Hi I'm Darron Gedge. I'm standing here on the summit of Mount Ngauruhoe. This
mountain is 2291 meters. It is the third highest mountain in the North Island. This
stratovolcano 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 6360
kilometers below the surface where temperatures of over 5500
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 the
aesthenosphere, 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
principal tectonic plates and 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 Sima 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 sial because they contain
a lot of the elements of silica, and aluminum. Continental crust is lighter
than oceanic crust, though it 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 is a mega quake like that in Japan and 2011. The plates move in one of three
ways: Plates can move towards each other. We call this a convergent zone,
also they're known as destructive or collision zones. Plates also can move
away from each other and these are called divergent zones. The other type
of movement is a transform which is when the plates move along side 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 heavier
oceanic plate subducts 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 Kermadec trench lies
on the boundary of the Pacific and Indo- Australian plates, and folded mountain
ranges such as the Kaimanawas 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 of the Indian in 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 through 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 of 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 Rangitoto Island, One Tree Hill and Mount
Eden