So what is the real scoop? Why does the Earth exhibit almost no impact craters while the Moon is covered in them? Here are some of the meteorite impact craters that we have on planet Earth. This one you may know, Meteorite Crater Arizona. It's a national monument or park or something.
Okay. made by an object about the size of an 18-wheel tractor trailer impacting here, making a hole in the ground. The kind of ridge around the edge is the debris that was thrown out from the impact.
It's not fairly small, about three quarters of a mile across, and it was made less than 50,000 years ago, and it looks like a pretty good impact crater. Now, let's look across here at this crater in Namibia, southwest Africa. It's roughly double the size, a mile and a half across, but notice what is so much different about it.
It's much shallower, isn't it? Well, it is nearly 4 million years old, and what's happened during the 4 million years? Blowing sand. in this desert environment has mostly filled it in.
Another million years or so, it'll disappear completely probably. This one down here, Chicxulub crater in Mexico, very famous. This is the impact that took out the dinosaurs 65 million years ago.
The crater is gigantic. 156 miles in diameter and in this weird image here this is the coast of the Yucatan Peninsula in Mexico so Gulf of Mexico, Mexico with some political boundaries shown. If you go to the coast of Mexico you don't see a giant hole in the ground.
This is what we call a seismic image. It was made by oil companies bouncing sound waves from the surface down into the rocks and getting reflections back to get an idea of the geometry of the rocks deep underground. This image is of a crater that is now about a mile below the surface. So whether it's land or water, up at the surface is pretty irrelevant.
Down there it's all rock and yeah this is a mile, buried a mile deep. Here's a quick look at one of the actual seismic cross sections showing the mile of sediments down to the floor of the crater and here are the two sides of the inner rim and here are the two sides of the outer rim. Here is, to the same scale, an interpretation of that seismic data, the inner crater rim and the outer crater rim. When this impact happened, 65 million years ago, this one was made by an object probably three or four miles across.
It slammed into the ground producing this giant hole nearly 160 miles wide and 40 miles deep. The consequences were enormous. The heat from the energy of the impact produced a fireball that incinerated about half the planet and the debris from the hole thrown up, I want to say into the air. Some of it was thrown far beyond air into space, some of it thrown so far that it left the planet, became meteors headed for other planets.
But that debris thrown into the air was carried around the world by the wind and blocked out all of the light from the sun for a period of time. We don't really know whether it was weeks, months or years, probably years. What happens when you block out all the sun?
Well, the temperature falls below freezing, and stays that way. What happens to plants? When there is no light, they need light to photosynthesize.
They die. What happens to animals that depend on those plants for food? They die too.
This event 65 million years ago triggered what we call a mass extinction. It's famous for the extinction of the dinosaurs, but that's just the tip of the iceberg. Well over 50 percent of all life on earth, species and individuals. disappeared in this incident. It's a very interesting geological story.
Geologists knew this had happened for at least 30 years before they found the crater. How did they know it had happened? They looked at layers of rock.
Layers of rock represent different ages, kind of like a stack of old newspapers. The oldest is at the bottom and the younger ones are at the top. And at rocks of precisely this age, all around the world, there's a very distinctive layer of dusty rock. It contains three key things.
It contains little spheres that were quite clearly melted drops of rock flying through the air, cooling and solidifying. before they landed. It contains shattered fragments of mineral crystals as if they'd been hit with a giant hammer and it contains an unusually high amount of a rare metal called iridium which is very scarce on earth but very common in many meteorites. Putting those three things together, the droplets of melted rock, the shattered crystals, the extraterrestrial iridium. Geologists were absolutely certain that there had been a major impact and the rocks below it, earlier than that, are full of fossils of all kinds of organisms.
The rocks above it have almost no fossils of only a few species. So the impact causing the mass extinction. It was many years later that this crater was discovered totally by accident. Oil company, hoping to find oil in this area, surveyed the entire region and when they saw this thing they said, aha this must be a dome, we will drill exactly in the middle and we will get lots and lots of oil. And they drilled down right here and they didn't find any oil.
What they did find was shattered rock, glass, glass made from melted rock that had cooled down and re-solidified rapidly, and yet more iridium. Here, in a drill sample from that attempted oil well, is the black glass formed when the rock melted by the heat of the impact solidified right on top of granite from deep deep deep down within the earth's crust. And here looking at the granite in a single crystal under the microscope you can see all the fracture lines, cracks, stress marks resulting from the massive stress as the impactor slammed into the rocks above. So here's the cause of the mass extinction.
Why can't we see it anymore? Well, in the 65 million years that have gone by, sediment has been deposited layer after layer after layer, and so yes, this thing is now buried a mile below the surface. Now here's the last crater I want to talk about, Manicouagan crater in Quebec in eastern Canada.
This is actually a photograph taken by astronauts. Looking out of the window of the Space Shuttle, you can just about see the horizon off in the distance here. Okay, you can see two things in this picture, light and dark.
It's wintertime. The dark is trees, the light is snow, and there's this very distinctive ring of snow. The snow is on the ice on top of a ring-shaped lake, Lake Manicouahagan.
Its size, about half of the other one, it's about 60 miles across, and much older. This one is over 200 million years old. Why is the lake shaped like a doughnut? When this formed, the ground was at least a thousand feet higher. The impact took place at this higher altitude.
Now, directly below a point of impact, the rocks are compressed and hardened by the force of the impact. But in a giant cone extending downwards and outwards, there's a region where the rocks inside are being pushed down and the rocks on the outside are coming up, down on the inside, up on the outside, resulting in shearing. So we have the rocks in this.
in this cone region are smushed and crushed and cracked making them softer. As time has gone by the level of the ground has gone down down down down down due to erosion and then a few thousand years ago in the last ice age glaciers moved across this area. A glacier is kind of like a giant bulldozer.
It couldn't make much impression on the hardened rock in the middle, but the bulldozing action of the ice scooped out and carried away the softer rock in the cone. And when the ice melted, the hole filled with water, making the lake, and in winter the lake freezes and snow lands on top of it, and here's what you see today. What is happening to these craters?
When they're new, they look pretty good, but as time goes by, they get filled in, buried, eroded away. In other words, they're disappearing due to geological activity. The Earth is dynamic. The Earth has all these processes going on, erosion, deposition, and so on, and Over time, the Earth's surface is recycled, renewed, and the old features simply disappear. On a much larger scale, that I can't even show in a photo, there's plate tectonics.
I think you guys know how plate tectonics work. Old plates disappear down into the interior of the Earth, where the material is melted and turns into new Earth crust. As old plates progressively disappear down into a subduction zone, whatever was on them is lost forever.
Okay, so the real difference is that the Earth is dynamic and constantly changing, recycling, reforming its landscapes, whereas the Moon is just sitting there, nothing other than impacts. has happened to it in several billions of years. Anyway, let's continue this on the next video.