A Geological Wonder in Tasmania: Cape Hauy

Geological Evolution of Cape Hauy, Tasmania

Rising stark and vertical above the crashing waves of Tasmania’s east coast, Cape Hauy is a dramatic sentinel of deep geological time. It showcases towering dolerite sea cliffs formed by Jurassic magma and sculpted by millions of years of erosion. Long before Cape Hauy’s dramatic columns plunged into the sea, ancient forces were at work laying down sediments, heaving magma through the crust, and carving the landscape. This narrative traces the formation and evolution of Cape Hauy’s geology from the Permian Period through to the present, highlighting how tectonic upheavals, volcanic intrusions, erosion, and sea-level changes shaped this iconic headland,

This is the geological story of Cape Hauy.

Permian Foundations: Glacial Seas and Sediments (299–252 Ma)

The foundations of Cape Hauy were laid in the Permian Period, 298.9 million to 251.9 million years ago, when Tasmania was part of the supercontinent Gondwana. During the Permian, southern Tasmania lay in a broad, shallow marine basin (the Tasmania Basin) that experienced frigid, glacial conditions. Ice sheets and glaciers extended into the sea, dropping rock debris as they melted. As a result, the lowermost Permian strata include glacial deposits – coarse tillites (ancient glacial rubble) and mudstones containing dropstones (isolated pebbles dropped from melting icebergs). These glaciomarine layers testify to a cold climate and mark the beginning of the sedimentary sequence underlying the Tasman Peninsula.

As the Permian progressed and the climate moderated, the basin accumulated thick layers of marine siltstones and sandstones rich in marine fossils. These sediments, remaining almost horizontal and undeformed, eventually reached hundreds of meters in thickness across what is now eastern Tasmania. By late Permian time, the sea floor around present-day Cape Hauy was blanketed by grey mudstones, siltstones, and sandstones, some containing fossil plants and animals that thrived in the cold seas. These rock layers formed a stable, layered foundation that would later host one of Tasmania’s greatest geological events.

Triassic Transition: Rivers, Swamps, and Basin Subsidence (252–201 Ma)

The Triassic Period brought a shift from glacial seas to milder, continental conditions in Tasmania. As Gondwana drifted and the climate warmed, the Tasmania Basin transitioned from marine to non-marine deposition. Rivers and swamps spread across low-lying areas, laying down sand and mud. On the Tasman Peninsula, the upper part of the sedimentary sequence consists of Triassic fluvial sandstones and shales, interbedded with thin coal seams. These rocks formed in freshwater environments – river channels, floodplains, and coastal swamps – indicating that by the Triassic, the sea had retreated and lush coal-forming swamps existed in the region. (Indeed, coal was historically mined in Triassic rocks on the Tasman Peninsula.)

By the end of the Triassic, the Permian–Triassic sedimentary pile (collectively known as the Parmeener Supergroup) had built a thick, relatively flat-lying platform of layered rocks beneath future Cape Hauy. Nearly 100 million years of sedimentation produced a stratigraphy of marine glacial deposits overlain by terrestrial sandstones and coals – a stable foundation awaiting tectonic upheaval. Little did those Triassic rivers and swamps know that magma was gathering below, poised to invade these sediments and dramatically change the region’s geology.

Jurassic Magmatism: The Great Dolerite Intrusion (201–145 Ma)

In the Jurassic Period, Tasmania was rocked by a monumental igneous event that would create the very cliffs of Cape Hauy. Around 175–185 million years ago (Middle Jurassic), as Gondwana began to break apart, massive volumes of magma invaded the crust of Tasmania. This molten rock, rich in iron and magnesium, intruded between the horizontal Permian–Triassic strata, forcing its way along bedding planes and through fractures. As it spread laterally and cooled beneath the surface, it solidified into dolerite (also known as diabase) – a hard, dark igneous rock.

The scale of this intrusion was enormous. The Jurassic dolerite in Tasmania covers over one-third of the island – an exposed area of about 30,000 km² with an estimated volume of 15,000 km³ – making it the largest exposure of dolerite in the world. The Tasman Peninsula was thoroughly intruded by this magma: almost horizontal Permian marine and Triassic non-marine rocks were intruded by Jurassic dolerite. The magma likely spread as huge sills (sheet-like intrusions up to 500 m thick) stacked within the sedimentary layers, as well as feeder dikes cutting across them. The weight and heat of the intrusion locally bowed up the overlying rocks and may have even uplifted the region’s surface. In fact, at around 165 million years ago, the Tasman Peninsula area appears to have been raised from beneath the sea as the dolerite forced its way in.

