Topic: Cliffs - Overview
CLIFFS, THEIR COMPOSITION AND HOW THEY ARE MOLDED INTO HOLLOWS AND PROMONTORIES
Cliffs are the separation points that keep the squabbling sea and grumbling land apart. Cliffs are the sacrificial pawns in a macabre game of chess that only the sea can ever win.
In time, cliffs give birth to sea-caves, arches and rock stacks. However, like the cliffs, they too eventually succumb to the sea. The topography of both the land and the sea-floor, combined with the coastline’s orientation relative to the prevailing wind direction, dictates how the sea sculpts the cliffs out into hollows (bays, inlets and coves) and promontories.
In Tongaporutu’s case, geologically speaking, this occurs at a breakneck speed. This is because the cliffs here, despite first appearances, are primarily composed of soft materials, such as fine-grained siltstone (often called papa), and sandstone. The top yellow-brown material (soil) is derived from sand and volcanic ash deposited in the last 125,000 years. Some cliffs, like those at Te Kawau Pa and Rapanui North are also highly folded.
Water is the primary carver of the coastline. It comes in the form of waves, rain and stream runoff. It is both a creator and a destroyer.
WAVES. Waves pounding into cliff bases, not only physically smash the cliff, they also generate shockwaves that radiate outwards and upwards through the internal structure of the cliff. Weak spots such as well defined fault-lines are particularly affected by these shock-waves. Wavequakes are the watery version of earthquakes. Stand on top of a cliff being shaken during a violent storm and you’ll know what I mean! Also, like earthquakes, they can instigate a ripple effect. The more powerful the wavequake, the further the shockwaves travel, the more likely they are to interact with other fault-lines and set up one or more daughter collapses.
RAIN. Raindrops come in different sizes. All things being equal, below a certain size, a longer period of time is needed to see any noticeable effect. However, once raindrops reach a critical size, they have sufficient mass/force to penetrate soft rock. All of this can occur within a single heavy rain bearing event. The longer the duration, the greater the erosion.
Rain also has another card up its sleeve: Liquefaction. Raindrops are a moving force. This has the effect of not only adding to the individual soil particle’s mass, (water being heavier than the air it replaces), but of adding a lubricating factor. Thus, when x number of soil particles are so binded together, then their collective mass is such that they act as a gigantic ‘whole’ and collapse downwards, usually along well-defined fault-lines. Adding to this is the shockwave generated by the raindrops hitting the cliff and travelling downwards. Because of the higher porosity, (air filled gaps), nearer the top, they can interact with wave generated shockwaves travelling upwards from the cliff’s base.
Though the prevailing weather front direction is from the south-west, the most destructive rain bearing events tend to come from the north-west. Thus the primary sea/wave carving direction is from the south-west, while the primary rain carving direction is from the opposite direction. Generally speaking, this leads to a ‘handedness’ in how the cliffs, arches and sea-caves, are carved and eroded out. More detailed information on this process is given in the relevant sections.
STREAMS. Streams flowing over cliffs on a permanent basis, (droughts notwithstanding), add to the erosion process. At site specific erosion hotspots, their action is highly caustic. They are both a destroyer and a creator, (as are waves and rain). On the one hand streams preferentially carve cliffs back, while on the other, they create rock stacks. Sometimes both processes occur at the same location.
Very dry conditions aren’t often thought of as being cliff carvers. However, extreme drying out can cause shrinkage and open up cracks. The tops of cliffs are particularly prone to this because the material is more loosely bound together. These conditions favour shavings and partial collapses. Dry conditions can also create large crack lines where promontories branch off from cliffs. Then, when it rains they close up. This shrink/expand cycle works in a similar way to the freeze and thaw cycle. Both of which are just as destructive. This is because both cycles instigate fatigue. Fatigue always increases over time and leads to eventual failure. Drought can perhaps thus be looked upon as the ‘silent carver’.
STATIC. Gravity can be seen as a constant force. It is always acting on the cliffs, irrespective of the presence or absence of other forces.
DYNAMIC. Wave shock. This includes an initial energy pulse – physical wave slamming into the exterior of the cliff. Physical waves also produce shockwaves that travel internally through the cliff.
