Meteorites — alien invaders

How do geologists investigate meteorite impacts? We are going to embark on an investigative journey, examining the critical evidence that can be used to identify meteorite impact craters, using examples from Western Australia. We will show how this evidence is pieced together to build a case to confirm that the craters and ancient buried or eroded features we see today were formed by meteorite impacts.

So, meteorites…what are they?

Well, they are potentially life-threatening, extremely destructive, extra-terrestrial visitors from space! Nevertheless they have also been potentially life-bringing with theories of life on Earth being seeded by meteorites, as well as them delivering our first water.

The history of meteorite craters on Earth extends back to the very origin of the planet. However, most of the craters have been wiped away by tectonic activity and erosion, or have been covered by sediments, or are located in the ocean so are hidden – such as the Chicxulub crater at Yucatán Peninsula in Mexico. Most other rocky planetary bodies in our solar system lack the tectonic activity we have here on Earth; so bodies such as the Moon have surfaces that haven’t really been modified since they formed, and so are pockmarked with countless visible craters. There was a period of intense meteorite activity about 3800 million years ago called the Late Heavy Bombardment. During this time it is hypothesized that massive chunks of leftover debris from planetary formation collided with the Earth, Moon and other planets of the inner solar system.

However, some younger craters on Earth are visible and, as we shall see, there is also a wealth of indirect evidence for the existence of buried craters. In Western Australia, we have more than a dozen confirmed impact craters and several lines of evidence can be pieced together, like a detective story, to allow us to find them and also sometimes help us determine when they might have happened.

Why should we do this? Well, firstly they are pretty cool in themselves, but also locating meteoritic material can give us a wealth of information about the early solar system. We also want to learn as much as possible about them to help with mitigating the possible catastrophic effects of any future incoming missiles!

In the more than 4500 million years of Earth’s history so far they have had a dramatic effect on our planet and greatly altered the course of evolution. They have been responsible for wiping out certain species which then led to the rise of new ones. We have all heard about ‘the one that killed off the dinosaurs’ but without this untimely demise, mammals and humans might not have become the dominant species they are today.

Also, did you know here in Western Australia we lay claim to the oldest recognized meteorite impact structure on Earth! This is Yarrabubba, which is thought to have formed 2229 million years ago. 

We have 14 confirmed meteorite impact structures scattered over our State and many others that are for now termed ‘possible’ craters. The confirmed craters are shown on the map and you can click each crater pin to get a pop up with information on their diameter, age and whether they are exposed at the surface or not.

So what evidence can we piece together to confirm a meteorite impact origin for a feature?

At other locations, we can find distinctive types of rock that are unique to meteorite impacts. These rocks are formed by the extreme heat and force the meteorite produces as it slams into the Earth, shattering, fragmenting and melting the bedrock. 

Impact breccias are rocks that form as a result of meteorite impacts and they consist of rock and mineral fragments showing varying degrees of shock metamorphism, set in a crystalline or fine-grained matrix or groundmass.

At Yallalie meteorite impact crater, there is an impact breccia known as the Mungedar Breccia that is made up of a mix of bedrock fragments set in a fine-grained matrix. The angular fragments in the breccia are due to the target rocks being broken up by the force of the impact. These are then incorporated into the fine-grained matrix which consists of a mix of partially melted rock and very small particles. 

On a microscopic scale, we can see some of the minerals in the rock have a brittle fracture texture showing they were subjected to shock metamorphism. This produces permanent physical, chemical, mineralogical, or morphological changes in the minerals.

An impact melt breccia is similar to an impact breccia, but it has a fine-grained crystalline matrix, often with glassy or recrystallized melt particles. It is formed when the shock waves from the impact produce such high temperatures and pressures in the target rocks, that it causes them to disintegrate and partly melt. It is the presence of this melt component that distinguishes an impact melt breccia from breccias formed by other tectonic processes such as movement along faults. 

Here is an example of a thin section showing the texture of an impact melt breccia from Goat Paddock meteorite impact crater. We can see a clast of silica glass has formed within the finer grained matrix of the rock.

When the shock of a meteorite impact is large enough to cause widespread melting of the target rocks, this molten rock then solidifies into an impact melt rock. At Yarrabubba meteorite impact crater, the Barlangi Granophyre is an example of this type of rock.

The colossal force of a meteorite impact sends vast quantities of debris up into the air and forcibly expels it from the crater during its formation. It then falls back down to the ground as an aptly named ejecta 'blanket' because it blankets the surrounding area.  This video demonstrates the process in slow motion :

Ejecta is composed of whatever rock types were on the surface at the time of the impact as well as melted material, so they vary greatly in composition. This again means these rocks can be used as an indirect way of dating the impact. If we can accurately date them, then this provides a maximum age for the impact because they must've existed and been deposited before the impact took place. This is a photograph of the ejecta rim along Wolfe Creek meteorite impact crater.

One of the most compelling rocks that you can find at an impact site would have to be fragments of the meteorite itself! There are three broad types of meteorite: stony, iron and stony-iron.

