The Strangest Volcanic Rock in Victoria: Icelandite
The Rarest Lava in Victoria
There’s only one place in all of Victoria—just one—where a strange, steel-grey volcanic rock called icelandite erupted from the Earth. It didn’t happen often. It didn’t happen everywhere. In a state famous for its flood basalts and lava plains, this quiet hill near the town of Trentham broke all the rules.
That place is Spring Hill. If you didn’t know better, you’d drive past it without a second thought. It’s not tall. It’s not flashy. It blends seamlessly into the central Victorian landscape. But hidden within its slopes is one of the rarest volcanic signatures in Australia—a lava so chemically unusual, it has no twin in the state.
This isn’t just a curiosity. The existence of icelandite here tells us that something weird—something deep, ancient, and still not fully understood—happened beneath Spring Hill millions of years ago. And this lone area, unassuming and quiet, is the only visible evidence.
A Volcanic Landscape Built on Basalt
To understand just how strange this is, you need to know what normally happens in Victoria. Most of the state’s volcanic rocks belong to the Newer Volcanic Province—a sprawling field of basaltic lava that erupted over the last 4.5 million years. Before that, earlier volcanic phases—collectively called the Older Volcanics—also produced mainly basalt, born from direct melting of the upper mantle.
Basalt is the workhorse of volcanism. It’s low in silica, rich in iron and magnesium, and flows easily across the landscape in broad, flat sheets. Victoria is covered in the stuff—lava plains, cones, and maars that speak of tens of thousands of eruptions, all painting the same basaltic picture.
But Spring Hill doesn’t fit.
*Image depicts the Icelandite layer in Spring Hill.
The Outsider Rock
Spring Hill is made of icelandite, a volcanic rock that’s halfway between basalt and more evolved, silica-rich magmas like dacite. Icelandite has more silica and iron than basalt, making it thicker, slower, and more prone to forming blocky lava flows and domes instead of wide, runny sheets.
It was first described in Iceland, where unusual conditions along volcanic rift zones sometimes allow magma to evolve chemically before it erupts. That alone should tell you something: icelandite is rare even in the places it’s named after. It takes very specific conditions—like mantle enrichment, magma stagnation, and often crustal contamination—for it to form.
Which is why it’s so shocking that Victoria—a province dominated by quick, hot basalt—has exactly one known location where icelandite erupted, and that’s at Spring Hill.
*Image depicts waterworn Icelandite.
Not Just Unusual—Singular
What makes Spring Hill so compelling isn’t just that it’s geochemically weird. It’s that it happened only once, and we still don’t know exactly why. There are no other icelandite flows scattered across the Western Plains. There are no hidden cousins lurking beneath the basalt. Spring Hill is a geological orphan, its chemistry unmatched anywhere else in the state.
In the words of field geologists, this is a “one-off”—a rock type so uncommon that entire sections of the state’s geological maps must be updated just to accommodate it.
And if you’ve ever tumbled one of these rocks through a creek or polished a piece in your hand, you’ll know the colour gives it away: grey and muted when fresh, but blueish and smooth when water worn. It’s beautiful, durable, and completely unique. Like look at these specimens I found. They are gorgeous.
When Did It Erupt?
Spring Hill’s icelandite erupted between 8.3 and 5 million years ago, during the Late Miocene. That puts it in the Older Volcanics timeframe, just before the main pulses of the Newer Volcanic Province took off. While most of the eruptions around this time were basaltic, a few scattered regions in Victoria—like Newham, Trentham, and Creswick—record rare felsic episodes during this same window.
These felsic events appear to have come in waves, not continuous flows. The Spring Hill eruption seems to have been one of the earliest and most chemically evolved examples—more silica-rich than most, and isolated enough to stand alone on the state’s geological record.
Born of a Strange Mantle
But the reason icelandite appeared here and nowhere else? That’s where things get speculative—but the science offers some tantalizing clues.
The mantle beneath Spring Hill appears to have been chemically enriched, or in geological terms, heterogeneous. Unlike the uniform, depleted mantle that typically produces basalt, this zone had extra silica and trace elements—the leftovers of older tectonic processes, perhaps a long-forgotten subduction event or ancient plume activity that left a geochemical scar deep beneath central Victoria.
This mantle wasn’t normal, and the magma that rose from it wasn’t either.
