Coastal Creationism - Part 6: Igneous intrusion and thick-lipped mutant waves

Chris Buykx
Coastal Creationism

slab.jpgSlabs and hard rocks: why do they go so well together?

Slab! The word highlights the intimate relationship between hard rock and heavy waves. It’s the cold hard platform they lay your body out at the morgue, or it's the memorial stone they place on your grave…or it's the heaving mutant wave, breaking on almost dry reef that may have sent you there.

The relationship between slabbing waves and hard rock is actually more significant than first expected. It turns out that many of Australia’s thickest waves are breaking on some of the hardest rock reefs. For example, looking at the far south of Western Australia you have every slab-hunters go-to wave, The Right. A few hours along the coast you have every photographers dream mutant wave, Cyclops. There are a few other waves with similar origins but they shall remain nameless. 

If you explore the far south of WA, one soon becomes familiar with the texture of granite and gneiss. These outcrops are at their most impressive around William Bay, Torndirrip, and Recherche Archipelago. These rock were formed approximately 1.2 billion years ago, deep in the earth's crust as continents collided, forcing mountains up and pushing other rocks deep in to the mantle. The heat and pressure led to partial melting of the rock, later crystallised to form a rock of similar hardness to granite.

To keep it simple, if the rock retains some of the original layering and structure we call it a gneiss, if it fully melted and recrystallised we call it granite. The end result of these crystalline rocks are bubbles of incredibly hard rock. Amazingly, these rocks have remained virtually unaltered ever since, and as a result there are little or no cracks in these granite plugs. They have been weathering extremely slowly for hundred of millions of years, forming great smooth domes of hard rock. Some of the granites have been later covered by limestones in shallow marine coastal environments, however the reef at The Right and Cyclops are domes of solid granite.

adobestock_38400912.jpegSo why are the waves so steep and thick? Going back to Part 2: Complex Curves, we discussed that optimal reef shapes are convex. It turns out that granitic domes are steeply convex shapes when compared to almost all other reefs.

However, if you look around Esperance and Albany you realise that most of the time granitic domes are too steeply convex to create good surfing reefs. Swells tend to tend to either collapse (the lip turns inside out, leaving the wave without a workable face) or surge (the wave energy just washes up and over the reef).

The magic of these high profile and photogenic slabs is they push the envelope towards a collapsing wave, while still retaining the last surfable vestiges of a plunging wave. That is, they have a small area of workable face and a dramatically throwing lip. It can appear that all the wave is throwing lip which makes for spectacular, if improbable, viewing.

The improbability of these waves is highlighted by their rarity. It's a rare combination of steeply convex granitic domes with no joint cracks or veins to interrupt the smooth curve of the reef. The dome acts as a focus drawing the wave energy into the peak, forming a steepening A-frame, before the rapidly shallowing granitic dome trips up the base of the swell. The considerable speed of a deepwater, long period swell is unchecked, except that the base of the wave is almost stopped in its tracks as the water drains off the shallow dome. The result is all the focussed energy of the wave is drawn up, then thrown forward, creating a barrel of incredible breadth and thickness.

So why is it that the spectacular slabs are found here? I believe it is a combination of geology and oceanography. 

adobestock_38400886.jpegFirstly, the granitic rocks of the region tend to be exceptionally homogenous - that is, the rock exists as a single mass - and as it is without cracks or other weaknesses, the granite retains its large radius dome shape. Elsewhere, granite is usually jointed, resulting in the rock breaking up in to blocks and canyons, shapes that are unfriendly to good waves. The other reason is that the South Coast of WA is exposed to big, long period groundswells rising out of the Southern Ocean. It is this combination of large swells of long periods that interacts with a few steeply convex granitic domes to throw out incredibly thick lipped waves. In any other wave climate it just wouldn’t work.

For east coast surfers you can see the same processes at work on the NSW South Coast. In a small area centred on Bawley Point, a series of bubble-shaped volcanic intrusions have created several smoothly convex domes, with some convex parts of the dome below sea level, at the right depth in the surf zone. Several world class waves break off these hard rock domes. Looking around Bawley, you can tell it's not the same as the horizontally bedded siltstones and sandstones found further north and south. Known as the Termeil Essexite, it is geologically very interesting. It's a small intrusion caused by a volcanic balloon of magma that crystallised to rock forming nice rounded domes that in turn created the Point and Guillotines.

It's worth noting that the large diameter dome shapes are only formed in igneous rocks that do not have joint cracks. Looking at the Port Stephens north of Newcastle, there is bunch of igneous intrusion outcropping from Birubi Point north to Tomaree and across to Winda Woppa. However, these rocks have not formed any surfing reefs of note. This is because the rock is heavily jointed, vertical and horizontal cracks formed due to flexing in the earth's crust, causing the hard igneous rocks to break up into large blocks rather than weather in to smooth domes.

