Shattered Earth or Fractured Past

09/23/2008

A new research study looks at geological fractures in the walls of the Barringer Meteorite Crater.

When you stands on the aerial platform that extends over the north rim of the crater, two things are obvious:  (1) The impacting asteroid made a lot of rock disappear to create that vast empty hole in front of you and (2), when turning around to look at the crater wall, the same impact severely shattered the surrounding earth. 

Those disjointed crater walls were the focus of a recent study by Senthil Kumar and David Kring, who hoped the walls would provide clues for how the crater was excavated and, simultaneously, provide a measure of how similar fractures on Mars may provide conduits for water and help feed potentially interesting habitable zones. 

Analyses of the fractures in the crater walls revealed three different types: nearly vertical fractures that radiate from the center of the crater, nearly-horizontal fractures that also radiate from the center of the crater, and fractures that are concentric around the crater, like layers around the core of an onion.  All three types of fractures made it easier for rock to be excavated from the crater.  The concentric fractures along the crater walls also allowed additional debris to slide down and partially fill the crater floor at the end of the crater-forming process. 

Some of the fractures, however, predate the crater, which was carved from a huge segment of the Earth’s crust called the Colorado Plateau.  That plateau underlies the entire Four Corners region of Arizona, New Mexico, Colorado, and Utah.  Over millions of years, the plateau was tectonically uplifted.  That uplift is partly responsible for the Grand Canyon, because it forced the Colorado River to cut deeper into the Earth’s crust.  That uplift also stressed the Colorado Plateau, so the bedrock is cross-cut with a series of fractures.  In the red rock surrounding the crater and extending towards the Grand Canyon, those fractures are often spaced 1 to 2 meters apart. 

The pre-existing fractures were activated by the impact event.  The vertical and concentric fractures in the crater walls often parallel pre-existing fractures.  Preferential rock movement along those pre-existing fractures was particularly important in four areas around the crater, where differential uplift was very strong.  This produced a crater with a nearly square outline, rather than a circular one, confirming an idea first proposed by Gene Shoemaker and David Roddy.  In addition to taking advantage of existing fractures, the impact further shattered rock.  Geophysical measurements and models of the crater suggest the impact fractured the rocky crust to a depth of nearly two thousand feet beneath the crater floor. 

Interestingly, these fractures were (and continue to be) important conduits for water.  When it rains or snow melts, water flows through the fractures.  On Mars, similar fractures may provide sites for organic chemistry.  For astrobiologists, the fractured rock around Martian craters and the water that may be flowing through them are important targets for future exploration.  If life ever existed on Mars, water flowing around impact craters may have provided an important habitat.