COUPLING. This is where forces can be applied. A good example of this is wind and sea. The higher the wind speed, the rougher the sea – one force acts upon another to produce a greater overall force. This also affects RESONANCE. When the sea is relatively calm, it will resonate at a low frequency. The rougher the sea, the higher the frequency - the more energy the frequency carries.
MATERIAL BEING ACTED UPON
BEACHES. As everyone knows, standing on a beach in high wind is a most unpleasant experience. Sand gets blown all over the place, including into your eyes! Also, stand on a beach after a particularly virulent storm and you may wonder where all the sand has gone.
CLIFFS. As I have already mentioned, with the cliffs at Tongaporutu being composed of interbedded layers of sandstone and mudstone, it should come as no surprise that they too are highly susceptible to the twin erosional forces of water and wind.
To get some idea of what actually occurs, it might make things a little easier if one visualizes a cliff as being a series of thirds, starting at the bottom and ending at the top.
THE BOTTOM THIRD. When a wave slams into a cliff base, the resulting shockwave preferentially follows the line of least resistance. That is, it will extend further into the line of least resistance. What this equates to is that at the cliff, there is the maximum amount of resistance to movement. (It is the most stable). As the base cannot move, firstly because it is fixed to bedrock and secondly because the material is so compressed, it has no ‘give’. This induces brittleness that renders it highly prone to fracturing and carving. Also, all cliffs have natural fracture/fault- lines. When a wave slams into a cliff’s base, the shockwaves it generates will travel along these fault-lines. The more three-dimensional the surface, the more prone to fracturing the cliff will be. If the shockwaves are energetic enough, they will travel right up to the cliff’s top.
THE TOP THIRD. At the cliff’s top there is the minimum amount of resistance to movement. (It is the least stable). This is because, being less compressed than the bottom third, it has the maximum ‘give’. I call this the ‘wobble factor’. This renders the top third highly susceptible to oscillation. This is mostly induced by shockwaves travelling upwards from the base of the cliff. Adding to this, the top third is generally composed of a more loosely bound composite mix of volcanic ash, sand and plant created humus. This makes it highly vulnerable to rain induced ‘top down’ erosion.
THE MIDDLE THIRD. This section of the cliff is for the most part above the splash zone, but also for the most part, below the wobble zone of the top. This could lead one to believe that the middle third is therefore the most stable part of the cliff, but this may not be so. The bottom third of the cliff, though subject to fracturing, is usually quite hardened, (compressed), due to gravity. If it wasn’t, it couldn’t support the top two thirds for very long. The top third, though fairly loose and only subject to a relatively weak force of gravity compared to the base, is, like the base, more likely to lose on average, an insignificant amount of material at any one time, but for the loss to occur on a fairly regular basis. (There is a notable exception to this, namely on Beach One). The middle third of the cliff by contrast, is the most likely to experience a catastrophic collapse, but on an irregular basis that can span years before such an event occurs.
Why is this? The middle third is essentially the balancing act. How stable it is, is determined by the density ratio. That is, the ratio of mass to volume at any given point in time.
The bottom third has the most mass, while the top third has the most volume (space). If this space is filled with water, then the ratio of volume to mass shifts and you have the potential for destabilization. Equally, if the bottom third has a sizeable chunk carved off it, the density ratio changes.
Thus the primary reason why cliffs and rock stacks collapse is due to how the bottom third of the cliff and the top third of the cliff, impact, either separately or both together, on the middle third of the cliff.
Some of the cliffs at Tongaporutu have wall-like features. The main ones I have documented are located at the Gibbs Fishing Point. I call them the Wall and the Mega-Wall respectively. These cliffs are remarkably two-dimensional in nature. They have been compacted and super-hardened by the biggest waves on the coast. Because these cliffs are so flat and relatively smooth, water is able to more efficiently flow over them, thus minimizing any external damage.
At the other extreme are the ‘soil cliffs’ that form part of Beach One. These are composed of a higher ratio of loose soil and ‘rotten rock’ than most of the other cliffs on the Tongaporutu coastline. This makes them particularly vulnerable to rain induced erosion. (Liquefaction).
COLLAPSE FAILURE TYPES
CARVING. This has already been described, but to recap, waves cause direct carving of the cliffs.
