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juts sharply out of the surrounding landscape
in unmistakably volcanic fashion.
track, such as towering volcanic plugs,
occurs on private pastoral land.
This is Wedge Island, off Cape Hillsborough in Queensland, and the bay here is evidence of an ancient volcano that formed some 34 million years ago. It marks the northernmost point of the Cosgrove hotspot track, a chain of extinct volcanoes that extends southwards across eastern Australia through Queensland, New South Wales and Victoria, to the coast near Melbourne, and is now theorised to be located under Bass Strait near the northern Tasmanian coast. At more than 2000km, it is the world’s longest terrestrial line of intraplate volcanoes.
These types of volcanoes are unusual because they don’t occur at tectonic plate boundaries, where most volcanoes form. Instead, they occur within tectonic plates, forming above hotspots in the Earth’s mantle (see page 60). Volcano chains such as the Cosgrove hotspot track form when a tectonic plate moves over a hotspot. The track has puzzled geologists since it was discovered in the 1970s. It formed as the Australian crustal plate drifted north-northeast over a stationary mantle plume hotspot located some 3000km below the Earth’s surface. Recently, an international team led by Australian scientists has been able to explain why the volcanoes in this chain vary so dramatically in size and composition, and why they occur in certain places but not others.
THE ONLY CONSTANT in any landscape is change. Relentless transformation on a gigantically slow scale is happening around us all the time. Over millennia, giant boulders can wear down to soil, and rocks that were once deeply buried underground can be uplifted to the surface. But with volcanoes, the usual rules don’t apply. Consider Wedge Island, for example. Its oldest rocks are composed of limestone dating back 65–55 million years ago. About 34mya, fiery lava erupted here, enveloping the Earth and leaving behind thick, black, bubbly flows of basalt. Metres of white ash and boulders rained down on top, and thicker, stickier lava, or rhyolite, oozed across what would become Wedge Island. This all happened with dizzying speed, geologically speaking. But once the volcano’s final belches of ash and smoke died away, things slowed. Millennia ticked by with nothing but wind, rain, ice, and thaw to differentiate them. Vegetation grew, retreated, then grew again.
Crystals developed in the basalt bubbles. They formed geodes (spherical, nodule-like rocks with hollow centres coated in crystals) and thunder eggs (round, nodule-like rocks with solid or near-solid centres, usually composed of chalcedony, agate or quartz). The rhyolite weathered and eroded and the layers tip-tilted to form cliffs, creating the landscape we see today.
“I’M THE THIRD generation to [farm] this land,” says Kim Smith, who is washing down her horse trailer near her home tucked beneath the towering spire of nearby Pinnacle Peak. The peak is a plug formed by the Cape Hillsborough volcano, and Kim’s horses graze on its green lower slopes. “My grandfather bought this land in 1952; he started with dairy and small crops and then went to sugarcane,” says Kim, who now uses the land
geologists since it was
discovered in the 1970s.
shows unusual horizontal jointing that
looks like steps on its flanks.
Kim’s family has long loved the volcano that looms over the land for more than its agricultural benefits. “You know how the volcanic rock breaks geometrically?” she asks. “My grandmother had a garden edged with those stones.” That love has been passed down through the generations. “My daughter is eight and wants to be a geologist,” Kim adds. “She loves going to school and saying she owns a volcano!”
The volcanoes along the Cosgrove hotspot track are fascinating to Earth scientists too. “Plate tectonics theory predicts that most earthquakes and volcanoes will occur at plate boundaries,” says Associate Professor Rhodri Davies of the Australian National University’s Research School of Earth Sciences. “But Australia doesn’t sit on a plate boundary. So this is an intraplate volcanic province, and one of the largest continental examples on Earth. [Evidence of volcanic activity] can’t be explained by standard plate tectonic processes – there has to be another mechanism at play.” In this instance, that mechanism is a mantle plume.
“A mantle plume is a hot, buoyant volume of rock that rises all the way from Earth’s core to the surface,” Rhodri explains. “That may be hard to visualise, so I always use the example of a lava lamp where you heat the base. Think of that as Earth’s core, which is significantly hotter than the overlying mantle. It heats up some mantle rock that becomes buoyant enough to rise towards Earth’s surface. Almost 3000km later, it hits the top of the mantle – the base of the lithosphere. Like a lava lamp, you heat the bottom and spawn this buoyant lava that rises and hits the top. That’s how mantle plumes give rise to intraplate volcanoes.”
