Gosses Bluff: The Geological Story of an Ancient Impact Crater

Located in the heart of Australia’s Northern Territory, Gosses Bluff is a spectacular geological structure that reveals the aftermath of a colossal cosmic impact. About 142 million years ago – near the Jurassic-Cretaceous boundary – a meteorite or comet struck this region with unimaginable force. The impact created a crater roughly 20–25 kilometres across, though today only its eroded core remains. In this blog, we’ll explore the size of the impactor, the age and sequence of events during the cataclysm, the complex crater structure (central uplifts and rings), and the long saga of erosion that has sculpted Gosses Bluff into its present form. We’ll also examine whether this ancient impact left behind any mineral riches or chemical clues, and what it means for a crater to be “complex.”

A Cosmic Collision in the Outback

Around 142 million years ago, in the Early Cretaceous, central Australia was rocked by an explosive event. A large asteroid or comet – estimated to be on the order of a kilometre or two in diameter – hurtled toward Earth and slammed into what is now the Missionary Plain west of Alice Springs. To put that in perspective, the object was roughly the size of a small mountain, and it was traveling at tens of kilometres per second. Scientists estimate an impact velocity of about 70 km/s (252,000 km/h) for the bolide at Gosses Bluff. The energy released on impact was truly catastrophic – on the order of 10^5–10^6 megatons of TNT according to one calculation. In more familiar terms, that’s hundreds of thousands of times more powerful than the Hiroshima atomic bomb. The explosion would have been immediately devastating on a regional scale, and some researchers believe its shock was powerful enough to be felt around the globe.

Eyewitnesses (had there been any in the age of dinosaurs) would have seen a blinding flash and a towering mushroom cloud of superheated rock and dust rising high into the sky. The impactor itself was completely vaporized on impact – no large pieces of the meteor remain. In fact, no fragments of the meteorite have ever been found at Gosses Bluff, suggesting it was either entirely vaporized or composed of volatile material (leading some to suspect it may have been a comet). Any small metallic pieces that survived the blast would have weathered away in the millions of years since. What the impact did leave behind was a colossal crater and a host of shocked and melted rocks that tell the tale of this cosmic collision.

The Impact Unfolds: From Explosion to Crater

Geologists reconstruct the sequence of events during and immediately after the impact as follows:

Contact and Compression: In the first instants of impact, the asteroid/comet made contact with the ground, transferring its huge kinetic energy to the rocks. A shock wave raced outward from the impact point, compressing and heating the surrounding rocks to extreme levels. The pressure and heat were so intense that a significant volume of rock and all of the impactor were vaporized almost instantly. Temperatures likely spiked into the tens of thousands of degrees, and pressures shattered the bedrock like glass.

Excavation and Ejection: A fraction of a second later, the shock wave excavated a massive crater cavity in the bedrock. Material was blasted outwards and upwards – millions of tons of rock were thrown into the air. Some debris was ejected beyond the crater’s vicinity, raining down across the landscape for many kilometres. At the same time, a transient cavity (akin to a giant bowl) opened in the ground. Based on geological evidence, the initial excavation may have punched down to a depth of several kilometres. The rim of the growing crater was pushed up and outward by the force of the explosion. We can imagine the scene: ground shaking, a fireball roaring upward, and chunks of rock the size of houses being flung across the desert.

Modification – Rebound and Collapse: Within minutes of the impact, the over-steepened crater began to modify itself under gravity. The shock-compressed crust rebounded – like a trampoline releasing – causing the centre of the crater floor to spring upward. This is a common process in large impacts: the earth’s crust behaves almost like a fluid in those moments, leading to a central uplift of deep rocks. At Gosses Bluff, the rebound was so significant that rocks from 3–4 km beneath the surface were thrust upward to ground level. Meanwhile, the initially steep crater walls slumped inward. The rim collapsed in sections, sending landslides of debris into the crater. These adjustments enlarged the final crater diameter. When the dust settled, the crater was roughly 20–25 km across – a complex crater structure with a raised central region and a broad outer rim.