Crucially, as the molten dolerite slowly cooled deep underground (perhaps 1–2 km below the surface), it contracted and fractured into columnar joints. These fractures formed a geometric array of vertical, hexagonal columns – nature’s penchant for hexagons, much like in cooling basalt lava flows, but on a larger scale. Each column grew as the magma cooled inward from its top and bottom surfaces, cracking in a perpendicular direction to the cooling fronts. The result was a vast formation of polygonal rock pillars hidden within the earth. Later, these joints would dictate how the rock breaks apart, setting the stage for Cape Hauy’s pillars. At the time, however, the dolerite remained buried, forming a dolerite plateau beneath whatever cover of sedimentary rock and soil remained on the surface.

The Jurassic dolerite intrusion in Tasmania was part of a much larger event across Gondwana. Equivalent tholeiitic dolerites and basalts are found in Antarctica, South Africa, and South America from the same period, all related to the breakup of the supercontinent in the Karoo-Ferrar large igneous province. In total, several million cubic kilometres of magma were emplaced, an event so vast it has been hypothesized to contribute to global environmental change (the Toarcian extinction) around that time. Locally, this magmatism transformed Tasmania’s geology: the once-sedimentary landscape gained an indurated dolerite spine, endowing features like the Tasman Peninsula with the hard rock that would later form sheer cliffs.

Breakup of Gondwana: Rifting and Uplift (Cretaceous, 145–66 Ma)

Following the Jurassic, Tasmania entered a phase of tectonic fragmentation as Gondwana continued to split. In the Cretaceous Period, the Australian plate began rifting away from Antarctica and other remnants of Gondwana. Rifts opened to the east of Tasmania: around 83 million years ago a spreading ridge propagated south of Tasmania, eventually opening the Tasman Sea and isolating a sliver of continental crust (the Lord Howe Rise) from the Tasmanian coast.

This tectonic upheaval affected the Tasman Peninsula and Cape Hauy region in significant ways. The stretching and thinning of the crust induced faulting and block movements. The once-continuous Jurassic dolerite sheet was broken by faults as the land warped under extensional stresses. Segments of the Tasman Peninsula’s block were likely uplifted or tilted, while others subsided. Geological evidence indicates that some areas of southeastern Tasmania also experienced minor igneous activity during this time – for example, small syenite porphyry dykes and sills around 100 million years ago have been found intruding older rocks near Hobart. Though no major volcanic eruptions occurred on the Tasman Peninsula itself in the Cretaceous, these intrusions hint that the region’s crust was under strain and partial melting.

By the late Cretaceous, Tasmania’s broad geologic architecture was set: the dolerite-cored highlands (including the Tasman Peninsula) stood near the newly formed eastern continental margin. It’s possible that the Tasman Peninsula became a coastal highland at this stage – one side facing the nascent Tasman Sea. With time, erosion began to exploit any weaknesses. The overlying Permian and Triassic sediments, much softer than dolerite, started to wear away faster. The stage was now set for exhumation of the dolerite and the sculpting of the dramatic coastline.

Cenozoic Exhumation: From Buried Sill to Sea Cliffs (66–2.6 Ma)

During the Cenozoic Era between 66 and 2.58 million years ago, the land that includes Cape Hauy rose and fell with gentle epeirogenic motions, and climate and sea-level changes accelerated erosion. After the breakup of Gondwana, Tasmania experienced phases of uplift that helped raise the Jurassic dolerite closer to the surface. By the late Neogene, approximately 10 million years ago), geologists infer a significant regional uplift in eastern Tasmania. This uplift, combined with steady erosion over tens of millions of years, stripped away much of the remaining Permian-Triassic cover from the dolerite. The once-buried dolerite sill was gradually exhumed, emerging as the backbone of the Tasman Peninsula’s topography.

Interestingly, there is evidence of renewed volcanism in Tasmania during the early Cenozoic: portions of the Tasman Peninsula were actually covered by Tertiary basalt lava flows after the Jurassic dolerite had formed. These basalt flows (likely from volcanoes or fissures in Tasmania’s east) capped some areas with a fresh layer of igneous rock. However, on the high-energy coast near Cape Hauy, any basalt cover has been mostly removed by erosion, re-exposing the tougher dolerite beneath. The presence of Tertiary basalt remnants indicates that even after the Jurassic, magmatic processes continued to influence the landscape in bursts.

With uplift came the down-cutting of rivers and the onslaught of weather. The exposed dolerite, now at surface, underwent unloading – the release of pressure as overlying layers were removed. This caused the dolerite to fracture further (exploiting the columnar joints formed during cooling), creating more angular blocks and deep vertical cracks. Rain, wind, and vegetation attacked these fractures from above, breaking the rock along joint planes. On the flanks of the peninsula, streams carved valleys along weaker zones, and slopes shed angular boulders of dolerite. Meanwhile, the edges of the land met the relentless force of the sea.

