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| Outcrops from the North Island of New Zealand, displaying discrete layers of geology called strata. Each outcrop has a different story to tell through the stratigraphy, providing clues to past environments; depositional environment; climactic conditions; changing sea-levels; and a record of steady long-term geomorphic processes interspersed with occasional catastrophic natural events. Left: Coastal cliffs at Waihi Beach, South Taranaki. Source: Phil Pollock (2018); Centre: Coastal cliffs at Waiotahe Beach, Eastern Bay of Plenty. Source: Ilmars Gravis (2018); Right: Coastal cliffs at Otarawairere Beach, Ōhope. Source: Ilmars Gravis (2018). |
Aotearoa Rocks welcomes today's guest writer Phil Pollock. Phil holds a bachelors degree in Earth Science and is passionate about all things geology. Based in New Plymouth under the shadow of Mt Taranaki/Egmont Volcano, he has a particular interest in fossils and the evolution of marine life in Zealandia. He can often be found in the ‘middle of no-where’ hunting and collecting fossils or rock samples to add to his collection (much to the annoyance of his wife). As a long-time resident of Taranaki, Phil has had extensive opportunities to explore this dynamic and active landscape. His previous post introduced us to the giant fossil crabs of North Taranki. In this post Phil gives us an insight into the dynamic geology of the Hawera area, and the stories of a constantly changing landscape and catastrophic events told in the outcrops of this area. Read on for a riveting geological detective story that will take you from 4 million years ago to the present.....
A geologist is often asked “how do you know that?” when interpreting geological features, landscape forming processes, or the age of geological features. In fact, when acting in a professional capacity it is important we are able to justify our interpretations and conclusions. In this post I will explore how a geologist reaches their conclusions, and show how geology is very much an observational science. Clues in the rocks allow a geologist to piece together events that have played out over great spans of time, as well as environments in which they took place. In utilizing a few basic key principles, the geologist can note these clues and gather evidence much like a detective at the scene of a crime, and deduce the sequence of events that have taken place over millions of years.
To demonstrate some of these concepts, and "laws of geology" we'll assume the role of geological detectives and examine a coastal cliff located near the South Taranaki township of Hawera.This particular cliff begins its story on a calm sea floor four million years ago, and ends with a series of distant mega-violent events before recording the enhanced erosion of the last glacial maximum 20 000 years ago. Let’s look more closely and begin at the bottom.
The rocks we observe at the bottom are the oldest. This may seem obvious and this is known in geology as the ‘law of superposition’. This law states that the rocks at the bottom of a sequence are the oldest and rocks at the top are the youngest. However it is important to realise that this law only applies in normal circumstances. That is to say, only rocks that have been laid down sequentially and have not been affected by tectonic forces that can thrust older strata over younger rocks.
These rocks are known as the Ohawe Sandstones. They are a grey silty sandstone, rich in fossils. The fossils are often found whole and in their life positions. The species and condition of fossils, as well as the sediments they are in, provide clues to the depositional environment. Because the fossils are whole and there is a silt content to the sandstone we can assume that deposition took place in a relatively low energy environment such as the middle of a sheltered bay.
The time of deposition can also be determined by both macro and micro fossil assemblages. This technique is an example of ‘relative dating’. For example, the fossil Mesopeplum crawfordi pictured above, appears in the fossil record 5.3Ma and disappears at 3Ma. Therefore, the sediment must have been laid down somewhere in between those times. Indeed we know that the Ohawe Sandstone is around 4 million years old and belongs to a period of time called the Pliocene Epoch
At the top of the Ohawe Sandstone we observe fossil shells belonging to Barnea similis (a type of burrowing barnacle). However, these are actually from the younger strata found above and they have bored into the older rocks. They are therefore younger than all other fossils found in the Ohawe Sandstone. As it turns out, much younger. Let’s move up the cliff a little.