Immediately after formation, Gosses Bluff would have featured a towering central uplift (a mountain or ring of rock rising at the crater’s centre) surrounded by a deep circular crater basin. Around the basin’s edge rose the crater rim – a ring of uplifted, shattered rock perhaps a few hundred meters high – marking the crater’s boundary at roughly 22 km in diameter. The central uplift and the collapsed terraces of the inner walls likely formed an intricate pattern of peaks and valleys inside the crater. Any life for tens of kilometres around would have been obliterated, and the landscape was turned into a scene of utter devastation: shattered bedrock, hot dust and steam, and a gaping bowl where flat land had been moments before.

The Structure of a Complex Crater

Gosses Bluff is classified by geologists as a complex impact crater, meaning it is not a simple bowl-shaped pit but instead has internal features like a central uplift and ring-like structures. On Earth, craters above a certain size (a few kilometers in diameter) transition to this complex form due to the collapse of the transient cavity under gravity. For context, the famous Meteor Crater in Arizona (about 1.2 km wide) is a simple crater with a bowl shape and no central peak or ring. By contrast, Gosses Bluff – with a final diameter on the order of 20 km – experienced enough inward collapse and rebound to develop a pronounced central uplift and an altered, shallower profile. Complex craters like Gosses Bluff typically have terraced rims, central uplifts, and flat floors filled with breccia (or broken rock) deposits.

Central Uplift: A Mountain Born of Rebound

The central uplift of Gosses Bluff is the most striking feature that remains today. Right after the impact, this uplift would have been a dome or ring of rock rising from the crater floor. In the case of Gosses Bluff, the uplifted core consists of ancient sedimentary rock layers (Ordovician to Devonian sandstones and shales) that were residing deep underground before the impact. The force of the rebound literally pushed these deep rocks up to the surface. Evidence for this comes from the orientation of the rocks: today we find steeply tilted and even overturned strata in the central ring of hills. Rocks that once lay 3–4 kilometers beneath the pre-impact surface are now exposed and even flipped on end – a clear indicator of the immense upward thrust during the crater’s formation.

At Gosses Bluff, the central uplift took on a roughly circular or multi-peaked form about 4.5–5 km in diameter. We can think of it as a mountain ring raised in the middle of the crater. Initially, this central uplift may have stood quite high. It’s possible that the central peaks rose to elevations comparable to or even above the original rim of the crater. They were composed of massive blocks of rock fractured by the shock. Geologists have found megabreccia – gigantic, jumbled blocks of rock tens of meters across – capping parts of the central uplift, likely the remnants of rocks that were snapped up and thrown about during the rebound. Some of these blocks were rotated upside-down as the ground surged upward. The central area would have been a chaotic highland of broken rock, freshly heaved up from the depths.

Surrounding the central peaks, there was likely a ring-shaped trough or basin within the crater. As the central uplift rose and the walls collapsed, an annular (ring-like) depression tends to form between the uplift and the outer rim in large craters. This annular zone at Gosses Bluff would have been filled with a mix of debris: impact breccia (crushed rock), fallback material from the explosion, and slides from the collapsing rim. Indeed, geophysical surveys of Gosses Bluff have identified annular breccia troughs corresponding to a deformed outer ring about 24 km in diameter. In simpler terms, there was an outer ring of disturbance – the true crater rim – and inside it a ring of down-dropped, broken material encircling the central uplift.

The Vanished Rim and Outer Ring

The outer rim of the crater, at ~22–24 km diameter, was the final edge of the impact structure. This rim would have been characterized by an upturned lip of rock, where layers of the earth were bent upward and outward by the force of the explosion. Just outside the rim, debris from the impact would have blanketed the surrounding terrain. Right after the event, the rim might have stood a few hundred meters above the surrounding plain (by analogy with other craters of this size). The presence of shatter cones and intensely shattered rock has been recorded in outcrops around what would have been the outer parts of the crater, confirming that the disturbance extended far beyond the current hills. However, at Gosses Bluff today, this outer crater rim is no longer visible in the landscape – it has been removed by the relentless processes of erosion (as we will explore next). What we see now as a “crater” is actually the central uplift area; the true rim lies many kilometres beyond the visible hills, detectable only by subtle clues such as a circular gravity anomaly and remnants of breccia deposits.