Quaternary to Present: Carving of the Coastline (2.6 Ma–today)

The final touches on Cape Hauy’s scenery have been applied during the Quaternary Period, within the past 2.58 million years, as oscillating sea levels and relentless wave erosion sculpted the coastline into its present form. Over the past 2 million years, Earth’s climate cycled through ice ages and warm interglacials. During glacial periods, sea levels dropped by up to 120 metres, expanding Tasmania’s land area – at times even reconnecting it to mainland Australia via land bridges. In those low-sea-level phases, the shoreline would have been further out to sea, and Cape Hauy’s cliffs were likely not pounded by waves but exposed to subaerial weathering. Conversely, in warmer interglacial stages (like today), rising seas flooded the coastal plains and attacked the cliffs at Cape Hauy’s base.

With each rise of sea level, powerful waves and saltwater began eroding the foot of the dolerite cliffs. The ocean preferentially exploited any lines of weakness – the vertical joints and faults in the rock. Over time, surf erosion widened joint-bound fractures and ate away at any remaining fringes of softer sedimentary rock. This process gradually isolated slender dolerite columns from the main cliff. Famous rock spires adjacent to Cape Hauy, such as the Candlestick and the Totem Pole, are pinnacles of dolerite that were cut off from the headland by wave action following joint planes. These sea stacks, about 70–100 m tall, are essentially detached columns that illustrate how far marine erosion has progressed. What began as continuous cliffs has been partitioned by the sea into freestanding pillars and clefts.

Marine erosion also undercut the cliffs, sometimes causing sections of the columnar rock to collapse into the sea. The sheer faces and hexagonal outlines of the columns remain remarkably crisp despite millennia of wave attack – proof of the dolerite’s toughness and the relatively recent onset of marine erosion on a geological timescale. In contrast, where Permian sandstone does crop out near the Tasman Peninsula’s coast (for example, at Eaglehawk Neck north of Cape Hauy), the ocean has carved those softer rocks into caves, arches, and blowholes. The nearby Tessellated Pavement at Eaglehawk Neck is a flat Permian rock surface uniquely etched by erosion along fractures, showing the stark difference in erosion style between sandstone and dolerite. At Cape Hauy, virtually all the softer sedimentary layers have been stripped away, leaving only the resistant dolerite to face the full force of the Tasman Sea.

Today, Cape Hauy stands as a narrow promontory fringed by 300-meter-high cliffs of columnar dolerite plunging into the sea (nearby Cape Pillar reaches similar heights, ~300 m, making these some of the tallest sea cliffs in the southern hemisphere). Cape Hauy’s summit and flanks are draped in thin soils and hardy vegetation, but the rock beneath is the Jurassic dolerite sill, still jointed in neat polygons. The broader context of the Tasman Peninsula is evident in every direction: Cape Hauy’s geology mirrors that of its sister capes (Pillar and Raoul) – all are products of the same Jurassic intrusion and subsequent erosion. The peninsula as a whole is essentially a dissected dolerite plateau. It is attached to mainland Tasmania by the narrow Eaglehawk Neck isthmus, where the dolerite gives way to weaker sediments, explaining the low-lying, eroded nature of the neck. On the peninsula’s edges, however, the dolerite “core” is exposed as soaring cliffs and columns. In essence, Cape Hauy fits into the puzzle as one spectacular edge of the Tasman Peninsula’s dolerite massif, shaped into a peninsula by the sea.

Epilogue: The Ever-Evolving Landscape

From the icy seas of the Permian to the basalt lava of the Tertiary and the waves of the present, Cape Hauy’s geology records a dynamic Earth story. The geological evolution of Cape Hauy can be summarized as follows: Permian–Triassic sediments formed a substrate; a Jurassic dolerite intrusion injected new rock and strength into the landscape; Cretaceous rifting fractured and elevated the region; Cenozoic uplift and volcanism exposed the hard dolerite; and Quaternary waves sculpted the cliffs and columns we marvel at today. Each chapter – sedimentation, intrusion, erosion – added a layer of complexity and beauty.

What we see now at Cape Hauy is a snapshot in time. The relentless ocean will continue to gnaw at the headland, likely creating new sea stacks and collapsing others. Yet the fundamental structure – the dolerite columns – will endure far into the future, slowly yielding to gravity and waves. In a broader sense, Cape Hauy is a prime example of Tasmania’s geologic heritage: the island’s signature Jurassic dolerite landforms, born of an ancient supercontinent’s rifting, and revealed by ages of erosion. It stands as a majestic intersection of deep time and dynamic processes, a place where one can visually trace Earth’s history written in rock layers and columns rising above the sea. The grandeur of Cape Hauy’s cliffs is indeed “carved by time,” a tribute to the geological forces that have shaped not just the Tasman Peninsula, but Tasmania itself.

Here's the link to the video we made on Cape Huay on the OzGeology Youtube channel: