Surface Features of The Alpine Fault New Zealand’s Alpine Fault How did it form?The Earth is composed of four layers (pictured below); theinner core, outer core, mantle and crust. The inner core, the inner most layerthat makes up the earths centre, is a sphere with a radius of 1,220 km, formedof mostly an iron-nickel alloy. Although its temperature is estimated to beclose to 5,400 C°, well above iron’s melting point of 1538 C° ornickel’s 1,455 C°, the inner core is solid, due to the immense pressure at thecentre of the earth keeping the core pressed together.
The outer core a spherethat surrounds the inner core, and is much larger than it at 2,300 km thick. Similarly,to the inner core, it is mostly made of iron and nickel. The only majordifference between the outer and inner cores is that the outer core is aliquid. The more relevant of earth’s layers to the formation offault lines are the mantle and crust. The mantle is the thickest of Earthslayers, at around 2,890 km thick. By far the largest of the Earth’s layers, itcomprises around 84% of the earth, compared to the inner and outer core whichmake up 15% of the Earth’s volume.
The composition of the mantle varies widely,with the only real definition of what makes up the mantle as rock. Most of therock in the mantle is silicate. Most of this rock is solid, or, closer to thecore, semi liquid, again because of the pressure on the rock solidifying therock despite it being over the melting point of most rocks.
During the formation of the earth, earth was constantlybeing hit by meteorites that contributed to its size. Because of the heatgenerated by the impacts, the earth’s surface was not solid, but rather a magmaocean. Once the accretion of new meteorites and planetoids by earth’s gravitationalfield began to slow, this magma ocean cooled, solidifying into a primordialcrust. Since then, that crust has been changed into a completely different’secondary’ crust. The primordial crust was slowly destroyed by erosion, or,more often, plate tectonics. The crust developed into two separate sections:oceanic crust and continental crust. Oceanic crust contains more mafic rocks. Themost common of these is basalt, and all mafic rocks are rich in iron and magnesium.
This is as opposed to the rocks more commonly found in the continental crust;felsic rocks. The most common felsic rock is granite, and felsic rocks are richin lighter elements like silicon and oxygen. Because of the relative density ofthe oceanic crust, this crust rests lower on the mantle than the continentalcrust; therefore, water pools in the basins (which the difference in heightbetween the types of crust forms) to create oceans.Although the upper section of the mantle is just as solid asthe crust, it is defined as separate because of the chemical and mineraldifferences between the composition of the mantle and the curst. However, forgeological purposes, these two ‘different’ layers are often classed as onecontinuous whole; the lithosphere. Underneath the lithosphere is the upper partof the mantle that, when under pressure, will flow and move rather than breakor deform. When rock is heated, it expands. This means that warmer rockis less dense.
Therefore, rock from low down in the mantle, where thetemperature is higher, rise as they develop buoyancy as they expand. At thesame time, rocks that have been in higher positions in the mantle slowly sink asthey cool, contract, and gain density. This process creates convection currents(pictured below), in which there is a constant flow of rock heating, rising,cooling, falling and heating again. Because all of the rock cannot be moving upwardsand downwards in the same place, convection cells of circular motion areformed. These slow-moving currents of solid rock not only provides much of the movementnecessary for the plate tectonic process, but also creates the rock cycle.
In the same way that older rock higher in the mantle which havehad more time to cool become denser and sink, so, when the earth was still new,older, colder sections of the crust slowly sank into the mantle. When thishappened, the surrounding crust was weakened. Over millions of years, this wasrepeated enough that the entire lithosphere has fractured into differentpieces. These pieces form the 8 major tectonic plates and the many minor plates.
The lines where plates meet are called fault linesThe entire lithosphere rests on the less rigid, slow-movingasthenosphere. This means that the tectonic plates slowly move in the directionthat the warmer rock beneath them is moving. Because of this, the way theconvection currents are moving dictates how the tectonic plates move. As shownin the picture above, the currents are usually the same; they move in circlesgoing the opposite direction to wherever the upwelling magma is. This meansthat at the point where magma rises to the surface, the plates also separate.This is why volcanoes are common at some fault lines, specifically the oneswhere plates are separating, but not fault lines where plates are colliding. Atthese fault lines, one plate must dive beneath the other, a process usuallyknown as subducting. The areas of the lithosphere which are subducting beneath anotherplate are known as subduction zones.