COLLAPSE TYPES There are basically two types of collapse. ROCK CLIFF COLLAPSE which can be further defined as shavings, partials, full cliff face collapse and the biggest of all, a total cliff section collapse. The latter two fall into the category of catastrophic or abrupt failure, whereas shavings and partial cliff collapses would be more wear and tear or erosion type failure.
Another type of collapse is LIQUEFACTION. This is where loose soil or a mix of loose material flows like a liquid avalanche. The soil cliffs of Beach One as I have just mentioned, are particularly prone to this type of collapse. During extreme rain events, these cliffs literally ‘melt’ in gargantuan soil bleeds.
You can also have a combination of the two as in the massive (and only) cliff section collapse that I recorded on the Three Sisters Beach in November 2008. My first impression upon seeing it from Pilot Point, was that it looked like a giant flood; not of water, but of soil/boulders.
CRACK GROWTH. This is concerned with the stress factors that impact on the cliffs and in particular the fault-lines that honeycomb the cliffs. Over time they are continually being subjected to rain, drought, wave smashing, wave bounce and wind. In short, the cliffs and their inherent fault-lines suffer cumulative damage resulting in a failure end point.
LOADING can be added to this. All cliff collapse types alter the loading on the surviving parts of the cliffs and their fault-lines as the cliff’s mass and energy is re-distributed. This in turn will reconfigure their failure end point.
Lastly, we have the effects of climate change to consider. The warmer the climate, the greater the level of thermal expansion, the more severe the weather, the higher the wave impact zone, equals the rate of coastline retreat. In other words, the warmer the climate, the faster the rate of coastline retreat.
TE KAWAU PA. Highly folded rock cliffs.
Te Kawau Pa is at the northern end of the Tongaporutu coastline. It is a stand alone site.
The rocks that form the cliffs at Te Kawau Pa are more elaborately folded than further south. Also, there is a greater variety of types and textures. The undercut light grey layer at the base of the cliffs is fine-grained siltstone – often called papa and it is softer than the overlying sandstone. Folds in the sandstone formed before the sediments consolidated into rock, while they were still being carried into deep water. Above the sandstone is yellow-brown material derived from sand and volcanic ash in the last 125,000 years. Whatever their origins, none of these rocks are able to withstand the assaults meted out by the highly energetic Tasman Sea.
Generally, the interbedded mudstone (silt) and sandstone rock stratas form narrow bands, but in certain places the individual rock stratas can be quite wide. If one uses water as an analogy, then the narrow interbedded bands would represent showers, wider spaced bands more general rain, while particularly broad bands would be akin to extreme flooding events.
The height of the cliffs range from medium to high. Also, the upper soil type strata is particularly crumbly here. There is one major cave system. It has two entrances, one of which has a huge, open bowl on the northern side. Just south of the cave (looking from north to south), is a chain of rocks tumbling down to the low tide mark. I call these Chameleon Rocks, one of which has a through keyhole. There are also two rock stacks, Lion Rock and the Sphinx. (I later learned that ‘Lion Rock’ is Te Kawau Pa proper). Near the northern boundary is a large passageway cave cum arch. This leads round to the outer northern boundary. It is an impassable bluff which people fish from. The beach itself is usually well endowed with sand.
Another geologic feature at Te Kawau Pa, is the mudstone (papa) rock strata formations associated with the base of the Keyhole and the base of Lion Rock, have geometric patterned cracks. That is, these specific mudstone rock strata’s exhibit fractal honeycombing - geometric shaped bricks that combine to form a whole. On the one hand this makes them highly vulnerable to the smashing actions of waves where mudstone bricks are carved off on a regular basis. A key point to note, excuse the pun, is because of the numerous fracture-lines, the force being applied by a pounding wave is fractally diluted relative to the fracture-lines. That is, instead of one large force radiating out unhindered across a large plain area, the force’s full destructive power is reduced. Thus, while individual ‘geometric mud bricks’ are carved off on a semi-regular basis, dependent upon the Tasman Sea’s energy state, the destruction incurred at any particular time tends to be on a small, localised scale.
This ‘fractal honeycombing’ has also been observed at Twin Creeks.