When large meteors, between 10 and 90 kilometers across collide with the Earth, the devastating effect can actually invert the Earth’s crust, according to U of T’s Dr. James Mungall. There have been three known examples of this phenomenon, including the impact that destroyed the dinosaurs 65 million years ago and produced the Chicxulub crater in the Yucatan Peninsula in Mexico.
Dr. Mungall of the Department of Geology studies a crater known as the Sudbury Igneous Complex, and the associated Onaping Formation. The Onaping Formation is thought to be the material that fell back down into the crater after the impact, but which originated in the lower crust. The force of the impact would have been so great, that the rocks in the lower crust would have actually melted.
“When all of the impact related processes were finished the gross layering of the continental crust, over tens of kilometers had been overturned in the area of the crater,” Mungall explains. Within a circular structure about 200 to 300 kilometers in diameter, the rocks of the lower crust ended up sitting on top of what used to be the rocks of the upper crust. This may have extended 40 kilometers down nearly to the depth of the mantle.
This process may help explain the earliest evolution of the Earth’s crust, since during the first half billion years after its formation, large impacts were far more frequent. During this Hadeon Eon, so named after its “hellish” conditions, the upper part of the Earth’s surface would have been constantly churned up, allowing only the survival of early microorganisms that lived deep within the crust.
In fact, it is still possible to find traces of this bombardment of the Earth in what is known as the late veneer of meteoritic material. “Rather than burying itself into the Earth and mixing itself into the Earth’s crust, the impactor actually spreads itself as a very thin sheet over the surface of the Earth,” says Mungall.
The meteor, or impactor, is vapourized by the heat of the impact as it strikes the Earth. The particles can stay in the atmosphere long enough for the crater to form below, and then later condense and add to the upper-most parts of the fallback material.
This whole process takes place in a matter of minutes. At the sizes and speeds involved in the collision, the rocks are melted, and the Earth actually behaves like a fluid. The heat created by the impact produces a huge explosion, so that the Earth under the point of impact gets pressed down, leaving a depression, the source of the crater.
These sorts of effects are also seen in craters on the moon and planets such as Mars. Although there have only been three large-scale collisions on Earth, there have been hundreds of smaller ones. They can be detected by the presence of the element iridium. Being rare in the crust but relatively abundant in meteorites, iridium serves as a kind of signature for the extraterrestrial origin of material. In the case of large impacts, such as that at Sudbury and in Mexico, the resulting plume of vapour and iridium can spread around the planet and cover the whole surface of the Earth.