From Cataclysm to Calm: Erosion and Exposure

The impact that formed Gosses Bluff was only the beginning of the crater’s story. In the tens of millions of years that followed, geological processes slowly transformed the crater’s appearance. Immediately after the impact, the freshly formed crater would have been a harsh bowl in the landscape. With time, however, erosion and sedimentation began to modify the site. Rainwater and wind worked on the fractured rocks; rivers might have carried sediment into the crater. It’s likely that the crater gradually filled in with sediments – sand, silt, and debris from the surrounding region – until the pit was no longer so deep. In fact, geologists believe that not long after its formation, Gosses Bluff’s crater bowl became choked with sediment up to the level of the surrounding land. The once prominent crater rim was steadily worn down and eventually erased. Within a few million years, the dramatic crater may have been difficult to recognize from ground level – it might have appeared as just an area of slightly different rock or a subtle depression in a relatively flat plain. Any lakes or wetlands that formed inside would eventually fill and dry as climates changed.

Over an immense span of time (millions upon millions of years), approximately 2 kilometres of rock was stripped off the top of the region by erosion. This is a staggering amount – imagine the entire landscape being planed down by water and wind, layer by layer. Gosses Bluff did not escape this levelling; in fact, it was ultimately over-excavated by nature, which is why today the site looks inverted compared to its original form. The softer outer parts of the crater and the infilling sediments were eroded away more readily, while the harder, shock-altered rocks of the central uplift resisted erosion. A river eventually breached the crater, likely entering from what is now the northeastern side, and cut through the sediments in the crater’s bowl. This washed out the softer fill from the centre of the structure. As the surrounding plains continued to erode and lower in elevation, the tough core of the crater began to stand out in relief. Essentially, the interior of the crater – once the lowest part of the structure – became a hill, because everything around it was worn down even more.

What we see today is the exposed heart of the impact structure. Gosses Bluff now appears as a circular ring of hills about 5 km across, rising up to ~180 meters above the surrounding plain. These hills are the central uplift – the broken, ancient rocks that were hurled upward by the impact and then left stranded as the world around them eroded away. The former crater rim, which was once far outside this central ring, is completely gone from sight. Only a trained eye (or a satellite image) can discern the faint outline of the original crater’s extent. In satellite photos, a subtle “greyish” circular pattern about 22 km wide encircles the central ring, hinting at the ghost of the outer rim. Geophysical measurements also detect a slight gravity low in a circular shape about 21 km wide – a sign of the less dense, fractured rock in the old crater area.

In summary, erosion has dramatically altered Gosses Bluff from a fresh crater to an inside-out relic of one. Initially the site was a deep hole with a raised rim; now it is a raised ring with a surrounding shallow depression. Geologists often refer to this outcome as inverted relief. The central uplift that was once the floor of the crater is now a small mountain range, and the original crater floor and rim have been worn down to nearly flat terrain. The process took tens of millions of years, involving periods of flooding, sediment deposition, and later uplift of the continent and arid climate that enabled wind and water to scour the land. By the Late Cretaceous or early Tertiary, an erosion surface had formed across this region (a gently undulating plain). Over time that surface too uplifted and incised, further exhuming the buried crater. Today, Gosses Bluff stands in the middle of a desert landscape, its ring of quartzite-and-shale hills a dramatic reminder of the ancient impact, accessible to scientists as a natural cross-section through a crater’s core.

Evidence of Impact: Shocked Rocks and Geologic Clues

How do we know Gosses Bluff was formed by a meteor impact and not, say, by a volcano or other process? The evidence is abundant and decisive. In the 1960s, geologists began to suspect an impact origin, and the discovery of shatter cones in the rocks proved to be the smoking gun. Shatter cones are distinctive conical fracture patterns in rock that are only formed by the extreme pressures of a meteorite impact. At Gosses Bluff, shatter cones are found in abundance within the central uplift rocks, all pointing back toward the centre of the structure – exactly as expected if a massive explosion occurred there.