Next time you're exploring the coast and you come across a smooth dome of crystalline rock, think about what that shape would do to a long period groundswell if it was in the surf zone.

Coastal Creationsim is an eight part series written by Chris Buykx. Chris is a geologist, traveller and lifelong surfer. Specialising in eco-tourism, his passion is interpreting nature and the environment. Chris is a resident of Sydney’s Northern Beaches though he's currently doing a lap of Australia with his family. Read past articles:
Part 1: Basic reef shapes
Part 2: Complex curves
Part 3: The good, the bad, and the ugly of coasts
Part 4: Sedimentary sequences and superior shapes
Part 5: Sand dunes and limestone

Comments

Sheepdog's picture
Sheepdog's picture
Sheepdog Wednesday, 25 Nov 2015 at 11:47am

I've really enjoyed this series.... fantastic stuff...

50young's picture
50young's picture
50young Wednesday, 25 Nov 2015 at 12:32pm

Love reading series all information is enlightening, keep it coming

caml's picture
caml's picture
caml Wednesday, 25 Nov 2015 at 2:20pm

Chris I thought you had left us because you didn't reply to me last thread . It has occurred to me that you maybe able to help me on my search for a big wave break . I think you maybe able to provide information that will help . Any chance you could reply to the questions last story ?Please

Chris Buykx's picture
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Chris Buykx Wednesday, 25 Nov 2015 at 2:39pm

Hey Caml, I was waiting for this story to get published as it expands on a lot of what we were talking about. Yes there is definitely need for an article or three on offshore bathymetry and how it relates to lensing swell on to a reef. I have not really begin to study bathymetry heading offshore as its the sort of thing you could spend the rest of your life doing. One of the big challenges relates to how a change in swell direction can be either really good or really bad. I think the key to understanding those perfect numbers of swell direction, size and period for a particular break will be based on old fashioned data collection. Keep a record and notes over a few seasons and then start to look closely at the bathymetry and the shapes that matter will reveal themselves. I will be doing some homework and see if I can distill the important working concepts in to an article.

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Chris Buykx Wednesday, 25 Nov 2015 at 5:12pm

sure, would love to help in the hunt. Ask Stu for my contact details or look me up.
Where you are proposing does have some prospects, just the local geology N of the Pieman is not prospective. There is some potential south of there and I will do some homework to narrow down the prospects. The local Abalone blokes would be the best to give further advice.

caml's picture
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caml Wednesday, 25 Nov 2015 at 10:18pm

Chris thanks I talked to craig & requested your contacts . I never said tassie specifically . Someone else did .

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caml Wednesday, 25 Nov 2015 at 10:27pm

Wa , sa, vic were the first areas of interest .

indo-dreaming's picture
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indo-dreaming Thursday, 26 Nov 2015 at 4:12pm

Yeah the West Coast of Tassie might not be rich in set ups with mostly rip type beaches in the corners of any bays, but i definitely wouldn't write it off for the kind of waves your looking for, i use to surf that coast as a grommet and there is the odd magic reef and that coast gets an amazing amount of swell (and wind) most days we were always searching for the smallest wave to surf and even then it was often way overhead…Zeb Critchlow would be a guy to try to get in contact with he grew up there one of the rare locals and believe he is into those kind of waves these days, i think he lives down Hobart now though one of those guys that surfs Shippies.

tonybarber's picture
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tonybarber Thursday, 26 Nov 2015 at 4:29pm

Thanks Chris for an interesting series. I am sure this will help lead to the next phase of identifying possible point breaks, sand banks and rock shelves for future modification purely for the surf.

caml's picture
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caml Thursday, 26 Nov 2015 at 9:53pm

Ok thanks indo yes that area would certainly have the rare clean 25ft day.

caml's picture
caml's picture
caml Friday, 27 Nov 2015 at 5:54pm

May have found an example of chris's convex reef a reef shape but this one is concave . It all looks good on the chart depth look good rising to 4-5 m . Deep water then a good looking triangle ramp . So I ask a mate if there's a wave there and he said theres not . So I have another look on the chart & see that it maybe not a convex ramp that Chris has stated is simply the best shape for optimum surfing waves . This probably isn't so interesting for most but is this the truth of what I suspect in regards to a concave shape of ramp

caml's picture
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caml Saturday, 28 Nov 2015 at 8:09pm
caml wrote:

May have found an example of chris's convex reef a reef shape but this one is concave . It all looks good on the chart depth look good rising to 4-5 m . Deep water then a good looking triangle ramp . So I ask a mate if there's a wave there and he said theres not . So I have another look on the chart & see that it maybe not a convex ramp that Chris has stated is simply the best shape for optimum surfing waves . This probably isn't so interesting for most but is this the truth of what I suspect in regards to a concave shape of ramp

Convex shape is the best but what happens when it concave but still coming up to a shallow reef ?