Sitting on top of the Ohawe Sandstone is a shell-bed approximately half a metre thick containing shells in a friable sandy matrix. The Rapanui shell-bed is where our burrowing barnacle friends lived. The fossils and the coarse-grained matrix again give clues to the environment and time of deposition. The sandy matrix and the disarticulated and jumbled shells tell us that this was a high energy environment such as a tidal zone. But hang on, the fossil assemblage is modern! In fact, we know that the Rapanui shell bed is only 120 000 years old for reasons that will be explained later. That means we seem to be missing four million years of time!
The rocks that we would expect be there were either not deposited in the first place, or far more likely, have been eroded away. This develops a feature known as an ‘unconformity’. Because the Ohawe Sandstone is dipping slightly to the southeast and the overlying beds are more or less horizontal, it is more correctly termed an ‘angular unconformity’. The Rapanui shell bed atop of the unconformity also marks the beginning of another important geological feature: the bottom of a marine terrace.
Gradual rates of uplift in this part of the country, coupled with rising and falling sea levels during the ice ages, has produced a series of geomorphological features known as ‘marine terraces’. The Rapanui shell bed marks the base of the Rapanui Marine Terrace, one of the youngest in the sequence.
Marine terraces form at times of high sea level during inter-glacials, such as in the warm period that the Earth is now experiencing. During these warm periods, high sea levels cut a flat platform in existing rock and if that surface is then uplifted in the following glacial period with its associated low sea levels, the platform is preserved as a marine terrace. The sequence of successively higher marine terraces found inland of the coast in this region of New Zealand preserves a record of uplift over the last half million years. The terrestrial sediments (sands, volcanic deposits, loess, and soils) that form on top of marine terraces are known as cover beds.
Directly above the Rapanui shell bed the cover beds begin with a sequence of thin gravels inter-bedded with thicker beds of sands. At the bottom of the sequence the gravels are small and rounded and are composed of greywacke and argillite. Their shape tells us they have been transported fluvially by streams and/or longshore drift. Some may have been deposited by storm surges. The horizontal beds of sands in between the gravel beds are also evidence of this. Occasional layers of sands also begin to show cross-bedding which is indicative of the formation of sand dunes. Slightly higher above this layer the lithology of the gravels change to andesite. Here is our first evidence of large eruptions from Mount Taranaki.
Andesite is the most common type of lava erupted from subduction related volcanoes, and its has a distinct diagnostic mineralogy. The most abundant minerals are plagioclase feldspars, pyroxenes, and amphiboles (hornblende) with smaller amounts of trace minerals. Mount Taranaki andesite has a higher concentration of hornblende than the andesite volcanoes found in the Taupo Volcanic Zone (Ruapehu, Tongariro etc.) which makes it easily identifiable.
Also making an appearance in the gravel and sand beds are thin beds of tephra (any type of volcanic air-fall deposit) and volcanic ash. They typically over-lie organic matter that represent vegetation buried by ash fall. We can date the formation of the cover beds by means of several different methods of radiometric dating. These methods provide dates that are more accurate than those provided by relative dating previously described above. They are therefore known as ‘absolute dating’ techniques.
Radiometric dating relies on the decay of atomic nuclei. Some atomic nuclei are less stable than others and these nuclei break apart to form more stable ones. This process is what we know as radioactivity. By this means a particular isotope of an element can change into another by a series of radioactive decay stages, gaining or losing particles and thereby changing their atomic mass and/or number as they go. The atomic number is the number of protons in an atomic nuclei and it is this that defines a given element. The atomic mass is the sum of protons and neutrons in an atom, so if neutrons increase or decrease, but the number of protons remain constant then a new isotope is formed.
The original unstable isotope is called the parent product, and the decayed isotope is known as a daughter product. The time taken for half of the atomic nuclei in a given sample to decay is known as the half-life. Since rates of decay and therefore half-lives are well known and constant, the ratio of parent and daughter products can then be measured to give an accurate age of the formation of the original parent product. Some of the most common decay series include uranium to lead, potassium to argon, and rubidium to strontium. Carbon dating is another form of radiometric dating but is restricted strictly to forms of carbon isotope. It is vitally important to use the correct form of radiometric dating for a given sample and to account for any possible contamination of that sample. This is especially important in regards to carbon dating.
