New Zealand’s Hidden Truths: Using Maps and Topography to Understand the Landscape.
The geological history of New Zealand is dynamic and chaotic. It is part of a much larger and wider continental shelf called Zealandia, which slopes gently beneath the ocean to as low as 200 meters below sea level. Although submerged, this underwater continent has major variations in topography, and in some cases, the land has risen high enough to form islands.
New Zealand itself is a part of an archipelago of over 600 islands. The 2 major land masses of the north and south islands, are the most extensive, and these formed due to major geological processes that were, and still are, occurring beneath the country in a tectonic zone known as ‘the ring of fire.’
In this region, two major tectonic plates are crashing against each other. The Australian plate is on the north-west side, and this is pressing against the Pacific plate which lies to the south-east.
With the power of maps, we can uncover more about the islands geological past and understand the processes that are shaping the landscape, which we observe in topography.
New Zealand’s southern island straddles the Australian-Pacific fault line. The southern Alpes, which stretch 600 kilometers south to north, sit directly where these two plates are colliding. As the fault line moves further north, it splits eastwards in several directions, heading towards the Marlborough Fjordlands. The main fault line heads past Kaikoura and eventually out to sea (figure 2).
Figure2: The Pacific -Australian plate boundary. In the South Island the two plates compress against each other and the Pacific plate is uplifted above the Australian plate. Northwards the fault breaks off and moves out to sea, and here the Pacific plate is subducting below the Australian plate
As the Pacific plate and Australian plate push against each other, this causes compression and major earthquakes occur in the southern island. Ruptures do not just create sideways movement but also vertical, as the Pacific plate is slowly uplifting over the Australian plate. As a result, the Southern Alpes, which are now thousands of meters high, continue to rise today at a rate of approximately 1 to 2 meters per century, which is extremely fast on a geological timescale. The Pacific plate has uplifted 20km since mountain building began 23 million years ago, but the mountains today still sit below 4000 meters, due to the powers of erosion.
Topography shown from maps demonstrates the vastly destructive and creative effects this geologically young country is having on its surrounding landscape, as the changes in elevation are extensive.
Figure 3: The fault line, running through the Southern Alpes can be seen here, and its effects on the western coastline topography are apparent, as land on the western (top) side lies at a lower elevation compared to the ever-rising Southern Alpes on the east (bottom) side, which are continuously thrust upwards by the Pacific Plate.
The powers of tectonics crush, bend and shape the rock, but these processes are not the only forces at play in this region but are in fact highly interactive with external erosional processes, which are predominantly driven by water, ice and gravity.
Glaciers which have grown and receded several times over the past 2.5 million years (the Quaternary Period) have carved the many deep river valleys and lake basins that can be seen today, many of which are now national landmarks within the southern island.
At their most extensive, when New Zealand was 4.5C degrees colder, glaciers blanketed the entire Southern Alpes, from the Fjordlands in the south to the Marlborough Sounds in the north, and even as far as the Canterbury Plains in the east, where they deposited glacial outwash.
As snow accumulated and the ice sheets grew, the thickness and weight of glaciers would increase. Due to their sheer mass, glaciers acted like giant rivers of ice, sculpting and carving through the landscape. As they picked up rock debris they would cut through the mountains, carving deep U shaped valleys, known as moraines.
Some river valleys in the western coastline, stretching out to the ocean, are 1000s of meters high and extend to as deep as 400 meters below sea level in some areas. These massive steep valleys were formed by previous glaciers, which then later receded as sea level rose, infilling the valley, as can be seen at Milford sound, in Fjordland National Park.
Figure4: map showing the differences in elevation in Milford Sound. Notice the many purple shaded peaks of the steep-sided valleys.
Figure5: Steep-sided river valleys of Milford Sound. The main river inlet looking west towards the Tasmanian sea. The measured height on the southern slope (left side) here is 1235m. The river width where measured is approximately 3.2 km.
Ice has advanced and retreated rapidly many times during the past 2.5 million years, and the erosion of ice has continuously been sped up by the powers of water.
The Southern Alpes today, happen to sit in the path of the pacific westerlies, where massive amounts of water are precipitated over the mountains and dumped on the Southern Alpes every year.
Because uplift rates are so high, and because the mountains receive so much water (10 meters of rain or more per year) the powers of rivers and avalanches are merciless. As the mountains continue to uplift, rivers cut down through the rock, as steep waterfalls, which continually erode the mountain sides. Avalanches as well, frequently break off and tumble down the steep surfaces, taking rock debris with them.
This is why the landscape is so jagged in these mountains, as the erosive force of water is constant and powerful.
On the eastern side of the mountains, rivers have carried huge bed loads of gravel and sand down the sides of valleys.
Mixed bedload of sediments are moved through mountain passages and these have been transported down large rivers and gullies. In some cases, the alluvium has built up enough to form alluvial fans, which can be seen from satellite imagery.
Figure6: Mountain sediment being transported along rivers and mountain channels, which are slowly being moved eastwards.
Eventually, this mixed bedload of gravels and sand gets carried to lower elevations onto the floodplains in big rivers which have disbursed sediment across the eastern landscape in a large relatively flat area, which is now known as the Canterbury Plains.
Today the Canterbury Plains are a highly fertile land area, and extensively used for agricultural purposes.
