hi i'm darren gage i'm standing here on the summit of mount narahoe this mountain is 2291 metres it is the third highest mountain in the north island this strato volcano is actually part of the tongariro complex and last erupted in 1975 behind me here is its crater you can see sometimes the sulfur on the crater walls to explain how volcanoes like this form it is first necessary to have an understanding of the underlying processes that take place deep inside the earth the center of the earth is a very hot metallic core containing iron and nickel much of this information is based on the study of meteorites the earth's core is six thousand three hundred and sixty kilometers below the surface where temperatures of over five thousand five hundred degrees celsius seismic waves do not pass through this layer and that is how scientists have inferred that it is a liquid surrounding the core is the mantle which is about 2890 meters below the surface the mantle is composed mainly of silicate rocks which contain a lot of magnesium and iron in the outer part of the mantle called vestenosphere semi-molten rock moves slowly in convection currents convection currents are generated by heat upwelling from the core the thick hot magma rolls up to the surface and sinks again as it cools above the asthenosphere is the lithosphere or crust which is made up of seven principle tectonic plates in numerous other smaller plates these plates float on the mantle due to the action of convection currents on the boundaries of where plates meet huge forces mean that they can form features such as volcanoes folded mountains and deep ocean trenches earthquakes also take place here a well-known pattern of volcanic and seismic activity is called the pacific ring of fire there are two types of crustal rock that lie on the plates oceanic crust is mostly basalt rock covered with layers of sediments these dark heavy rocks are called sema because they contain a lot of silica and magnesium the oceanic crust is a thin five to ten kilometers thick the continental crust is the layer of granitic sedimentary and metamorphic rocks which form the continents in the continental shelves these rocks of the continental crust are referred to as soil because they contain a lot of the elements of silica and aluminium continental crust is lighter than oceanic crust vote is considerably thicker on average between 35 to 40 kilometers but it can be over 70 kilometers thick under some mountain ranges convection currents in the mantle cause the earth's plates to all move slowly usually by as little as one or two centimeters a year except of course when there's a mega quake like that in japan in 2011. the plates move in one of three waves plates can move towards each other we call this a convergent or our zone also be known as destructive or collision zones plates also can move away from each other and these are called divergent zones and the other type of movement is a transform which is when the plates move alongside each other convergent margins occur between plates which are moving towards each other this can form a subduction zone or a continental collision a subduction zone is formed when an oceanic and a continental plate collide the heavy oceanic plates of ducks or slides below the continental plate and is eventually destroyed new zealand is located close to a major subduction zone to the east of the north island the pacific plate is sliding under the indo-australian plate causing volcanic activity in the north island during subduction the two plates grind together and friction generates heat along the collision zone at shallow depths the rocks of the continental crust begin to melt these molten rocks collect as pockets of magma magma may return to the surface during a volcanic eruption deep marine trenches and fold mountain ranges are typically associated with subduction zones in new zealand this occurs about 140 to 180 kilometers to the west of a plate boundary between white island and mount taranaki in keeping with typical subduction zone characteristics the kumudik trench lies on the boundary of the pacific and indo-australian plates and folded mountain ranges such as the co-manoas are also present a continental collision is formed when two plates containing continental crust collide this can result in the formation of mountain ranges such as the himalayas which result from the collision off the indian and the eurasian plate divergent margins occur between plates which are moving apart magma rises from the mantle to fill the gap between the two plates this forms a mid-ocean ridge mid-ocean ridges typically consist of chains of underwater volcanoes new oceanic crust is formed from volcanic activity and then gradually moves away from the ridge this process is called seafloor spreading the mid-atlantic ridge is formed where the north american plate is moving away from the eurasian plate this has created a long chain of subterranean mountains as well as the volcanically active iceland transform margins occur where plates slide past each other along transform faults neither creating nor destroying land this has happened in new zealand where northwest nelson has become separated from southern new zealand along the alpine fault the main effects of a conservative plate boundary are earthquakes which can be fairly violent and frequent sometimes volcanic activity is not linked to plate margins as in the case of the auckland volcanic field this is believed to be due to the presence of hot spots places are localized heat under the earth's crust that then finds their way to the surface a mantle hot spot exists about 100 kilometers below auckland the auckland volcanic field has about 60 volcanoes including iconic landmarks such as rangitota island one tree hill and mount eden [Music] [Music] good