So, by applying several forms of radiometric dating to closely overlying tephras, along with other dating methods (both relative and absolute) demonstrating a correlation, we know that the Rapanui Marine Terrace formed around 120 000 years ago. Let’s continue up the cliff a little more as we continue to apply our geological detective techniques to reading these rocks.
As we continue up the gravel/sand beds we observe more frequent layers of tephra interspersed between the marine beds, reflecting more frequent eruption events. Taranaki Volcano is capable of very large eruptions, demonstrated by the occurence of at least 47 Taranaki derived tephras detected as far away as Auckland. Tephras deposited at that distance from the eruptiuon source would only be possible with towering eruption columns that would have reached well in excess of 40 km in height. However, these eruptions are not the most dangerous hazard posed by this still active volcano. Evidence for what that hazard is lies directly above the sand beds.
Above the sands we find a two meter thick bed of broken andesite known as a breccia. The term breccia is used to refer to angular fragments of rock that have not been subject to erosion removing the sharp angular edges.The clasts in this breccia range in size from 20 cm to over one metre in size. This is separated from a similar two meter thick bed by a thin very fine grained deposit that has distinctive flow lines. Above the second brecciated bed is another thick fine grained bed that fines up toward the top. Within this layer are rip-up clasts of country rock such as mudstone and the remains of trees. The top of this deposit is the same as the bed that separates the two breccias. What could be the explanation for these deposits?
Andesite volcanoes are formed by layers of lava flows, lahar flows, pyroclastic flows, and ash, creating an inherently unstable structure known as a composite cone or stratovolcano. Over time weathering will effect the components of theses layers, and increasing steepness of the cone as it builds up may utilmately lead to partial collapse, with or without an eruption event. These produce what is known as a debris avalanche flow. In fact, most of the Taranaki peninsula was formed by these events from Taranaki Volcano, and the earlier forming cones of Pouakai and Kaitake volcanoes.
To date approximately 14 debris avalanche events on the Taranaki ring plain have been defined, with the vast majority of these originating from Mt. Taranaki itself. This equates to roughly one event every 14 000 years. Flows from Pouakai and Kaitake cones have mostly been covered by the Taranaki flows and are therefore largely undetectable. These flows are catastrophic and truly devastating events, with debris flow deposits reaching the coast at some point on the ring plain. The next one to occur could possibly have the very real potential of wiping entire townships from the map. Given the eroded state of the current cone coupled with its steepness, the potential for a catastrophic collapse event is very real.
The deposits we observe in our outcrop are the result of two very closely spaced collapse events. The bottom flow and overlying thin bed are known as the Waingongoro Formation while the overlying beds belong to the Waihi Formation. (The term "Formation" refers to a unit of rock, or combinations of rocks that share characteristics or mode of origin). The brecciated beds represent the basal section of the flows while the finer deposits represent the top and runout sections of the flows (sometimes called hyperconcentrated flow deposits). The Waihi event appears to be the larger of the two events. However, an alternative explanation for its thickness may be that the Waingongoro event filled in depressions in the landscape and therefore facilitated the flow of the Waihi event so that more of it made it to the coast. The flows are thought to have been emplaced at sometime around 70 and 60 thousand years ago respectively. Let’s continue up, we are nearing the top.
As we continue towards the top of our outcrop we observe another thin section of sands directly above the Waihi Formation, which grades into a reddish brown soil. This is loess (pronounced lurse, derived from the German word Löss), and it represents wind-blown dust that accumulated during the last glacial maximum 20 000 years ago. During glacial periods sea level falls dramatically, the climate is colder and dryer, and there is less vegetation covering the land. Erosion is enhanced and great quantities of dust are moved about the landscape by the increased winds of the period, in time forming thick deposits blanketing the landscape. In other parts of the country beds of loess are separated by layers of darker soils that formed during warm periods (interglacials).
The loess then grades into a rich dark soil that is known as the Egmont black loam. This soil belongs to a group that are classed as allophanic soils. Allophanic soils are derived from volcanic parent materials and their rich mineral content make them prime soils for pasture growth. This is why the Taranaki area is one of the principle dairy regions of New Zealand.
