The Ritland Crater - A chapter in Earth's historybook
The Ritland Crater is a unique natural phenomenon in Hjelmeland, a peaceful and rural municipality nordeast of Stavanger. Near the Ritland farm lie the well-preserved remains of a meteorite impact crater, created 500 million years ago by a 100-150 m wide meteorite. The crater is part of an accessible and popular hiking area near the border of the Vormedalsheia protected landscape area.
In and around the crater, you can find evidence of 500 million years of geological history. You can also find traces of the impact itself, fossil-rich mountain rock and unique mountain plants in e.g. Øyastølmyra natural reserve.
The discovery of an impact crater
In 1985, Stavanger geologist Fridtjof Riis came for the first time to the Ritland area and was immediately intrigued by the unusual geology of the area. He couldn't explain what he found in Ritland by known geological processes, like mountain building and erosion.
In 2001, he could compare the Ritland area with the Gardnos crater in Hallingdal, the first known meteorite impact crater in Norway. There, he realized that the Ritland geology could be explained by a meteorite impact.
He then collaborated with scientists from the Institute for Earth Sciences of the University of Oslo, who had also studied the Gardnos crater. The Norwegian Reasearch Council financed the in-dept investigation of the Ritland Crater through a project grant to the University of Oslo. During one of their field trips, they found a small area with a special type of rock, called suevite.
Suevite is created during an impact event, but there are other rock types that can look just like suevite, so the impact theory was not yet completely proven. But in 2008 they saw shocked quartz under the microscope, quartz grains with a changed crystal structure, which formed the final piece of evidence for an meteorite impact in Ritland.
They presented their results on scientific meetings and published a full research report in 2011. Also in 2011, the Ritland crater was included in the Earth Impact Database, a global scientific database of proven meteorite impact craters.
A shocking geological story
When the Ritland meteoroid hit the earth, the Ritland area was a shallow sea with a clay bottom. The meteoroid was about 100-150 m wide and traveled with a speed of 20 km/s towards the earth. A tremendous amount of energy was released when the meteoroid hit the ground. The meteoroid evaporated and disappeared and a crater 2,7 km wide and 400 m deep was created.
The mountain rocks in the wide area where broken by the impact and the temperature became so high, that the rocks near the place of impact melted and created a special type of rock, called suevite. Water and rocks were thrown up in the air for more than 3 km and flung more than 4 km away from the impact site. The mountain rocks contained small grains of quartz mineral, whose very crystal structure was shocked and changed by the force of the impact. There's only one natural event that can create this kind of changes and that's a meteorite impact!
Afterwards, the crater was first filled with stone and rocks that slided down from the steep crater edge. Later the crater was filled with clay and sand, and maybe other sediments. About 200 million years after the impact, the western mountain chains in Norway were built and the crater was pressed more than 4 km under the ground by the overlying mountains. Due to the heat at these depths, the clay layers were transformed to shale, sand to sandstone and the broken mountain rocks to hard rocks, like granite. Later in geological history, glaciers and rivers partially removed the top layers and erosion took away the major part of the crater edge and shale inside the crater. But the eastern part of the crater was preserved, just like we can observe it today.