they carry the continents with them.
What are the characteristics of plate tectonics?
Large building blocks are -
Continents
Oceans
Lithospheric plates
Small building blocks are -
Hotspots (Hawaii)
Island arcs (Japan)
What are the three main types of plate boundaries?
Divergent plate boundaries:
Plates move apart, creating new ocean floor.
Convergent plate boundaries:
One plate with oceanic crust at the front collides with another plate with continental crust at the front. The denser oceanic crust subducts beneath the lighter one.
Transform plate boundaries:
Two plates slide past each other.