Finally, our geological detective story ends at the top of the cliff. By observing the clues left in the rocks we now have a very good picture of not only how these cliffs formed, but also the timing and environments in which they formed. As you can see, geology requires observation and interpretation. The more clues that can be gathered, the more accurate that interpretation will be. However, like all science, new techniques and improving technology will allow us to refine those interpretations further over time and to plan for future hazards.
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| Views of coastal cliffs at Waihi Beach, Hawera. The strata consists of Pliocene marine deposits with overlying Quaternary terrestrial and volcanic deposits spanning the last 4 million years. Source: Phil Pollock (2018). |
These rocks are known as the Ohawe Sandstones. They are a grey silty sandstone, rich in fossils. The fossils are often found whole and in their life positions. The species and condition of fossils, as well as the sediments they are in, provide clues to the depositional environment. Because the fossils are whole and there is a silt content to the sandstone we can assume that deposition took place in a relatively low energy environment such as the middle of a sheltered bay.
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| Well preserved macro-fossils typical of the Ohawe Sandstone. Left to right: Crassotrea ingens (oyster); Mesopeplum crawfordi (scallop); and Alcithoe haweraensis (gastropod). Source: Phil Pollock (2018). |
The time of deposition can also be determined by both macro and micro fossil assemblages. This technique is an example of ‘relative dating’. For example, the fossil Mesopeplum crawfordi pictured above, appears in the fossil record 5.3Ma and disappears at 3Ma. Therefore, the sediment must have been laid down somewhere in between those times. Indeed we know that the Ohawe Sandstone is around 4 million years old and belongs to a period of time called the Pliocene Epoch
At the top of the Ohawe Sandstone we observe fossil shells belonging to Barnea similis (a type of burrowing barnacle). However, these are actually from the younger strata found above and they have bored into the older rocks. They are therefore younger than all other fossils found in the Ohawe Sandstone. As it turns out, much younger. Let’s move up the cliff a little.
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| Above: Barnea similis (a type of burrowing barnacle) visible in the top layers of the Ohawe Sandstone. Below: Detail from above, showing associated burrows preserved in the Ohawe Sandstone. These fossil shellfish are in fact much younger than the sandstone they are found in, as they have burrowed down from the younger strata above. Source: Phil Pollock (2018). |
Sitting on top of the Ohawe Sandstone is a shell-bed approximately half a metre thick containing shells in a friable sandy matrix. The Rapanui shell-bed is where our burrowing barnacle friends lived. The fossils and the coarse-grained matrix again give clues to the environment and time of deposition. The sandy matrix and the disarticulated and jumbled shells tell us that this was a high energy environment such as a tidal zone. But hang on, the fossil assemblage is modern! In fact, we know that the Rapanui shell bed is only 120 000 years old for reasons that will be explained later. That means we seem to be missing four million years of time!
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| The Rapanui shell-bed. This approximately half-metre thick shell bed, laid down 120,000 years ago is found lying directly atop the 4 million-year-old Ohawe Sandstone, forming an unconformity. The disarticulated and jumbled shells within a coarse sandy matrix indicate a high-energy depositional environment. Source: Phil Pollock (2018). |
The rocks that we would expect be there were either not deposited in the first place, or far more likely, have been eroded away. This develops a feature known as an ‘unconformity’. Because the Ohawe Sandstone is dipping slightly to the southeast and the overlying beds are more or less horizontal, it is more correctly termed an ‘angular unconformity’. The Rapanui shell bed atop of the unconformity also marks the beginning of another important geological feature: the bottom of a marine terrace.
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| Three common types of unconformity, all representing a significant time gap in the geological record. Left to right: A disconformity arises when erosion but no tilting of the older layer occurs before deposition of sediments. An angular unconformity is similar, but tilting, faulting or folding of the older beds has taken place before resumption of sedimentation. A nonconformity is created when igneous or metamorphic rock has been uplifted and eroded before sedimentation has resumed. Source: www.typesof.com (2018). |
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| An example of an angular unconformity formed by near-horizontal marine and volcanic sediments (5 MYA) laid down over near-vertical beds of Torlesse greywacke and argillite basement rocks at Otarawairere beach at Ōhope in the Eastern Bay of Plenty. Source: Ilmars Gravis (2018). |
Gradual rates of uplift in this part of the country, coupled with rising and falling sea levels during the ice ages, has produced a series of geomorphological features known as ‘marine terraces’. The Rapanui shell bed marks the base of the Rapanui Marine Terrace, one of the youngest in the sequence.
