Rift Valleys - Constructive
...the major example of this is the East Africa Rift Valley...
Extreme heat and pressure forces up a magma plume, which causes the crust to uplift, extend and buckel. The crust faults and some sections would collapse back down, creating horst and graben.
Sometimes magma comes through the surface and form volcanoes. Water from the horst often falls and collects in Graben to form large akes
Mid-Oceanic Ridges:- constructive
...The Mid-Atlantic Ridge...
Convection currents:- currents create drag within the magma which creates drag, that then pulls the plates apart.
Transform Faults:- different parts of the plate move at this causes faults between thse two areas
Speed of Divergence:
- A slow rate of divergence creates a wide ridgeg, deep and well defined rift valley with inward facing fault scarps
- Intermediate rate- less defined ridge, smooth outline
- Rapid rate - smooth crest with no rift
Oceanic-Oceanic Convergence:- Destructive
...ie the Mariana Volcanic Islands and Trench where the pacific subducts under the philippine plate
Where oceanic crusts meet, the denser or faster moving is subducted under the other plate. The downward warping forms a trench
Subduction increases the pressure so can trigger earthquakes where the plates meet
As the oceanic crust descends, the heat and friction cause it to melt in the Benioff zone. This creates plutons of magma that rise to the surface through faults and weaknesses in the crust.
These plutons create andesitic lava so create stratovolcanoes which are explosive. The volcanoes erupt under water but may grow to be above sea level and create an island arc.
Oceanic-Continental Convergence: Destructive
...the Andes - Nazca and South America Plates converge...
When oceanic and continental crust meet, the denser oceanic crust is subducted beneath the continental plate. The downward warping of the oceanic crust forms a trench.
The subduction causes an increase in pressure which triggerss earthquakes where the plates meet.
Sediments that have accumulated on the continent are deformed by folding and uplift to form mountain ranges
The oceanic crust moving into the mantle becomes hotter and the crust melts in an area called the Benioff Zone. This molten material is the plutons of magma and rises to surface through fractures in the crust.
They create andesitic lava and compostive volcanoes as well as Island Arcs if they occur offshore - eruptions tend to be explosive
Continental-Continental Convergence: Destructive
Convection currents in the Asthenosphere cause two continental plates to collide as they have similar densities and are lighter than the underlying layers.
As they collide, their leading edges and layers of sediment are compressed and folded up to form ranges of fold mountains and high plateaus - layers forced downwards create deep mountain roots in the lithosphere.
No subduction means no volcanic activity but the plate movement triggers powerful earthquakes...ie the Himalayas as the Indian and Eurasian plates collide
...where two crustal plates slide past each other and the movement of the plates is parallel to the plate margin, there is no creation or destruction of crust.
As the margins are passive, there is no subduction and therefore no volcanic activty
The movement of the plates, however, creates stress between the plates edges and, as sections of the plates rub past each other, the release triggers shallow-focus earthquakes
The best known example is the San Andreas Fault in California, whre the pacific and north american plates more parallel to each other. Both plates are moving in the same direction but not at the same speed. Stresses set up by this movement cause transform faults to develop, running at right angles to the main fault line.
...vulcanicity is normally associated with plate margins but, in the centre of the Pacific Ocean, we find the volcanic Hawaiian Islands that are not connected with any plate boundary.
It is believed that this volcanic area is caused by a localised hot spot within the pacific plate. A concentration of radioactive elements inside the mantle may cause a hot spot to develop. From this a plume of magma rises to eat into the plate above - where lava breaks through to the surface, active volcanoes occur above the hot spot.
The hot spot is stationary, so as the pacific plate moves over it, a line of volcanoes is created. The one above the hot spot is active and the rest fomr a chain of islands of extinct volcanoes. The oldest volcanoes have put so much pressure on the crust that subsidence has occured.
This, together with marine erosion, has reduced these old volcanoes to sea mounts below the level of the ocean. The line of Hawaiian Islands and ages are evidence that the pacific plate is moving northwest. This is furtherproof that the Earth's crust is moving as suggested by Wegener