Meteor Crater, also known as Barringer Crater, was excavated by an iron meteorite 54 yards wide 50,000 years ago. It's 3/4 mile in diameter and 570 feet deep. Credit: NASA

Yesterday we touched on Mars and moon craters. Earth also had the pleasure of meteorite and asteroid pummeling over its 4.6 billion-year history. While it’s easy to spot craters on the moon with little more than 10x binoculars, finding them on our planet takes scientific sleuthing. Most have long since been erased by water and wind erosion, glaciers, volcanoes and the ever-changing jigsaw puzzle of our planet’s crustal wanderings known as plate tectonics.

Can you spot the crater? The Sudbury Basin in Ontario, Canada is the large, tilted oval across the center of the picture. It's 39 miles long x 19 miles wide. Sudbury, the second largest crater on Earth, formed during an impact 1.85 billion years ago . Originally round, its shape has been distorted by geological processes. A second crater called Wanapitei (upper right) formed 37 million years and is now a lake. Credit: NASA

To date 182 confirmed craters or impact structures have been discovered on terra firma. Most blend into the landscape and wouldn’t be known if it weren’t for telltale shock features found in local rocks. Others, like Meteor Crater near Flagstaff, Arizona, stick out like a sore thumb.  In only a small minority have fragments of the impactor been found around the craters – Meteor Crater, Kamil (Egypt), Henbury (Australia), Sikhote-Alin (Russia), Wabar (Saudi Arabia) and Campo del Cielo (Argentina) to name a few.

Iron-nickel fragment from Meteor Crater. The metal crystal pattern is peculiar to iron meteorites and not found in earthly iron. Photo: Bob King

The pieces are almost always iron-nickel meteorites. Though much rarer than stony meteorites, irons are more resistant to weathering than rocky meteorites and more likely to survive passage through the atmosphere intact.

Without meteorites as clues, how do scientists know whether a suspected Earth crater has an extraterrestrial origin? This or that hole or depression might just as well be an ancient lake bottom or sculpted by a volcanic explosion.

No meteorites were found at the Tenoumer crater in Mauritania in Africa but shocked rocks and frothy impact melt glass like the 4-inch wide slice (inset) clinched its extraterrestrial origin. Click photo to learn more. Credit: NASA and Bob King (inset)

The key discovery was made by geologist Eugene Shoemaker in 1960 when he uncovered strongly shocked forms of quartz called coesite (KOH-site) and stishovite in sand and rocks in Meteor Crater in Arizona.

The 1.1 mile diameter Lonar Crater in India is filled with a saltwater lake. Shocked quartz as well as impact breccia mixed with glass (inset) were found within the crater's rocks. Lonar formed between 35,000 and 50,000 years ago. Click photo to learn more. Credit: Wiki and Bob King (inset)

Under the extreme pressures and temperatures experienced during a large meteorite impact, quartz is transformed into these new minerals. No volcano nor any other force on the surface of the Earth, except the detonation of a nuclear bomb, has the power to alter quartz in this way.

Sand-sized quartz grain from the Chesapeake Bay impact structure showing two sets of shock lines at angles to one another. Credit: U.S. Geological Survey

Scientists could now use shocked quartz as a litmus test to identify impact craters. And since the mineral abounds on Earth, all they need to do is gather pieces from a suspected impact crater and bring them back to the lab. There they examine the specimen under a powerful microscope for tiny crystals of coesite and stishovite.

Meteoric impact also leaves behind “tracks” as multiple sets of shock veins within quartz crystals (above).

Space station astronauts and surveillance satellites find occasional new craters from orbit that are later confirmed by ground expeditions. Even regular folks have spotted potential impacts from the comfort of their homes and offices using Google Earth.  The recent Kamil crater in Egypt was discovered in satellite imagery by a former museum curator in 2008. An expedition to the remote hole in 2009 turned up 1.6 metric tons of iron meteorite fragments!

Beautiful shattered rock called breccia in specimens from Lake Wanapitei (left) and Gardnos Crater in Norway. Click to learn more. Photo: Bob King

Along with shocked quartz, meteorite impacts blast, tumble and mix local rocks into a tutti-frutti of fragments that are compacted over time into breccias (breh-chuhs) . The heat and pressure of a strike also melts rocks, forging curious varieties of glass called impact melts.

Breccias and impact-made glasses are known collectively as impactites. Some impactites even contain bits of the original impactor. While not as exotic as meteorites, these altered rocks represent Earth’s inaction with meteorites and are fascinating materials in their own right.

Year 2000 map of Earth impact craters. Several of the featured craters are shown. Click to visit the Earth Impact Database.

Tomorrow we’ll look at another type of impact debris on Earth – the enigmatic tektites.