At fault lines with subduction zones,often deep trenches are formed, with mountain ranges behind them, caused by thelowering plate on one side, and the plate on the other side rising above it. Otherfault lines consist of more violent collisions between plates, resulting in thesubduction zone being sheared off of the rest of the plate. This results in thecolliding plates both lifting. This is what is currently happening at the boundarybetween the Indian and Asian plates, creating the Himalayas.-Uplift There are many more fault lines than tectonic plates, becausefault lines can occur in any place where rock is under stress, and each faultline is different, but there are broad categories that define faults. Firstly,faults can be divergent, convergent or transform. Divergent faults are createdwhere rocks spread apart from each other.
When this happens in the EarthsCrust, magma rises from beneath the crust, and usually forms volcanoes. Thiscreates new crust. To counteract the creation of new crust, This means that, when viewed with the fault line in avertical position, the right-handed plate will move toward the viewer.
The Alpine Fault consists of the meeting between the Pacificand Australian plates across almost the entire South Island of New Zealand. Itbegins slightly off the southwest corner of the South Island, runs along theWest Coast before splitting into several smaller faults in the northeast of theIsland. The Alpine Fault is a right-lateral strike-slip fault (pictured below).
In this case, the right-handed plate is the Pacific Plate, which is movingsouth while the Australian Plate moves north. This means that the Southern Alpsmove down the west coast at an average rate rate of about 3cm every year. Thismovement is not constant, however.
The Pacific and Australian Plates are mostlystuck in place, straining under the pressure of being forced toward and pasteach other. When the plates finally move past each other, they violently jumpforward, and cause earthquakes aboveground. These earthquakes happenapproximately once every three hundred years, where the plates move about 10metres past each other along the alpine fault.As well as moving across each other, the two plates are alsomoving towards each other.
North of theAlpine Fault the heavier, oceanic Pacific plate is subducting beneath theAustralian Plate. This causes the crust of the Australian Plate to thin out asit stretches over the Pacific Plate, and this makes it easier for magma topenetrate the lithosphere. This is the reason for the North Island’s numerousvolcanoes. Along the Alpine Fault the reverse happens, with the AustralianPlate subducting beneath the Pacific Plate. Most of the motion is transform,with the Pacific Plate moving South by the Australian Plate, however, and theAustralian Plate only subducts beneath the Pacific Plate along the Alpine faultslowly, especially in comparison to the speed it subducts beneath Fiordland tothe south. The Mt ArthurMount Arthur is part of the Arthur Range located in KahurangiNational Park. The entire region around Mt Arthur, known as the Tablelands.
Thetablelands are formed of limestone, while Mt Arthur itself is made almostentirely out of marble. Mt Arthur is well known for its deep cave systems and theglacial features near its summit that have formed basins and sinkholes all aroundthe peak. The Alpine Faultis about 100 km away from Mt Arthur, but Mt Arthur was still created by it.(More general Mt Arthur details) The reason that MtArthur is mostly marble can be attributed to the internal process known as therock cycle. The rock cycle shows how rocks transition between the three maintypes of rock; igneous, sedimentary and metamorphic. The most important fact ofthe rock cycle is that rock is constantly moving. Over long periods of time,rock that at one point was on the bottom of the seabed can be pushed lower intothe earth, and then move back up to form mountains. This movement is mainly caused by plate tectonics, inwhich subduction zones take rocks down, and seafloor spreading and upliftproduces new rock.
Rather than being explained as a continuous cycle, therock cycle can be described simply by how each type of rock is created. Sedimentaryrocks are caused by the processes of weathering and erosion. When rocks areexposed to the atmosphere, small grains, or sediment, are swept off or brokenaway from the main body of rock. In most cases, with sedimentary rock, thesegrains settle as sand on the seabed (although the same process can happen onoland, sand is the most common form of eroded grains). These grains are buriedbeneath other grains, and undergo lithification, in which the grains are fusedtogether by pressure, which creates solid rock, known as sedimentary rock. Igneousrock is created when rocks are pushed beneath the surface to the point thatthey become magma.