Figure7: Major rivers flowing off the eastern slopes of the side of the mountains. Rivers combine and bring alluvium eastwards towards the flat plains of Canterbury where elevation flattens and deposits are disbursed.
The North Island is dominated by processes of volcanism. The central region stretching northwards is called the Taupo Volcanic Zone where numerous volcanoes and calderas have been formed. This is due to subduction in the Pacific Ocean east of the north island. Here the Pacific plate is sinking beneath the Australian plate. As this goes deeper into the underlying mantle, pressure builds underneath the Australian plate. Eventually, the crust melts, and due to heat and pressure, magma rises up and spews out onto the Earths surface.
This process is known as back-arc volcanism and the Northern Island has been heavily shaped by this process during the Quaternary Period.
Types of volcanism vary greatly in the North Island. There are several strata (conal shaped) volcanoes and 3 Crater Volcanos (Calderas).
Figure8: highlighted areas within the dotted zone represents the Taupo Volcanic Zone. Most of this area sits on an area of higher elevation due to the thick ignimbrite deposits that built up here following numerous eruptions. In the central and north regions lie lake Taupo and lake Rotorua, two Calderas that have both erupted violently in the past.
The strata volcano Ruapehu is the most active volcano in the north island and is also the tallest, standing at 2797m. Its crater lake, which usually sits below a layer of ice, on top of the mountain can get very hot. The volcano ejected blocks of andesitic lava during a previous 2007 eruption, which fell in violent mudflows down the mountain. 1.4 million cubic meters of rock and mud was deposited into the Whangaehu river on the eastern side.
Figure9: Larhar channels coming off the eastern slope of Mt Ruapehu, following the 2007 eruption.
Just north of Ruapehu lies Ngauruhoe and this is also highly active and is, in fact, the youngest volcano that is a cone of the Tongariro Mountain Complex. Looking at figure 10, it appears the scree slopes are steep and covered in materials from recent eruptions. The Tongariro Complex itself consists of Mt Tongariro and Nghauroe as well as other cones and craters, and there are numerous volcanic features such as hot springs, loose tephra and solidified lava flows, within the Tongariro Alpine region.
Figure10: Steep Scree slopes of Ngauruhoe, located north of Mt Ruapehu.
North of this Ngauruhoe and the Tongariro complex, lies the giant Lake Taupo. Spanning an area of 616 square Kilometers, this is the biggest lake in New Zealand by far.
Figure11: Lake Taupo total surface area found using the Google Earth Pro polygon tool.
Although covered by water today, Lake Taupo is in fact, a giant caldera that was formed by a super volcano, which erupted 27 thousand years ago.
This event, known as The Oruanui eruption, generated pyroclastic density currents, which can travel up to 700km per hour sideways, and this would have caused devastation across the surrounding landscape. There was also pyroclastic ashfall covering 430 square km during this eruption.
Ignimbrite deposits of up to 200 meters thick settled within the central northern area of the north island, although the eruption blanketed most of New Zealand. Even the Chatham island 1000km away received 18 centimeters of ashfall.
These highly explosive eruptions alongside strata volcano eruptions are responsible for much of New Zealand's topography and geology. The Taupo plateau is formed entirely of volcanic deposits and much of the North Island is dominated by volcanic rocks and loose sediments, as can be seen from the geological map (figure 12).
Figure12: Geological Map of New Zealand. The north island is dominated by volcanic sediments, whereas the southern island predominantly consists of Greywacke (hard sandstone) and Schists. Some much older Paleozoic deposits lie to the northwest in the Southern Island.
To the East of this Volcanic Zone lie the Axial Mountain ranges, which are composed of Greywacke (sandstone and mudstone). The fault line, which continued on from the Southern Alpes is also having an effect here and has produced rugged topography and uplift along this region and numerous mountain ranges, although these hardly ever extend above 1500m. The effects of uplift are far greater in the Southern Alpes on the South Island.
On the north-west side of the North Island lies Auckland and this sits directly in a volcanic lava field. The city has numerous hill cones where scoria has previously spilled. The figure below shows four major volcanos that have erupted in the past.
Figure13: Various cones of the Auckland Volcanic field. The field has 48 volcanoes in total. The google image shows 4 well-known volcanos, situated within the city. Digital elevation exaggeration is used to clearly show the location of each volcano.
In other areas of Auckland, eruptions blew out low lying craters such as Panmure Basin, which is now infilled as an estuary.
Unlike volcanoes within the Taupo Volcanic Zone, the volcanos within Auckland have only ever erupted once, and the field now lies dormant, although its possible that eruptions could occur again.
The take-home message
Using geological maps, satellite imagery, elevation models and measuring tools, we have been able to look at the geological research of New Zealand and link this to what we see when observing the landscape from above.
The power of maps is astonishing and essential when it comes to understanding the processes that shape the geological features of New Zealand, and why it looks the way it does.
New Zealand is extra special in this instance because the forces at play are constantly altering the landscape from such a diverse array of processes. New landforms are being formed and reshaped by volcanoes, earthquakes, uplift, and subduction, and yet just as quickly, water has continually destroyed these formations, and carved deep valleys and jagged terrain that can be see all over the country. New Zealand is in a constantly changing state of creation and destruction.
See more detailed topographic maps of New Zealand here