Striking story

15.12.2008
The Ritland crater at Hjelmeland outside Stavanger is a geological showcase. Professor Henning Dypvik and the author of this article have now established that it was created by a meteorite.
  • Fridtjof Riis, NPD/International Research Institute of Stavanger (Iris)

Panoramic view of the Ritland crater from the north.

1. Panoramic view of the Ritland crater from the north. The figure below shows the crater with geological eyes. Its rim is shown by a red line, the peneplain with black. Crushed basement rock – red hatching, landslide deposits – orange, shale – green, marine sandstone – yellow, and nappes – purple.

Panoramic view of the Ritland crater from the north.

 

METEORITE STRIKE

The idea that the Ritland formation could have been formed by a meteorite impact occurred to the author in 2000 after visiting the area several time without understanding all the distinctive rocks and structures on view.

Fossils in the crater have been known since the 1950s, and have subsequently been investigated in detail.

I resolved to map the distribution of the various sediments and crushed rocks in a bid to establish connections between them. This work convinced me that Ritland was an ancient crater.

In recent years, I have collaborated with professor Henning Dypvik at the University of Oslo in seeking microscopic evidence that this structure really was formed by an impact.

Now that the evidence has been found, it remains to publish these findings in a scientific journal – and to continue the investigations.

Great interest is shown by the international community studying meteorite strikes when new structures are identified, and many advanced analysis methods can be applied to learning more.

But Prof Dypvik and I hope and believe that the crater, which has now been established with certainty, will appeal to geologists and geology students who can use it to learn more about their subject and its dimensions.

 

Geology is the study of how the Earth’s crust has developed since the planet was born more than four billion years ago. This progress can be viewed at many scales – from microscopic processes in a grain of quartz to the formation of mountain chains and sedimentary basins covering hundreds of kilometres – and over times from seconds to tens of millions of years.

With a little guidance, the outcome of many such processes can be observed in a compact area at the Ritland crater north-east of Stavanger. It provides an opportunity to learn a lot of geology in an interesting way.

This formation was created when a meteorite struck the Earth more than 500 million years ago. Its exact age remains unknown, but shales deposited at the bottom of the crater contain fossils dated to the middle Cambrian – making it certain that the impact occurred before then.

Measuring just over two kilometres in diameter and about 350 metres deep, the crater is one of just 175 confirmed meteorite strikes worldwide and the second in Norway. Its big brother, Gardnos in the Hallingdal valley to the east, has a diameter of about five kilometres.

 

Scenic

Ritland lies in a nature conservation area, and the view across the crater provides a scenic experience in itself. But knowing something about the geological history adds extra dimensions in space and time.

Layers of shale accumulated here for millions of years after the impact, but the crater’s form can be detected by anyone who tries to visualise how the landscape would look without these deposits.

That will also reveal how considerate the glaciers have been to geologists. The crater walls have resisted erosion, and are mostly intact.

Rock and sand stripped from the crater wall before the sea covered the area can be found in layers of varying thickness around the margins of the formation. These sediments have been compressed to create additional resistant rocks. Known as breccia by geologists, these are easy to identify because they comprise many rock fragments.

Above the shale come layers of sandstone filled with the burrows of fauna which lived on the seabed some 500 million years ago. So the geological part of the view is not an unspoiled crater, but a long history which began with the impact. It is fascinating to view this formation and think of the energy unleashed when the meteorite struck.

Anyone who wants to learn more about those forces can take a look at the crushed basement rock around the crater edge and immediately beyond. They indicate the shock involved.

Those interested in how a geological basin gets filled can also study the sedimentary rocks which have accumulated here over a long time and in changing environments.

 

Bombarded

The Earth is constantly being bombarded by dust and rock fragments from outer space, which are slowed down by the atmosphere, heated until they glow and form shooting stars.

Collisions with large visitors are less frequent. Most come from the area between Mars and Jupiter, and hit the Earth with a speed to roughly 25 kilometres per second.

A large chuck of rock hitting the ground at this speed has the same effect as a bomb. Craters on a par with Ritland are formed by a meteorite measuring about 100 metres in diameter.

Since the energy released will be greater than the biggest hydrogen bomb ever exploded, it is fortunate that such impacts are rare. The interval is estimated at 50 000 years.

Even larger – and even less frequent – collisions have the potential to change conditions for life itself, because the energy unleashed will ignite massive fires and hurl so much dust into the air that sunlight is reduced for many years.

