Ever since it was first discovered, in the early years of SCUBA diving, divers have recognised it for what it is: a barren arch. Recent attempts to rename it in more gentle terms, are doing a disservice to this most unique of all archways. Already many years ago I figured out what is happening here, and since then kept a watchful eye out for changes that could confirm my conclusions. Essentially there is a beach in the sheltered northern cove, consisting of large round boulders of 0.5 to 1.5m diameter. During mild storms, these stones move down the slope outside the cave, being mopped towards its entrance at about 40m depth. During cyclonic storms (hurricanes) coming from the north-east, these stones are swept through the cave to be spit out at the shallow end. Those that don't make it, roll back inside the cave along a hollow groove (a debris gully) on each side. In the process, wherever they collide with the inside of the cave, all life is ground off. After a large storm, one can find many new round stones on top of the more embedded ones, outside the archway in the shallow northern cove. The diagram shows the basic idea.

Let's first look at the archway with some pictures.
 

f012536: a yacht shows the commanding walls of the southern, deep cove. Underwater these walls are as steep as above.

f012537: a look at the deep end of the archway. It is close to 40m deep here, and large round blocks are waiting for the next storm.

f012600: looking back through the shallow end of the archway. The sil is braking at low tide. It's usually rough here. Big rocks are spit out here.

f024907: a debris gully is found at the shallow end of the cave, ground deep and hollow by rolling stones. Nowhere else around the Poor Knights such a  feature is found.

f024910: inside Barren Arch, the walls show that sessile life is frequently removed and also to what height. Only carpet sponges survive.

f024918: two deep grooves return the rolling stones to the deep end, but in the berm in between, life is more bearable as one can find sessile life that survived some 10-15 years.

f012601: round stones in the shallow cove show that these are frequently moved. Life on it is scarce. There was another layer of round stones on top, but these have recently been moved to the deep end.

The beaches we know are all around the main islands, after waves travelled over the sloping continental shelf, and in the process lost much of their energy. But here at the edge of the continental shelf, storm waves arrive with undiminished strength, enough to move unusually large 'sand grains'.

Here are some facts that point in the right direction:

• this is the only place at the Poor Knights with such large round stones. In fact, it is the only one I know on NZ's coast.
• round stones become round by being rubbed either against other stones or against a hollow anvil. This must happen frequently. Stones cannot become round by lying still.
• since the last ice age, the water stopped rising, about 6000-8000 years ago. So these stones must have been ground to rounded forms in a relatively short time. It means that the grinding happens relatively frequently, and also that it happens with good force. Assume two hurricanes per decade, then over 6000 years there have been 1200 occurrences of storms large enough to circulate the stones.
• the debris gully at the shallow end is the only debris gully I have seen in NZ. One can find it also in Niue where unusually large tropical cyclones can occur, and Niue does not have a wave-tempering continental shelf either. It also shows the size of debris returned after stones have been thrown into it.
• the inside of the arch is a gradually rising ramp (beach) with a debris gully on both sides.
• the outside of the arch is a gradually sinking ramp (beach) returning the rounded stones back to the deep entrance.

How fast must water flow? To estimate the water speed necessary for moving such large boulders, we need to extend the work of Heezen and Hollister, as shown on right. These scientists actually measured what water speeds were needed to shift sand particles of a large range of sizes (vertical scale). We've extended the graph upward by extrapolating it, and one can see that a round block of 1m (1000mm) diameter can be moved by a water speed of about 1000cm/s (right-hand red curve) or 10m/s or 10x3600m/h or 36 km/h. The block settles down at about one quarter that speed (leftmost red curve). 36km/h may not seem a lot, but this should be the water's speed at the entrance to the cave, 40m deep. Because water velocity decreases rapidly with depth, the water's speed at the surface must be many times 36km/h, at least 100km/h, and this becomes quite respectable. We've called this arch a hell-hole, and now you may understand why. You can also understand, that once a boulder moves up the inside ramp, it moves into shallower water, picking up water with higher velocity, until at the exit, it may travel at over 100km/h, enough to be spat far out.

Why only here? Why do we find this type of arch only here and nowhere else? The answer must be found in the topology of the area. I think that the deep steep wall at the southern cove forms an almost ideal parabolic reflector to throw wave fronts back and concentrate them onto the archway's deep entrance. In doing so it could focus and amplify waves by at most three times. In the picture a black dashed curve shows the idea. It is known that vertical walls reflect waves back without absorbing any energy. Ironically, the life attached to such a deep and steep wall, experiences this as shelter, reason why an exposed steep wall like Macro City can have sensitive life growing on it. Remember for a moment that the arch must have been formed at some time in the past, as it is not likely formed by under-earth flows of lava. It could well be that the wave reflector also created the archway in a relatively short time (say 1000 years), after which it stabilised.