Slabs 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.

So 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. 

Firstly, 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