HOW COME? Where Earth's craters come from

How did the craters on the Earth's surface form? asks a reader.

Say "crater," and most people imagine a huge hole, carved out of the ground by an it-came-from-outer-space impact. And in fact, like the other planets and moons of our solar system, Earth endured billions of years of bombardments before late-arriving humans stumbled upon (or into) the first craters.

The surface of a rocky, 4.5 billion-year-old planet like ours is an imperfect record of the past. Plates carrying the planet's crust have slammed together, creating mountains. Moving glaciers have worn mountains into hills. Hot, liquid magma has welled up from inside the Earth, exploded in volcanoes, and cooled into solid rock -- often leaving craters behind.

But most craters are blasted out of our planet's surface, by objects hurtling in from space. One such rocky object dropped out of Earth's sky about 20,000 years ago, over what is now Arizona. Today, about 35 miles east of Flagstaff, an enormous crater -- 570 feet deep, and 4,000 feet across -- marks the impact. (Set at the bottom, New York City's 663-feet Trump Tower would only protrude about eight stories above the crater's rim.)

Scientists think that the meteorite that carved out the 4,000-feet crater was itself about 150 feet across. How did it leave such an enormous hole? An impact by a falling meteorite, asteroid or comet can do serious damage to the face of a planet: A 100-feet meteorite, traveling at 34,000 mph, would hit the surface with the destructive power of four million tons of TNT.

What happens in an impact? When, say, a meteorite slams into the solid surface of a planet or moon, a high-speed jet of debris shoots up from the ground. Underneath the impacting object, rocks flatten, expand, and crack into pieces, flying in all directions. This "ejection blanket" of pulverized rocks can be seen just outside a crater's rim.

But as years pass, a crater's shape can change. Walls may sag; magma may seep up through cracks and harden, adding new layers of rock. Wind may blow in debris, while rains and floods may slowly fill the crater with water.

So the Earth's surface isn't a pristine record of past impacts, big or small. But satellite photographs reveal circles in the landscape -- the footprints of ancient craters. These include the many faint circles on the plains of central Canada, as well as circular crater lakes, like Lake Bosumtwi in Africa's Ghana. So far, about 182 impact craters have been identified on our long-battered planet.