What is a Meteor Crater?
In the most simplistic of terms; a meteor crater is the result of the damage and destruction that has been caused after a meteor has crashed into a planet’s surface. The size and speed with which this extra-terrestrial body is hurtling towards the unfortunate planet will have a determining factor on the scale of impact crater that is left behind.
If you study images of surfaces on other planets and their lunar landscapes, within our own solar system, you may feel that meteor impacts are far more common away from Earth. Alas, this could not be further from the truth! Our planet is just as prone to such potentially catastrophic events, it is just that there are certain forces at work on Earth that mean that the evidence for these occurrences becomes lost forever.
The planet Earth is subjected to a number of forces, which over time, will help to disguise the impact craters that were formed by the meteors crashing into this planet’s crust. Although volcanic activity is thought to have decreased significantly over time, molten lava flows can still be responsible for covering up original impact crater evidence. However, this is a relatively rare phenomenon compared with other forces that are to blame.
Continental plate tectonics would have to be the biggest culprit. Despite the fact continental plates move at an agonizingly slow pace (literally a couple of centimetres per year on average), over millions of years, mountains are forced up as plates collide together and rocks are known to metamorphosize as a result of the changes that occur within the Earth’s crust.
Weather erosion will also play a hand here, as will sedimentary and sand deposition that will invariably cover over the original crater over a sufficient period of time.
Such forces are not at work or have long since stopped on other planets and their moons, therefore more meteor impact craters are visible. You generally tend to find that the smaller the body (e.g. our moon), the more craters are visible as any volcanic or plate tectonic forces will stop earlier. An average rate at which a planet can be expected to be bombarded by meteors has been formulated and this helps to determine the ages of any given planetary surface.
Most meteors that enter our planet’s atmosphere are small enough to be disintegrated through the shear heat that they encounter upon entry. This makes for rather fascinating spectacles that are known as either solitary shooting stars or even more breathtaking meteor shower displays. However and this is a big HOWEVER – there are occasions where the meteor or asteroid is large enough to sustain this intense heat and these destructive bodies will continue to head towards our planet’s surface.
Given the premise that only the largest meteors and asteroids survive this process, you can always pretty much guarantee that the collision is going to be a significant event. Couple with this the fact that such bodies will be traveling at ridiculously high speeds, you soon realize that there is going to be one almighty explosion when it lands. In actual fact, meteors will usually be traveling at around 12 to 20 kilometres per second when they hit. In real terms, we can never begin to imagine this type of speed.
When you start to contemplate meteors the size of fridge freezers you soon appreciate what damage is likely to result. Meteors have and do get much larger than this though. Even today, there are still scars on the planet’s surface, which have survived all of the geological forces, that prove that we have been hit by significant meteors and even asteroids in our turbulent past.
Worryingly, some craters have been discovered that are known to have been gauged out by asteroids which would have measured up to several kilometres across. These types of events have been linked with what is known as mass extinction events and it was this kind of occurrence that was thought to have been responsible for the demise of the dinosaurs, 65 million years ago.
Such apocalyptic impacts from asteroids create forces that we can never begin to imagine. Strata is thrown high up into the planet’s atmosphere and this will lead to the world becoming dark for many months at a time and it also leads to grains of the molten strata literally being displaced for many thousands of miles. To the other side of the planet in the most extreme cases. In addition to the shear force that is most certainly generated through such events, any life present on the planet, at these times, struggles beyond belief to cling on to their existence. Indeed, more often than not, they will not make it!
The morphology of a meteor impact crater is very simple. In fact, you can even gain an idea of how this happens yourself; by throwing a large stone into loose sand. You will notice that there is an immediate central uplift where the stone fell initially. Then a further rim around this area where the displaced sand has spread out to.
This is exactly the same where meteor impacts are concerned. But the force of the impact and the heat energy involved will obviously be on a much larger scale. There will be a central uplift area and then there is also the wider crater which has taken the brunt of the force from the rock (often literally molten) being violently displaced. It’s a very simple equation here – the larger the meteor was and the faster it was traveling, the bigger the resulting crater will be.
Meteor impact craters hold vast amounts of interest for astronomers, geologists and many other people alike. But that said, it is also important to bear in mind the freak level of force that would have been required to create the feature in the first place. Whilst huge asteroid impacts are a very rare phenomenon, they do happen. What’s more, statistically the planet will be hit again… it is just a scary matter of time!
Christopher L. Shelby, M.D.