Marine terraces form at times of high sea level during inter-glacials, such as in the warm period that the Earth is now experiencing. During these warm periods, high sea levels cut a flat platform in existing rock and if that surface is then uplifted in the following glacial period with its associated low sea levels, the platform is preserved as a marine terrace. The sequence of successively higher marine terraces found inland of the coast in this region of New Zealand preserves a record of uplift over the last half million years. The terrestrial sediments (sands, volcanic deposits, loess, and soils) that form on top of marine terraces are known as cover beds.
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| Example of marine terraces that can be that can be found stretching inland between between Manawatu and Taranaki. The oldest terraces are the highest in the landscape due to gradual uplift over the last 500,000 years. Uplift can also be sudden and immediate due to massive tectonic events. For instance, the recent earthquake in Kaikoura caused uplift of approximately two metres at the coast. This view also shows how terraces are eroded and cut by rivers and streams. Source: Ballance (2009). |
Directly above the Rapanui shell bed the cover beds begin with a sequence of thin gravels inter-bedded with thicker beds of sands. At the bottom of the sequence the gravels are small and rounded and are composed of greywacke and argillite. Their shape tells us they have been transported fluvially by streams and/or longshore drift. Some may have been deposited by storm surges. The horizontal beds of sands in between the gravel beds are also evidence of this. Occasional layers of sands also begin to show cross-bedding which is indicative of the formation of sand dunes. Slightly higher above this layer the lithology of the gravels change to andesite. Here is our first evidence of large eruptions from Mount Taranaki.
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| Cross bedded sands interbedded with coarse sandy gravel beds. Cross bedding is associated with dune formation. Source: Phil Pollock (2018). |
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| Gravel and sand beds above the Rapanui shell-bed. The small rounded clasts consist of greywacke and argillite (basement rocks) with minor andesite (volcanic rocks). The rounded shape of the clasts tell us they have been transported by water. Andesite increases and greywacke/argillite decreases as we move up the sequence. Source: Phil Pollock (2018). |
Andesite is the most common type of lava erupted from subduction related volcanoes, and its has a distinct diagnostic mineralogy. The most abundant minerals are plagioclase feldspars, pyroxenes, and amphiboles (hornblende) with smaller amounts of trace minerals. Mount Taranaki andesite has a higher concentration of hornblende than the andesite volcanoes found in the Taupo Volcanic Zone (Ruapehu, Tongariro etc.) which makes it easily identifiable.
Also making an appearance in the gravel and sand beds are thin beds of tephra (any type of volcanic air-fall deposit) and volcanic ash. They typically over-lie organic matter that represent vegetation buried by ash fall. We can date the formation of the cover beds by means of several different methods of radiometric dating. These methods provide dates that are more accurate than those provided by relative dating previously described above. They are therefore known as ‘absolute dating’ techniques.
