Tectonics

Geography notes

Tectonics

EQ1

Seismic hazards are caused by rock stress and displacement; volcanic hazards are caused by weaknesses in the earth’s crust. Earthquakes occur when pressure exceeds the strength of the rock and it fractures. The sudden release creates seismic waves.

Earthquake zones are found along plate boundaries with 70% being in the Pacific Ring of Fire. Intra-plate earthquakes between plates can occur, oceanic fracture zones are along mid-ocean ridges, continental fracture zones are along mountain ranges and there are old fault lines. At intra-plate earthquakes, weaknesses in the plate occur due to its movement over a spherical object. Old rocks are still shifting. Hotspots are hot thermal plumes that rise from deep within the earth that melt rock to form magma which can rise through cracks.

Hotspot volcanoes are mid-plates fed by the underlying hot mantle plumes. These are hotter than the surrounding mantle. Hawaii is an example. They form in the middle of the Pacific Ocean. Magma rises as its lighter and erupts on the sea floor making the volcano grow until it emerges. The plume of rock pushes upwards; pressure drops and rocks become molten, melt and push through the crust. As plates move over the plume, lava creates a succession of new volcanoes migrating along with the plate. Alternative hot spot theory is that plates have scars from previous collisions and when these pass over they melt easily.

Fracture zones are a belt of activity. 50 volcanoes erupt annually and these are mostly along tectonic boundaries.

Divergent plate boundaries move apart from one another so magma rises through the cracks and shield volcanoes form. Friction can cause earthquakes as the plates slide past one another. These can create mid ocean ridges when 2 oceanic plates pull apart and earthquakes have a shallow focus and low magnitude. Volcanoes are less explosive, like along the Indian and Eurasian plate. Divergent boundaries make an underwater earthquake occurred. On land it can form rift valleys like the great East African rift valley as rising magma causes the continental crust to bulge and fracture so the lines drop down between these.

Convergent plate boundaries move towards one another and collide. The dense oceanic plate is subducted beneath the continental plate. Friction builds up and creates an earthquake when the plate slips or the plate melts as it is dragged down towards the hotter core where sea water makes the magma less dense so it erupts. Composite volcanoes can form which are dangerous and explosive. Earthquakes are common at the Benioff zone where plates get locked due to pressure and break away after the relief. An example is the Philippines and Pacific plate. Deep ocean trenches can form Fold Mountains and chains or submarine volcanoes if the continental plate pushes up. 2004 Indian Ocean tsunami was caused by 2 convergent oceanic plates and Nepal 2015 was 2 continental plates. When 2 oceanic plates meet an island arc forms like Montserrat or 2 continental plates meeting makes a fold mountain.

Conservative plate…