An Impact from the Past
Scientists have discovered geological evidence that a large impact event occurred in the southern part of Canada’s Alberta province, near the hamlet of Bow City. The event happened tens of millions of years ago, and very little is left of the ancient impact structure today.
To the naked eye, the area around Bow City shows no signs that an object from space once came screaming out of the sky and smashed into the ground at high speed. However, when studying geophysical logs routinely obtained during petroleum drilling in the area, Dr. Paul Glombick then at the Alberta Geological Survey spotted ring-like structures in the underground ‘topography’ of different rocks layers that surrounded a central uplift, or peak, in the middle. To the eyes of a geologist, this is one sign of a crater. The next step was to determine what type of crater it might be.
“We went through a round of looking at all the other possibilities that it could be (volcanic, karst, tectonics, etc.), but none made any sense to form such a large structure,” said Doug Schmitt, Canada Research Chair in Rock Physics at the University of Alberta and co-author on the study. “The seismic data that allowed us to peer into the earth and see more detailed depth ‘profiles’ showed slumping normal faults at the outside edges together with thrust up rocks in the centre that are very distinctive of impact structures.”
The team is still in the relatively early stages of their investigation, and further studies could provide more details about the impact and its resulting crater.
“We have not detected any of the nail-in-the-coffin evidence, such as highly shocked minerals yet,” said Schmitt. “We may need to drill into the central zone to see that. However, it could be that we are below the level at which we might see such highly shocked material.”
Their search for evidence is difficult, because so much of the crater structure and its features have been wiped away with time.
“The real value of this is that it can provide some constraint to modelers on how such features actually form,” said Schmitt.
Because Earth is active, the planet is actually a very difficult place to hunt for old impacts. Many of the telltale signs used to identify them, such as craters, are wiped off the surface of the Earth by wide-ranging processes, from weather and erosion to plate tectonics and glaciers. So much of the structure in Alberta is now lost that its difficult to tell exactly when the impact happened or what caused it (e.g. an asteroid or a comet).
According to Schmitt, erosion and other natural processes have likely erased the opportunity to determine the impactor’s composition for certain. In order to identify the culprit, the team would have to drill into the ground and search for melt sheets or geochemical clues.
What the scientists do know is that the impact left a crater roughly 8 kilometers wide. Using this as a starting point, the team can make some estimates about the size of the object and the force with which it struck the Earth. According to Schmitt, the object would have been 300-500 meters in diameter if it were an iron-nickel meteorite. For a stony meteorite, it would need to be larger than 1 kilometer. This is assuming that the object was traveling at 30 kilometers per second, and coming straight down from the sky (rather than striking the surface at an angle).
The resulting explosion of the impact was big enough to level a modern-day city. In fact, if the same event occurred today, the city of Calgary would suffer large amounts of destruction and extensive damage would also occur as far away as Edmonton.
“We are roughly estimating that the energy released would be about 200 times that of the largest H bomb ever released during underground tests, and I think that is probably a very conservative number,” said Schmitt.
The study, “The Bow City structure, southern Alberta, Canada: The deep roots of a complex impact structure?” was published in Meteoritics and Planetary Science.
The research was supported by the Alberta Geological Survey, NSERC and the Canada Research Chairs program. Legacy seismic data was generously donated by a number of petroleum companies in Alberta.
Impact craters on Earth are important study sites for astrobiologists. These structures can help scientists understand the role of impacts in the history and evolution of life on Earth.
Objects from space may have delivered numerous materials – from water to amino acids – that were important for the origin of life. Impacts have also been linked to extinction events that altered the course of life on our planet.
Recent studies have shown that impact events can provide habitats for microorganisms that live deep below the surface of the Earth. An impact cracks rocks deep below the ground, opening up new spaces for microbes to colonize and mixing in new materials that the organisms can use to survive. Think of it like tilling the soil for planting crops – but on a very large scale. During periods when the Earth was bombarded by numerous impacts, these underground habitats could have been a refuge for life when surface environments were rendered uninhabitable.