Mantle plumes are stationary, but the Australian continent moves northwards at about 7cm a year. So, over millions of years as the continent moved over the plume, volcanoes erupted in turn along the line now known as the Cosgrove hotspot track. The plume first hit the Australian continent at Cape Hillsborough about 34mya, and, as the track progressed southwards, more volcanoes formed. Those at the track’s southern end are millions of years younger than those in the north.
Not only is it the Earth’s longest continental track, it used to be – until recently – the most mysterious. Rhodri was the lead researcher on a paper published in the scientific journal Nature, in 2015, that solved the track’s most baffling conundrum: the volcanoes it created vary dramatically in size and composition. Some are enormous, some are almost unnoticeable, and there’s even a 700kmlong gap where there are no volcanoes at all.
Rhodri and his colleagues determined that the explanation for this was to do with the thickness of the lithosphere, the Earth’s rigid outermost layer. “Australia contains some of the largest lithospheric thickness variations on Earth,” he explains. The team noticed that where the track enters northern NSW, a mineral called leucitite was present in the rocks. They deduced that leucitite appeared where the lithosphere was only just thin enough for magma to erupt through it.
“The larger volcanoes occur in regions where the lithosphere is thin,” Rhodri explains, although ‘thin’ is a relative term because here it means less than 130km deep. “And our hypothesis for why that occurs is that the underlying plume is able to rise to shallow depths, melt extensively, produce a lot of magma and a large volcano. The leucitites occur exclusively in regions of intermediate lithospheric thickness. So there we argue that the plume rises to depths where it can only melt to a small degree, and those melts are such low volume that they can only build small, low-volume eruptions.” Where the lithosphere is thicker than 130km, there are no volcanoes at all.
VISIT KEVIN SMALLEY at his home near Smalleys Beach (named after his family, who arrived in 1925), at Cape Hillsborough, and you’ll see the results of a lifetime of rock-collecting piled up in the yard on trestle tables, in crates, on windowsills and around the front door.
Kevin is a rockhound. “Ever since I was old enough to crawl on hands and knees I’ve been interested in rocks,” he says. “I’m all self-taught – I’ve never worked in the field.” He’s passionate about the beauty and rarity of the local rocks, and they all come courtesy of the volcano. He displays rock after rock, plenty of which he literally found in his own backyard.
Master Reef Guide
It’s been two decades since I last visited this celebrated dive site on Ribbon No. 3 Reef, east of Cooktown, on the outer edge of the Great Barrier Reef (GBR). During that time this World Heritage-listed natural wonder has suffered a series of environmental threats, ranging from over-fishing to agricultural run-off. Many of these are now better managed. But the reef continues to face its biggest threat by far – climate change. Well beyond the realm of local management, this global phenomenon has caused four major coral bleaching episodes since 1988, killing huge swathes of coral and degrading the habitat associated with it (see AG 142). At the time of going to press another major bleaching was underway.
A TIME TO HEAL
as the giant Currowan fire roared over Mt Scanzi and down the hill to Tallowa Dam, on the western fringes of Kangaroo Valley, about 180km south-west of Sydney.
March of the spider crabs
A SINGLE GIANT SPIDER CRAB can be hard to see. It barely exceeds 15cm across, despite its common name, and its triangular upper shell is covered in spines, hairs and knobs that make it blend into an ocean fkoor background. It will even make itself more inconspicuous by placing living sponges, hydroids and algae onto its shell from the surrounding temperate reef environment where it lives. But when this species comes together en masse, in aggregations that can exceed 50,000 individuals, it’s difficult to miss.
REVEALED BY FIRE
AARON MORGAN is understandably excited about recent international recognition for his homeland. A Gunditjmara man from south-western Victoria, he was in Azerbaijan in July last year, when the state’s Lake Condah and the Tyrendarra fish traps were inscribed on the World Heritage List for their extraordinary cultural and historical values. It was confirmation that these were two of Australia’s most important Aboriginal sites.
The EGG MAN OF CHANGI
A sense of frustration was growing in Whakatane, fuelled by outside control swooping in and sidelining local pilots, quarantining tour boats without properly cleaning them of corrosive ash, and suggesting – wrongly – that there was a criminal inquiry into White Island Tours. The town’s diffuse anger found focus on delays in the recovery.