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| Two closely spaced layers of tephra. The brown material under each tephra is organic matter from decayed vegetation (lignite). Source: Phil Pollock (2018). |
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| Microscope detail of one of the above tephra. Plagioclase feldspars are visible as light coloured minerals, darker minerals are amphiboles (hornblende). These are typical minerals found in deposits from andesitic subduction-related volcanism. Source: Phil Pollock (2018). |
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| Three different tephras showing variations in colour and grain size, providing clues about chemistry of magma and eruptive conditions Left: Rhyollitic pumice, light in colour and weight due to high silica content of magma.Air bubbles in pumice indicate a gaseous explosive eruption in the Taupō Volcanic Field; Centre: Very dark scoria, with a high iron and magnesium content and low content of silicates. From Auckland Volcanic Field. Basaltic chemistry and air bubbles in scoria are typical of effusive eruptions of the AVF; Right: Andesitic tephra formed from a combination of andesitic ash and pumice. Colours and combination of ash and larger fragments are typical of an andesitic eruption in the Tongariro Volcanic Centre. Source: Ilmars Gravis (2018). |
Radiometric dating relies on the decay of atomic nuclei. Some atomic nuclei are less stable than others and these nuclei break apart to form more stable ones. This process is what we know as radioactivity. By this means a particular isotope of an element can change into another by a series of radioactive decay stages, gaining or losing particles and thereby changing their atomic mass and/or number as they go. The atomic number is the number of protons in an atomic nuclei and it is this that defines a given element. The atomic mass is the sum of protons and neutrons in an atom, so if neutrons increase or decrease, but the number of protons remain constant then a new isotope is formed.
The original unstable isotope is called the parent product, and the decayed isotope is known as a daughter product. The time taken for half of the atomic nuclei in a given sample to decay is known as the half-life. Since rates of decay and therefore half-lives are well known and constant, the ratio of parent and daughter products can then be measured to give an accurate age of the formation of the original parent product. Some of the most common decay series include uranium to lead, potassium to argon, and rubidium to strontium. Carbon dating is another form of radiometric dating but is restricted strictly to forms of carbon isotope. It is vitally important to use the correct form of radiometric dating for a given sample and to account for any possible contamination of that sample. This is especially important in regards to carbon dating.
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| Example of radioactive decay series, showing Uranium 238 decaying via several different elements and isotopes to Lead 206. By measuring different ratios of parent and daughter products, the number of half lives passed can be determined, and therefore an age can be ascertained. Alpha and beta refers to two common decay processes. Alpha emission wgich lowers the mass number and beta emission which increases the atomic number. Source:
https://glossary.periodni.com/dictionary.php?en=decay+series
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So, by applying several forms of radiometric dating to closely overlying tephras, along with other dating methods (both relative and absolute) demonstrating a correlation, we know that the Rapanui Marine Terrace formed around 120 000 years ago. Let’s continue up the cliff a little more as we continue to apply our geological detective techniques to reading these rocks.
As we continue up the gravel/sand beds we observe more frequent layers of tephra interspersed between the marine beds, reflecting more frequent eruption events. Taranaki Volcano is capable of very large eruptions, demonstrated by the occurence of at least 47 Taranaki derived tephras detected as far away as Auckland. Tephras deposited at that distance from the eruptiuon source would only be possible with towering eruption columns that would have reached well in excess of 40 km in height. However, these eruptions are not the most dangerous hazard posed by this still active volcano. Evidence for what that hazard is lies directly above the sand beds.
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| Tephra sample typical of those found in the younger beds towards the top of our outcrop. This heavier tephra consists of grains averaging 1-2mm diameter? Source: Phil Pollock (2018). |
Above the sands we find a two meter thick bed of broken andesite known as a breccia. The term breccia is used to refer to angular fragments of rock that have not been subject to erosion removing the sharp angular edges.The clasts in this breccia range in size from 20 cm to over one metre in size. This is separated from a similar two meter thick bed by a thin very fine grained deposit that has distinctive flow lines. Above the second brecciated bed is another thick fine grained bed that fines up toward the top. Within this layer are rip-up clasts of country rock such as mudstone and the remains of trees. The top of this deposit is the same as the bed that separates the two breccias. What could be the explanation for these deposits?
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| Brecciated andesite and overlying deposits forming layers, or "strata", near the top of the coastal cliffs at Waihi Beach, South Taranaki. Strata in the lower half of the view are mostly obscured by vegetation. Source: Phil Pollock (2018). |
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| Detail from above showing a piece of carbonated wood and a small rip-up clast of mudstone that is typical of the beds we observe directly above the brecciated andesite. Source: Phil Pollock (2018). |
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| In-situ rip-up clast, approximately 1 metre in diameter in the upper portion of the fine-grained flow deposit. A rip-up clast is a piece of rock that has quite literally been ripped up from the ground and transported by a high-energy pyroclastic flow or debris avalanche. Source: Phil Pollock (2018). |
Andesite volcanoes are formed by layers of lava flows, lahar flows, pyroclastic flows, and ash, creating an inherently unstable structure known as a composite cone or stratovolcano. Over time weathering will effect the components of theses layers, and increasing steepness of the cone as it builds up may utilmately lead to partial collapse, with or without an eruption event. These produce what is known as a debris avalanche flow. In fact, most of the Taranaki peninsula was formed by these events from Taranaki Volcano, and the earlier forming cones of Pouakai and Kaitake volcanoes.