At Veevers meteorite impact crater, well over 30 individual iron meteorite fragments have been found with a combined weight of just under 300 grams. The majority of the fragments weighed between 2 and 8 grams, but one larger fragment was 36 grams. This might sound small, but the majority of meteorite fragments found are small because often most of the meteorite burns up on entry to the atmosphere. The surviving fragments then slam into the surface producing a huge amount of heat and energy like a colossal explosion. Some of the meteorite vaporizes immediately; the majority of it melts, and only a minor amount remains solid and disintegrates into many small fragments that are explosively thrown out into a plume of debris and scattered around the crater.

There is a common misconception that a large meteorite is buried beneath an impact crater. It is reported that Daniel Barringer, the original owner of Meteor Crater (formerly known as Barringer crater) in Arizona, USA, spent 27 years and the majority of his fortune trying to find the buried meteorite. He wanted to mine the iron from the meteorite, but after drilling to a depth of 419 metres and finding nothing, he eventually realized it had vaporized on impact.  

At Dalgaranga meteorite impact crater, stony-iron meteorite fragments have been found.

This type of meteorite is the rarest, constituting only about 1% of falls. The samples have been further classified as belonging to the mesosiderite group, of which only 0.7% of total meteorite falls are this type.

Shale balls are meteorite fragments that have been partially or wholly converted into iron oxides by weathering.

At Wolfe Creek crater, the surviving meteorite fragments have been weathered over time by wind, water and temperature changes. This caused them to crack, which allowed water to seep in and rust their iron component to form significant amounts of these unique balls. They can be found on the southwestern side of the crater either welded to the top of the laterite, or within the ejecta units, and they are up to several decimetres in diameter.

Some of the rocks found at craters have distinctive structures that are only formed by extreme shock waves such as those generated by a meteorite impact or a nuclear explosion, so they are pretty compelling evidence of a meteoritic history if found.

Shatter cones, such as these seen at Shoemaker meteorite impact crater, are distinctive conical-shaped arrangements of cracks and scratches present in rocks that have been subjected to intense shock waves.  

They are usually found in groups with dimensions that range in size from a few millimetres to several metres in length. The apices of the cones point towards the origin of the shock waves, so the impact would have been located to the left hand side of the image here. 

Horse-tail shatter features are the scratched features visible on the surface of shatter-coned rocks. The scratches or striations form a fan-like pattern and radiate out from a central apex. They are called ‘horse-tail’ fractures because they resemble a horse’s tail. 

Other structures are microscopic and can only be seen when the minerals in a rock are viewed under a microscope. Here we can sometimes see evidence for things such as recrystallization, hydrothermal alteration and metasomatism. These processes occur under various geological conditions due to very high temperatures and pressures variously altering the existing rocks; however, in the absence of other known events of this kind, the changes can add evidence for an impact having produced them. 

Planar deformation features (PDFs) are distinctive features in the crystal structure of silicate mineral grains like quartz, which can only be produced by extreme shock metamorphism, such as that resulting from a meteorite impact. 

They consist of multiple sets of very narrow planes of glassy material arranged in parallel layers. They can be seen here in the Teague Granite from the central uplift area of Shoemaker crater (first image) and in the quartz grains in the melt breccia at Goat Paddock crater (second image).

When quartz is in an unaltered state it doesn't usually have any cleavage planes, so seeing these structures provides compelling evidence for a meteorite impact.

Sometimes we can use geophysical data to show us what a structure looks like buried beneath what might be a seemingly featureless surface. 

Aeromagnetic images show changes in the magnetic properties of the rocks that were a result of an impact. Typically a negative (lower than expected) magnetic reading over the impact site is revealed due to the rocks beneath the surface being brecciated and broken up by the intense shock waves from an impact. Positive (higher than expected) magnetic readings are often seen right in the centre of the impact structure where there is uplift and remagnetization of the rocks beneath the surface there.

At Yallalie meteorite impact crater, there is a series of circular positive and negative magnetic anomalies centred on the crater. The alternating rings of anomalies represent a series of faults created as the crater walls collapsed inwards during the impact to create a complex type crater.

You can see that just looking at the ground in the image on the left, there is nothing visible to suggest anything has happened here, no obvious crater or evidence of the area being slammed into by a meteorite! However, when you see the magnetic signature left behind in the rocks as shown by the graphic on the right, it enables us to reveal a glimpse of what the rocks look like buried beneath the featureless surface. You can click and drag the slider to the left or right to see either image in full.

Gravity images show changes in the density of the rocks as a result of an impact. Like the magnetic images, they also commonly reveal alternating rings of positive (higher than expected) and negative (lower than expected) density anomalies centred on the impact site.

At Woodleigh crater, the processed gravity image on the left reveals a series of subtle alternating rings of positive and negative gravity anomalies. The image on the right shows a 3D version of the gravity anomalies which has been enhanced to highlight the buried structure of the impact crater. 

