Lecture Two - Divergent boundaries or spreading centres (rifting and drifting) Flashcards
Define convergent, divergent and transorm plate boundaries.
Divergent - Plates move apart, creating new ocean floor.
Convergent - One plate with oceanic crust at the front collides with another plate with continental crust at the front. The denser oceanic crust subducts under the less dense continental crust.
Transform - Two plates slide past eachother - can creat a strike slip fault.
Describe a Wilson Cycle.
The lithosphere is thinned by the heat of the asthenophere underneath it.
Upwelling mantle (asthenosphere) has hot rocks coming up and pushing against the lithosphere.
This drives the plates apart, causing rifting.
Asthenospheric mantle melt causes volcanism.
The ocean can invade the rift craters, creating a mini ocean.
The continents split apart.
Oceanic lithosphere is created.
MOR is formed.
There are weakness zones in the part of the lithosphere between the land/ocean transition.
When the plates change direction and move back towards eachother, the oceanic plate subducts under the continental plate in this weakness zone.
The passive margin where the land met the sea becomes an active margin, where subduction occurs.
Volcanism can be caused due to subduction.
Mountain belts can form when the two continents hit eachother.
What is rifting and how does it occur?
Rifting occurs when continents separate.
The continent streaches and thins (extends) to form a rift valley at the surface, and hot asthenosphere rises from below (mantle upwelling) causing volcanism.
When necking of the asthenospher occurs, and then possible dyke swarms, this is when new oceanic crust is formedfrom MOR basalts.
What are sea floor stripes and how are they formed?
Sea floor stripes are formed at MORs.
When magma from the asthenosphere is deposited on the lithospheric surface, forming new oceanic crust, the magnetite minerals in the lava crystalise parallel to the direction of the magnetic field.
When the magnetic field changes, so to does the direction these megnetite crystals are aligned.
This leaves a record of the megnetic fields direciton on the sea floor which are symmetric stripes around the centre of the MOR.
What is happening in Iceland regarding the MOR?
There is a MOR cutting through Iceland, causing volcanism and the continent to slowly split apart.
There is also a large transverse fault line connecting the MOR.
What is the difference in depth between earthquakes which occur at fault lines and at subduction zones.
Earthquakes on fault lines are generally more shallow.
Earthquakes at subduction zones can be shallow to deep, depending on where in the subduction zone they occur.
How can the rifting that occured between a once single continent (now two seperate continents) be studied.
E.g. The rifting that would have occured between Australia and Antarctica.
By studying the now passive margins of the Australian and Antarctic coasts.
These passive margins would have once been part of the rift zones formed when the two continents separated from one another.
What is a stable shield?
A very old continental region with very little geological/seismic activity. Such as Western Australia.