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| Map showing some of the major flows originating from Mt. Taranaki volcano. These include debris avalanche, lahar, pyroclastic flows, and block and ash flows. As can be seen in the map, many of these events have reached as far as the coast. Source: Zernak et al., A medial to distal volcaniclastic record of an andesite stratovolcano: detailed stratigraphy of the ring-plain succession of south-west Taranaki, New Zealand. Published International Jouranal of Earth Sciences (2011). |
To date approximately 14 debris avalanche events on the Taranaki ring plain have been defined, with the vast majority of these originating from Mt. Taranaki itself. This equates to roughly one event every 14 000 years. Flows from Pouakai and Kaitake cones have mostly been covered by the Taranaki flows and are therefore largely undetectable. These flows are catastrophic and truly devastating events, with debris flow deposits reaching the coast at some point on the ring plain. The next one to occur could possibly have the very real potential of wiping entire townships from the map. Given the eroded state of the current cone coupled with its steepness, the potential for a catastrophic collapse event is very real.
The deposits we observe in our outcrop are the result of two very closely spaced collapse events. The bottom flow and overlying thin bed are known as the Waingongoro Formation while the overlying beds belong to the Waihi Formation. (The term "Formation" refers to a unit of rock, or combinations of rocks that share characteristics or mode of origin). The brecciated beds represent the basal section of the flows while the finer deposits represent the top and runout sections of the flows (sometimes called hyperconcentrated flow deposits). The Waihi event appears to be the larger of the two events. However, an alternative explanation for its thickness may be that the Waingongoro event filled in depressions in the landscape and therefore facilitated the flow of the Waihi event so that more of it made it to the coast. The flows are thought to have been emplaced at sometime around 70 and 60 thousand years ago respectively. Let’s continue up, we are nearing the top.
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| Mapped and extrapolated extent of two separate but very closely occuring debris flow events The left map shows extent of deposits from the Waingongoro event and the right map shows the Waihi debris flow deposits. The star on each map indicates the location of type sections. Source: Zernak et al. (2011). |
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| Stratigraphic column showing the relationship between deposits from the two events. A stratigraphic column is a graphical representation of strata and their relationship as observed at a particular location, providing information such as thickness of strata, and geological description of each layer of strata. This allows the recording and communication of a great amount of detail. Source: Zernak et al. (2011). |
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| Layer of loess on top of sandy gravels outcropping at the Awatere valley, south of Blenheim. Loess typically shows this colour, has a gritty sandy texture, forms poor soils, and can be prone to erosion. Source: G.R. Roberts, Natural Sciences Image Library of New Zealand. Ref. Go11869Rbt.tif Retrieved from teara.govt.nz (2018). |
The loess then grades into a rich dark soil that is known as the Egmont black loam. This soil belongs to a group that are classed as allophanic soils. Allophanic soils are derived from volcanic parent materials and their rich mineral content make them prime soils for pasture growth. This is why the Taranaki area is one of the principle dairy regions of New Zealand.
Finally, our geological detective story ends at the top of the cliff. By observing the clues left in the rocks we now have a very good picture of not only how these cliffs formed, but also the timing and environments in which they formed. As you can see, geology requires observation and interpretation. The more clues that can be gathered, the more accurate that interpretation will be. However, like all science, new techniques and improving technology will allow us to refine those interpretations further over time and to plan for future hazards.
Click here for a free downloadable guide to New Zealand Geology, by Peter Ballance, thanks to the Geoscience Society of New Zealand
