The moon’s surface is riddled with craters ranging in size and structural complexity, and billions of years ago before life emerged, the Earth looked the same way.

“The bottom line is, everything that happened on the moon happened on the Earth,” said David Kring, crater expert and team leader for Center for Lunar Science and Exploration. “The Earth used to look just like that.”

But Earth has several things the moon doesn’t — an atmosphere and liquid water that cause erosion. And the trump card, plate tectonics, that recycles much of the planet’s crust over millions of years and smooths away blemishes left by cosmic impacts. As a result, there are only around 160 known impact craters in existence today (though there are surely more that haven’t been discovered).

Craters come in two flavors: those that aren’t caused by asteroids or comets, impact craters, are formed by powerful volcanic explosions.

Such outbursts can be violent enough that once the eruption is over, the volcano collapses in on its empty vacant magma chamber and forms a caldera, or volcanic crater.

Lake Toba in Sumatra, the largest volcanic structure on Earth, is an example of an enormous caldera that has filled with water over time.

Whereas volcanic craters arise from deep inside the planet, impact craters originate in outer space.

When a meteor makes it through Earth’s atmosphere without burning up, it strikes the ground faster than the speed of sound.

“Something we don’t understand very well on the geological side (of crater formation) is, we still find it difficult to determine the trajectory of impacting objects for most impact craters,” Kring said. “We’re still searching for a clue to deduce that.”

But no matter at what angle it makes contact, the enormous amount of kinetic energy the projectile carries immediately transfers to the target rock it hits, triggering powerful shock waves.

Although craters look like imprints of a giant fist smashing the ground inward, impact shock waves have the opposite effect, which planetary scientists divide into three phases.

The compression stage of crater formation involves that initial exchange of energy between the projectile and the impact area.

During the excavation phase, the massive shock wave causes the projectile to simultaneously melt and vaporize, spewing plumes of searing hot rock vapor miles high into the atmosphere. The force can catapult chunks of molten and solid rock hundreds of miles from the impact site — this material is known as ejecta flow.

And so far, the crater formation process has only lasted a few seconds.

During the final modification phase, the remainder of ejecta partially refills and rings the crater site, and debris forms a rich mineral composite called breccia.

Larger, more forceful impact events will form complex craters in which the rock at the center of the crater rebounds from the downward pressure of the shock wave and uplifts into a mound-like formation.

But the environmental effects of impact crater formation go far beyond forming benign basins.

For instance, the famous Chicxulub crater in Yucatan, Mexico, is thought to be the site of the meteor impact that instigated the K-T event, which wiped out the dinosaurs in a mass extinction that affected much of life on Earth.

On Mars, meteor storms 100 million years ago may have literally shaken the Red Planet to the core and destroyed its magnetic field.

Even the crater-covered moon might be a chip off old Earth’s block, an enormous shard shot into orbit following a giant impact event.

Given such drastic, far-reaching outcomes of space rock impacts, Kring said that studying crater formation holds the answer to understand not only how life on Earth began but also how it could be wiped away again in a future, perhaps inevitable, K-T event.

“There will be another Chicxulub-size impact event,” he said.

Since tectonic plate movements has erased much of Earth’s crater record, the answers to the lingering questions about crater formation and timelines lie in the “exquisitely preserved” craters on the moon.

But until NASA returns to the lunar landscape, researchers must rely on shockwave simulators, mathematical models and the well-worn geological formations on Earth to estimate how and when another impact event might occur.

“Where we’re really going to get the answers – the gold standards of answers – is when we go back to the moon,” Kring said.

Posted by: Soderman/NLSI Staff
Source: http://news.discovery.com/earth/how-are-craters-formed.html

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