Riding a Plume or Tapping a Ghost?
One hypothesis suggests that the unusual Spring Hill magma may have been influenced by a plume-like upwelling beneath the Bass Strait region. As Australia drifted northward and the lithosphere stretched, the base of the continent may have passed over a stationary heat anomaly in the asthenosphere—a kind of weak hotspot that locally melted the mantle in odd ways.
Other models argue it was more about inherited crustal architecture—deep fractures and fault zones created during the rifting of the Tasman Sea millions of years earlier. These structural pathways may have allowed magma from this enriched mantle patch to rise, stall, and evolve—creating icelandite in the process.
Either way, something strange happened under Spring Hill that didn’t happen anywhere else.
Xenoliths: Fragments of the Crust
As the icelandite magma rose, it didn’t come up clean. It tore fragments of the Castlemaine Group from the Ordovician-aged crust—chunks of ancient marine sediments that had been sitting quietly for over 450 million years.
These fragments, called xenoliths, are preserved in the lava. Some show signs of recrystallisation, others of partial melting. Their presence tells us the magma interacted heavily with the crust before erupting—assimilating silica, modifying its chemistry, and leaving behind physical evidence of its journey.
This kind of interaction helps explain why Spring Hill’s icelandite is even more evolved than it might otherwise be. The combination of a strange mantle and active crustal contamination was the perfect recipe for something rare.
What Did the Eruption Look Like?
The eruption of icelandite at Spring Hill was most likely effusive, meaning gentle lava flows, but with the potential for a brief explosive opening phase if the magma was gas-rich. Icelandite is more silica-rich than basalt, making the lava thicker and more viscous, which means it didn’t flow far or fast. Instead of producing widespread lava plains, it would have emerged slowly, forming blocky, stubby lava flows or even small domes near the vent.
The presence of vesicular textures, meaning the cavities in the rocks you see here where the Spring Hill icelandite is pitted with gas bubbles frozen in place—suggests the magma was gas-rich, pointing to a more explosive eruption style than previously assumed. While the magma’s high silica content would have made it viscous and resistant to flowing, the trapped gases would have increased internal pressure during ascent. This likely led to violent degassing near the surface, causing explosive bursts as the lava erupted. These explosions could have scattered ash, scoria, and volcanic bombs around the vent, especially during the initial phase. After this more dynamic start, the eruption probably transitioned into a thick, slow-moving effusive phase, with the remaining degassed magma extruding to form blocky lava flows and domes. The combination of high viscosity and high volatile content makes Spring Hill’s eruption especially unusual in Victoria—a rare hybrid of explosive and effusive behavior, tied to an equally rare magma type.
The absence of thick ash deposits or widespread pyroclastic material suggests that any explosive activity was limited and short-lived. Field evidence, like closely spaced horizontal joints in the rock and glassy textures, supports a scenario where the lava cooled slowly in place, forming a rugged, compact volcanic mound rather than a broad cone. In short, Spring Hill’s eruption was a geochemically unusual event—thick, sticky lava oozing out from solitary vents scattered around a local region, leaving behind a rare rock type found nowhere else in Victoria. And speaking of rare rock types, I’ve picked up a few nice specimens of the Icelandite that are available on my website, if you’d like to pick up the rarest volcanic rock in Victoria for yourself to keep, check out the link in the pinned comment below.
Why It Matters
Spring Hill’s icelandite isn’t just a curiosity—it’s a geochemical and tectonic clue. It tells us that Victoria’s mantle isn’t as uniform as it might seem. It hints at deep processes that left no other trace, and it challenges our understanding of how volcanic provinces evolve over time.
More importantly, it shows how even in a region dominated by a single rock type—basalt—local anomalies can break the rules. These anomalies aren’t noise in the data; they’re the most informative parts of the story.
A Singular Hill with a Singular Story
There is no other place in Victoria where you’ll find icelandite. No twin peaks. No matching flows. Just this one modest area, quietly holding the key to a deeper, stranger past.
In a state built on a basaltic foundation, Spring Hill is a singular exception—a reminder that sometimes, the most interesting places are the ones that don’t fit the pattern at all.
And maybe, just maybe, there are more surprises still buried in the hills and creeks of Victoria—waiting for someone curious enough to ask why a rock looks just a little too blue.
Here's the video we made on this on the OzGeology YouTube Channel:
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