Oct 6

Oct 6 Geology of Meteor Craters

Which is which?

About 1,000 tons of meteoric material falls to earth every day. Why don’t we notice? Well, much of it falls in remote areas or into the oceans. Also, most material that lands on earth has been broken into bits so small they aren’t noticed.

A meteoroid is the rock that is hurling through space. It becomes a meteor the burns up when it enters the earth’s atmosphere, and we see those as “shooting stars” in the sky. When the rock survives the trip through the atmosphere and manages to impact the earth, it is a meteorite.

Meteorites can be chunks of asteroids or comets. Asteroid material is stronger and denser, therefore more likely to survive the trip through the atmosphere and hit the earth.

What typically reaches the earth is the size of a marble or smaller, the portions that didn’t burn up when it entered the atmosphere.

A Long History of Meteorite Impacts

There are 190 craters all over the globe that have been confirmed to be formed by meteorite impact.

Mass Extinction

The impact of a 6-mile wide asteroid 65 million years ago contributed to the extinction of about 75 percent of marine and land animals on Earth at the time, including the dinosaurs in what is known as the Cretaceous-Paleogene extinction event. The impact created the 180-mile-wide Chicxulub Crater on the Yucatan Peninsula in Mexico and caused immediate massive earthquakes and tsunamis, as well as global wildfires.

A Long, Long Winter

Dust and particles of debris from the collision got into the upper atmosphere and blocked the sun's rays. Plants had trouble getting enough light to photosynthesize, causing a wide-scale collapse of the food web. Meanwhile, melting of rocks at the impact site released carbon dioxide into the atmosphere, resulting in greenhouse warming. Increased acid rain from the reaction of water with the sulfur dioxide and carbon dioxide added to the atmosphere by the impact.

More Extinction

The 62-mile diameter Popigai crater in Russia’s Siberia is the result of impact approximately 35 million years ago and is thought to be a factor in the Eocene-Oligocene extinction event.

A Well-Preserved Crater

One of the most intact impact craters is the Barringer Meteor Crater in Arizona. It’s about 4,000 feet in diameter and was formed by the impact of a piece of iron-nickel metal approximately 160 feet in diameter. The meteorite and the rocks that were impacted underwent extreme melting and some were even vaporized! Imagine vaporizing iron! It is only 50,000 years old and so well preserved that it has been used to study impact processes.

Until the early 1900s, it was thought that all craters were formed by volcanoes. Daniel Barringer was one of the first scientists to study a crater (Meteor Crater in Arizona) as a possible impact crater. After 26 years of study, his theory was becoming accepted by scientists. Not until 1960 was the theory proven. Along with the proof came an estimate tat an event of this size should occur about once every 50,000 years. (Wait, that impact happened 50,000 years ago… Yikes!)

Astronaut Practice

Impact craters on the moon are well-preserved because of the lack of atmosphere and erosional factors. Because the Meteor Crater in Arizona is so well-preserved, it is used by astronauts to train for their moon explorations.

Mineral Evidence

Extreme compression forms two minerals, coesite and stishovite, from silica. These minerals in craters help prove that the craters were formed by meteorite impacts rather than volcanic eruptions.

More Modern Impacts

A large meteoroid into Earth’s atmosphere in 1908 in a remote part of Siberia in Russia, but didn’t quite make it to the ground. It exploded in the air a few miles up. The force of the explosion was powerful enough to knock over trees for hundreds of miles. Scientists think the meteor itself was about 120 feet across and weighed 220 million pounds.

In 1992 a 27-pound stony meteorite was viewed as a fire ball streaking across the sky from Kentucky to New York. When it landed, it punched a hole in the trunk of a car. In 1947. About 150 tons of fragments reached the ground in Siberia. The largest intact fragment of that meteorite weighed nearly two tons.

In 2013 a house-sized meteorite entered the atmosphere at over 11 miles per second and blew apart 14 miles above above Chelyabinsk, Russia. The explosion released the energy equivalent of more than 400,000 tons of TNT. The shock wave blew out windows and damaged buildings over 200 square miles. More than 1,600 people were injured in the blast, mostly due to broken glass.

Is the Sky Falling?

So, why don’t large meteorites hit the earth now? Well, they do, just not very often by our human timeline. After all, what’s “now” compared to 4.5 billion years?

Meteorites that are car- or house-size only land about every 1,000 years. Larger impacts, with solid meteorites close to a mile in diameter, only occur every million years. It’s a good thing they don’t occur more often, because that “impact winter” would devastate life as we know it.