Microscopic examination of the rocks reveals further hallmarks of an impact. Grains of quartz exhibit planar deformation features (PDFs) – tiny parallel shock fractures within the crystal structure – that are indicative of sudden high-pressure shock metamorphism. Such features cannot be produced by normal earth processes; they require a violent event like a meteor impact or nuclear explosion. Geologists have also identified high-pressure mineral phases and melting evidence. For instance, some of the sandstone at Gosses Bluff was momentarily melted into glass by the impact. In one outcrop (aptly named Mount Pyroclast), a rock known as impact melt breccia is found – a mix of fragmented rock that was partially melted and then solidified. In that rock, quartz grains were transformed into glass and even into an uncommon mineral called tridymite (a high-temperature form of silica) before cooling. These shocked rocks later partially recrystallized; scientists have found that the glassy shocked quartz at Mount Pyroclast turned into a form of solid-state glassy quartz (called diaplectic quartz) upon cooling. All of these transformations speak to the intense heat (>1700°C) and pressure of the impact moment.

The impact also induced chemical changes in the target rocks. When shale layers were vaporized and melted, they released hot, potassium-rich fluids that moved through the rocks. As a result, unusual mineral assemblages formed as the crater cooled – for example, pumice-like frothy rocks composed of sanidine (a potassium feldspar) with veins of zeolites and hematite have been identified in the uplift. These are essentially hydrothermal minerals, evidence that the impact generated a short-lived hydrothermal system in the crater. Such systems can circulate hot water and fluids for thousands of years after an impact, altering the minerals in the rock. Geochemically, the rocks at Gosses Bluff show these telltale signs of shock and heating, but no significant ore deposits (e.g. metallic mineral concentrations) have been tied to the impact.

In terms of geochemical anomalies, one might wonder if Gosses Bluff left any trace in the global record – for example, an iridium-rich layer (since meteorites often contain iridium). The impact occurred ~142 million years ago, but no known global extinction or boundary layer is associated with it, and any ejecta that settled in distant sediments has not been clearly identified. This impact was huge on a local scale, but modest compared to the truly global-scale impacts (like the Chicxulub impact that ended the dinosaurs at 66 Ma). Thus, Gosses Bluff’s global geochemical signature, if it exists, is subtle. Locally, geologists searching within the structure haven’t found preserved meteorite fragments or pervasive metal enrichment – as noted, the projectile was likely vaporized. They have, however, noted small gravity and magnetic anomalies over the site: a slight gravity deficit (because shattered rock is less dense) and some magnetic highs corresponding to the cooled impact melt pockets. These geophysical clues further confirm the presence of an impact structure beneath the surface, matching what is seen at other confirmed craters.

It’s also interesting that exploration companies once drilled two wells in the center of Gosses Bluff in search of petroleum, given that big impacts can create structural traps for oil. Those wells found the expected broken and upturned rock layers, but no significant oil or gas accumulations were discovered. The absence of commercial resources might be disappointing to prospectors, but it’s not surprising – while some large craters (like the larger, buried Chicxulub or Chesapeake Bay craters) can host oil or gas in their breccia-filled cavities, Gosses Bluff’s remains are relatively shallow and have been heavily eroded and uplifted over time. In essence, aside from its scientific riches, Gosses Bluff is not known to harbor any notable mineral wealth created by the impact. Its riches are instead geological: in the form of shocked rocks and a ready-made natural laboratory for studying impact processes.

The story of Gosses Bluff is a grand tale written in rock – from the fleeting moments of a cataclysmic impact to the slow transformation by erosion into the form we see today.

For the general observer, it may look like just a curious ring of cliffs in the outback, but to the geologist, it tells a vivid story: of an inferno unleashed from the sky, of rock behaving like liquid, of mountains rising in minutes, and of the slow healing and reshaping of Earth’s surface. In its present form, Gosses Bluff is both an awe-inspiring natural monument and a rich source of information about hyper-velocity impacts. By studying places like Gosses Bluff, scientists better understand not only Earth’s past (and the role impacts may have played in shaping life’s evolution), but also the processes that would occur if – or rather when – another large meteoritic impact occurs in the future. Gosses Bluff’s geological story is truly one of destruction and resurrection: an ancient cataclysm carved out a giant crater, and over eons, Earth’s ceaseless erosion has sculpted that crater’s heart into the enduring ring of rocky hills we see today.