 

Shockwaves

Twenty-five kilometres per second is an unimaginably high speed (figures 2 and 3), and the meteorite impact sends shockwaves through the basement rock.

Close to the collision site, the energy generated will melt and vaporise the rock or pulverise it into small fragments of dust which are hurled into the air.

Further away, the bedrock will be crushed and extensively fractured. The mix of fragmented rock and glass on the rim of the crater, known as suevite, is typical of such impact sites.

Microscopic examination reveals the forces involved through the deformation of individual grains of quartz and feldspar, which have developed shock lamina (the thick parallel stripes in the illustration).

Such “shocked quartz” only forms under sudden huge pressures (five to 10 gigapascals – GPa). Quartz grains with such lamina indicate a meteorite strike, because that is the only way pressures of this kind can be generated in the Earth’s crust.

The shocked quartz and glass in the figure were identified this spring, and confirmed the hypothesis that the Ritland formation is the result of an impact.

A hiker without access to a microscope can still gain an idea of the forces involved by observing how the bedrock has been totally crushed up to a couple of hundred metres from the crater.

 

Microscope image of shocked quartz grains.

Microscope image of shocked quartz grains.
Photograph of polished rock with glass fragments.

 

Cauldron

The Ritland crater lies like a huge circular cauldron (figure 2) on the sub-Cambrian peneplain, which was a vast, lifeless flatland when the meteorite struck. Plants had yet to leave the seas.

This peneplain, discernable today in the mountains of Rogaland (the county which embraces Stavanger and Hjelmeland), was subsequently covered with shale (phyllite) and sandstone.

Because the basement rock is more resistant to erosion than the shales, however, the peneplain is exposed as shelves and flat surfaces in the present landscape. Its remains form many of southern Norway’s mountain plateaux.

Available geological observations indicate that the crater was formed on dry land. No crushed sedimentary rocks have been found within it, for instance.

 

 

The stratigraphic sequence in the mountains east of the Ritland crater, with the basement surface (peneplain) shown by a red line and the base of the nappes in yellow.
 

 

Processes

The crater was formed in the space of seconds and, once the dust had settled, normal geological processes continued their work (figures 5 and 6).

Sediments were eventually deposited inside the formation, its steep walls collapsed and rainwater running down them created small streams which carried sand and rock into the basin. These deposits are visible at many points in the crater, and its bottom is largely covered by such ancient sediments.

Thick landslide deposits, with rocks several metres wide, can be found at various points around the margin – but only internally, so that geologists and hikers can use them to define the extent of the crater.

 

Exposed

How long this formation lay exposed to wind and weather is unknown, but it cannot have been many tens of millions of years or the basin would probably hold even more landslide deposits.

What can be established is that the crater was still several hundreds of metres deep when a big change occurred in the landscape. The sea invaded the land during the Cambrian, and submerged the whole peneplain.

The mountains east of Ritland contain a metre-thick layer of fine-grained sandstone deposited over the flooded flatlands before they became so deeply drowned that little sediment reached them.

The only deposition took the form of clay, and this really is one of the slow geological processes. A piece of shale one centimetre thick at Ritland took more than a 100 years to form.

These layers of clay had a high content of organic material, are called alum shales in the Oslo area and have developed into oil shales around the Baltic. At Ritland, they were buried so deep that the organic material has been converted to tiny particles of carbon (graphite).

The way this two-kilometre-wide circular basin was entirely filled with clay sediments over many millions of years can be explored along the crater rim. At one point, it is possible to see how rocks large and small have fallen into the clay soup.

Eventually, coarser sands began to be deposited over this region and gave rise to the sandstones which lie like a bench 20-30 metres thick, clearly visible in the landscape. The crater was now filled with sediments.

What happened after this deposition phase is unknown, because many pages in the geological history book have been ripped out here. A complex of rocks belonging to the thrust belts in the Caledonian mountain chain overlie the sandstones.

Mountain chains are built when two continental plates collide at the rate of a few centimetres per year. The forces acting in such collision zones compress basement rock and sedimentary strata to create folding and overthrusts (nappes).

The Caledonian orogeny – as this process is called – buried the Ritland crater to a depth of more than 5 000 metres. Pressure and temperature rose, converting the clays to shale and the sand to hard sandstones.

 

Sandstones deposited in water at the bottom of the area

Sandstones deposited in water at the bottom of the crater before
the sea submerged the area.

 

The basement rock is pulverised into fragments large and small for several hundred metres beyond the crater rim.

The basement rock is pulverised into fragments large and small
for several hundred metres beyond the crater rim.


Topics: Geology