Chris Buykx's picture
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Chris Buykx Saturday, 28 Nov 2015 at 6:43pm

Caml, I have been thinking about the best way to approach your broad scale search and I think that high resolution bathymetry offers the best chance.
If you are looking for a big wave, then you need deeper water and a large reef, and probably well offshore. In the bigger picture, once you get offshore in to deeper water and larger features, the geology is much the same from Cape Naturaliste all the way round to Port Lincoln. That is: large granitic plugs and occasional limestone on top.

Look at the shapes that work in the conditions you want (Cow Bombie?) and then look for something similar. You are probably aware that super deep water will help and a broad flattish continental shelf does not.

The only other suggestion is that big cliffs can sometimes create surprise big waves. Major cliff collapses can create cliff-base boulder fields that sometimes get shaped in to decent big waves. This is more likely if the coast is shaped in to a point at a good peel angle to the swell train. It helps if the rocks are hard - unfortunately many of the limestones are not - but sometimes the magic can happen!

caml's picture
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caml Monday, 30 Nov 2015 at 2:42pm

Hi chris , well I use navionics and the quality is decent but obviously could be better if I paid for info but I was told that the free navionics is as good as anything for wave searching . What I have done is studied about ten of the best big wave breaks then search for similar bathymetry along australias coast . I planned to discuss the details further via email because it is best to keep certain words off public forums . Certain areas are unlikely to have the correct bathymetry so those areas could be eliminated . Like how u mentioned a certain part of tassie . The offshore bombie off esperance dont appear to be xxl paddle waves ,albany same , great aust bight too shallow , etc . The swell net team said you maybe busy so I haven't written yet , & there's been surf . But I was planning to contact you further . Thank you .

caml's picture
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caml Saturday, 28 Nov 2015 at 8:15pm

So we could say having done the searching via navionics I then ask chris for help , possibly he maybe able to point out something that I don't already know . Say we find that the majority of big wave breaks have certain things in common , maybe areas that are prolific for big reefs at certain depths made of certain type of rock , then we find certain things in common to areas of the Australian coast .

Chris Buykx's picture
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Chris Buykx Sunday, 29 Nov 2015 at 3:37pm

Caml, the challenge with your search is that the scale is different to regular breaks. The swells you are looking for much larger and longer period and so the shapes are relatively different.
Yes you are still looking for a convex lozenge of reef at a good peel angle to the swell train, however convexity is relative. If the swell wraps on to a ramp, the orthogonal of the swell still follows a convex profile, despite the ramp having a flat profile. A 6 metre swell at 18 seconds will wrap very differently to a 3 metre swell at 14 seconds.
Also one of the greatest influencers of good big waves is tongue of reef stretching out a long way in to deep water. This has the effect of lensing the swell on to the apex of the reef. Mavericks is the most famous example of this. I will write an article on this effect but probably not for a while.
Send me coordinates of the spots you are thinking about and I will cross reference with what I can discern about geology etc.

southey's picture
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southey Sunday, 29 Nov 2015 at 9:35pm

I don't think so . ( in regards to co ordinates ) .
Not just big waves rely on Deep bathymetric focusing , small waves can to . What I think is more Important is long period swells which are generally the case with big waves . But certain places like long clean lines to work even small wave spots . Generally reef breaks can and do handle lumpier / less organised swells . But the ones that require focusing tend to also want only one dominant swell running at the time or infact only one existing swell . These spots tend to work best when the ocean has gone flat and a new swell fills in under a continuation of unchanged offshore breezes . Especially at these locations that like long period swells .

caml's picture
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caml Monday, 30 Nov 2015 at 1:22pm

Hmmm what do I do here ? Have I failed to explain that I already know the recipe for big waves ? Hopefully I haven't confused the issue with my question about non convex lozenge shapes . Can we just change the subject to the question about a concave lozenge or non convex ramp ? I will say it again , found a reef that looks perfect on navionics & ask a mate if theres a wave there and apparently theres not much so is it possible that this is a concave rather than convex shape ?

thermalben's picture
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thermalben Monday, 30 Nov 2015 at 1:35pm

Camel, I'd be a little cautious on Navionics data. It's great across regions that have been well profiled (i.e. along popular boating/shipping routes), but a lot of areas you're likely to be looking at probably have a very coarse bathy profile. This could affect the way the bathy data is presented (and the way it's interpreted), as it's likely to miss out some key features. 

Chris Buykx's picture
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Chris Buykx Monday, 30 Nov 2015 at 1:41pm

A concave shape can still make a wave break, just it there is a risk it will be forced in to a surging wave without a rideable face.
If the reef looks like it has the goods in terms of shape, depth etc. but still doesn't appear to break with the expected waves - have a look further out to sea. Is anything that may be diverting swell away? A deepwater rise can both lens a swell on to one reef and deflect swell away from another nearby.

caml's picture
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caml Monday, 30 Nov 2015 at 11:00pm

.