The gravity images show a peak positive anomaly due to the uplift of denser rocks in the centre of the crater (a), surrounded by a negative anomaly ring that represents the low density rocks and ejecta that infilled the crater after it formed (d). In between this are alternating rings of anomalies that represent the faults associated with the formation of a complex crater (b, c). Surrounding all of these is a positive anomaly ring that marks the crater rim (e).

If we could peel away the upper surface layers of rock, we would see something like the cross-sectional artist's impression shown below. It shows a geological interpretation of the buried impact crater created using geophysical data, and it gives us a window into what structures are hidden beneath the surface.

Seismic images provide a vertical cross-section of the different layers of rock beneath the surface and can also reveal buried structures like faults. They can show where rocks have been uplifted or slumped at the centre of a crater in the classic ‘sombero-style’ shape of a complex type crater. 

Here we have the seismic profile from beneath Yallalie crater. It shows the uplifted central peak, the surrounding crater walls that have undergone slumping and collapse along faults, and the location of the crater rim (outer extent of the crater). The movement along the faults would have allowed the central area to be pushed up and the surrounding outer areas to slump down around it.

Drilling is often carried out by mining companies during their exploration phase and it can be a valuable tool to travel back through time and see what has happened in the area and left its mark in the rocks. 

Drilling can show us the rocks buried beneath the surface that are related to a meteorite impact such as impact breccias, or rare minerals. At Hickman crater, drilling in the centre of the crater intersected an impact breccia that wasn’t exposed or visible at the surface and this helped confirm that a meteorite was responsible for the structure. 

Furthermore, dating of the rocks (related to an impact) that are otherwise buried at depth can help us determine when the impact occurred.  

We can also find evidence of an impact producing a gap in the rock record, where the target rocks at the time were completely vaporized and are missing from that specific location, but can be seen in drillcores from the surrounding area. This was the case with the Yallalie impact structure that was drilled by Ampol Exploration in 1990. 

The drillhole results show a massive gap of millions of years in the rock record beneath the Yallalie impact crater compared to what was expected by comparing to other drillholes in the surrounding area (Dandaragan Trough).

This allows us to infer those ‘missing’ rocks (the Coolyena Group) were the target rocks that went up in smoke — literally!! It therefore also tells us that the meteorite impact happened some time after these rocks were deposited.

There are two exceptionally rare elements or minerals on Earth, which can however be found in much higher concentrations associated with meteorites — they are iridium and reidite.

Iridium is a very hard, white, transition metal of the platinum group and is the most corrosion-resistant metal known. It is one of the rarest elements in the Earth’s crust, but is found in much higher concentrations in meteorites. It was actually the anomalously high iridium levels in a narrow horizon of clay around the world that was one of the key pieces of evidence that proved a global-scale meteorite impact, about 66 million years ago, was responsible for the demise of the dinosaurs. The timing of this correlated with the meteorite event that formed the Chicxulub crater at Yucatán Peninsula in Mexico.

Geochemical analysis of drillhole samples of the impact breccia beneath Hickman crater uncovered its enrichment in iridium and provided strong evidence for a meteorite impact. The iridium levels in the breccia were 9–85 parts per million (ppm), compared to 1 ppm or less in the surrounding rocks. The samples also contained very high levels of nickel, between 295 and 2381 ppm, compared to the surrounding rock levels of under 3 ppm. The high nickel levels suggest that an iron meteorite was responsible for the impact.

Reidite (ZrSiO ₄ ) is an exceptionally rare mineral that forms when the relatively common mineral zircon is subjected to extreme pressures (over 30 gigapascals or approximately 300 000 atmospheres) and temperatures. It has only been located on Earth fewer than 10 times and then only in extremely small quantities — all specimens found have been associated with meteorite impacts and we have one of these localities here in Western Australia at Woodleigh crater! Lenses of reidite (coloured blue) were found within zircon crystals (green) of the Precambrian granitic gneiss bedrock that were recovered from the Woodleigh 1 drillhole.

Researchers used the discovery to suggest the meteorite impact here must have been a large one, potentially with a crater diameter of over 100 km, in order to generate the extreme pressure required to form reidite. This then opens up the possibility that the impact at Woodleigh could have been associated with one of the mass extinction events that occurred between 359 and 200 million years ago (the estimated age of the crater).

There are accounts from around the world of people witnessing meteorite fragments falling from the sky and then recovering the meteorite pieces. While these are only small fragments and don’t create a lasting meteorite crater, they are extremely exciting and provide evidence of how the larger scale process works. One notable encounter took place on Binningup Beach, just south of Perth, on 30 September 1984. Two sunbathers, Kathleen Clifton and Theresa Davies, were startled by a whistling noise and a loud thud as a meteorite fragment crashed to Earth right beside them. The fragment of black rock was roughly the size of a large potato and had gouged a small crater in the sand not more than 4 metres from where they had been lying. 

Science tells us there are undoubtedly many more as yet undiscovered meteorite impact craters on Earth. You never know, if you know what to look for, you might well find a meteorite fragment, or even discover the next meteorite impact crater...so keep an eye out!