This magma then rises towards the surface in two differentways; extrusive and intrusive. Extrusive igneous rock rises quickly and is expelledas lava. It then quickly cools aboveground, into rocks such as obsidian orbasalt. Intrusive igneous rocks rise slower, and cool beneath the surface. Thisforms rocks that are less fine-grained, most commonly granite. Metamorphic rockis changed physically or chemically by high pressure or temperature, usuallydeep below ground. The extreme stress often re-crystallises the rock into adifferent mineral. An example is graphite changing into diamond under highpressure.
-Rock Cycle Mount Arthur itself is made from marble, which is a metamorphicrock. Marble is created when carbonate rocks are metamorphized. In the case ofMount Arthur, and in most cases, the rock marble formed from was limestone. Limestoneitself is a sedimentary rock. Unlike most other sedimentary rocks though,limestone was not formed from the compressed grains of other eroded rocks.
Limestone is made of calcite grains, which are usually from the skeletons ofmarine animals, especially coral. Over millions of years, the grains of coraland other skeletons that settled onto the seabed near New Zealand compressed and concreted intolimestone. This rock then underwent either high pressure or high temperatureand its grains recrystallized to form marble.
This marble uplifted from beneath theseabed to become a mountain.-Limestone/Marble FormationThe other major internal process that formed Mount Arthur ismountain formation.-Mountain Formation – Folding/Faulting-Uplift-Earthquakes? -Glacial cirques-Water——How were the caves formed? —– Franz Josef Glacier Franz Josef Glacier is located on the West Coast of theSouth Island. A glacier is made of snow that has settled and, over time,solidified into denser ice. Eventually, the ice becomes heavy enough that gravityslowly drags it downwards. At such a large thickness, rather than actingbrittle as ice usually does, the body of ice flows downwards plastically.
FranzJosef Glacier, specifically, is an alpine glacier. As opposed to a continentalglacier, which covers a large, relatively flat plain and usually empties intothe sea, alpine glaciers travel down valleys in the sides of mountains and endwell above sea level. Franz Josef Glacier itself is 12 km long, and rather thanemptying into a terminal lake, as some glaciers do, the meltwater from thebottom of the glacier empties as a river.Glaciers form through a combination of the right meteorologicaland geological conditions.
Alpine Glaciers are formed from snowfields near thetop of mountains, in which the snowfields do not melt over summer, and soaccumulate over time. Eventually, the snow is packed thick enough that itstarts to solidify into dense ice. This ice builds up and eventually overflowsthe area it has formed in. It then slowly moves down the side of the mountain.If a glacier is formed in a basin or a plateau, it is usually known as an icefield, often with glaciers flowing out of the overflow points.
A glacier isalso separate from an ice cap, which will form anywhere where the climateconditions are right. They are not bounded by geological features and so willform over and around mountains. There are three main conditions needed for glaciers to form.The first of these is cold temperature. Without extremely cold temperature, snowwill not fall, and any snow that does fall will melt too quickly to form aglacier. The temperature needed for glacier formation can only be found at highaltitude or latitudes close to the pole.
The second condition is consistent snowfall.Even in areas with cold temperatures, without moisture in the air, snow willnot fall and so glaciers cannot form. The final necessary condition is thatoutside factors do not remove the snow. This means that the snowfield must besheltered from the wind and not be in an unstable location prone to avalanches.Franz Josef Glacier travels down from a snow field in the mountainsaround Mt Cook. The glacier begins at 2700 metres above sea level and travelsdown until it melts at 240 metres above sea level. In the Southern Alps, thesnow line (the height below which snow will melt quickly) varies between 1600and 2700 metres above sea level. Snow would not have been able to collect withoutthe height caused by the Southern Alps, which were themselves caused by theAlpine Fault.
The weather needed for snow to fall is common in New Zealand.Th climate conditions needed for snow are simply cold temperature (caused bythe height of the Southern Alps) and moisture. In New Zealand, moisture is mostlybrought into the atmosphere by the westerly winds which blow commonly acrossthe country. These westerly winds, commonly known as the Roaring Forties,travel around the latitudes between 40 and 50 degrees, uninterrupted except forTasmania, New Zealand, and the lower edge of South America.
The Southern Alps aredirectly exposed to these winds.-A glacier is-Franz Josef is this kind of glacier-details of Franz Josef-Glaciers are caused by-Franz Josef was caused by –what you’re doing now – main point.