My exploration and documentation of craters (presently only on this planet).
One of the impact shock effects I noticed in my crater explorations was the fractured rocks around the perimeter of the structure where there should have been solid rock outcrops. Though this phenomenon IS NOT firm evidence of an impact but is a feature of impacting and would indicate that further investigation of the structure is warranted.
The Barringer Crater is one of the youngest impact sites on this planet and the effects of the impact still remain in situ. On the rim of the crater I noted fracturing of this country rock by the impact shock wave. Note that the country rock at this point was uplifted approximately 45 metres from its original position over the surrounding plain. Erosion has not yet exposed the fractured rocks buried outside of the crater.
An overturned rim sequence is also present at the rim of the Barringer Crater and is now recognized as one of the hallmarks of an impact crater.
In 2008 I was very fortunate to have had the opportunity to explore the Pingualuit Impact Structure in Northern Quebec. While hiking outside the crater rim I noted the effects of the impact on the exposed bedrock. The target rock surrounding the Pingualuit Crater consists of a mélange of metamorphosed, Archean plutonic rocks cut by rare basic dykes (Shoemaker, 1962). This image shows an in situ sample of this bedrock that was completely fractured by the impact. The rim of the crater is visible as the small hill 6 kilometres distant on the horizon. The distance gives you an appreciation of the energy required to fracture this rock from that distance.
This block of exposed fractured bedrock is on the rim of the Brent Impact Structure. It has the characteristics of rock exposed to the shock of an impactor 396 million years ago! The force of the explosion is estimated to have been equivalent to the explosion of 250 megatons of TNT. I was fascinated to see the effect first hand, a wall of bedrock with this amount of damage!
When approaching the Manicouagan Impact Structure from the south, I was specifically on the lookout for any changes in the local geology that may have been caused by the impact. Upon entering the inner fracture zone of the crater I noted that some of the rock cuts along the highway changed from solid granite faces to fractured walls. The rocks in this image were fractured by the energy release from a large meteorite impact, approximately 40 kilometres from this spot.
The magnitude of fracturing of the country rocks in the Manicouagan structure increased towards the centre of the crater, the point of maximum shock effect. The fragmentation increased to where the energy from the impact caused the rocks to melt. These melted rocks remain today as the central peak of the crater (the island in the image). The “smaller” fragmented rocks surrounding the “melt rock” central peak were easily evacuated by glaciation and erosion. The annular moat around the Manicouagan central peak (the circular lake in the image) is what remains after the country rocks that experienced “maximum” fracturing were removed. This circular moat is an impact indicator.
This Sudbury Crater fractured bedrock is outside of the Sudbury Igneous Complex (SIC), north-west of Windy Lake on highway 144 approximately 30 Km from the centre of the crater. At the time of impact this fractured rock was several kilometres underground. It has since been exposed by 1.8 billion years of erosion. When driving into the SIC from the north this is the first indicator of an impact event. The fracturing of these footwall rocks illustrates the deformation of the local bedrock that immediately followed impact. Shatter cones (an item I will describe in a later article) are also found in this area.
During our exploration of the Presqu’ile impact structure, we climbed a series of rapids to arrive at this site where we noted fractured rocks and shatter cones (a firm indicator of an impact). These shatter cones and fractured rock surfaces occur within meta-basalt and rhyodacite rocks 5 km east of the crater, Lac de la Presqu’ile. The cones have angles of ~90° and their apparent position is vertical (Grieve). The discovery of these shatter cones at this site confirmed that an impact formed the Presqu’ile structure. Shatter cones will be documented in the next section.